Galileo SAR is Europe’s contribution to the COSPAS-SARSAT network, a distress alert detection and information distribution system best known for detecting and locating emergency beacons activated by aircraft, ships and hikers.
By providing COSPAS-SARSAT with the coverage capacity of the Galileo constellation equipped with SAR transponders, Europe is helping to reduce the detection delay of a distress signal from up to several hours to 10 minutes.
A return link, a signal informing the person in distress that the signal has been received and localized, will be added to the system by the end of 2018.
Beacon Awareness Day
The Galileo SAR launch day, April 6, is Beacon Awareness Day in the United States. It’s also named 406 day. 406 stands for 4/06 — the date in U.S. format — and the 406-MHz frequency of the SARSAT beacons.
For Twitter and social media, special hashtags #406day, #406day17 and #savedbythebeacon already exist. The program has added the hashtag #getabeacon to complement it.
The following video about the program focuses on maritime operations, which account for 75 percent of the alerts.
Coming to the Rescue
With Galileo, the time to identify the location of a beacon signal is reduced from several hours to a few minutes. At sea, this makes SAR rescue operations easier thanks to a narrowed “search box,” since the vessel in distress has less time to drift.
On land, the quick acquisition of a precise position enables rescue teams to more quickly reach the operation zone and assist the victims.
In the air, Galileo contributes to fulfilling International Civil Aviation Organization (ICAO) requirements for implementing the next-generation emergency management system Global Aeronautical Distress and Safety System (GADSS). In particular, it enhances location of an airplane in distress, which will be mandatory on Jan. 1, 2021.
The Search And Rescue transponders on Galileo satellites can pick up signals emitted from any 406-MHz distress beacon anywhere in the service coverage area and transmit this information to the dedicated ground stations (MEOLUTs). The SAR/Galileo infrastructure is interoperable with GPS and GLONASS SAR transponders.
Once the beacon is located by the MEOLUTs, the location data is sent to the COSPAS-SARSAT mission control centre (MCC), which distributes it to the relevant rescue centres. The rescue centres, under the responsibility of national competent authorities and administrations, then coordinate the required rescue efforts.
Improving COSPAS-SARSAT
Galileo plays an important role in the Medium Earth Orbit Search And Rescue system of COSPAS-SARSAT (MEOSAR), and provides a ground segment coverage of 40 million square kilometers over Europe as a contribution to MEOSAR global coverage.
Thanks to the advanced European technology used, integration of Galileo into COSPAS-SARSAT improves the system by:
enabling faster detection and localization of distress signals anywhere in the service coverage area, reducing the delay between beacon activation and distress localization
making it easier to find the source of a signal by significantly boosting precision in comparison to the current situation
increasing availability and improving detection of signals in difficult terrain or weather conditions.
The Galileo Search And Rescue service is one of the three services launched in December 2016 with the Initial Services. The SAR service represented just 1 percent of total Galileo program costs, but should result in thousands of lives being saved, according to the head.
By Peter Steigenberger, André Hauschild, Steffen Thoelert and Richard B. Langley
Between Feb. 7, 05:02 UTC and Feb. 8, 12:30 UTC, 2017, all seven operational GPS Block IIR-M satellites were consecutively subject to short periods of unavailability. These official outage periods, when the satellite signals were set unhealthy and deemed unusable, were announced ahead of time through Notice Advisories to Navstar Users (NANUs). An overview of the outage periods and the corresponding NANUs for each satellite identified by their pseudorandom noise code (PRN) assignment and space vehicle number (SVN) is provided in TABLE 1.
Table 1. GPS Block IIR-M satellite outage periods and corresponding 2017 NANUs.
An analysis of the measured signal-to-noise-density ratio (C/N0) from several tracking stations of the International GNSS Service (IGS) indicates that the satellites’ transmit powers were increased during the outage periods. The effect is visible in the plots in FIGURES 1 and 2, which show C/N0 of the L1 C/A-code over time for satellite passes on the three consecutive days Feb. 6, 7 and 8, 2017.
Figure 1 shows the results for PRN 17 as tracked by a Septentrio PolaRx4TR receiver (USN8) located in Washington, DC. The pass on the outage day Feb. 7 is plotted in blue. Obviously, the receiver is configured to not track unhealthy satellites, since no observations are available during the outage period. However, a clear increase in the C/N0 is visible from about 50.5 dB-Hz before the outage to approximately 52 dB-Hz after the outage. The C/N0 level of the day before is similar to the level prior to the outage. The C/N0 level on the following day is very similar to the C/N0 after the outage, which indicates that the satellite continues to transmit with an increased power.
Figure 1. Plot of L1 C/A-code C/N0 over time for consecutive satellite passes of satellite PRN 17 (SVN 53) tracked by a Septentrio PolaRx4TR receiver located in Washington, DC, on Feb. 6–8, 2017. The satellite’s unhealthy period on Feb. 7 is indicated by the gray shaded area.
The plot in Figure 2 shows the same analysis, this time for PRN 05 and for a Leica GR10 receiver (KOUG) located in Kourou, French Guiana. This receiver continues to track the satellite during the unhealthy period. The distinct step in C/N0 is clearly visible shortly after the satellite is set unhealthy. Also, this satellite continues to transmit with increased power during the pass on the following day. The same observations as in Figure 1 and Figure 2 can also be made for all other Block IIR-M satellites and other receivers.
Figure 2. Plot of L1 C/A-code C/N0 over time for consecutive passes of satellite PRN 05 (SVN 50) tracked by a Leica GR10 receiver located in Kourou, French Guiana, on Feb. 6–8. The satellite’s unhealthy period on Feb. 7 is indicated by the gray shaded area.
The difference between the measured C/N0 before and after the unhealthy period is typically 1–2 dB-Hz depending on the receiver and the satellite (see TABLE 2). On average, a power increase of 1.5 dB with a scatter of ±0.25 dB among the various satellites is suggested by the measured data.
Furthermore, it may be noted that different receivers respond with a different change in C/N0 for a given change in transmit power. At the average 1.5 dB power increment, C/N0 changes between 1 dB and 2 dB are reported by the different types of receivers. This indicates manufacturer-specific algorithms for C/N0 estimation that impact the use of measured C/N0 as a reliable indicator of received signal power strength.
Table 2. Changes in C/N0 (dB-Hz) obtained from differences of days before and after the increase of the transmit power.
It is interesting to notice in this context that NANU 2017005 issued Jan. 19, 2017, states that “The 2d Space Operations Squadron (2 SOPS) periodically conducts configuration changes on GPS satellites to assess current capabilities, validate future capabilities and ensure continued interoperability.”
Furthermore, the Civil GPS Service Interface Committee Executive Secretariat released the following statement on Jan. 25, 2017: “Beginning 25 January 2017, Air Force Space Command (AFSPC) will conduct a limited duration test implementing an increase of the L1 C/A power level on the GPS Block IIR-M and IIF satellites (19 vehicles).”
However, no maintenance has been announced so far for any of the Block IIF satellites, and no obvious increase in the measured C/N0 could be found for these satellites. A repeated analysis for the Block IIR-M satellites on Feb. 22, 2017, confirmed that the L1 C/A-code power levels were still at their increased levels.
Measurements with the German Aerospace Center’s (DLR’s) 30-meter-diameter high-gain antenna at Weilheim, Germany, have been recorded to independently confirm the GPS Block IIR-M transmit power increase of the L1 C/A-code. FIGURE 3 shows the L1 spectral flux density for March 4, 2017 (blue line), and a previous measurement taken on Dec. 7, 2015 (red line). The sharp peak in the middle of the spectrum represents the C/A-code. A clear increase of the power in the measurement of March 2017 compared to Dec. 2015 is visible. Further analysis of the high-gain antenna data yields a power increase of about 2 dB.
Figure 3. L1 spectral flux density of PRN 29 (SVN 57) for Dec. 7, 2015 (red, normal C/A-code power level) and March 4, 2017 (blue, increased C/A-code power level).
However, the M-code flux density with main lobes near 1565 and 1585 MHz is reduced in March 2017 compared to Dec. 2015, whereas the P(Y)-code signal strength remains essentially unaltered. The total transmit power in the L1 frequency band is the same for both time periods. Therefore, the analysis reveals a redistribution of transmit power from M-code to C/A-code for the Block IIR-M satellite PRN 29 (SVN 57).
Authors Peter Steigenberger, André Hauschild and Steffen Thoelert are from the German Aerospace Center (DLR).
Richard B. Langley is from the University of New Brunswick and authors the monthly Innovation column for GPS World magazine.
RF optical solutions maker Foxcom has introduced a range of products to serve the GPS/GNSS repeater market.
Foxcom launched an Iridium repeater in September 2016 and is now offering advanced GPS/GNSS repeater solutions globally.
The firm’s repeaters have been designed to cover a wide range of commercial and military applications, such as:
aircraft hangars
time distribution in data centers
GPS distribution in tunnels
police and fire stations
manufacturing and test facilities
GPS L1 and GLONASS signals are passed through the repeater to the interior space. This means that satellite navigation devices will always receive a signal when indoors, eliminating any satellite acquisition delay when leaving the building.
Foxcom offers a choice of coax or optical solutions that have been optimized to meet the needs of customers worldwide, including.
Optical GPS/GNSS Repeater. Foxcom’s GPS/GNSS optical repeater solution is for retransmitting GPS/GNSS signals indoors. The repeater system provides seamless coverage inside a hangar or a large facility enabling the testing of navigational systems.
GPS/GNSS Distribution in Tunnels. Foxcom’s redundant GNSS Time Distribution System (TDS) ensures failsafe global satellite navigation signal transmission in tunnels.
GPS/GNSS Distribution for Data Centers. Foxcom’s optical redundant GNSS Time Distribution System (TDS) ensures failsafe synchronization in data centers by transmitting fully redundant GPS/GNSS signals. By deploying Foxcom’s optical GPS/GNSS link, networks of data centers at multiple locations can be accurately synchronized.
GPS Optical Link | GL7222. Foxcom’s Sat-Light/Gold L-Band Interfacility Link offers a high performance, alternative to conventional coaxial-cabled systems. The Gold GPS Link covers the frequency range of 1100 to 1600MHz and supporting both L1 and L2 GPS bands. The Gold Series GPS link is compatible with wide range of active GPS antenna and is equipped with voltage selectable GPS antenna powering.
GPS Repeater Kit. Foxcom’s GPS repeater solution is for retransmitting GNSS and GLONASS signals indoors. The repeater system provides seamless coverage inside a hangar or a large facility enabling the testing of aircraft navigational systems. The kit consists of an active repeater, indoor/outdoor antennas and 3 x 30 foot coax cable.
Coax-based Iridium repeater. Iridium satellite telephones are used all over the world. They generally can’t operate indoors, because the structure of the building blocks the ingress and egress of the signal. When it isn’t practical or safe to leave the building to make a call, a repeater system overcomes this barrier.
Iridium repeaters are used in a wide range of situations, including underground civil defense/military bunkers, oil rigs/ships, large buildings and any other underground facilities.
Foxcom’s coax-based Iridium repeater can be used when the distance from outdoor to indoor antennas is short. For example, when used in an aircraft hangar the ODU and IDU may be just a few meters apart. The cost of the coax-based kit is significantly lower than that of the optical version.
The new coaxial repeater system merges the ODU and IDU into one combined unit removing the optical fiber interfaces. The single IP65 repeater unit is roof-mounted and comes as a kit with antenna set and the required cabling.
The European Global Navigation Satellite Systems Agency (GSA) has selected Eutelsat Communications for the development, integration and operation of the next-generation EGNOS payload on a future Eutelsat satellite.
Credit and copyright: GSA.
Eutelsat and GSA have concluded a long-term contract valued at €102 million covering the preparation and service provision phases for the EGNOS GEO-3 payload that will be hosted on the Eutelsat 5 West B satellite that is due for launch end of 2018.
The new payload marks a replenishment of current EGNOS capacity and is scheduled to start service in 2019 for a duration of 15 years.
With the addition of the EGNOS payload, Eutelsat is further optimizing the Eutelsat 5 West B satellite that was commissioned in October 2016 on a design-to-cost basis from Airbus Defence and Space and Orbital ATK. Airbus Defence and Space is building the satellite’s commercial Ku-band payload and the EGNOS payload while the platform is being manufactured by Orbital ATK.
The EGNOS GEO-3 payload on Eutelsat 5 West B will comprise two L-band transponders that will act as an augmentation, or overlay to GNSS messages. Data from GNSS measurements received by an interconnected ground network of positioning stations across Europe will be transferred to a central computing centre where differential corrections and integrity messages will be calculated and then broadcast by Eutelsat 5 West B to users.
The new payload will be the first step towards the deployment of the EGNOS next generation, EGNOS V3. This new generation of EGNOS will augment both Galileo and GPS and is planned to be qualified by 2022. EGNOS V3 will provide a higher level of performance and robustness than the current EGNOS legacy services, as required by the growing use and reliance on such services.
Established in 1977, Eutelsat Communications specializes in communications satellites. The company provides capacity on 39 satellites to clients that include broadcasters and broadcasting associations, pay-TV operators, video, data and internet service providers, enterprises and government agencies.
Higher-Accuracy Positioning to Improve GNSS Network Performance and Reliability
Trimble has introduced version 3.10 of its Pivot Platform software, a modular solution for real-time GNSS infrastructure management, ranging from a single-base GNSS continuously operating reference station (CORS) to a full real-time network (RTN), serving thousands of end-users worldwide.
Version 3.10 provides improvements to network performance and office and field productivity. The new features and capabilities include:
Galileo support provides access to five GNSS constellations — GPS, GLONASS, BeiDou, QZSS and now Galileo — allowing end-users to expect improved positioning accuracy and fix performance from the 50 percent increase of accessible satellites;
GPS L5 support utilizes all available L5 third-frequency GPS observations to enable end users to further improve field productivity;
Code Bias Calibration client and server improvements provide a higher availability of network-modeled RTK corrections to allow field users to reduce dependency on station biases;
Sparse Network supports Galileo and BeiDou. Sparse Network, a Trimble technology, enables RTN operators to achieve the benefits of a full network-processed GNSS constellation even if the network is not fully covered with multi-constellation CORS.
Dynamic Station Coordinates (DSC) module improvements minimize the impact of erroneous reference station coordinates to improve system performance.
“Trimble continues to transform the way our customers manage their real-time GNSS infrastructure by making networks more robust and easier to manage,” said Mark Richter, director of marketing for Trimble’s Advanced Positioning Division. “Accessibility to the Galileo constellation and the addition of the L5 third-frequency observations in particular, makes Trimble’s Pivot Platform significantly more versatile to improve functionality and performance for end users in the field.”
Trimble Pivot Platform version 3.10 is available now from Trimble’s Distribution Network and Trimble Sales Representatives. Customers with a valid software maintenance agreement receive the new version at no additional cost.
Here’s a panorama in broad strokes across the range of GNSSs, garnered from top system spokespersons at the Munich Satellite Navigation Summit. It’s been several years since breaking news was aired at this annual late winter/early spring event, but it’s always good for a wide-ranging update, recalibrating levels, so to speak.
GPS. With 31 operational satellites (24 is baseline) and an estimated 3 billion receivers in use worldwide, what more needs to be said about the gold standard? Its best week ever for accuracy logged a signal-in-space performance average of 45.3 centimeter. The next-generation ground control system OCX “survived quite a struggle” and has emerged from Nunn-McCurdy breach, back on track and seemingly ready for future action. Or at least for future pre-certification tests. SV1 of the GPS III generation has completed all tests and is in storage, awaiting the first GPS III launch in spring 2018. SV02 and 03 are in assembly and integration, SV04 thru 08 are in box-level assembly, and 09 and 10 are on contract. Technical challenges with payload have been resolved.
(Click to enlarge.) Galileo satellite top-level block diagram. OHB Systems AG as prime contractor and Surrey Satellite Technology (SSTL) have teamed for production of the navigation satellites. OHB is responsible for the concept, the satellite platforms and the satellite-level inegration and test. SSTL supplies the satellite payloads and supports OHB on system level. OHB also supports the customers during launch preparation and in-orbit testing. (Image courtesy OHB)
Galileo. With 18 on-orbit satellites (15 operational), the European GNSS can be termed a coming thing. Performance statistics are based on only 11 of these satellites however; the four most recently launched in November 2016 are not yet included. Nevertheless, the system is logging 80-centimeter ranging accuracy. Eight more await launch: four in 2017, and four in 2018. The constellation is broadcasting the Open Service, the Public Regulated Service, and the Search and Rescue (SAR) signal. The SAR service will officially launch in early April — on April 6, because 406 MHz is the Emergency Position Indicating Radio Beacon frequency. Galileo has improved the historic SAR location performance from 3 hours to 10 minutes. The Commercial Service is still in preparation, and will be available in 2020. Spoofing is seen as a very real threat to GNSS overall by the Galileo authorities, as exemplified by the recent bloom of amateur spoofers encouraged by Pokemon go.
GLONASS. The Russian system will undertake three or four launches this year; one of them will be a triple-satellite launch. There have been several disruptions to efforts to decrease the offset between GLONASS system time and Universal Coordinated Time but the initiative perseveres. English versions of four system interface control documents (ICDs), to include the new CDMA signal, are promised for Q2 2017; Chinese versions are coming, too. Russian-language ICDs are available at glonass.aic.ru.
BeiDou. With the addition of three new satellites in the past year, China’s system is enjoying improved system performance. Hydrogen clocks are succeeding rubidium clocks, bring an order-of-magnitude improvement in timing accuracy. A BeiDou white paper was published last June, and a revised ICD appeared in November.
In the massive Chinese mass market, 30 percent of smartphones sold in China now have BeiDou capability; that’s out of a 700–800 million total. Huawei multi-function chip LX1101 is a key driver behind this. Unistrong has released a phone with RTCM input for professional use, blurring the line between mass and professional markets.
Six to eight satellites will be launched this year, and 10 to 12 in 2018. BeiDou is in a “very ambitious and aggressive race with time to complete the global system.”
ICG. The United Nations’ International Committee on Global Navigation Satellite Systems will meet in Japan in December of this year, in China next year, and in India in 2019. This can be interpreted as vigorous international interest and “a desire to advance and promote their respective systems’ visibility” worldwide. All pertinent documents can be found at unoosa.org.
EGNOS. The European Geostationary Navigation Overlay Service has two operational geosynchronous Earth-orbit satellites (GEOs) in operation, plus one in test and one in deployment, ready to swap in. It is extending its Ranging and Integrity Monitoring Stations (RIMS) to several new countries, notably Israel and the Ukraine. EGNOS.v3 is coming and will introduce dual-frequency (L1 and L5) service, and also Galileo with GPS, for multi-constellation corrections. The new system’s qualification is planned for 2022.
QZSS. This year, Japan’s Quasi-Zenith Satellite System will launch the second and third of the figure-eight inclined geosynchronous orbit (IGSO) satellites of the Michibiki type, to become operational in 2018. A GEO bird will also be launched. A seven-satellite system is the ultimate goal.
Among other announcements of note made during the course of the Summit, although not by the GNSS operators’ spokespersons:
(Click to enlarge.) Key features of the Galileo satellites.
• OHB, the Galileo satellite manufacturer, said its customer has decided to refurbish the clocks on eight satellites in preparation. “Satellite navigation is nothing but comparison of very precise clocks.”
• Airbus announced a new concept for train positioning integrity: “virtual valises” to correct train position that will replace or augment current trackside valises that are very expensive to build and maintain.
• Munich Aerospace (munich-aerospace.de), a public-private non-profit venture between DLR, the German space agency, Bauhaus Luftfahrt and two technical universities, will mount a Ph.D-level education and research program for 70 individuals, with candidates from 27 nations. This will be located in “the Bavarian Silicon Valley.” It will also undertake a global effort with several other organizations.
• One of the above technical universities, the Federal Armed Forces University in Munich, announced that it is investigating Lidar for potential use in an asteroid mining project for future space exploration. It also has underway initiatives concerning Lidar + GNSS and inertial + GNSS for autonomous vehicles.
By Stephen Mackey, Hadi Wassaf, Karen Van Dyke, Christopher Hegarty, Karl Shallberg, John Flake and Terence Johnson.
OOBE Levels associated with LTE signal power used in testing. Source: Stephen Mackey, Hadi Wassaf, Karen Van Dyke, Christopher Hegarty, Karl Shallberg, John Flake and Terence Johnson.
The Adjacent Band Compatibility Assessment evaluated the adjacent radiofrequency band power levels that can be tolerated by GPS and GNSS receivers, to advance the U.S. Department of Transportation’s understanding of the extent to which such power levels impact devices used for transportation safety purposes, among other applications. The paper describes the testing approach and data analysis used to develop interference tolerance masks (ITMs) based on a 1-dB carrier-to-noise-ratio (CNR) degradation. DOT and other participants tested 80 GPS/GNSS receivers in an anechoic chamber. Four types of testing were conducted which involved a linearity test, 1-MHz Bandpass Noise, 10-MHz Long Term Evolution (LTE), and effects of third order intermodulation.
This paper also presents the resulting ITMs and puts forward a recommendation for the bounding ITM for each GPS/GNSS receiver category. Given a particular use case scenario, the significance of these bounding ITMs is that they provide information that is necessary for the downstream analysis to determine the maximum Effective Isotropic Radiated Power (EIRP) that can be tolerated in the adjacent radiofrequency bands on a per category basis. The paper discusses acquisition results as they relate to the 1-dB CNR degradation limit, and a cross comparison for some of the receiver results between radiated and conducted tests incorporating the appropriate antenna characterization data.
SBG Systems has released a new generation of its advanced and compact inertial navigation systems. The Ekinox 2 series features new accelerometers and gyroscopes, enhancing attitude accuracy by a factor of two over the original Ekinox.
Photo: SBG
The Ekinox series is a line of tactical grade MEMS-based inertial navigation systems, first released in 2013. The latest improvements come from a complete redesign of the in-house inertial measurement unit (IMU), integrating cutting-edge gyroscopes and accelerometers.
With higher accuracy for the same form factor and price level, Ekinox 2 Series is designed for industrial-grade vehicle navigation, equipment motion compensation and data georeferencing. It provides a 0.02-degree roll and pitch, 0.05-degree heading and a centimeter-level position.
Applications for the Ekinox 2 include hydrography, mobile mapping and antenna tracking. With new accelerometers, this new generation has also significantly improved its resistance to vibration. Finally, the addition of the BeiDou constellation improves signal availability in Asia.
Compact and light-weight, the Ekinox Series has been designed to simplify installation operations. Configuration is made with an intuitive embedded web interface where all parameters can be displayed and adjusted. For example, users can choose a profile (vessel, plane, car, etc.), and the 3D view will provide a visualization of settings such as the sensor position, alignment and lever arms.
The Ekinox 2 Series is ITAR Free. The product line will be available during the second quarter of 2017.
All across Europe, the number of smart cities is multiplying. To tackle their growing needs and to guarantee efficient city planning and maintenance, many cities are engaged in massive investments in such key areas as street lighting, road maintenance, traffic and waste management.
In parallel, public transportation is continuously evolving in terms of coverage, comfort and technology.
Within this context, the exploitation of Galileo and its integration with other sensors is key to developing concrete solutions for current and future smart-city planning. Along these lines, the Horizon 2020-funded GHOST (Galileo Enhancement as Booster of the Smart Cities) project is designing, developing and validating an intelligent system for vehicles that equips existing public transport fleets with a Galileo-enabled camera and connects these vehicles to a web portal.
The GHOST system equips existing public transport fleets with a Galileo-enabled camera and connects these vehicles to a web portal. (Photo: GSA)
The system automatically takes pictures of predefined points of interest (POI) based on the accurate position of the vehicle — provided by Galileo. All images are sent to a processing server capable of detecting such anomalies as potholes or a burnt-out street light. The system then uses the web portal to report these findings to the relevant authorities.
“At this point, GHOST is designed primarily for reporting street lighting anomalies and road deteriorations, monitoring public garbage collection and detecting double parking infractions or disabled parking spots occupied by unauthorized vehicles,” said Project Coordinator Claudia Maltoni. “In addition to these basic functions, we have also identified more advanced services, such as spotting bus-lane and congestion-charging-area violations, which will be implemented at a later date.”
A user-focused system
The GHOST system’s key differentiator is its use of Galileo positioning, which gives it the capability to take autonomous snapshots with an error range of 1 to 10 meters (depending on the size of the POI). In densely populated urban environments, such a level of service is only possible with the combined use of Galileo, inertial sensors and Kalman filters.
The GHOST system’s key services:
reporting street lighting anomalies and road deteriorations
monitoring public garbage collection levels
detecting double parking infractions or disabled parking spaces occupied by unauthorized vehicles
monitoring timely collection of garbage.
Another unique feature is a free smartphone application that citizens can use to collect geo-localized snapshots. “Whenever an individual user sees an anomaly within a city’s infrastructure, all they have to do is snap a picture with their smartphone,” explained Maltoni. “This level of engagement not only enhances the overall system, but also empowers individual users to play a key role in urban upkeep.”
Improving urban efficiency
By taking advantage of the many vehicle movements happening in cities every day, GHOST proposes a competitive way to improve the efficiency of monitoring a city’s operations and infrastructure. Once finalized, the system will enable faster detection of double parking or road deterioration and help reduce traffic, accidents and pollution.
“Thanks to our field tests and favourable lab results, we are already setting up the next phase of the project, with the aim of taking the system’s technology to the next level,” concluded Maltoni. “This includes providing real-time, onboard image processing so that the system can handle such dynamic scenarios as bus-lane infractions and congestion-charging enforcement.”
The project is working to bring GHOST technology to market. Coordinators are busy making key contacts with interested public administrations, garbage collection companies and traffic police departments. It is also working to ensure that the system complies with all European regulatory standards, such as those related to circulation or privacy.
On March 20, PRN 123 (now in test) will be introduced in the operational platform, and on March 21, PRN 136 will be moved from the operational platform to the test platform.
Users equipped with non-(E)TSO-certified SBAS receivers (such as those used in agriculture, surveying, mapping and maritime, but not in aviation), it is recommended that users reassess the equipment configuration after the change, to ensure that both operational EGNOS GEO satellites (PRN 120 and PRN 123) are configured in the equipment.
More details on this change are available in the official Service Notice #15.
Depending on the receiver, users can check equipment manuals or contact product manufacturer/dealer. Guidance is provided on the EGNOS website on how to configure an EGNOS receiver for some of the most common equipment used in agriculture.
A SpaceX Falcon 9 stands ready for launch from Cape Canaveral Air Force Station, Fla. The Air Force awarded a second contract for GPS III Launch Services to SpaceX.
SpaceX has won a second contract from the U.S. Air Force for launch services to deliver a GPS III satellite to its intended orbit.
SpaceX was awarded the $96,500,490 firm-fixed-price contract over the United Launch Alliance. ULA — a joint venture of Lockheed Martin Space Systems and Boeing Defense, Space & Security — did not compete for the first GPS III launch contract. That contract, worth $82.7 million, is expected to orbit a GPS satellite aboard a Falcon 9 rocket in May 2018.
According to the contract announcement, SpaceX will provide launch vehicle production, mission integration, launch operations, spaceflight worthiness and mission unique activities for a GPS III mission. The contract is being overseen by the Air Force’s Space and Missile Systems Center (SMC), Los Angeles Air Force Base, California.
Work will be performed at Hawthorne, California; Cape Canaveral Air Force Station, Florida; and McGregor, Texas. It is expected to be complete by April 30, 2019.
“The competitive award of the GPS III Launch Services contract to SpaceX directly supports SMC’s mission of delivering resilient and affordable space capabilities to our nation,” said Lt. Gen. Samuel Greaves, leader of SMC.
A technique developed by researchers at the University of Illinois at Urbana-Champaign distinguishes a reflected non-line-of-sight (NLOS) signal of a particular satellite from the LOS signal and characterizes the NLOS signal as coming from a virtual mirror-image satellite in the direction of the signal reflection point. By using information on the position and orientation of the reflector, the NLOS signal can be treated as an additional LOS signal.
By Yuting Ng and Grace Xingxin Gao
INNOVATION INSIGHTS with Richard Langley
THIS ARTICLE IS ABOUT VIRTUAL SATELLITES. No, we don’t mean physical objects that are almost satellites. That’s the common everyday meaning of the word virtual. We mean it in the sense used in computing to describe something that is not physically present but made to appear so by software (and perhaps aided by hardware). The word was first used in this sense by computer scientists in the 1950s in the term virtual memory to describe a memory management technique. It is now widely used in computing, most commonly as virtual reality. But what is a virtual satellite then?
As we all know, GPS satellite signals are quite weak. The antenna of a standard GPS receiver needs to have a clear line-of-sight (LOS) view to the satellites for successful signal tracking and position determination. Buildings and other structures will block signals coming from certain directions. In built-up areas, this can result in fewer LOS signals than the minimum of four needed for unaided positioning. Even with four or more LOS signals, the receiver-satellite geometry may be poor resulting in a large dilution of precision and poor positioning accuracy as a result. It is true that augmentations such as wheel sensors and inertial measurement units coupled with dead reckoning may permit an acceptable level of positioning accuracy for some kinematic applications, but the accuracy will degrade over time if satellite blockage continues unabated. And yes, multi-GNSS can help in these situations with receivers availing themselves of additional LOS signals from the GLONASS, Galileo, and BeiDou systems and in Japan, QZSS. But Galileo, BeiDou and QZSS are still in development with a variable number of satellites available at a given location during the day. Is there anything else that can be done to improve the availability of GPS signals?
In fact, there are often more GPS signals arriving at a receiver’s antenna than just the LOS signals. These are non-line-of-sight (NLOS) signals that bounce off nearby structures before arriving at the antenna. We call the phenomenon multipath and, as we have discussed before in this column, multipath typically reduces positioning performance when the NLOS signals from a particular satellite combine with the LOS signal to distort a receiver’s standard correlator outputs thereby biasing pseudorange and carrier-phase measurements. Various techniques have been developed to reject multipath signals at the antenna or in the receiver while others have been developed to lessen the effect of these signals and so minimize their impact on position solutions. On the other hand, non-positioning GPS applications have been developed to use reflections from the Earth’s surface to measure snow depth, ground moisture content, and ocean-surface roughness. But could we somehow use multipath signals to improve positioning applications rather than degrade them?
In this month’s column, we look at a technique developed by researchers at the University of Illinois at Urbana-Champaign that distinguishes a reflected NLOS signal of a particular satellite from the LOS signal and characterizes the NLOS signal as coming from a virtual mirror-image satellite in the direction of the signal reflection point. By using information on the position and orientation of the reflector, the NLOS signal can be treated as an additional LOS signal, albeit from a ghost satellite. The authors have demonstrated that the technique works well in practice and in one difficult positioning environment, obtained an improvement in horizontal position accuracy of 40 meters — a reflected blessing indeed.
Building obstructions and reflections present serious challenges to GPS receivers operating in urban environments. In such environments, buildings may obstruct GPS signals, leading to reduced GPS signal availability. In addition, buildings may reflect GPS signals, resulting in reception of non-line-of-sight (NLOS) signals. NLOS GPS signals are delayed versions of the line-of-sight (LOS) signals. As such, they lead to pseudorange errors, resulting in positioning errors. Conventional approaches treat NLOS GPS signals as unwanted interference to be rejected or mitigated.
Conventional approaches reject NLOS GPS signals at multiple stages of GPS signal processing. Antenna-based approaches include the use of right-hand-circularly-polarized (RHCP) antennas and controlled reception pattern antennas (CRPA). Correlator-based approaches include the use of the narrow correlator, the double-delta correlator, the multipath estimating delay lock loop (MEDLL) and the vision correlator by various receiver manufacturers. In addition, receiver autonomous integrity monitoring (RAIM) approaches reject pseudoranges with inconsistent positioning residuals.
Besides rejecting NLOS GPS signals, conventional approaches also make use of robust filtering and joint signal tracking techniques to mitigate the effects of these signals. Robust filtering techniques include the use of Bayesian filters such as Kalman filters and particle filters. Joint signal tracking techniques include vector tracking and direct position estimation (DPE). A list of existing approaches addressing NLOS GPS signals is provided in TABLE 1.
TABLE 1. Approaches for rejecting and mitigating NLOS GPS signals.
In contrast to conventional approaches that reject or mitigate the effects of NLOS GPS signals, we propose transforming NLOS GPS signals from being unwanted interference to becoming additional useful navigation signals. In addition, we provide a navigation solution under reduced GPS signal availability.
RELATED WORK
In our approach to using NLOS GPS signals, we make use of DPE and 3D map-aided positioning. The following sections provide an overview of these techniques.
Direct Position Estimation. DPE is an unconventional joint signal tracking and navigation technique that directly estimates the GPS receiver’s navigation parameters from the GPS raw signal. It does so by directly comparing the expected signal reception of multiple potential navigation candidates against the actual received signal. The navigation solution is then estimated as the navigation candidate with the highest overall correlation between the expected and the actual received signal. This overall correlation is an accumulation of signal correlations across all available satellites, with replica signal parameters aligned to the candidate navigation parameters. In this manner, DPE jointly uses signal correlations from all available satellites to produce a robust navigation solution.
3D Map-Aided Positioning Techniques. State-of-the-art approaches use available 3D maps to predict NLOS signal reception. Apart from rejecting and/or mitigating the effects of NLOS pseudoranges, state-of-the-art approaches leverage the benefits of NLOS pseudoranges, constructively using the affected pseudorange measurements through special treatment of NLOS paths during trilateration. Using 3D building models, they model NLOS paths as LOS paths from satellites to virtual receivers located at receiver mirror-image positions. However, these approaches are limited by the issue of reduced signal availability due to multipath fading in addition to building obstruction. Under reduced signal availability, the navigation solution obtained via trilateration is degraded. With further reduction in signal availability — the number of available pseudorange measurements reduced to fewer than four — conventional calculation of the GPS navigation solution via trilateration with four unknowns is not possible.
In contrast to state-of-the-art approaches addressing NLOS signal reception at the GPS pseudorange measurement level, we directly address and constructively use NLOS signals at the GPS signal level via DPE using NLOS signals.
OUR APPROACH: DPE USING NLOS SIGNALS
We first model NLOS signals as LOS signals to virtual satellites at satellite mirror-image positions, as shown in FIGURE 1. This approach is similar to using virtual transmitters for multipath-assisted wireless indoor positioning. We calculate these satellite mirror-image positions and velocities using knowledge of building reflection surfaces estimated from available 3D maps.
FIGURE 1. NLOS signal transformed from being (top) an unwanted interference to becoming (bottom) an additional LOS signal to a virtual satellite at the satellite mirror-image position.
We then integrate these NLOS signals into GPS positioning via DPE. We modify the expected signal reception used in DPE to include NLOS signal information, as shown in FIGURE 2. Our approach deeply integrates this information and accurately describes the actual received signal.
FIGURE 2. Overall correlation in DPE, with the NLOS signal treated as an additional LOS signal to a virtual satellite at the satellite mirror-image position.
In addition, our approach provides a navigation solution under reduced signal availability. FIGURE 3 shows a block diagram of our approach.
FIGURE 3. Block diagram of DPE using NLOS signals and involving calculation of satellite position, velocity and time (PVT) and batch correlation using a fast Fourier transform (FFT).
IMPLEMENTATION AND EXPERIMENT RESULTS
We implemented DPE using NLOS signals with commercial front-end components and our software platform, PyGNSS. We conducted an experiment in front of the 53 meters by 40 meters wind tunnel located at NASA’s Ames Research Center, Mountain View, California (see FIGURE 4).
FIGURE 4. Experiment setup in front of the 53 meters by 40 meters wind tunnel located at NASA’s Ames Research Center, Mountain View, California. (a) data collection equipment; (b) wide-angle photograph of the wind tunnel’s air-intake port.
The material of the vertical surface of the wind tunnel’s air-intake port is a metal wire mesh with a grid spacing of 1.8 centimeters by 1.8 centimeters, as shown in FIGURE 5. This grid spacing is approximately one tenth of the carrier wavelength of the GPS L1 signal; the mesh wire radius is much less than the grid spacing. Thus, the vertical surface of the air-intake port acts as a reflector of GPS L1 signals.
FIGURE 5. Metal wire mesh on the vertical surface of the wind tunnel’s air-intake port. (Left) close-up photograph showing the grid spacing of 1.8 centimeters by 1.8 centimeters; (right) photograph from another perspective showing wire mesh covering the entire vertical surface of the air-intake port.
We estimated the normal vector and a point on the wind tunnel’s reflection surface using a geo-referenced 3D point cloud available on line through the National Oceanic and Atmospheric Administration’s (NOAA’s) Data Access Viewer tool. We refined the estimate using iterative closest point map-matching with a lidar scan (FIGURE 6).
FIGURE 6. Building reflection surface estimated from NOAA Data Access Viewer (DAV) point cloud, refined using map-matching with a lidar scan.
We then determined possible LOS and NLOS paths from satellite elevation-azimuth plots. Plotted in FIGURE 7 are the satellite positions, the satellite mirror-image positions and the building reflection surface. An NLOS path to a satellite exists if the corresponding LOS path to the satellite mirror-image intersects the building reflection surface. In our experiment, LOS paths exist to satellite PRNs 5, 7, 27 and 28 and an NLOS path exists to satellite PRN 5. Thus, both LOS and NLOS signals from satellite PRN 5 are present. This is verified by examining the amplitude of the in-phase prompt correlations over time. Only the in-phase prompt correlations of satellite PRN 5 exhibit a sinusoidal behavior characteristic of having both LOS and NLOS signals, as shown in FIGURE 8.
FIGURE 7. Elevation-azimuth plot with satellites highlighted using green boxes and satellite mirror-images highlighted using red boxes. The 3D point cloud of the wind tunnel’s air-intake port is plotted using grey dots. The path to the mirror-image of satellite PRN 5 passes through the surface of the wind tunnel. Thus, an NLOS path to satellite PRN 5 exists. In addition, LOS paths exist to satellite PRNs 5, 7, 27 and 28.FIGURE 8. Only the in-phase prompt correlation of satellite PRN 5 exhibits a sinusoidal behavior characteristic of having both LOS and NLOS signal components.
We then performed DPE, including the signal correlation contribution from the NLOS path to satellite PRN 5, where the NLOS path is represented as a LOS path to the satellite mirror-image. The overall correlation result, including the signal correlation from the NLOS path to satellite PRN 5, is shown in FIGURE 9. The color of the position markers, plotted using Google Maps, represents the overall correlation amplitude. Red indicates a high overall correlation amplitude and blue indicates a low overall correlation amplitude. The navigation solution is directly estimated as a correlation-weighted mean of the navigation candidates.
FIGURE 9. Normalized overall correlation with contributions from all satellites, including the satellite mirror-image of PRN 5.
The result, as compared to that estimated using pseudoranges from scalar tracking followed by trilateration, is shown in FIGURE 10. DPE using NLOS GPS signals demonstrated improved horizontal positioning accuracy by 40 meters.
FIGURE 10. DPE using NLOS GPS signals demonstrates improved horizontal positioning accuracy by 40 meters. This is in comparison to the navigation result obtained using pseudoranges estimated from conventional scalar tracking followed by trilateration.
CONCLUSION
In summary, we proposed DPE using NLOS signals to mitigate the issues of NLOS GPS signal reception and reduced GPS signal availability in urban navigation. We modeled NLOS signals as LOS signals to virtual satellites at satellite mirror-image positions. In this manner, NLOS signals are transformed from being unwanted interference to becoming additional useful navigation signals. We then created expected signal receptions to include NLOS GPS signal information at multiple potential navigation candidates and use DPE for positioning. Finally, we experimentally demonstrated a reduction in horizontal positioning error by 40 meters. This is in comparison to the navigation result obtained using pseudoranges estimated from conventional scalar tracking followed by trilateration.
ACKNOWLEDGMENTS
The authors thank the Safe Autonomous Flight Environment (SAFE50) and the Unmanned Aircraft System Traffic Management teams at NASA’s Ames Research Center, where the lead author was hosted for the summer of 2016, for their equipment support. The authors also thank Akshay Shetty for collecting and map-matching the lidar scan to the geo-referenced 3D point cloud.
This article is based on the paper “Direct Position Estimation Utilizing Non-Line-of-Sight (NLOS) GPS Signals” presented at ION GNSS+ 2016, the 29th International Technical Meeting of the Satellite Division of The Institute of Navigation, held Sept. 12–16, 2016, in Portland, Oregon.
YUTING NG received her B.S. degree in electrical engineering and her M.S. degree in aerospace engineering from the University of Illinois at Urbana-Champaign (UIUC) in 2014 and 2016, respectively. Her research interests are advanced signal processing, satellite navigation systems and radar.
GRACE XINGXIN GAO is an assistant professor in the Aerospace Engineering Department at UIUC. She obtained her Ph.D. degree in electrical engineering from the GPS Laboratory at Stanford University in 2008. Before joining UIUC in 2012, she was a research associate at Stanford University.
FURTHER READING
• Authors’ Conference Paper
“Direct Position Estimation Utilizing Non-Line-of-Sight (NLOS) GPS Signals” by Y. Ng and G.X. Gao in Proceedings of ION GNSS+ 2016, the 29th International Technical Meeting of the Satellite Division of The Institute of Navigation, Portland, Oregon, Sept. 12–16, 2016, pp. 1279–1284.
“Mitigating Jamming and Meaconing Attacks Using Direct GPS Positioning” by Y. Ng and G.X. Gao in Proceedings of IEEE/ION PLANS 2016, the Position, Location, and Navigation Symposium, Savannah, Georgia, April 11–14, 2016, pp. 1021–1026, doi: 10.1109/PLANS.2016.7479804.
“Evaluation of GNSS Direct Position Estimation in Realistic Multipath Channels” by P. Closas, C. Fernández-Prades, J. Fernández-Rubio, M. Wis, G. Vecchione, F. Zanier, J.A. Garcia-Molina and M. Crisci in Proceedings of ION GNSS+ 2015, the 28th International Technical Meeting of the Satellite Division of The Institute of Navigation, Tampa, Florida, Sept. 14–18, 2015, pp. 3693–3701.
“On the Maximum Likelihood Estimation of Position” by P. Closas, C. Fernández-Prades and J. Fernández-Rubio in Proceedings of ION GNSS 2006, the 19th International Technical Meeting of the Satellite Division of The Institute of Navigation, Fort Worth, Texas, Sept. 26–29, 2006, pp. 1800–1810.
“NLOS Correction/Exclusion for GNSS Measurement Using RAIM and City Building Models” by L.-T. Hsu, Y. Gu and S. Kamijo in Sensors, Vol. 15, No. 7, 2015, pp. 17329–17349, doi: 10.3390/s150717329.
“GPS Multipath Detection and Rectification Using 3D Maps” by S. Miura, S. Hisaka and S. Kamijo in Proceedings of ITSC 2013, the 16th International IEEE Conference on Intelligent Transportation Systems, The Hague, The Netherlands, Oct. 6–9, 2013, pp. 1528–1534, doi: 10.1109/ITSC.2013.6728447.
“Urban Multipath Detection and Mitigation with Dynamic 3D Maps for Reliable Land Vehicle Localization” by M. Obst, S. Bauer and G. Wanielik in Proceedings of IEEE/ION PLANS 2012, the Position, Location, and Navigation Symposium, Myrtle Beach, South Carolina, April 23–26, 2012, pp. 685–691, doi: 10.1109/PLANS.2012.6236944.
• Virtual Transmitters
“Simultaneous Localization and Mapping in Multipath Environments” by C. Gentner, B. Ma, M. Ulmschneider, T. Jost and A. Dammann in Proceedings of IEEE/ION PLANS 2016, the Position, Location, and Navigation Symposium, Savannah, Georgia, April 11–14, 2016, pp. 807–815, doi: 10.1109/PLANS.2016.7479776.
