GPS World

Category: Survey

  • Position: 20 Kilometers, Heavy Construction

    World’s Longest Immersed Tunnel, 40 Meters Underwater

    By Anna Jensen, Dirk Hermsmeyer, Bastian Huck, Jürgen Rüffer, and Peter Skjellerup

    The Fehmarnbelt Positioning System between Denmark and Germany includes a geodetic basis, four permanent GNSS stations, and a real-time kinematic (RTK) service for construction of a road and rail causeway between the islands of Fehmarn, Germany, and Lolland, Denmark, across the Fehmarnbelt, a 20-kilometer stretch of open water in the Baltic Sea. This homogeneous, consistent, coherent, highly accurate GNSS-based positioning system exemplifies comparable systems and services that can be established for any major construction site or infrastructure project. Now in use for environmental, geotechnical, and geophysical investigations, it provides cost-efficient operations and facilitates the precise navigation of large, costly offshore equipment.

     

    A fixed road-and-rail link across the Fehmarnbelt body of water in the Baltic Sea will by 2020 connect the German island of Fehmarn and the Danish island of Lolland. It will provide a critical time- and cost-efficient trade and traffic link between north-central Europe and Scandinavia.

    Geophysical and geotechnical pre-investigations have been completed as well as an environmental assessment of the fixed link. Initially proposed as either a bridge or a tunnel (Figure 1), an immersed tunnel is now the preferred solution. It will be placed in a trench excavated on the sea floor, and covered with a layer of stones. It will be the longest immersed tunnel in the world at 17.6 kilometers, excluding peninsulas on both sides to be constructed for easier entrance to the tunnel. The strait is 20 kilometers wide at the site. The immersed depth is up to 40 meters.

    During planning and construction of the fixed link, it is very important to be able to perform reliable positioning with high accuracy. This requires a well defined geodetic basis — a 3D reference system and a reference frame for GNSS positioning, a height system and a geoid model for working with heights, and a map projection for plane maps and drawings. The ability to determine positions with high accuracy in real time within the project area is also very important. Therefore a carrier phase-based GNSS positioning service, a real-time kinematic (RTK) service, has been established.

    Altogether, we refer to the geodetic basis and the RTK service as the Fehmarnbelt Positioning System (FBPS), and the geodetic basis as the Fehmarnbelt Coordinate System (FCS). In this article we describe the geodetic basis and the RTK service, including four new permanent GNSS stations established for the purpose.

    Geodetic Reference Frame

    The reference system for the FCS is the International Terrestrial Reference System, realized by the ITRF2005, the newest and to date most accurate realization of the ITRS.

    Four permanent GNSS stations were established around Fehmarnbelt during the autumn and winter of 2009/2010: two on Fehmarn and two on Lolland (Figure 2).

    After establishment of the GNSS stations, seven days of GNSS data were collected in February 2010. Coordinates for the stations were determined by the National Survey and Cadastre-Denmark, using the Bernese GPS software. Data from six GNSS stations of the network of the International GNSS Service (IGS) was included in the data processing, and these stations with coordinates in the ITRF2005 were used as reference stations. Hereby, the ITRF2005 was introduced in the Fehmarnbelt area, and a reference frame for positioning in three dimensions has been established.

    Height System and Map Projection

    The height difference between Germany and Denmark is known from a 1987 hydrostatic levelling between Puttgarden and Rødbyhavn. For the Fehmarnbelt Fixed Link, precise levelling has been carried out between the connecting points of the hydrostatic levelling and stable point groups further inland. Levelling points with a large displacement since 1987 were eliminated, and the hydrostatic levelling was then used for transfer of the height difference between Germany and Denmark.

    The next step was determination of present mean sea level (MSL) in the Fehmarnbelt and establishment of a project-specific height system with the zero-level as close as possible to the actual MSL of Fehmarnbelt. In this area of the Baltic Sea, a slow rise of MSL relative to the neighboring land is taking place, and therefore water-level data from Heiligenhafen on the German mainland, and from Puttgarden and Rødbyhavn, was analyzed in cooperation with the Danish National Survey and Cadastre and the Danish National Space Institute.

    Analyses of the last 20 years of water-level data show an increase in the water level of approximately 2 millimeters per year at Rødbyhavn. Data from Heiligenhafen was also analyzed; as Heiligenhafen is not directly adjacent to the site, the time series was not used directly for establishing the MSL datum but instead used as an independent control.

    Water-level data was used for estimation of the present MSL in Fehmarnbelt, and the zero level for the FCS Vertical Reference 2010 (FCSVR10) coincides with MSL at Rødbyhavn in 2010. The zero level of FCSVR10 thus deviates from both the German and the Danish height systems.

    The Danish National Survey and Cadastre conducted precise levelling to determine FCSVR10 heights to the four new permanent GNSS stations, and determined FCSVR10 heights to a number of existing height benchmarks on Fehmarn and Lolland. Local land uplift on Fehmarn and Lolland causes differences between the FCSVR10, the national German DHHN92 height system, and the national Danish Vertical Reference 1990 height system. Differences between the height systems are not constant values but vary within the area, so it is very important to use the geoid models when converting heights for high-accuracy applications.

    To determine heights relative to MSL with GNSS it is necessary to utilize a geoid model. The Danish National Space Institute performed new gravity readings to supplement the existing gravity database. Then all existing gravity data from the area was used for development of a local geoid model for the Fehmarnbelt. The geoid model is fitted to the height system FCSVR10 and to the ITRF2005 by the four new permanent GNSS stations, and the model can be used for conversion between MSL heights and ellipsoidal heights.

    The last item of the geodetic basis is the definition of a map projection, using a transverse Mercator projection. The projection is fitted to the area to obtain a scale factor as small as possible within the construction area. Also, a false Easting value was chosen to provide FCS Easting values within the construction area which are different from Easting values of the ITM, UTM, or Gauss-Krüger projections used in Germany and Denmark. Table 1 gives the defining parameters for the map projection.

     

    Permanent GNSS Stations

    The four permanent GNSS stations are established as geodetic-grade stations, as shown in the photo. Individually calibrated GNSS choke ring antennae are mounted on 3-meter tall concrete pillars, with foundations 3 meters into the ground at stations 1, 2, and 4, with predominantly silty glacial till of stiff consistency at about 0.70 (stations 1 and 2) and 1.70 meters (station 4) below soil surface. At station 3, foundations for the antenna monument are built 9 meters into the ground. Soil conditions are sandy at this location to about 7 meters below soil surface, where stiff glacial till is met. In geotechnical investigations and analyses carried out before establishment of the GNSS stations, the glacial till at the station locations was rated as a good to very good foundation ground, with little tendency to settlement.

    The concrete antenna monuments are surrounded with about 0.30 meters of styrofoam for thermal insulation. The monument head is bevelled with an angle of 30° from vertical, reflecting GNSS satellite signals striking the monument head underneath the antenna away from it, to further minimize signal multipath effects.

    The GNSS reference station receivers are capable of processing GPS and GLONASS L1 and L2, GPS L5, and Galileo E1, E5a, E5b, and Alt-BOC frequency band signals. Galileo signals can be processed when Galileo satellites are available; a firmware update on the receivers will be required. In view of the long-term demand for the FBPS (until 2020 or longer), its compatibility with Galileo signals in particular makes the system future-proof.

    GNSS reference station receivers, access points to power grids, and uninterruptible power supply are mounted in cabinets adjacent to the antenna pillars. Additional equipment in each cabinet comprises an industrial PC, Internet router, GSM/UMTS router, satellite communication equipment, transmitting and receiving radio modems, and a heat exchanger to cool the in-cabin room if required.

    At each station, a radio mast of about 10 meters height carries a satellite dish for wireless Internet access, and a Yagi antenna to broadcast GNSS correction data into the proposed construction area in the Fehmarnbelt. Radio masts are located directly north of the GNSS antennae.

    RTK Service

    To ensure accurate GNSS positioning, an RTK GNSS service has been established, based on GNSS data from the four new permanent GNSS stations (primary stations) as well as four GNSS stations located further away in Germany and Denmark (secondary stations), which existed previous to our work. Figure 3 shows the locations of the eight stations used for the RTK service. The stations relay GNSS data to the control center, which derives and transmits RTK correction data to surveyors in the project area with RTK rovers.

    The RTK service has been developed with focus on robustness, with two control centers at different addresses in Germany. Three different communication carriers provide data communication between the GNSS stations and the control centers, and RTK correction data is distributed to users in two different ways, via ultra-high frequency (UHF) radio and mobile Internet. Figure 4 shows the communication lines of the RTK service.

    FBPS RTK users who wish to receive RTK corrections via UHF radio require a UHF radio modem and antenna, in addition to an RTK rover. The four primary GNSS stations broadcast RTK correction data on four separate radio frequencies. By switching their radio modem to one of the frequencies, users receive the correction signal from the control center via the respective station. RTK corrections via UHF radio can be used where radio signals from one of the four primary GNSS stations can be received.

    From the users’ point of view an advantage of using UHF radio over using a mobile Internet connection is that the UHF connection is free-of-charge and can be collected from four different sources.

    Users who wish to receive RTK corrections via mobile Internet must connect via General Packet Radio Service (GPRS) and require a GPRS modem, antenna, and a subscriber identity module (SIM-card) in addition to their RTK rover. GPRS connections will be charged according to tariffs of the respective mobile phone network provider.

    Figure 5 shows areas of signal coverage. Areas 1 and 2 are covered by UHF radio and mobile Internet. Area 3 is covered by mobile Internet.

    The FBPS RTK service generates and broadcasts RTK corrections in two different modes: master-auxiliary corrections (MAX) mode, and virtual reference station (VRS) mode. MAX and VRS are two different calculation methods to generate RTK corrections in a standard format defined by the Radio Technical Commission for Maritime Services (the RTCM format). The version used for the FBPS RTK service is the RTCM version 3.1.

    With MAX corrections, the RTK rover does not send its position to the reference network software. The GNSMART reference network software calculates and sends MAX corrections to the rover. These contain the measurements from a master station and correction data from the auxiliary reference stations. The rover individualizes the corrections for its position, which means it determines the best suitable RTK corrections. RTK data in MAX mode can be received by users of RTK rovers via both possible types of connection, UHF radio and GPRS.

    With the VRS concept, the user’s RTK rover transmits its approximate position to the control centre, which returns to the rover observations or corrections of an individual VRS near the user’s position. Data is transmitted back and forth between the RTK rover and the control center. Therefore a two-way communication link must be established with VRS. Because the UHF radio connection is one-way, GNSS correction data in VRS mode can be received via digital cellular phone (GPRS) only. For data transmission via GPRS, the FBPS RTK service uses the networked transport of RTCM via Internet protocol (NTRIP).

    Multiple RTK rovers (that is, multiple users) can receive RTK corrections from the FBPS simultaneously with any of the connections described above, while every user may select his or her favourite connection type. The RTK service can be used with any commercially available geodetic GNSS receiver that is capable of processing RTK data.

    System Test and Results

    The RTK service was established during the spring of 2010 and was run in test mode May 12–July 31 to test system accuracy, signal coverage area, and signal availability.

    Accuracy. An error budget of the RTK service is provided including all known error sources and latencies in the system, and a description of how these errors are handled. The accuracy obtainable by end users is better than 1.0 centimeters in the horizontal and better than 1.8 centimeters in the vertical. Values are provided as one sigma, and are valid during normal ionospheric activity. Applying an RTK rover and RTK corrections received from the FBPS RTK service, users inside the coverage area can determine the coordinates of a marked survey point repeatedly with these accuracies.

    System inspection is carried out monthly. Part of monthly inspection is the visit of marked control points with an RTK rover. ISO 17123-8:2007 (ANSI, 2007) standard procedures are applied to determine control point coordinates.

    Coverage Area. The RTK service coverage area shown in Figure 5 is defined as the geographic area where the described accuracy can be obtained for end users at any time. Test measurements of UHF radio signal strengths from the four primary GNSS stations have been carried out onshore Lolland and Fehmarn, as well as offshore across the Fehmarnbelt (see photo). Modelled UHF radio signal broadcasting areas are closely verified during these tests.

    Availability. The positioning system and the RTK service are designed using necessary technology, redundancy, and back-up to ensure that the system is operational and available in the entire coverage area for more than 99 percent of the time. Availability is defined as the time where all elements of the positioning system are available for end users and where the described accuracy can be obtained for all users within the coverage area. Availability is evaluated in percent of time per day: the system must be available for at least 23 hours and 45 minutes per day. During the first year of operation it is accepted that RTK correction data from the system are available to end users for 97 percent of the time or more per day.

    A control segment has been established to constantly monitor RTK service accuracy and the availability of the system. The control segment is installed in such a way that all relevant output and data streams from the GNSS stations are available through the system’s website.

    Evaluation of availability is carried out automatically by the control segment, and an overall evaluation of availability is performed every month. Results from evaluation of availability during the test operation are listed in Table 2. During test operation, the required availability of 97 percent per day during the first year of operation was reached on all days. Availability only fell below 99 percent, as is the required availability during following years, for 5 out of 81 days (5.6 percent) of the test period.

    Conclusions and Outlook

    System tests results regarding accuracy, coverage area, and availability show that the positioning system and the RTK service fulfil all specifiecation requirements.The first RTK user was registered in July 2010, and the complete system is now being used for environmental, geotechnical, and geophysical investigations.

    User benefits of the FBPS include:

    • ensured consistent and uniform geodetic reference throughout the planning, construction and operation phases of the Fehmarnbelt Fixed Link, available to all stakeholders at any time;
    • seamless, real-time data flow from the point measurement at the construction site into computer-aided design (CAD) or geographic information systems (GIS);
    • simplified geodata transfer across interfaces between project stakeholders and project phases;
    • cost efficiency, reducing costs in both surveying and data management, particularly in precise operation of large, expensive offshore equipment, including during critical procedures in the construction phase.

    The positioning system for the Fehmarnbelt Fixed Link is an example of a homogeneous, consistent, coherent, and highly accurate GNSS-based positioning system. Comparable systems and services can be established and used for any major construction site or infrastructure project.

    Acknowledgments

    This work is funded by Femern A/S. The authors acknowledge contributions from the National Survey and Cadastre, Denmark, Danish National Space Institute, Land Survey Office of Schleswig-Holstein in Germany, German Federal Agency for Cartography and Geodesy, Richter Deformationsmesstechnik GmbH, Günther Steimann, and Ohms Nachtigall Engineering GbR. Also Mr. and Ms. Thomsen, Stadt Fehmarn, Mr. Henriksen, and Mr. Boserup for permitting establishment of FBPS GNSS stations on their property.

    Establishment, operation and maintenance of the GNSS stations and RTK service was entrusted by Femern A/S to AXIO-NET GmbH, with ALLSAT as subcontractor for implementation of the four GNSS stations (both companies in Hannover, Germany). Ramboll Arup JV was entrusted by Femern A/S with project coordination and geodetic consultancy, using AJ Geomatics as subcontractor. More information about the fixed link is available, and more on the RTK service.

    Manufacturers

    The RTK service is based on GNSMART software (GEO++ GmbH). The permanent GNSS stations are equipped with Leica Geosystems AR25 antennas and GRX1200+ receivers.

    Anna Jensen is owner and CEO of AJ Geomatics in Denmark. She holds a Ph.D. in geodesy and has worked with research and development within GNSS and geodesy for more than 15 years.

    Dirk Hermsmeyer holds a Ph.D. from the University of Hannover, and is a project management professional. He previously worked at ALLSAT and is now with the Chamber of Commerce in Lübeck, Germany.

    Bastian Huck is head of operations and quality management with AXIO-NET. He is a university-level geodesist and certificated project management practitioner with 10 years of experience in RTK projects.

    Jürgen Rüffer is co-owner and CEO of ALLSAT and AXIO-NET. He is a university-level geodesist, a publicly certified expert for GNSS positioning at the chamber of engineers in Germany, working with GPS and GNSS since 1977.

    Peter Skjellerup is chief advisor on geotechnology with Ramboll Denmark. He has worked with ground engineering for many years, and holds a M.Sc. in physics-geophysics from the University of Copenhagen.

    Note from author Anna Jensen (2/27/13):

    “Since publication of the article, the opening year for the Fehmarnbelt tunnel has been changed to 2021.”

  • GPS Surveying/Mapping Current Events

    Trimble Navigation has made a fair number of strategic acquisitions in the past ten years. Spectra-Precision and Tripod Data Systems were acquired early last decade. Applanix, Seco Manufacturing are some you’ve heard of, but there’s been a fair number of companies that you’ve never heard of, typically ones that allow Trimble to entrench themselves deeper into their core vertical markets (engineering, construction, GIS, MRM, etc.). Trimble has always strived at providing a complete solution (hardware, software, sensors, etc.) and it’s one of the reasons they’ve been so successful.

    Within the past 30 days, they announced two acquisitions that are higher profile and you may have noticed.

    The first acquisition was Measurement Devices, a UK-based company specializing in laser rangefinders. The acquisition is not surprising as the ground-based (terrestrial) laser scanning business is growing. Actually, I should clarify, I’m not sure it was an acquisition or what kind of acquisition it was since there’s been no press announcement on it that I’ve seen, but it doesn’t matter. Obviously, something happened because this week Trimble announced the Trimble LaserAce 1000 handheld laser rangefinder, which is clearly based on MDL technology.

    Trimble LaserAce 1000

    The second acquisition was a bit more surprising to me and some of you, but probably a smart move on Trimble’s part. Trimble announced they acquired certain assets of OmniSTAR’s land applications business. OmniSTAR also has a significant offshore client base (oil & gas) so apparently that wasn’t included in the sale. The acquisition does include OmniSTAR’s land business for North/South America, Europe/North Africa/Middle East/India, Asia Pacific, and South Africa.

    The OmniSTAR acquisition is pretty smart, at least for the medium-term. Trimble has been quietly (until now) growing their GPS correction service business. Their VRS Now service, a subscription-based RTK Network, provides both RTK and decimeter corrections in many parts of the world already. OmniSTAR will only enhance Trimble’s subscription offering. In the short-term, they will have a strong portfolio in the real-time corrections business with Deere/Navcom being the only other major player offering satellite-based world-wide subscription services. However, the Deere/Navcom system (StarFire) is focus on agriculture and doesn’t have much support from receiver manufacturers/integrators outside of the agriculture market like OmniSTAR does. With Trimble’s acquisition of OmniSTAR’s land business, Deere/Navcom might look at the non-ag markets differently. It will be interesting to watch.

    The longer-term competition for real-time decimeter correction are the public (free) SBAS such as WAAS (North America), EGNOS(Western Europe/No Africa), MSAS (Japan), and GAGAN (India). They are all slated to implement the new civil L5 signal. Once that happens, albeit 5-10 years from now, decimeter accuracy will be at your fingertips, free of charge, if you’re using an L1/L5 capable receiver and in an SBAS coverage area.

    Speaking of Deere/Navcom, just this week they showed signs of non-agriculture life by taking a step to enter markets outside of agriculture with the introduction of their pole-mount SF-3040 GNSS receiver. Although somewhat of a “me too” product, it does include the capability of accessing their StarFire network, which makes it unique.

    Deere/Navcom’s SF-3040 Pole-Mount GNSS Receiver

     

    Seeing how OmniSTAR seems to be a popular subject this week, newcomer Geneq added another OmniSTAR receiver to their product like this week. Claiming to be the smallest GPS L1/L2 OmniSTAR receiver in the world, they introduced the SXBlue III-L GPS that’s able to use OmniSTAR’s HP and XP corrections services. If you recall, a few months ago, Mike Whitehead and I collected 24 hrs. of OmniSTAR HP-corrected data as part of some experimenting we did for the January webinar. I ran the data through a rigorous statistical software program that randomly tested the accuracy of the data. The horizontal accuracy (at the NSSDA 95% confidence level) was 9cm.

    Geneq SXBlue III-L GPS

     

    LightSquared Saga

    I feel I need to keep you up-to-date on what’s going on with LightSquared. As crazy as it sounds, I could see the FCC pushing this through unless the GPS community makes a lot of noise. Bear in mind, I don’t think it’s an ‘all or nothing” deal. LightSquared is not going to rollover. For sure, the testing will show it jams GPS to some extent. I’m confident of that. At the end of the day, I think they will push for some sort of compromise, a compromise that would likely mean that GPS functionality would be degraded, possibly signal strength degradation. The high-precision users (sub-meter and below) will take the hit because those receivers try to squeeze as much from GPS as possible, so a few dB of signal strength is very important.

    On April 21, we are hosting a free webinar entitled “LightSquared and GPS: Our Story So Far”. I’ll be on the webinar dicussion panel as well as some people who are a lot more intelligent than me. My role is to bring a high-precision user community perspective to the discussion. If you want to gear up on the LightSquared issue, the webinar is a good opportunity.

    To help visualize the issue, following is a graphic I lifted from the Federal Communications Commission (FCC) website. I’ve inserted the GPS center frequencies (L1, L2, L5) as well as frequencies that LightSquared wants to use. If radios worked with nice, clean lines, we’d be in good shape. LightSquared would stay below 1559 MHz and GPS would stay above 1559 MHz. But it doesn’t work that way. High-precision GPS receivers use a wide radio front-end for improved performance. It can be as much as 25 MHz wide. 1575 MHz (GPS L1 center frequency) minus 25 MHz = 1550 MHz. LightSquared base stations are broadcasting at 1,500 watts. A certain amount of noise is going to invade the 1559-1610 MHz range that GPS uses. Furthermore, mobile devices built to use LightSquared’s signal may also invade the 1559-1610 MHz range. The water starts to become muddy very quickly. Bear that in mind when viewing the chart below.

     

    Source: FCC

     

    Click here
    to view the latest article from GPS World on LigthtSquared and GPS.

    Lastly, it’s not too late to take action. Following is a response I received from Oregon U.S. Senator Jeff Merkley after contacting his office about my concerns.

    I haven’t heard anything more since I received this letter on March 25, 2011, but I trust Mr. Merkley’s staff is querying the FCC about this. The more attention we draw to the issue, the better.

    Thanks, and see you next time.

    Follow me on Twitter at http://twitter.com/GPSGIS_Eric

     

  • On the Edge: Making Peace: Surveyor’s GPS Device Serves as Mediator in Kenyan Land Dispute

    By Noah Kertich

    Early morning on February 2, 2011, I went to work in my job as a road surveyor in the Bungoma District of Kenya. Here, land disputes are common, though the government is trying to reduce the conflicts by issuing land titles and certificates.

    I carried with me a small handheld GPS, the Magellan Explorist 100. While I was using it, a stout man in early fifties approached me and introduced himself as a surveyor, too. He was very interested in the way I was walking around with the “gadget,” trying to locate a control point. He asked me how the gadget worked. I explained it to him, showing him how its easy to use in general boundary surveys. He was satisfied, and we exchanged contacts and parted.

    A month later, he called me for help. When I asked him what was wrong, he told me there were a group of land owners, or members, who were about to kill each other in a dispute over a 128-acre farm they had bought. These members had each contributed money to buy a single parcel with the intention of subdividing it fairly. They were engaged in a disagreement about the boundaries and the subdivision of the farm. The gentleman asked me if I could take a survey of the farm sometime in the next few days. Concerned about the conflict, I answered, “Yes, in hours not days.” Still, it wasn’t until two days later that he could assemble the members of the disputed farm and called me to mark the boundaries for them.

    I arrived at the farm with my Magellan GPS and my laptop. To my dismay, I found that some of the members were armed with crude weapons, ready to fight each other. I asked them to be peaceful and wait for just a few hours while I surveyed the site.
    I started picking the boundary corners of the farm all around the permiter. I was through with that task in less than 35 minutes. This parcel of land was to be divided into 18 pieces. I uploaded the data manually to my laptop, then I did the subdivision using AutoCAD Land Development 2000.

    After two hours and fifteen minutes, I called the members and told them to ready themselves to be shown the boundaries of their property.

    I walked around the property with them, guided by my handheld GPS, to each boundary beacon. After one and a half hours, the warring members were shaking hands and laughing, saying “So, it was that easy!”

    The dispute had ended, and was solved peacefully. My small Magellan Explorist 100 acted as a peace mediator.

     

    Noah Kertich is a surveyor with H Young Construction EA Ltd., which is under contract with the World Bank in conjunction with the government of Kenya. Kertich graduated from the Kenya Institute of Surveying and Mapping in 2004 and received a diploma in photogrammetry and GIS from Icaros Geosystems, Israel, in 2008.

  • Webinar Brief – A Closer Look at L5: The Future of High-Precision GNSS

    Yesterday I conducted a webinar titled “A Closer Look at L5: The Future of High-Precision GNSS.” Preparing for it was quite interesting, so I thought I’d share some of the slides I produced (and had produced) for the webinar. I think you’ll find them interesting.

    The webinar was focused on discussing the value of the new L5 civilan frequency for GPS/GNSS receivers. An interesting challenge in preparing for the webinar was my attempt at estimating what a satellite constellation of satellites (GPS and others) broadcasting at least L1 and L5 would look like four or five years from now. The point of it was to illustrate that a useful constellation of satellites broadcasting L1 and L5 (as well as L2C) is potentially only four to five years away.

    In that timeframe, there are potentially 30 satellites that would be healthy and broadcasting navigation signals on the L1 and L5 frequencies that we can use. How is that possible?

    Both GPS and Europe’s Galileo support the new L5 civil frequency (as well as L1). The U.S. has already launched one of the new GPS model IIF satellites. The IIF is currently healthy and broadcasting three civil frequencies; L1 C/A, L2C and L5. There are 11 more of the IIF satellites being built. It’s estimated that all 11 will have been launched into their orbits by ~2015. On the other hand, the first 18 Galileo satellites have been contracted to be built, and it’s estimated that the 18 will be launched into their orbits by ~2015. The Galileo satellites are designed to support L1 and L5 (as well as others). That’s a total of 30 satellites broadcasting L1 and L5.

    In an ideal world and in the best interest of the civilian user community, the Americans and Europeans would coordinate orbits planes/slots of the 30 satellites so they would be in an optimal configuration (steady # of visible satellites, reasonable PDOP) for the user community. But, I seriously doubt that’s going to happen.

    So, the next best thing is to attempt to estimate what an “uncoordinated” constellation of 30 GPS/Galileo satellites would look like in 2015 (assuming the launch schedules hold). Fortunately, our friends at the Galileo Supervisory Authority (GSA) have already mapped out the orbit plane/slot data for the 18 satellites. Without that data, none of these projections would have been possible.

    GPS was a little tougher to estimate. The U.S. Air Force doesn’t have (or at least they don’t share) a long-range plan for where the next 11 IIF satellites are going to be inserted in the GPS constellation. They look out one satellite at a time. That’s understandable because the health of the GPS constellation changes over time. However, the U.S. Air Force does present a “watch list” of the weaker satellites in the constellation so we have some idea of where the new ones are going to be placed.

    Once we compiled the information from the Galileo folks and our projections on where the next 11 IIF GPS satellites will be inserted, we were able to come up with some interesting plots I’d like to share with you.

    All of the following satellite visibility plots are based on my location in Portland, Oregon, USA, and with a 15º elevation mask. Using a 15º elevation mask is pretty conservative so the plots are pretty conservative if you’re working in an open-sky environment like in agriculture.

    The first plot is of the 12 GPS IIF satellites only. You can see there’s an average of about three IIF satellites in view between 6 a.m. and 8 p.m. Thanks to Analytical Graphics, Inc. for help generate the following plots.

     

     

    The next plot is of the 18 Galileo satellites. You can see there’s an average of 4-5 Galileo satellites in view between 6 a.m. and 8 p.m.

     

     

    The next plot is of both the 12 GPS IIF satellites and the 18 Galileo satellites. You can see there’s an average of 8 GPS IIF and Galileo satellites in view between 6 a.m. and 8 p.m.

     

     

    Finally, the last plot is of the 12 GPS IIF satellites, 18 Galileo satellites, and the 19 remaining legacy GPS satellites (broadcasting L1 and L2). You can see there’s an average of 12 GPS IIF, Galileo, and legacy GPS satellites in view between 6 a.m. and 8 p.m.

     

    For a different perspective, here are 3D orbit plots of the 18 Galileo satellites and the 12 GPS IIF satellites.

    3D orbit plot of 18 Galileo satellites

     

    3D orbit plot of 12 GPS IIF satellites

    There are several more plots similar to these in my webinar for different locations around the world including London, Rio de Janeiro, New Dehli, Perth, and Bangkok. In the webinar presentation, I also provide more details about the benefits of L5. You can view a recording of the webinar by registering here. After registering, you’ll receive an e-mail with instructions on how to view the webinar.

    Thanks, and see you next time.

    Follow me on Twitter at http://twitter.com/GPSGIS_Eric

  • Trimble Enters into Definitive Agreement to Acquire OmniSTAR Assets for Land Applications

    Trimble today announced that it has entered into a definitive agreement to acquire certain assets related to the OmniSTAR GNSS signal corrections business from Fugro N.V. The acquisition is expected to significantly expand Trimble’s worldwide ability to provide corrections services for land based agriculture, construction, mapping, and geographic information system (GIS) and survey applications.

    Trimble and Fugro also entered into a multi-year service agreement which includes Fugro’s ongoing operation of its correction network and satellite service broadcast systems that power the OmniSTAR service. Fugro’s offshore marine business is unaffected. Closing of the transaction, anticipated in the first quarter, is subject to certain closing conditions. Financial terms were not disclosed.

    OmniSTAR provides space-based GNSS correction services that can improve the accuracy of a GNSS receiver for precise positioning applications. These are the four levels of OmniSTAR service:

    • VBS offers sub-meter positioning.
    • XP service delivers better than 20-centimeter accuracy.
    • HP service delivers greater than 10-centimeter accuracy.
    • The new OmniSTAR G2 service combines GPS-plus-GLONASS-based corrections to provide decimeter level positioning.

    Trimble pioneered RTK technology in the early 1990s, which enabled high-accuracy corrections for field applications. RTK is now recognized as the industry leading technology for centimeter-level positioning. To further improve accuracy, Trimble subsequently introduced VRS technology in 2000 and shortly after that Trimble VRS Now Service.

    “With the addition of the OmniSTAR services and our strong relationship with Fugro, we will offer a full range of high-precision positioning capabilities which now includes satellite-delivered corrections,” said Patricia Boothe, general manager of Trimble’s newly-formed Positioning Services Division. “Today, our agriculture customers use OmniSTAR services to perform planting, harvesting, variable rate application and many other operations. Our expanded portfolio will provide not only farmers, but also surveying, construction, and GIS professionals with more options to satisfy their particular accuracy, delivery, and financial needs.”

    “Trimble and OmniSTAR have enjoyed a long standing relationship,” said John Waits, president of OmniSTAR. “The transfer of land-based GNSS signal corrections assets marks the next phase of our efforts to bring a broader range of positioning services to our combined customer base, on land and offshore. The OmniSTAR and Fugro teams remain committed to providing industry leading corrections services for customers who own a variety of GNSS receivers.”

    The OmniSTAR business will be reported as part of Trimble’s Engineering and Construction segment.

  • I’m Buying A New RTK Receiver. What Should I get?

    In light of this weeks webinar, A Closer Look at L5: The Future of High-Precision GNSS, and spurred by an email from a reader about how to sift through all the GPS/GNSS receiver choices, following are my thoughts if you’re looking to purchase an RTK receiver today.

    First of all, an email from a reader succinctly sums up the challenge:

    I currently utilize static GPS / GLONASS receivers in my day to day operations and I am looking at buying a couple more receivers (an RTK setup). To be honest, I am totally confused as to what technology I should buy.
    Specifically, I don’t know whether or not it is worth buying receivers that have L2C and L5 capabilities. It seems that vendors are not very well educated on what these options can do for you, and how many satellites are up and running that provide these signals. It is my understanding that L2C is simply a civilian code that is carried on the L2 frequency, and that it provides an almanac and atmospheric correction information. I don’t even know if receiving L2C will help me as a surveyor, or if it is more designed for autonomous use in navigation. It sounds like L5 will be of great advantage once the constellation has enough SVs that broadcast it.  It also seems like Galileo will be extremely helpful for surveyors, but who knows when that will be available. Basically, I don’t want to spend thousands of extra dollars for “bells and whistles” that are not yet operational from a practical standpoint, and that won’t be in the near future.

     

    He’s right. There are a lot of moving parts these days in the world of GPS/GNSS. Not only are GPS/GNSS receivers steadily improving (better, smaller, faster, cheaper), but the GNSS themselves (GPS, GLONASS, SBAS) are changing too. Making a decision of which “bells and whistles” to pay for and which ones to pass up is not so easy.

    Let’s break it down and see if we can clear things up.

    It used to be that when looking to purchase a dual frequency GPS receiver, the choice was simple because RTK receivers came in one flavor, L1/L2.

    Do I want RTK (real-time centimeter positioning) or am I satisfied with post-processing the GPS data?

    Either way you went, it was an straight-forward decision.

    Today, that is not the case. If you choose RTK, there are many options available:

    -GPS L1 or GPS L1/L2?

    -add GLONASS?

    -add L2C?

    -add L5?

    -add Galileo?

    The pricing of these options can be substantial. The reader’s letter goes on…

     

    I have a vendor that is pushing an L2C capable receiver on me for more money than a standard dual frequency dual constellation receiver.  The other option is to spend about $13K more and get the L2C, L5, and Galileo ready receiver.

    If you look at what the manufacturer’s are offering for GPS/GNSS RTK receivers, it seems there are generally four choices:

    1. GPS L1

    2. GPS L1/L2

    3. GPS L1/L2 + GLONASS

    4. GPS L1/L2 + GLONASS + L2C + L5 + Galileo

     

    GPS L1

    longer initialization (issue when working around trees)

    short baseline length

    Really should have the same base/rover receiver (SBAS), not really suited for RTK Network usage.

     

    GPS L1/L2

    Legacy, proven technology.

    Upside…less expensive, entry level dual frequency RTK

    Downside…GPS “brownouts”, susecptible to semi-codeless sunset

     

    GPS L1/L2 + GLONASS

    Eliminates the GPS “brownout” problem.

    Increased cost, although some manufacturers include it.

    Doesn’t support future signals

    Suscpetible to semi-codeless sunset.

     

    GPS L1/L2 + GLONASS + L2C + L5 + Galileo

    Eliminates the GPS “brownout” problem.

    Ready for future signals

    downside…future singals aren’t available yet.

    Increased cost

     

  • LightSquared Saga, and Recent Solar Activity

    This week I’m following up on my article from a couple of weeks ago about the potential effects of LightSquared’s plans. As a user of high-precision GPS receivers (particularly GPS L1 sub-meter, but also dual-frequency), you should be particularly concerned about this issue. I’ll tell you why. Also, I have a note on recent the solar activity.

    LightSquared

    The reasons you should be concerned about LightSquared’s plans are two-fold:

    1. Consumer GPS receivers and professional-grade GPS receivers designed for higher performance (mapping, surveying, etc.) aren’t necessarily designed the same way. High-performance GPS receivers use a wider bandwidth radio design.

    For example, the GPS L1 frequency is 1575.42 MHz. Many high-performance GPS receivers use a wide bandwidth radio that scans +/- 20 MHz from 1575.42 MHz. That equates to a range of 1555 MHz to 1595 MHz. LightSquared’s frequency spectrum is 1525 MHz to 1559 MHz. Clearly, there’s overlap, which is another word for interference. On top of that, LightSquared plans on a broadcast strength of 1,500 watts from a tower located down the street. The GPS broadcast signal strength is about 30 watts from a satellite located some 19,000 kilometers away in outer space. Who’s going to win that battle?

    I’m not an aerospace engineer or an RF (radiofrequency) engineer, but I don’t think it takes one to see the potential impact of LightSquared’s service on high-performance GPS receivers. At the very least, it warrants an in-depth technical study.

     

    2. Neither the policymakers nor LightSquared know about or understand the user community of high-performance GPS receivers comprised of hundreds of thousands of high-end GPS receivers. They think the GPS user community is comprised of auto navigation and mobile-phone users. They don’t understand that we are the infrastructure people. We use GPS in a way that they don’t understand, but is so critical to our infrastructure. It’s not their fault, but you can’t assume they know, so it’s up to us to inform them. You have to speak up.

    Here’s a perfect example. Click on the following link to view a report presented by LightSquared last week in Taipei, Taiwan, at a 3GPP conference.

    “Final Report on Overload Characteristics of GPS Receivers in Proximity to LightSquared’s L-band Terrestrial Base Stations (BTS) and User Equipment (UE)”

    The best part about this report is the following statement from the Executive Summary:

    “Although results have been provided to date of a limited number of devices (6), LightSquared proposes to close the study at this stage as a more comprehensive study, covering a wider variety of GPS receivers than those involved in cellular applications, has now been initiated under the auspices of the FCC [2].  This study will be conducted by a cross-industry group led by LightSquared and USGPSIC, the reports of the study having complete public visibility.”

     

    Granted, I understand the Taipei conference was focused on the impact of LightSquared’s plan on mobile phones using GPS, but if this is the extent of their testing, it’s alarming. Furthermore, it’s relatively easy to acquire and operate an inexpensive consumer GPS receiver. Can you picture LightSquared attempting to test a sub-meter GPS L1 receiver or a RTK setup? GPS, GLONASS, SBAS, DGPS, real-time, post-processing, and the myriad of receivers on the market need to be tested. Although it’s likely not possible to test all equipment on the market, it’s not prudent to leave anything to chance. If, one year from now, you wake up and find out your $10,000 RTK receiver doesn’t work like it used to, it will be too late to do much about it. It takes very little time to voice your concern now to your elected officials so the appropriate attention is given to high-precision users.

    The good news is that Trimble Navigation is involved, along with the Federal Aviation Administration, with the U.S. GPS Industry Council and will be working closely with LightSquared in a Technical Working Group to better understand the impact that LightSquared’s system would have on GPS. Trimble and the FAA aren’t the only parties involved in the working group, but they are the parties that understand the needs of the high-precision user.

    The Technical Working Group’s first report is due March 15, 2011. Time is short, so don’t delay.

    Use these guidelines to take action. It is a call to action from Dr. Joe Paiva, veteran of surveying since the 1980s with whom many of you are familiar.

     

    Solar Activity

    As you’ve probably heard, we’re entering the next solar cycle, which is due to peak in May 2013.
    I want to periodically touch on this subject as the solar activity is going to increase over the next few years, and if the solar activity (geomagnetic storms, not sunspots) is severe enough, it will have an effect on GPS accuracy and tracking. Regardless of what you’ve heard in the mainstream media in recent months, the last event serious enough to affect GPS operations was in December 2006. That’s not to say that things aren’t heating up.
    But the recent activity does highlight the fact that “the Sun has become, somewhat suddenly, more eruptive,” according to Joe Kunches, of NOAA’s Space Weather Prediction Center. “We’ve been fortunate so far, in that the terrestrial effects — and impacts to GPS — have been very minimal. The most obvious sign of this has been the brilliant auroras up north.”
    “The video shows a large prominence eruption — billions of tons of plasma being strewn off the Sun. Some of it is drawn by gravity and rains back to the surface — the rest of it escapes. It’s the blown-away plasma that forms the coronal mass ejections that, when properly pointed, go by the Earth and cause problems for GPS,” said Kunches.
    Click on the following image to view a 15-second video of a solar flare that occured on February 24, 2011.
    Credit: NASA/GSFC/SDO

    From NASA:

    When a rather large-sized (M 3.6 class) flare occurred near the edge of the Sun, it blew out a gorgeous, waving mass of erupting plasma that swirled and twisted over a 90-minute period (Feb. 24, 2011). This event was captured in extreme ultraviolet light by NASA’s Solar Dynamics Observatory spacecraft . Some of the material blew out into space and other portions fell back to the surface. Because SDO images are super-HD, we can zoom in on the action and still see exquisite details. And using a cadence of a frame taken every 24 seconds, the sense of motion is, by all appearances, seamless. Sit back and enjoy the jaw-droppi
    ng solar show.

     

    March 17, 2011 Webinar: A Closer Look at L5: The Future of High-Precision GNSS

    Last year, the first GPS IIF satellite was launched. It became the first GPS satellite to broadcast the new L5 civilian signal/frequency. At 1176 MHz, it is further separated from L1 and L2 and located in the protected Aeronautical Radionavigation Services band, so there is no possibility of commercial interference like we see today with the LightSquared controversy. The availability of GPS L5 will usher in a new era of inexpensive, accurate GNSS receivers and will be the future of high-precision GNSS receivers, and quite possibly single-frequency receivers. I will also discuss the international support of L5 from other GNSS in development such as Galileo, Compass, QZSS, as well as SBAS (WAAS/EGNOS/MSAS).

    I’ll be presenting some interesting new material in the webinar such as graphics illustrating how many satellites (GPS and others) are projected to be broadcasting L1 and L5 just four years from now. It will be well worth 60 minutes of your time.

     

    Thanks, and see you next time.

    Follow me on Twitter at http://twitter.com/GPSGIS_Eric

     

  • Iraq on the Map: Installing Reference Stations for Accurate Engineering

    By Anas Malkawi

    Edge-HARNS-installation
    The team installs a HARNS in the southern province of Basra. Since 2005, Iraqi engineers have attempted to recover HARNS, but many were destroyed by locals who thought they indicated buried treasure.

    As a geodetic surveyor, I served in the U.S Army for 10 years. During that time, my team and I developed a nationwide GPS infrastructure system called the Iraqi Geospatial Reference System (IGRS). We installed Continuously Operating Reference Stations (CORS) and High Accuracy Reference Network Stations (HARNS), the first Iraqi owned and maintained system of its type.

    As a native Arabic speaker, my role was to train the Iraqi engineers to install additional CORS, as well as update and maintain the IGRS as a part of the International GNSS Service (IGS) network to sustain the accuracy of engineering and mapping projects. The IGRS was critical to other major infrastructure projects in the effort of rebuilding the battered nation, such as telecommunications, public works, and natural resource management to name a few.

    Some of the CORS we installed have Virtual Reference System (VRS) capability, a technology newly developed to establish real-time corrections in the field by using CORS as a base station for real-time kinematic (RTK) data collection.

    Key coordinators for the installation included Wisam Al-Hassani of the Iraq Ministry of Water Resources, Paul McKenzie of the Canadian Army, Linda Allen of the U.S. State Department, and myself, representing the U.S. Army, in addition to representatives from National Geodetic Survey (NGS), National Geospatial-Intelligence Agency (NGA), and Trimble Navigation.

    In addition to developing the IGRS, we performed several critical projects to assist in the rebuilding efforts as well as providing force protection, navigation, and mapping. My topographic engineering unit was responsible for providing coalition forces with GIS analysis, map production, and geodetic surveys.

    Edge-GPS-in-Haditha-Dam
    GPS equipment collecting data on a reference benchmark used to monitor the deformation of the Haditha Dam.

    For my second tour in Iraq (2007–2008), I was the platoon sergeant, which is equivalent to a project manager in a surveying firm. During the 15-month deployment, my team performed various survey projects including: 10 airport obstruction surveys, a dam deformation survey, more than 30 artillery and target-acquisition radar surveys, base-camp designs, site layouts, and ground-truth data collection for photogrammetry and remote sensing projects. We also established a nationwide database of all survey control stations in Iraq. The CORS was installed using Trimble NetRS receivers and Zephyr geodetic antennas. Trimble GPSNet and GPSBase software were used to process the continuous satellite data, for inclusion in the worldwide CORS network for public use. Field survey operations were conducted using Trimble 5700 GPS equipment.

    Traveling in Iraq was a major obstacle for survey operations. We had a choice: either fly on helicopters or drive military vehicles. Flying in helicopters with survey equipment was a challenge because we could never fit all our personnel and equipment. However, it was much safer than ground transportation through the dangerous roads of Iraq. In one incident, we were building a bridge in Baiji to help Iraqis and coalition forces cross the Tigris River after the original bridge was destroyed during the 2003 invasion. Our vehicle hit an improvised explosive device (IED). Some of the survey equipment was damaged, but we went back the next day and eventually built the bridge.

    Anas Malkawi served 10 years in the Army as a geodetic surveyor and senior technical engineer. He is currently enrolled in Old Dominion University’s Civil Engineering program while working at Transocean International Corporation as the Iraq program manager.

    Edge-IGRS-plan-map
    The initial plan of IGRS and placement of CORS/HARN through the Southern provinces.
    Edge-Airport
    Soldiers establish geodetic control for an airport aeronautical survey.
    Edge-Navaid-Survey
    Soldiers survey airport navigational aids that require high geodetic accuracy.
    Edge-IGRS-new-CORS-plan-meeting
    Malkawi discusses installation of Iraqi operated and maintained CORS with Al-Hassani.
    Edge-crash
    The result of traveling in military vehicles over roads infested with IED.
    CORS-coordination-team
    Key coordinators for the installation of the first Iraqi owned and maintained Continuously Operating Reference Station (CORS.) From left are Hussein, Malkawi, McKenzie, and Allen.
    Edge-Grp
    The 2005 U.S./British IGRS Team. Despite the difficulties, the soldiers I am honored to have served with stayed motivated and performed exceptionally every day by providing accurate data that saved lives.

     

     

  • Webinar Follow-up Q&A: SBAS, DGPS or Post-Processing? Which Should You Use?

    Last week, I conducted a webinar along with Dr. Michael Whitehead titled “SBAS, DGPS or Post-processing? Which Should You Use?” It was one of the best webinars I’ve conducted to date. More than 600 people registered. We barely squeezed it into 65 minutes and could have kept going for the better part of two to three hours, given the subject matter to cover and the number of questions we received before and during the webinar. Thank you for attending, if you did. If you weren’t able to you, can download it by registering here. After registering, you’ll be provided a link to download it.

    I knew that only having 65 minutes would be a serious issue for the webinar because the discussion could take many worthwhile tangents. And it was. But alas, we stuck to the presentation agenda, stayed on schedule, and were able to address several audience questions.

    We had a lot of questions before and during the webinar. As customary, I’d like to address some of those as well as present the poll results here. First, the poll questions and results with accompanying pie charts to illustrate the results.

     

    Poll #1: For those of you who use post-processing, what are the reasons you use it?

    Total votes: 117

    Gakstatter comment: This is an interesting spread with no clear dominating reason. Based on data I’ve seen and data we collected, I’m not convinced that post-processing is more accurate. If it is, is it worth the extra 10%, 20%, or ??% accuracy? I understand the votes for more reliable corrections. There’s something to say for reverse processing (forwards and backwards).

     

    Poll #2: For those of you using post-processing, from where do you access GPS base station data?

    Total votes: 129

     

    Gakstatter comment: These answers don’t surprise me. National and regional CORS have become very prolific in the past 10 years.

     

    Poll #3: For those of you who use real-time DGPS/SBAS, what is the reason you use it?

     

    Total votes: 110

    Gakstatter comment: These answers surprised me a little. I thought more people would vote for “less complicated.” Does that percentage of users really need corrected coordinates in the field? Why? E-mail me a quick answer if you have a chance.

    Poll #4: For those of you using real-time DGPS/SBAS, from where do you access DGPS/SBAS corrections?

    Total votes: 129

    Gakstatter comment: This answer doesn’t surprise me at all. I suspect RTK networks will increase due to their continued proliferation and different levels of accuracy offered.

    Poll #5: When I purchase GPS/GNSS equipment in the future, I will likely select equipment that utilizes the following correction method (select all that apply):

    Total votes: 144

    Gakstatter comment: This was the only multi-answer poll. People could select more than one answer. These answers were surprisingly close. That surprised me. It didn’t surprise me that SBAS was the leader. It surprised me that post-processing is still as predominant as it is. If you have a chance, e-mail me a quick explanation as to why you will use post-processing in the future.

    Before diving into some audience questions, I’d like to clarify the slide illustrating the post-processing plot shown below.

    During the webinar, we were discussing PPP (precise-point positioning) when this slide was displayed. This data was not corrected via PPP, but rather post-processing the pseudorange data, which is the equivalent of L1 SBAS and L1 DGPS. The point was to show how SBAS/DGPS accuracy compares to post-processing. In the real world, you won’t post-process 24 hours of data. Some of you will post-process only a few minutes of data per session in cases where you need to turn off the receiver and travel between points. In other cases, users will keep the receiver tracking between points, allowing reverse processing to work more effectively.

    On to the Questions

     

    Question #1: Will there ever be a way in which the position of a rover can become fixed by using two fixed base stations?

    Gakstatter comment: SBAS does this already. SBAS’s consist of a number of base stations within the coverage area (e.g., WAAS has 38). Data from many base stations is used to compute the correction information sent to an SBAS-enabled GPS receiver.

    I’m assuming your reasoning is to improve position integrity.

    Another method of accomplishing this is by post-processing against more than one base station or switching between DGPS beacon stations. If they differ significantly, then you might want to compare against a third base station.
    Question #2: At what point in time will the strength of the GPS signal be increased? To what strength will this occur? 500 times more powerful? What improvements in signal reception will be experienced? Indoor my house reception?
    Gakstatter comment: The GPS broadcast strength is increased with new GPS satellite model. For example, the current Block IIF satellite broadcasts the new L5 signal about four times stronger than L2C. While no one can be sure yet as to how much this will improve indoor positioning, there will be some marginal improvement in conditions where GPS doesn’t operate very well today. Also helping will be the improved code and error-correcting techniques that should make operating in difficult conditions a bit better, especially where there are a mixture of satellites with strong and weak signals.
    Also, it raises the issue of a viable L5 single frequency receiver, which should outperform the L1 C/A single frequency receivers of today.
    Question #3: NAD83, WGS84, ITRF differences, how to make the best choice?
    &nbsp
    ;
    Gakstatter Comment: I don’t think there is an incorrect choice, except maybe that NAD83 is a 2D system and will eventually give way to a 3D system, but that won’t happen in the U.S. for many years.
    Otherwise, it’s a question of matching disparate data sets. Probably the #1 question I hear from users is “why doesn’t my GPS data line up with my basemap?” The answer is almost always a difference in datums. Many papers have been written on this. Click here for a good PowerPoint presentation created by Dave Doyle of the National Geodetic Survey.
    Question #4: Are there any open source post-processing software programs available?
    Gakstatter Comment: Mike suggested looking here….http://gpspp.sakura.ne.jp/rtklib/rtklib.htm
    Question #5: If a person uses real-time correction satellites, is there a need to post-process?
    Gakstatter Comment: It’s rare that someone would do both, but not out of the question. For example, one might rely primarily on real-time corrections and record raw data for post-processing in case there is a problem receiving the real-time corrections. The opposite is true, too. One might rely primarily on post-processing and use real-time corrections as a back-up in case there is a problem with post-processing.
    Caveat emptor: There are probably datum differences between the sources of real-time and post-processing corrections. This needs to be reconciled when combining data that has used the two sources.
    Question #6: Is it possible to post-process data without using a DGPS?
    Gakstatter Comment: Yes, all that is required for post-processing is the ability to record raw observation data.
    Question #7: Are there geographic areas in the U.S. that are not covered by NGS CORS stations?
    Gakstatter Comment: No, not for pseudorange (L1) differential corrections. The distance to the base station will vary depending on where you are located and thus may affect your accuracy to some degree, but the density of CORS in the U.S. is such that you will never be more than a couple of hundred kilometers from a base station and likely much closer.
    A side note: Back in the mid-1990s, I remember experimenting with post-processing software we were developing. At that time, I tried post-processing data collected in Oregon with a base station located in Atlanta, Georgia. This was a 2,500 km baseline. It produced a result, albeit not one I would necessarily trust. The only limitation is that the two units must track common GPS satellites. With that length of baseline, it’s possible that only half of the satellites tracked may be in common.
    Question #8: What is the ideal distance range from a CORS station to your site to use post-processing?
    Gakstatter Comment: Ideally, as close as possible. The further you are from a base station, the more potential error will be introduced due to atmospheric differences between the two locations. As stated above, the density of CORS (at least in the U.S. and many parts of the world) are such that the nearest base station is quite near and likely no more than a couple of hundred kilometers away.
    Question #9: What is the trade-off between short observation time (couple of minutes) to position accuracy when using post-processing?
    Gakstatter Comment: Ok, remember we are talking about pseudorange corrections (as opposed to carrier phase). Given that the receiver has been tracking satellites for a period of time (let’s say two minutes), the observation times only need to be a few seconds for each feature to be mapped.
    For example, if you are mapping utility poles and don’t turn off the receiver between poles, you only need a few seconds (5-10 seconds) of data for each pole and average it for the final coordinate. Think about if you’re mapping a road centerline. You’ll likely record data while moving, so each second you are recording a new position.
    Question #10: What about the vertical correction? I see in the slide an antenna carried in a backpack. Is the antenna placed at ground level for point? Is there a constant correction required?
    Gakstatter Comment: Vertical accuracy is typically worse than horizontal accuracy by a factor of 1.5-2.0 due to the inferior satellite geometry, especially in areas of hilly terrain and/or trees/buildings where the horizon is blocked. Good geometry for vertical positioning requires tracking a number of GPS satellites that are low on the horizon.
    Question #11: What is the future of DGPS? I heard Coast Guard beacons were going away?
    Gakstatter Comment: The beacon stations operated by the U.S. Coast Guard are not in jeopardy and never have been. Neither have the marine beacons in the other 40+ countries that broadcast GPS corrections. However, the U.S. Department of Transportation operates 29 inland stations in the U.S. which have faced budget challenges the past few years. In April 2008, the U.S. DOT issued a policy decision to continue operating the 29 inland sites. Construction of seven sites remains that would allow the Nationwide DGPS to reach Initial Operating Capability (IOC), which would provide coverage to 99% of the continental U.S. No budget has been approved for the construction of those seven sites.

     

    Question #12: Can you briefly explain the difference between DGPS & RTK?
    Gakstatter comment: Here are a couple of good websites that explain each of these techniques. Essentially, DGPS is a real-time GPS positioning technique accurate to about 30 centimeters at the very best. RTK is a real-time GPS positioning technique accurate to about 1 centimeter.
    Question #13: How much time do you need to get the position from the base station for real-time DGPS?
    Gakstatter comment: Assuming both receivers are already tracking satellites, your receivers will begin using the base-station corrections as soon as the data link is made between the two.
    Question #14: Can you comment on advantages (if any) of using corrections from a network RTK service for DGPS corrections. Any advantages on eliminating base separation?
    Gakstatter comment: I’ve heard that DGPS corrections are optimized within an RTK Network. However, I need to research this a bit further to better understand the true advantages, if any.
    Whitehead Comment: A virtual base station (VBS) solution could be formed using the network. Thus differential GPS could exhibit the same advantages using such a network that RTK does (cancellation of atmosphere errors). The software would have to support this.
    Note though that if close to one of the Reference Stations in the network, it is probably best to just use the nearest Reference station as this will best cancel the atmosphere errors. When in the middle the network, the VBS solution would use surrounding reference stations to provide a good approximation of atmospheric errors and then output a correction that looked like it originated from a reference station (virtual station ) near to the users receiver.
    Question #15: What is up with PRN 135? Still on station?
    Gakstatter comment: Communication has be re-established with WAAS PRN 135 and is being tested by its owner, Intelsat, as well as the Federal Aviation Administration (FAA). See a detailed article by clicking here. The latest information I heard is that it’s currently at 93°W longitude undergoing testing. If the testing is successful, it will be re-located back to 133°W longitude and brought back into WAAS service. A timeline has not been published, but I’m guessing within the next 30-60 days.
    Question #16: We used to hear that your point accuracy degraded as the distance from the base station increased. One reason we used to post process. Is this still a factor?
    Gakstatter Comment: Due to advancements in GPS technology, it’s not as much of an issue as it used to be. I think this is illustrated in the results we achieved in our 24 hr test data.
    Ten years ago, it would be hard to find a GPS L1 receiver that would receive DGPS corrections from a beacon station 184km away and still achieve sub-meter horizontal accuracy at the 95% confidence level.
    I’m not saying the distance is negligible. There still the issue of tropospheric, ionospheric and satellite orbit errors as you move farther away from the base station. But, it’s certainly less of a factor than it was before.
    Whitehead Comment:
    Question #17: If we use WAAS correction, does it really help to try to use a post-processing type of software afterward? So far we just use WAAS correction.
    Gakstatter Comment: One of the reasons we collected data using several sources of real-time corrections and also showed the results of post-processing was to illustrate the differences between the two.
    If you follow proper procedures, there’s no reason to think that accuracy obtained using WAAS will differ significantly from accuracy obtained using post-processing. This is assuming that you’re using a single-frequency GPS receiver and post-processing using pseudorange corrections and not carrier-phase processing. Some receivers like the Trimble GeoXH are actually dual-frequency receivers and so data from it will likely surpass the accuracy of WAAS if you’re using its dual-frequency antenna and equivalent post-processing software.
    By proper WAAS procedures, I mean letting it track for five minutes upon initial start-up to allow it to download a current ionospheric map.
    Question #18: Does SBAS use 1 receiver and no base station? Expensive?
    Gakstatter Comment: SBAS uses 1 receiver and a lot of base stations. You just don’t have to pay for the SBAS base stations (or to use them.) The signal, like GPS, is provided free of charge.
    SBAS consists of a network of base stations (WAAS has 38) and communications satellites that broadcast corrections to users on the ground (or aviation users in the air).
    Question #19: How far north in Alberta is WAAS coverage available and useful?
    Gakstatter Comment: The primary concern would be visibility of the WAAS GEO satellite that broadcasts the correction data. Following is a map that illustrates the coverage. The contour lines are degrees above the horizon for which the two WAAS GEO satellites are visible.
    Solid line = PRN 138, Dashed line = PRN 133
    Question #20: Do you have any comments about CDGPS in Canada/US?
    Gakstatter comment: Sadly, the CDGPS service is being decommissioned March 31. You can read about it here. 
    Question #21: I am hearing from my state specialists (NRCS) regarding the LightSquared issue. We are advising working through the PNT ExComm and our cooperating partners.
    Gakstatter comment: This is a potentially serious issue for GPS users. Click here for the latest news as of February 1.
    Question #22: Where do you find the DGPS beacon station list and what is available to you?
    Gakstatter comment: I’m not sure if this is 100% complete, but it’s the most complete list I’ve seen. Click here.
    Question #23: Are most mapping-grade GPS receivers (for example Trimble GeoXh) equipped off the shelf to receive beacon signals?
    Gakstatter comment: Some receivers are equipped off-the-shelf, others are not (such as the GeoXH) and require additional hardware.
    Question #24: In which areas is it possible to use corrections from OmniSTAR?
    Gakstatter comment: Click here to view worldwide maps of OmniSTAR coverage.
    Question #25: Was the Garmin set to WAAS?
    Gakstatter comment: Yes, during the 24-hour data collection session, the Garmin unit was receiving WAAS 100% of the time as far as we could tell. The purpose of the 24-hour test period was to able to randomly sample data during that period to arrive at the accuracy statistics we presented. I randomly sampled the dataset several time
    s (averaging 10 seconds worth of positions 200 times) and the results were consistent with what we presented.
    Question #26: How does post processing account for ionosphere or troposphere errors if receiver is geographically far away from the base station? If not, does DGPS and WAAS provide better accuracy and integrity?
    Whitehead comment: Post Processing using a CORS station would take the nearest station and do differential GPS which cancels common errors in ionosphere and troposphere (ionosphere and troposphere are both temporally and spatially correlated) so if the CORS station is close, there will be good cancellation. If the receiver is far, the algorithms could use a troposphere model to account for the differential troposphere (as was done in the Presentation for BeaconT) and this would probably cancel troposphere so that remaining errors were sub-decimeter level. Differential Ionosphere errors could also be easily modeled with good results. It is likely that the performance could be made to easily surpass SBAS.
    DGPS would suffer from the same effects as does post processing, and maybe even more so since a model of differential atmosphere errors is rarely used. SBAS will likely provide better accuracy in situations where you are far from a base station.
    Question #27: What is Beacon T?
    Gakstatter Comment: While collecting data to present at the webinar, Mike noticed there was a bias in the beacon measurements. The beacon station is located ~184km away at about 7,000 ft elevation while the test site was at about 1,000 ft elevation. Initially, Mike wasn’t modeling the troposphere difference between the base and rover.
    To model the troposphere, Mike said he used a troposphere model to figure out troposphere in both locations, and then subtract the two. Although the models are not necessarily that accurate in an absolute sense, the differential tropo between the two locations is fairly accurate using the models. This differential tropo allows the receiver to correct the tropo in the base station differential to make it appear as if it originated in the rover location. Mike said he could’ve done the same for the ionosphere, but he didn’t since that is it usually less of a factor. After using the modified tropo model (Beacon T), the height bias was around 1/2 meter, which could be attributable the ionosphere. The horizontal bias is small, as you can see in the results.
    Using this troposphere model resulted in a significant improvement over the original solution.
    Question #28: Why is VBS better than WAAS?
    Gakstatter Comment: It surprised me too. The receiver used was the same that was used for beacon and WAAS. I contacted OmniSTAR for their opinion.
    John Pointon of OmniSTAR responds: “There have been incremental improvements in the VBS service over the years, mostly improvements in modeling and processing. We have added two or three extra reference stations but that hasn’t been the most critical improvement, just helped in some specific areas. These, combined with the relatively benign solar environment, result in VBS accuracy which, although not equivalent to our dual-frequency and multi-system solutions, is consistently better than either Beacon or WAAS.”
    Whitehead Comment: In the past, we’ve seen similar performance from both OmniStar VBS and WAAS.  Different atmosphere conditions and different locations can affect the performance of both. We’ve seen situations where WAAS is better.  It is probably fair to say that OmniStar is more focused on accuracy, whereas WAAS is focused on integrity.  It may be wise to do a comparison in the particular area where you operate.  Note, however, that in the US, OmniStar is referenced to NAD83 whereas WAAS is references to ITRF so positions reports between the two can differ by several meters.
    Question #29: When I look at your scatter plot, I have to ask if short-term point averaging is really effective at achieving more accurate positions?
    Gakstatter Comment: I think it’s well accepted that you are wasting time by occupying a point for 180 seconds. That said, there’s something to be said for letting the receiver track satellites for a period of time (1-2 minutes) before storing 5-10 seconds of data. Of course, if the receiver is already tracking satellites, then it’s not necessary to wait. The idea is to let the measurements settle down and take advantage of carrier-phase smoothing if the receiver uses that technique.

    Question #30: Could you go into PPP a bit more? How does it work?

    Gakstatter Comment: We opened a can of worms by discussing PPP. It’s an entirely different subject that I will cover in a future article. In the meantime, you can read Dr. Richard Langley’s article on PPP here.

    Question #31: How do you test the accuracy of SBAS collected data?

    Gakstatter Comment: In the U.S., it’s easy. Find a local survey mark using the National Geodetic Survey website. Printout the ITRF coordinates of the survey mark. If they aren’t on the datasheet, you can convert from NAD83/CORS96 to ITRF using the HTDP program. Compare the coordinates output by your GPS receiver to the coordinates of the survey mark.
    If you’re located outside of the U.S., look for a similar government agency in your country that maintains a record of survey marks. It’s vital that you are comparing coordinates referenced to the same datums.

     

    Question #32: Will there be any disadvantage if we use a EGNOS corrections in Kuwait, if we receive EGNOS?

    Whitehead Comment: Kuwait is outside the EGNOS coverage zone, so satellites to the south may not even have Clock and Orbit correctors available, which means the Receiver could not compute a correction for these satellites.  Unless the receiver can mix differentially cor
    rected ranges with non-differentially corrected ranges, it would likely drop the satellites in the south that had no corrections. This would then reduce PDOP and thus accuracy. Mixing differentially corrected ranges with non-differentially corrected ranges may give worse accuracy than no corrections at all since the SBAS system may have clock or other biases relative to GPS.
    By the way, I wish the SBAS providers would get together and share data so that they each could provide world-wide orbits and clocks. Then it would matter less if you were outside the coverage area.
    Gakstatter Comment: I’ve heard that EGNOS is planning an expansion to the south and east, so Kuwai may eventually be within the EGNOS coverage footprint. Also, you’ll want to monitor the progress of India’s GAGAN system, which is a similar SBAS. It’s possible you might fall within the GAGAN extrapolated footprint for non-aviation users.
    We covered most of the questions posed by the audience. If we didn’t address yours or didn’t provide a complete enough answer for you, please e-mail me and I’ll do my best to answer you.
    As I mentioned above, we had quite a few questions about PPP. It’s a technology that’s worthy of further coverage and discussion. Look for a future article on it.
    Thanks, and see you next time.
    Follow me on Twitter at http://twitter.com/GPSGIS_Eric

     

  • 2011: The Year for Galileo

    Back in December 2006, I wrote about the momentum of Galileo (Europe’s planned satellite navigation system) in an article discussing GNSS trends. Galileo has been discussed off and on for well over a decade and was a hot topic for a number of years. In fact, back around 2001, the U.S. really didn’t want the European Union to embark on the project. While there was not a clear policy against Galileo, certainly the sentiment was questioning the creation of another satellite navigation system when GPS already exists that’s free for everyone to use. Ok, it probably wasn’t that simple, but you get my point. No bueno from the U.S. at that time.

    The following is an EU slide that illustrates why the EU wants to develop its own satellite navigation system similar to GPS:

    Source: European Commission – Montpellier, France – October 2010

     

    Then, in 2004, the U.S. government abruptly changed its tune. It really doesn’t matter why and I’m not sure I’d believe the answer if I was given one, but President George HW Bush instituted a new policy that encouraged international cooperation. The U.S. SPACE-BASED POSITIONING, NAVIGATION, AND TIMING POLICY issued in 2004 stated, among other things, that the United States shall:

    “Seek to ensure that foreign space-based positioning, navigation, and timing systems are interoperable with the civil services of the Global Positioning System and its augmentations in order to benefit civil, commercial, and scientific users worldwide. At a minimum, seek to ensure that foreign systems are compatible with the Global Positioning System and its augmentations and address mutual security concerns with foreign providers to prevent hostile use of space-based positioning, navigation, and timing services;”

    Also in 2004, the U.S. and European Union signed the landmark GPS-Galileo Agreement that established a basis of cooperation. This was great news for the GNSS user community. More satellites and more signals usually equates to better performance.

    The next policy update after 2004 was last year (2010) and it was simply titled “NATIONAL SPACE POLICY“. The sentiment regarding international cooperation was the same, if not leaning more towards cooperation:

    “Engage with foreign GNSS providers to encourage compatibility and interoperability, promote transparency in civil service provision, and enable market access for U.S. industry;”

    After the 2004 GPS-Galileo policy was published, the question from the civil user community was, “When are we going to have satellites in orbit broadcasting signals we can use?”

    The answer to that question wasn’t easy, and took longer to answer than anyone predicted, including myself.

    Now, we have the answer.

    Unlike GPS and GLONASS, Galileo is a civilian project, not a military-funded one. I’m not saying GPS and GLONASS were easy to fund, but the core application was defined (military use), and the funding required to develop and maintain GPS and GLONASS is drawn from the military budget. Furthermore, the European Union is comprised of 27 member countries. The political dynamics are, obviously, very complex.

    The Galileo funding modeling initially was to be a public-private partnership (PPP). Part of it would be funded with public money and part of it would be funded by a consortium of companies. But, that wasn’t so easy. How much funding would each contribute? What’s the return on investment? How would it generate revenue? Would there be a tax receiver sales? Would there be a user charge?

    We’re not talking about small sum of money. We’re talking about several billion Euros just to get it off the ground.Think about it, how much money has the U.S. military spent to develop GPS? $30-$35 billion for development, deployment and long-term maintenance. Granted, Galileo will cost a lot less than that, but it’s still a healthy sum that no company would be willing to gamble without a solid return-on-investment (ROI) argument.

    Eventually, the PPP (Private-Public Partnership) funding model was abandoned and in late 2007, and as described in a January 2008 GPS World article:

    “European officials responsible for the EU budget said they had found funds for Galileo, proposing to draw unused money originally earmarked for natural resources programs this year and next. The move would provide some €2.4 billion ($3.3 billion) for Galileo — the budgetary shortfall left with the dissolution of the public/private partnerships — over the course of the next six years. The following month, European parliamentarians agreed with the plan, but felt it didn’t go far enough. They boosted proposed funding for Galileo, increasing the money set aside for the program in 2008 to €739 million ($1.06 billion), up from the much more modest €151 million under the transport officials’ original proposal for next year.

    Not all were sold on public funding for Galileo. But in November, European officials said they had ironed out their differences. At the 11th hour came heated debate about how Galileo funding and contracts would be awarded among member states and their respective aerospace companies. Eventually, a final accord was reached. Europe anticipates spending €3.7 billion on Galileo through 2013.”

    (Updated figures: €2.1 billion for IOV and €3.4 billion for FOC)

    That was three years ago. The EU folks have been working hard since then, but talk is cheap and people stopped talking about Galileo with the exception of a few information spikes here and there. There was nothing else to say until now.

    2011 is the Year for Galileo

    Galileo will likely meet a major milestone this summer, launching their first two satellites for in-orbit validation. But unlike the two Galileo test satellites already in orbit (GIOVE-A and GIOVE-B), these satellites will be part of the planned 30-satellite operating constellation.

    For you Galileo naysayers, the EU is past the point of no return. Eighteen satellites are contracted. There is no reversing the process. And, if I were to place a bet, it’s very unlikely to stall at 18. That would be sort of like building a structure, but not finishing the interior.

    Although I haven’t seen a detailed launch schedule or control segment plan, the latest Galileo public document I’ve read (European Commission – Montpellier, October 2010) presents the following timeline:

    2011/2012 – In-Orbit validation: Four IOV satellites and ground segment (based on European Commission presentation from October 2010).

    2014/2015 – Initial Operating Capability for early services — 18 satellites (based on European Commission presentation from October 2010).

    2019/2020 – Full Operating Capability — 30 satellites
    (based on mid-term review released January 18, 2011)

    2014 Will Be the Year of Cheap GNSS Accuracy

    I believe the magic year for GNSS will be 2014. That’s when GNSS receivers are going to be very interesting.

    Why?

    It’s no secret that I think the new L5 signal is a game-changer. Last summer I wrote an article titled “What’s Going to Happen When High-Accuracy GPS is Cheap?”  It’s all about L5.

    L1/L5 dual-frequency receivers are going to be cheap, and accurate. Today, dual-frequency (L1/L2) receivers are thousands of dollars. L1/L5 receivers will be a fraction of that cost because open signal specifications will lead to increased competition.

    As I mentioned in the article last summer, the GPS Directorate is planned to have 24 satellites broadcasting L5 by 2019. The beauty of Galileo is that it can cut that time in half and make it happen by 2014, only three years from now. Here’s how.

    Since Galileo supports L1 and L5 similar to GPS, you only need 12 x GPS satellites broadcasting L5 and 12 x Galileo satellites broadcasting L5 to have something close to 24 satellites broadcasting L5.

    The BIG question is if the U.S. and EU will coordinate orbit slots so the 12 x GPS and 12 x Galileo satellites are in a somewhat optimal 24-slot constellation instead of an uncoordinated configuration. The civil economic benefit from taking advantage of L5 as soon as possible would be substantial. Just this week, the EU issued a report stating that 6-7% of the GDP of EU countries is dependent on satellite navigation. Better accuracy enabled by L1/L5 will spur a mind-boggling number of new applications that will further broaden the GNSS user base and economic impact. It would also stimulate GNSS receiver development from a much broader range of GNSS receiver designers than we see today.

    With a combined GPS/Galileo constellation, not only will accuracy become cheaper, but availability will increase significantly. The new GPS 24+ 3 configuration is certainly a big help for high precision users with respect to availability. Can you imagine how much precise positioning availability will improve when 18 Galileo satellites (not to mention 30) are added to the mix? Last summer, the EU-U.S. Cooperation on Satellite Navigation Working Group C published a report entitled “Combined Performance for Open GPS/Galileo Receivers.” The report succinctly draws the following conclusion, with which I wholeheartedly agree:

    “The studies demonstrate and quantify the improvements that can be expected when using GPS and Galileo open services in combination under different environmental conditions. In all studied cases, the combination of GPS and Galileo led to noteworthy performance improvements as compared to single system performance. The most significant improvement is for partially obscured environments, where buildings, trees or terrain block portions of the sky. The increased number of satellites available provides robust performance even as some signals are blocked, which is reflected in a significant increase of positioning accuracy and availability.”

    Following are some data from the report that back up the conclusions on availability.

    Availability with a 15° elevation mask

    GPS only – 99.10%

    Galileo only – 100%

    GPS/Galileo – 100%

    Availability with a 30° degree elevation mask

    GPS only – 57.28%

    Galileo only – 75.02%

    GPS/Galileo – 98.93%

    Granted, you should take these numbers with a grain of salt. These are based on positioning with four satellites in view. The reality is that for high precision users, we need data from at least six satellites for robust positioning. But, I think the scale of improvement when going to GPS/Galileo constellation is obvious and will scale similarly when considering six satellite positioning.

    For all the reasons above, I’m putting my stamp on 2011 as being The Year of Galileo. Look forward to further coverage on Galileo in the coming months.

    Upcoming Jan. 26 Webinar: SBAS, DGPS or Post-processing? Which Should You Use?

    Speakers:

    Eric Gakstatter, Editor, Geospatial Solutions and Survey Scene newsletter &

    Dr. Mike Whitehead, VP of Technology at Hemisphere GPS

    Event Date: 01/26/2011 10:00 AM Pacific Standard Time, 5 PM GMT

    Tens of thousands of users around the world utilize GPS/GNSS receivers for mapping, surveying and navigating. Since autonomous GPS/GNSS typically does not provide the needed accuracy, users must rely on a source of GPS/GNSS corrections. There are three sources of GPS/GNSS corrections available to users who desire reliable GPS/GNSS accuracy in the sub-meter to three meter range: SBAS, DGPS and post-processing. Dr. Michael Whitehead, Chief Scientist at Hemisphere GPS, will join me in presenting a background on the three technologies as well as the strengths and weaknesses of each. I’ve known Mike for a number of years. He was an early innovator in the development of SBAS technology at Satloc as well as SBAS and DGPS receiver technology at Hemisphere GPS. He is one of the leading GNSS engineers in the world. I’m particularly excited about this event and promise a lively discussion that’s full of useful information, data and concepts that anyone using or considering using GPS/GNSS for mapping, surveying or navigating will find useful.

    Thanks, and see you next time.

    Follow me on Twitter at http://twitter.com/GPSGIS_Eric

     

  • To Post-Process or Not to Post-Process, that Is the Question

    If you’ve been around GPS mapping for any length of time, I’m sure you’ve heard of post-processing, and you may have even experienced it yourself. If you used GPS for mapping in the ’90s, you almost certainly post-processed your data. In fact, sometimes you had to pay for access to GPS base-station data for post-processing. That’s hard to imagine given the widespread, worldwide availability of GPS base-station data on the web today.

    SBAS (WAAS/EGNOS/MSAS) didn’t exist, and for real-time corrections and DGPS (beacon) coverage was spotty at best, but real-time commercial DGPS services like OmniSTAR, Landstar, and Satloc were around.

    One thing is for sure, no matter what, you have to have some source of corrections to collect GPS data for GIS mapping. It’s commonly referred to as differential GPS correction. Essentially, your GPS receiver needs to reference another GPS receiver (base station) that’s set up on a known position.

    Grafnav Post-processing software

     

    There are two primary methods in which to apply a correction to your GPS data: post-processing differential correction and real-time DGPS.

    Post-processing

    When you’re collecting GPS data that’s going to be post-processed, you need a GPS receiver (and software) that’s going to be able to record satellite observation data. Otherwise, data is collected as one normally would in the field, whether it’s utility poles, manhole covers, road centerlines or polygons of any sort.

    The accuracy of the GPS data while you’re in the field is autonomous GPS, so it could be several meters or even ten meters or more. You can’t use this type of method for navigating to a point with any sort of accuracy better than a few meters.

    After you’re finished collecting your GPS data for the day, you go back to the office and download your data to your computer. Post-processing requires special software. That software will allow you to search the Internet for the closest GPS base station(s) to use as a source of GPS corrections. In previous years, it was a laborious task to search for GPS base-station data that was recorded the same time as you were in the field (remember UTC vs. local time?). That’s not the case any longer as advanced post-processing software has made this a more automated process. The software will search for the closest base station and automatically select the appropriate files to download.

    It takes specialized software and training to utilize post-processing effectively.

    Real-time DGPS

    This is a method of receiving GPS corrections while you’re in the field. The GPS corrections are applied in real-time so your positioning is accurate. This is  useful when you want to navigate to a particular point very accurately. In the 1990s, there were a number of DGPS services, mostly commercial. One would pay a monthly or annual subscription fee to receive the DGPS corrections. During that time, the U.S. Coast Guard started developing a system by which it will install GPS base stations near the major U.S. waterways (coastlines and major rivers). It set up large towers that would broadcast the corrections via 300 kHz radio. Most importantly, it broadcast the corrections free of charge. One only needed a “beacon receiver” to receive the corrections. The system didn’t cover the entire U.S., but it opened the eyes as to what was possible in terms of a regionwide, or nationwide, DGPS network of base stations.

    The U.S. Coast Guard concept is still used today in more than 40 countries for DGPS marine navigation. The same GPS correction signal is also used by many people using GPS for mapping.

    Around the same time, the Federal Aviation Administration (FAA) began developing a system to improve GPS integrity and accuracy. They called it WAAS (Wide Area Augmentation System). It was the first SBAS in the world and, upon being declared operational in 2003, is in use by thousands of people for GPS mapping. SBAS is a regional system. WAAS only covers North America (U.S., Canada, and Mexico). It has spawned a number of similar and compatible systems such as EGNOS in Western Europe and MSAS in Asia with GAGAN under development in India.

    There are several advantages and disadvantages to both post-processing and real-time DGPS for GPS mapping. The primary advantage of post-processing is that you don’t have to worry about a wireless data connection in the field. The primary advantage of real-time DGPS is that you get much better accuracy in the field. There are many other factors you should consider when deciding which method to use.

    In fact, I think it’s an interesting enough topic that I’m conducting a webinar later this month that will address both of these methods. I’ve invited Dr. Michael Whitehead to join me. He’s the head technology guy at Hemisphere GPS and has worked extensively developing high performance GPS receivers. He was also the chief architect at Satloc back in the late ’90s.

     

    Webinar: SBAS, DGPS or Post-processing? Which Should You Use?

    Speakers:

    Eric Gakstatter, Editor, Geospatial Solutions and Survey Scene newsletter &

    Dr. Mike Whitehead, VP of Technology at Hemisphere GPS

    Event Date: 01/26/2011 10:00 AM Pacific Standard Time, 5 PM GMT

    Tens of thousands of users around the world utilize GPS/GNSS receivers for mapping, surveying and navigating. Since autonomous GPS/GNSS typically does not provide the needed accuracy, users must rely on a source of GPS/GNSS corrections. There are three sources of GPS/GNSS corrections available to users who desire reliable GPS/GNSS accuracy in the sub-meter to three meter range: SBAS, DGPS and post-processing. Dr. Michael Whitehead, Chief Scientist at Hemisphere GPS, will join me in presenting a background on the three technologies as well as the strengths and weaknesses of each. I’ve known Mike for a number of years. He was an early innovator in the development of SBAS technology at Satloc as well as SBAS and DGPS receiver technology at Hemisphere GPS. He is one of the leading GNSS engineers in the world. I’m particularly excited about this event and promise a lively discussion that’s full of useful information, data and concepts that anyone using or considering using GPS/GNSS for mapping, surveying or navigating will find useful.

     

    Thanks, and see you next time.

    Follow me on Twitter at http://twitter.com/GPSGIS_Eric

  • On the Edge: Five Big Ones in Ten

     

    Look back with me at the five 2010 GNSS events that most affected surveying, mapping, engineering, construction, and natural resource users. Each one had, or could have had, a significant effect on you and your work. Taking it from the top:

    GPS 24+3 Constellation. The most important event occurred a year ago, when the Air Force began implementing a new GPS 24+3 configuration. They had their military reasons, but the benefit for you and me is eliminating GPS brownouts — periods with fewer GPS satellites in view. When combined with obstructions such as terrain, trees, or buildings, they made GPS hard to use.

    It’s especially an issue with real-time kinematic (RTK) high-precision users because RTK technology is satellite-hungry. It needs six or more satellites to provide a robust position solution.

    The Air Force moved three satellites, SVNs 24, 26 and 30, from their original slots. SVNs 26 and 30 have already reached their destinations, and SVN 24 will do so this month.

    Three other satellites are being shifted slightly. SVN 55 found its new slot in December, while SVNs 46 and 56 start this month and should have completed their journeys by May/June 2011.

    By now, you should be seeing some improvements in GPS satellite visibility. Although you’ll see fewer peaks (high number of GPS satellites in view), you’ll also see fewer valleys (low number of GPS satellites in view). This should increase productivity for RTK users and those in obstructed environments such as tree canopy.

    First GPS Block IIF. Although it doesn’t really help users at this point other than being another satellite to enter service, the Block IIF satellite launched in May is the first to broadcast the third civil signal. L5 marks the beginning of a new era in high-precision GPS positioning. The Block IIF launch was the catalyst for my June column “What Happen When High Accuracy is Cheap?”

    This IIF is just a teaser though, and its fellows will launch at a snail’s pace. Remember though, it costs upwards of $200 million to launch a satellite and since there ares already 30+ operational GPS satellites in orbit, it’s hard for Congress and the Air Force to justify speeding up the launch schedule. The last target I heard was to have 24 satellites broadcasting L5 by 2019.

    GLONASS Growth. Despite the recent catastrophe, the Russian Federation was still able to launch seven new satellites in 2010, including a new K1 satellite that will test a new CDMA signal for better compatibility with GPS.With 21 operational satellites and three more coming in March, a consistent and healthy number of GLONASS satellites in orbit has given receiver manufacturers more confidence to develop GPS/GLONASS receivers. This year, we’ve seen several manufacturers integrating GPS/GLONASS into handheld receivers as well as OEM board products.

    User benefits are clear: more robust positioning and improved productivity due to decreased down-time.

    Solar Activity. The big news is no news: the sun was eerily quiet in 2010. If your GPS receiver didn’t work at times this year, it wasn’t due to solar activity. But it may ramp up in 2011.

    GAGAN, WAAS Failures. The Indian Space Research Organisation and the U.S. Federal Aviation Administration received a hard lesson in SBAS GEO management. In April, an Indian rocket launch failed, and one of the FAA WAAS satellites lost communication with its ground control.

    If you’re an SBAS user, don’t let it bring you down. SBAS is here to stay, and likely you were not affected by either incident — unless you work in northwest Alaska. A new U.S. SBAS satellite came online, and India is regrouping for more launches.

    Follow Eric on Twitter at GISGPS_Eric.