U.S. Coast Guard issues testing notice on GPS Week Number Rollover.
The GPS Directorate has released a Federal Register Notice announcing plans to execute a test in February to investigate legacy receiver week roll-over behavior and analyze any off-nominal behavior exhibited, according to a U.S. Coast Guard notice.
Photo: andrey_l/Shutterstock.com
The GPS week number rollover occurs in the GPS legacy navigation (LNAV) message every 1024 weeks due to the GPS week number being represented by only 10 bits within the LNAV message.
The next GPS week number roll over will occur 18 seconds prior to the 0000Z boundary (Coordinated Universal Time) between April 6/7 2019.
In most cases, any negative response from a GPS receiver caused by a problem accounting for the 10-bit week number roll over would likely affect the calendar conversion from GPS time to UTC date/time and could result in the GPS receiver thinking it had jumped backward in time by 1024 weeks to 21/22 August 1999.
To participate in the test, submit the answers to the nine questions in the Federal Register Notice to the SMC/GPE mailbox by Feb. 4. After the submission of the questionnaire, the team will schedule individual meetings with interested civil vendors to further discuss their participation in the test in more detail.
Satellite NTS-3 above Earth. (Illustration: Lt. Jacob Lutz, AFRL Space Vehicles Directorate)
Harris Corporation has been selected as the prime contractor to build Navigation Technology Satellite-3, the next-generation experimental positioning, navigation and timing (PNT) spacecraft. The satellite, called NTS-3, is expected to launch in 2022, with one year of experimental operations.
The Air Force Research Laboratory and the Space and Missile Systems Center selected Harris on Dec. 20, 2018, and announced it on Jan. 17.
PNT Testbed
As a unique testbed in geosynchronous orbit, NTS-3 will integrate several advanced technologies to demonstrate resiliency and new concepts of operation to include experimental antennas, flexible and secure signals, increased automation, and use of commercial ground assets.
Technologies matured and knowledge gained from NTS-3 are expected to transition to future generations of GPS and augmentation layers for national PNT capabilities.
Satellite NTS-3 closeup. (Illustration: Lt. Jacob Lutz, AFRL Space Vehicles Directorate)
“The National Defense Strategy tells us we must evolve our nation’s Position, Navigation, and Timing capabilities to be more resilient,” said AFRL Space Vehicles Director Col. Eric Felt. “NTS-3 is all about resiliency, and I am incredibly excited about the resiliency experiments our SMC, AFRL, and Harris team will be able to conduct with NTS-3’s innovative and flexible hardware, software, and waveforms.”
Agile Waveform Platform
In support of NTS-3, Harris plans to develop the Agile Waveform Platform, a digital signal generator that can be reprogrammed on-orbit, enabling operators to quickly develop and deploy new signals to meet rapidly-evolving needs on the battlefield.
Additionally, Harris’ electronically steerable phase-array antenna will support simultaneous broadcast of multiple waveforms in both Earth-coverage and spot-beam configurations.
NTS-3 will use Northrop Grumman Innovation System’s ESPAStar bus, building on AFRL’s EAGLE spacecraft that launched in April 2018.
The NTS-3 Space Experiment
Navigation Technology Satellite – 3 (NTS-3) was selected as the Space Vehicle Directorate’s next major integrated space experiment in 2015, and it represents AFRL’s first PNT flight experiment to prototype a more resilient PNT multi-layer architecture in accordance with the Space Enterprise Vision (SEV) and the Space Warfighting Construct (SWC).
Satellites NTS-1, 2 and 3. (Illustration: Lt. Jacob Lutz, AFRL Space Vehicles Directorate)
NTS-3 builds on a heritage of Department of Defense (DoD) satellite navigation (SATNAV) success that began in the 1970s with the predecessors of the modern GPS constellation. NTS-1 was developed by the Naval Research Laboratory (NRL) and launched in 1974 with two rubidium-vapor frequency standards that advanced the timing and navigation precision demonstrated by the earlier TIMATION satellites.
NTS-2 launched in 1977 as the first NAVSTAR GPS Phase I satellite, and demonstrated cesium frequency standards and a worldwide network for data acquisition. There has been no major DoD SATNAV developmental program for experimentation since then, until NTS-3.
In 2017, AFRL restructured NTS-3 to emphasize mission objectives to demonstrate disaggregated, resilient PNT in a multi-layer space architecture, as outlined by the SEV and the SWC. NTS-3 will provide space qualification for core technologies such as on-orbit digital signal reprogrammability and solid-state amplifiers. In addition to new signals, onboard experiments include improvements to timing accuracy and integrity, including ensembling to improve long- and short-term stability. NTS-3 will demonstrate key tactics, techniques and procedures (TTPs) for multi-layer PNT through all three segments of the SATNAV system: space, control, and user.
Ground Control
Braxton Technologies was selected in June 2017 to handle NTS-3 SATNAV ground control, while demonstrating and maturing innovative and affordable ground-based command and control capabilities to ensure resilient PNT in contested and denied environments.
Braxton experts also will demonstrate satellite ground-control technologies to inform future GPS ground-control systems. They will use the Multi-Mission Space Operations Center (MMSOC) open architecture standard, as well as the Air Force Satellite Control Network (AFSCN) for primary direct and secure communications with the future NTS-3 space payload.
Satellite NTS-3 in space. (Illustration: Lt. Jacob Lutz, AFRL Space Vehicles Directorate)
Ground control segment (GCS) objectives include commanding of multiple antennas to form high-gain regional beams in conjunction with traditional Earth coverage beams, and processing the subsequent impact on phase center bias and pattern variation.
The GCS will also incorporate commercial antennas for TT&C and experiment with automation of common functions to reduce the level of manual control that GPS requires. GCS development will emphasize cyber security and compatibility with Enterprise Ground Services (EGS).
Collaborators Wanted
AFRL/RV is seeking collaboration from industry, government agencies, and universities in developing experimental concepts and participating in the flight experiment.
The 2019 Munich Satellite Navigation Summit, which will take place March 25-27 in Munich, Germany, will offer a number of educational sessions to attendees.
One of the sessions will key in on the future use of the Galileo public regulated service (PRS). According to show organizers, this session will discuss the deployment of the Galileo ground- and space-segment — including the PRS relevant components — which will reach full operational capability in the next years. The session will also cover PRS-receiver developments and PRS testing.
Other sessions offered by the conference will include legal aspects on selected topics in the field of GNSS; augmented reality meets high-accuracy positioning; GNSS program updates; satellite and terrestrial navigation trends; and more.
According to organizers, the Summit is part of the efforts of the Bavarian government and the cluster on aerospace and satellite navigation to stimulate applications and services in this high-tech field.
The event, which is organized by the European Space Agency and ETH Zürich, will take place Sept. 4-6 in Zürich, Switzerland.
The event will bring together members of the European scientific community and their international partners involved in the use of GNSS — specifically Galileo — in their research. In addition, attendees will discuss opportunities where GNSS satellites can be used for scientific purposes.
According to event organizers, the colloquium will address five major areas of research, including:
Scientific applications in meteorology, geodesy, geodynamics, geophysics, space physics, oceanography, land surface and ecosystem studies, using either direct or reflected signals, differential measurements, phase measurements, radio occultation measurements, using receivers placed on the ground, in airplanes or on satellites;
Scientific developments in physics with a potential impact on future GNSS, particularly in testing fundamental laws of physics;
Aspects of metrology such as reference frames, on board and ground clocks, precise orbit determination and time and frequency transfer;
Scientific aspects of satellite navigation, positioning and its applications, such as signal propagation, tropospheric and ionospheric corrections, multi-constellation aspects, hybridisation with additional sensors and integrated navigation, precise positioning;
Transversal topics of interest to a wide number of scientific fields including collection of GNSS big data and GNSS scientific data archives; internet of things positioning for science; scientific payloads in GNSS satellites; novel disruptive technologies for science; the use of cubesats, HAPS, UAVs and autonomous vehicles for GNSS science; software receivers and low-cost SDR platforms; GNSS for space users and applications; and the topic of GNSS science and education.
The conference will be organized as a series of plenary talks, parallel half-day sessions and poster presentations throughout the duration of the event, event organizers add.
By Juan Vázquez, Elisabet Lacarra, Jorge Morán and Miguel A. Sánchez, ESSP SAS, and Julian Rioja and Jimmy Bruzual, Topcon Agriculture
The European Geostationary Navigation Overlay Service (EGNOS), a satellite-based augmentation system (SBAS), provides corrections and integrity information to GPS signals over Europe and is fully interoperable with other SBAS such as North America’s WAAS. Among its services is the internet-based EGNOS Data Access Service (EDAS).
EDAS gathers raw data from GPS, GLONASS and EGNOS GEO satellites collected by receivers at approximately 40 EGNOS ground stations distributed over Europe and North Africa. EDAS reformats and disseminates GNSS data in real time and through an FTP archive to EDAS users and service providers.
Additionally, EDAS provides differential GNSS corrections to the GPS and GLONASS satellites in view by the EGNOS system network through its Ntrip service.
The tests summarized in this article focused on the EDAS Ntrip Service, which can be used for differential positioning. An earlier test near Seville, Spain, concluded that these corrections could support pass-to-pass accuracies in the order of 20 centimeters in a consistent manner and with a high degree of repeatability.
To assess EDAS performance validity for agriculture applications, two additional tests were done in Lisbon, Portugal, and York, UK. These locations provide diversity with respect to the Seville test, especially in terms of distance from the farm to the selected EGNOS reference station (≈320 km in York and 40 km in Lisbon, versus the 110 km baseline of the test in Seville) and also geographically. In all tests, a real-time kinematic solution operated in parallel to the EDAS DGPS solution to provide the required reference for the post-processing of the recorded data. Nine different runs with a total of 78 passes were performed in these two campaigns.
Considering the results from the three tests, the pass-to-pass accuracy supported by EDAS DGPS corrections was below 10 cm for more than 60% of passes and below 20 cm for more than 85 percent of the passes. These figures exceed the earlier results and confirm that EDAS DGPS corrections can deliver pass-to-pass accuracies in the order of 10 to 20 cm in a consistent manner.
Cumulative distribution of P2P accuracy, in centimeters. (Chart: Topcon)
The stability of the results and the very good pass-to-pass accuracy levels observed in the York scenario, where baselines larger than 300 km were tested, deserve highlighting. For grain and dry soil cultivation, at least 1 meter (95th percentile) of absolute horizontal accuracy is required. It can be assumed that, within the area where EDAS DGPS supports sub-meter horizontal accuracies (up to 260 km from the selected EGNOS station, according to previous studies), EDAS DGPS corrections can also support pass-to-pass accuracies in the order of 10-20 cm.
Such performance levels are considered to be appropriate for most grain farm operations. In particular, the observed performance is sufficient to support the following precision agriculture applications:
The document contains the Galileo E6-B and E6-C codes specifications, including primary and secondary codes and their assignment to satellites, which is necessary for manufacturers who are developing Galileo E6-B/C enabled receivers.
The technical note represents the first step for these forthcoming Galileo services: high-accuracy service (HAS) and commercial authentication service (CAS) on E6-B/C signal.
By William Roberts, Joshua Critchley-Marrows, Marco Fortunato, Maria Ivanovici, Nottingham scientific Ltd., Karel Callewaert, Thiago Tavares, VVA Brussels, Laurent Arzel and Axelle Pomies, Telespazio France
The FLAMINGO initiative is developing the infrastructure, solutions and services to enable use of accurate, precise GNSS in the mass-market, operating predominantly in an urban environment. Whilst mass-market receivers are yet to achieve accuracies below one meter for standard positioning, the introduction of Android raw GNSS measurements and the Broadcom dual frequency chipset present such an opportunity.
FLAMINGO will enable high-accuracy positioning and navigation information on devices such as smartphones and internet of things (IoT) devices by producing a service delivering accuracies of 50 cm (at 95 percent) and better, employing multi-constellation, PPP and RTK mechanisms, power consumption optimisation techniques.
Whereas the Galileo High Accuracy Service targets 10-cm precision for professional users, FLAMINGO targets 50-cm precision for consumers. With accuracies of a few decimetres, a range of improved and new applications in diverse market sectors are introduced, including, but are not limited to, mapping and GIS, autonomous vehicles, augmented reality environments, location-based gaming and people tracking.
To obtain such high accuracies with mass market devices, FLAMINGO must overcome several challenges which are technical, operational and environmental. This includes the hardware capabilities of most mass-market devices, where components such as antennas and processors are prioritised for other purposes. We demonstrate that, despite these challenges, FLAMINGO has the potential to meet the accuracy required. Tests with the current smartphones that provide access to multi-constellation raw measurements (the dual-frequency Xiaomi Mi 8 and single-frequency Samsung S8 and Huawei P10) demonstrate significant improvements to the PVT solution when processing using both RTK and PPP techniques. Check out more information here.
Most observers missed the $5 million for a GPS backup technology demonstration in the U.S. Department of Defense appropriation passed in September. Congressional staff say it is included in an obscure research and development line item for “Electronics and Electronic Devices.”
This funding is in addition to the $10 million Congress provided for the project last fiscal year (note: since these are R&D funds, the monies remain available for three years after they are appropriated).
This additional funding is part of Congress’ long but accelerating march to establishing a terrestrial PNT system to backup and complement GPS, an effort with which the administration is struggling to keep pace.
Image: @SENTEDCRUZ
Members in both the Senate and House were surprised and concerned in 2009 when the Obama administration suddenly went against the advice of its departments, national advisory board, and virtually every technologist and engineer in government. That is when the administration decided to terminate plans to convert the old Loran-C system to eLoran as a complement and backup for GPS.
Congress’ concern was not completely allayed when, in a report Congress had mandated, the administration said that a wireless GPS navigation backup was not needed. Users could easily resort to paper maps and charts. The same report did admit that the need for wireless precise timing was another issue. The administration said it would study this, even as the Loran-C system was being terminated.
Subsequent hearings in Congress revealed ongoing concerns about the lack of a terrestrial capability. These were magnified by the nation’s major adversaries, Russia and China, retaining terrestrial Loran systems to inoculate themselves from the effects of disruptions to their space-based PNT systems.
More mixed signals from administration officials amplified Congress’ concerns and frustrations. These included:
The Department of Defense committing to establishing a terrestrial backup for GPS within the United States, then reversing its position just before its authorization bill was finalized. This reversal was not based upon technical or national security grounds; rather, that it “wasn’t the department’s job.” This reversal nullified almost two years of coordination and effort by Congressional members and staff.
A senior Department of Defense official at a hearing providing grossly inaccurate information about GPS resilience and backup systems. The official subsequently retired.
The Deputy Secretaries of Defense and Transportation in 2015 promising action to the chairman of the House Transportation Committee. In a December letter they said the administration would establish a GPS backup by first establishing an eLoran timing system, and then an eLoran navigation system. Aside from signing the letter, no further action was taken.
Congress’ growing skepticism about administration positions on this has led to a series of hearings, informal inquiries, demands for reports, and legislation. Together they chart a very deliberate effort to bypass bureaucratic infighting and confusion as much as possible en route protecting the nation with a terrestrial complement and backup for GPS.
Legislative action has included :
in 2015, halting demolition of Loran-C infrastructure pending a decision on a GPS backup system. (USCG Authorization Act)
in 2016, requiring the departments of Defense, Homeland Security and Transportation to identify requirements for a domestic GPS backup and report before the end of 2017. (National Defense Authorization Act/ NDAA)
in 2017, requiring a plan for a GPS backup technology demonstration by April 2018, completion of the project by June 2019, and authorizing $10 million for the program. (NDAA)
in March 2018, providing $10 million for the GPS backup technology demonstration (DoD Appropriations)
in August 2018, reaffirming Congress’ interest in the backup demonstration, requiring a progress brief by Dec. 1 2018, and authorizing another $5 million for the project (NDAA)
In September 2018, funding an additional $5 million for the backup demonstration (DoD Appropriations)
In December 2018, the National Timing Resilience and Security Act of 2018 was signed into law. It directs the Secretary of Transportation to establish a terrestrial, difficult-to-disrupt, wireless timing system to provide backup capability for GPS. A report on requirements and an implementation plan are due in June 2019, and system operation is mandated by December 2020.
Contacts with members and staff in the new, 116th Congress show that interest in this topic has increased. So has frustration with the administration missing many, if not most, of its deadlines for reports and briefings.
A recent GAO report that U.S. weapons systems are vulnerable to GPS spoofing; the need for a strong navigation and timing infrastructure for autonomous vehicles, drones, and intelligent transportation systems; and continued high visibility instances of deliberate GPS jamming and spoofing are all adding to concerns.
Also of note, Congressman Peter DeFazio (D-OR) has been named chairman of the powerful House Transportation and Infrastructure Committee. Rep. DeFazio has long believed in the need for action to provide a backup capability for GPS.
Congress is clearly set on a determined course. Perhaps the administration will catch up before it earns more of the Congress’ ire, and before a major disruption demonstrates the consequences of inattention to the entire nation and the world.
Features include enhanced ADS-B, SBAS and georeferenced charts.
Collins Aerospace’s Pro Line Fusion avionics upgrade for Pro Line 4-equipped Bombardier Challenger 604 series aircraft has been certified by the U.S. Federal Aviation Administration (FAA).
Working closely with Bombardier as the original aircraft manufacturer and Nextant Aerospace as the installation design certification lead, this sole all-in-one solution complies with pending mandates while modernizing the flight experience for pilots.
The Pro Line Fusion upgrade enhances the operational capabilities of the Challenger 604 aircraft to a similar level as that of the Challenger 605 and Challenger 650 jets equipped with Collins Pro Line 21 Advanced, while providing Challenger 604 operators with a solution to meet future regulatory requirements.
Among these enhancements, the upgrade replaces the factory-installed CRT displays with three 14.1-inch widescreen LCD displays with configurable windows. Features designed to improve situational awareness and reduce pilot workload for Bombardier Challenger 604 aircraft owners include:
A fully loaded package of baseline equipment for operation in modernizing global airspace — beyond ADS-B mandate compliance, offering SBAS-capable GNSS, localizer performance with vertical guidance (LPV) approaches, radius-to-fix (RF) legs and more
Geo-referenced electronic navigation charts that display own-ship aircraft position
The Challenger 604 Pro Line Fusion retrofit solution, which is already available for several Beechcraft King Air and Cessna Citation CJ aircraft, is part of our ongoing effort to provide owners with modern technology, enhanced situational awareness and compliance with airspace mandates,” said Christophe Blanc, vice president and general manager, business and regional systems for Collins Aerospace. “The Challenger 604 business jet is a highly-valued, long-haul aircraft that will be able to continue flying well into the future with this upgrade.”
The upgrade is available exclusively throughout Bombardier’s extensive network of service centers and Nextant Aerospace.
The European Space Agency (ESA) has received approval from the Galileo Security Accreditation Board to upgrade the global infrastructure running Europe’s Galileo satellite navigation system.
According to ESA, the resulting migration, set to start in February 2019, will incorporate new elements into the world-spanning system and boost the robustness of Galileo services delivered from the 26 satellites in orbit.
The system qualification campaign, which was run by the ESA Galileo project team in coordination with the WP1x system support team led by Thales Alenia Space in Italy, took more than a year to execute. It included more than 150 system tests — summing up to a total of 409 tests runs across Europe — in the various Galileo operational centers.
Galileo’s global ground segment. (Photo: ESA)
According to ESA, a major driver of this latest update was the growth of the Galileo constellation, which increased by 12 satellites through a trio of Ariane 5 launches in the last three years to become Europe’s largest.
The updated ground system incorporates a sixth telemetry, tracking and control station in Papeete, used to oversee Galileo satellite platforms, as well as an expansion of the number of antennas at the sites of uplink stations at Kourou in French Guiana, Reunion Island in the Indian Ocean and Noumea in French Polynesia.
In addition, receivers have been added to the Galileo sensor stations to ensure full redundancy.
“This marks the first update for Galileo’s operational infrastructure since it entered service,” said Edward Breeuwer, ESA Galileo system test and verification manager. “Galileo Initial Services began in December 2016, then last year we passed control of the system to our partner organization, the European Global Navigation Satellite System Agency, or GSA.
“This, therefore, marks a major step, but migration to the upgraded system should in principle be entirely transparent to Galileo users. We achieve this by taking advantage of the redundant elements of the Galileo system, taking them offline to update them while their operational counterparts continue to run.”
Mountainous areas present special problems for surveyors, overcome by the expanded availability of multi-GNSS. (Photo: Trimble)
Today’s GNSS satellites transmit on three or more carrier frequencies. The quality of the data in these signals from GPS, BeiDou, Galileo, GLONASS and QZSS reveals the expected measurement precisions. This article explores the noise of the range residual and ionospheric residual to indicate the oncoming capabilities.
Today, four GNSSs transmit various codes on various carrier frequencies: the USA’s GPS, Russia’s GLONASS, Europe’s Galileo and China’s BeiDou. Most of the carrier phase and pseudorange data are available using civilian GNSS receivers. Improvements in signal quality as well as reliability of the satellites are foreseen through the generations, as well as the introduction of new signals, such as L1C, L2C, L5 carrier and codes, and M-codes, on top of the existing L1-C/A code and the P(Y) code on both L1 and L2. Improvements are also seen in boosting the transmitting power.
This article investigates the use of two approaches to analyze the relative noise in the various carrier phase and pseudorange observable for GPS, BeiDou, Galileo, GLONASS and Japan’s Quasi-Zenith Satellite System (QZSS) augmentation. Two approaches analyze the relative noise in the observables: the range residual and the ionospheric residual. Both techniques can also be used to detect cycle slips.
Range Residual
UAV survey operations benefit from multi-GNSS receivers. (Photo: Septentrio)
The range residual is simply the change from one epoch to the next in the difference in the range calculated using the pseudorange and the range calculated by the carrier phase on a specific frequency. The pseudorange values are scaled using the wavelength to an equivalent range in units of the carrier’s cycles rather than meters. Equation 1 illustrates the range residual between the pseudorange ρ on a specific carrier frequency and the carrier phase observable φ, using the wavelength λ of the carrier to scale the pseudorange. The values of these observables are compared between adjacent epochs.
RR = (p/λ) – φ (1)
Two adjacent epochs are used, as then the integer ambiguity value, as well as the ionospheric and tropospheric errors, and satellite and receiver clock errors are the same, or negligibly different at such small (<1 s) epoch intervals. Therefore, these are all canceled out, and the resulting value is the measurement receiver and observable noise. The pseudorange observable will be significantly noisier than the carrier phase observable, therefore this method is a good way to calculate the measurement noise for the pseudoranges.
Ionospheric Residual
Surveyors work the Berezitovy mine in the North Amur region of Russia. (Photo: Javad GNSS)
If the carrier waves traveled only through a vacuum, then a phase observation from a specific satellite to a specific GNSS receiver could be scaled and converted to an equivalent phase measurement on another frequency using the frequencies of the carrier waves. However, as the signal passes through the ionosphere, systematic errors that are frequency dependent are introduced, so it is not possible to directly convert from one carrier phase value to another for a specific range measurement. The error is known as the ionospheric residual, and this will change slowly over time as the satellite passes overhead and the ionosphere being passed through changes, and also as the ionosphere slowly changes its characteristics over time, mainly due to the sun’s activities.
Equation 2 shows the calculation, using L1 and L2 carrier phase readings and corresponding frequencies, used to calculate the ionospheric residual. Again, the difference in the ionospheric residual values between adjacent epochs is used, as in the same way as the range residual values, external noise sources are eliminated.
(2)
Results
The results presented here are a subset of a much larger set. Figure 1 illustrates the range residuals for L1 and L2 as well as the L1L2 ionospheric residual for PRN32 (Block IIA satellite).
Figure 1. L1 range residual (left) L2 range residual (center) and L1L2 ionospheric residual (right) for GPS PRN32 (Block IIA) satellite. (Charts: Authors)
Figure 2 illustrates the L1 and L5 range residuals and the L2 (C-code) L5 ionospheric residual for PRN01 (Block IIF satellite).Both figures’ data are for the complete passing of the satellites from horizon over and back down again.The data for PRN32 is all that exists in the datafile, as this satellite only transmits L1 CA code and P(Y) code, as well as L2 P(Y) code, and corresponding carrier values.
Figure 2. L1 range residual (left) L5 range residual (center) and L2 (C code) L5 ionospheric residual (right) for GPS PRN01 (Block IIF) satellite. (Charts: Authors)
PRN01 is a block IIF satellite, and data for L1 CA code, L2 P(Y) code as well as L2 C-code, L5 code, and corresponding carrier phase values are recorded in the datafile.The block IIF satellites can result in four range residual values and five ionospheric residual combinations.Figure 2 only illustrates three of these combinations.The data were obtained from the Curtin University GNSS repository on Sept. 1, 2015, gathered at a 1-Hz epoch interval; 29,908 epoch of data were gathered for PRN32, and 26,073 epochs for PRN01.
It can be seen from these figures that the L1 range residuals are similar in characteristics for both PRN01 and PRN32.The values are noisy at the start and the end of the time series, indicating that the CA code is more prone to noise at low elevations.Comparing these to the L2 (PRN32) and L5 (PRN01) range residuals, we can see that both the L2 and L5 range residuals are not as prone to low elevation noise. Also, the two L2 and L5 range residuals are visually similar in characteristcs.By comparing the L1L2 and L2L5 ionospheric residuals (Figure 1, right, and Figure 2, right), we can see that the L1L2 combination is slightly noisier than the L2L5, in particular at low elevation angles.
If we compare BeiDou ionospheric residual results, we can see the comparison of noise on the three ionospheric residual combinations, B1B2, B1B3 and B2B3, as well as the results from the three types of satellite orbits, ie MEO, IGSO and GEO. Figure 3 illustrates the ionospheric residual results for PRN07 (IGSO) for the three frequency combinations, from data gathered on a static pillar located on top of the University of Nottingham Ningbo China’s Science and Engineering Building.
Figure 3. Ionospheric residual results for BeiDou PRN07 (IGSO) for combinations B1B2 (left), B1B3 (center), B2B3 (right). (Chart: Authors)
Figure 4 illustrates the ionospheric residual results for PRN01 (GEO) for the three frequency combinations.
Figure 4. Ionospheric residual results for BeiDou PRN01 (GEO) for combinations B1B2 (left), B1B3 (center), B2B3 (right). (Chart: Authors)
Figure 5 illustrates the ionospheric residual results for PRN12 (MEO) for the three frequency combinations. Here it can be seen that the B2B3 combination is generally less noisy than the B1B2 and B1B3. In addition to this, it can be seen that when the MEO and IGSO satellites are at lower elevation angles, the observables also become noisier. The GEO satellites have a constant elevation angle, and do not experience this phenomenon.
Figure 5. Ionospheric residual results for BeiDou PRN12 (MEO) for combinations B1B2 (left), B1B3 (center), B2B3 (right). (Charts: Authors)
Detailed Results
The data, gathered on a single GNSS receiver located at the University of Curtin’s GNSS research center, was downloaded in BINEX format and converted into RINEX 3.02 format using RTKLIB software. Software was developed by the authors in Matlab in order to interrogate the data files and implement the range residual and ionospheric residual algorithms. RINEX 3.02 format was chosen due to its compatibility with multi-GNSS and multi-frequencies.
Industrial UAV applications such as construction draw benefits from multi-GNSS receivers’ capabilities. (Photo: Skycatch, Swift Navigation)
Results are presented for both ionospheric residual and range residual results for various GNSS. These results have been calculated with varying elevation mask angles, ranging from 0° to 55° at 5° intervals. The RMS values of the resulting ionospheric residuals and range residuals were calculated and plotted against the respective elevation mask angle for each satellite and frequency combinations. This illustrates the influence of the elevation mask angle used on the results.
Typically, tens of thousands of epochs of data were used for every plotted point in the following figures. Further to this, not only are the results for the various frequencies and frequency combinations for the various GNSS illustrated, but also the various satellite types, MEO, GEO and IGSO, and various satellite Blocks for GNSS. GPS Block IIA (PRN04 and PRN32), Block IIR (PRN14), Block IIR-M (PRN31) and Block IIF (PRN01, PRN26, PRN25) data were all analyzed. Thus, the comparison of the various frequencies within each satellite system are illustrated, as well as the variations by comparing the various satellite constellation types and the various generations of GPS satellites.
Surveying accuracy is critical to roadway construction. (Photo: Leica Geosystems)
The BeiDou data illustrated are MEO (C12, C14, C11), IGSO (C09, C10, C07) and GEO (C01, C02). The data used were gathered on Sept. 1, 2015, in order to include GPS Block IIA satellites (PRN04 and PRN32). PRN32 was retired in June 2016, and PRN04 was taken out of active service in November 2015, but the satellite was reactivated in March 2018, this time broadcasting PRN18.
Figure 6 illustrates RMS of the range residual results for GPS (a), BeiDou (b), Galileo (c), GLONASS (d) and QZSS (e) respectively. These figures have been drawn so that the y-axis ranges are the same for each, hence illustrating the relative values.
Figure 6A illustrates the range residual results for GPS. It can be seen that the L1 CA code results are the noisiest, with PRN14 being the noisiest, followed by PRN31, PRN26, PRN01, PRN04, PRN25 and PRN32. It can also be seen with these results that lower elevation angle mask increases the noise level. Both the L2 and L5 code results are less noisy.
Figure 6A. RMS range residual results for GPS. (Chart: Authors)
Looking at the detail, the L5 code results is less noisy than the L2 and affected less than the L1 results by the changes in elevation mask angles used. Interestingly enough, the data file includes both the L2 P(Y) code and L2C code results. L2C only exists on the Block IIR-M and Block IIF satellites. The L2C code results are generally noisier than the L2 P(Y) code.
Figure 6B illustrates the results for the range residuals for the BeiDou satellites. Here it can be seen that the B1 code is affected more by low elevation mask angles than B2 and B3. It can also be seen that both the geostationary satellites’ B1 results stand out, with satellite C02 being noisier than C01. The B2 and B3 values for both these GEO satellites are bunched up with the majority of the other results towards the middle of the figure. The pairs of B2 and B3 results for the GEO satellites are close to each other in values, and the pairs of B2 and B3 results for the other satellites are also close to each other.
Figure 6B. RMS range residual results for BeiDou. (Chart: Authors)
It can also be seen that the range residual results for BeiDou are generally less noisy than than GPS, in units of cycles.
Similarly, for Galileo, Figure 6C, the E1 results are worst, and affected more by low elevation masks. Again, generally the Galileo results are seen to be improved over GPS. The GLONASS results, Figure 6D, illustrate that the L1C results are generally noisier, and then the L1P, followed by L2C and L2P. PRN09 is also consistently generally noisier than PRN10. Finally, Figure 6E illustrates the results for QZSS. Again, L1C is the noisiest and affected most by low elevation mask angles.
Figure 6C. RMS range residual results for Galileo. (Chart: Authors)Figure 6D. RMS range residual results for GLONASS. (Chart: Authors)Figure 6E. RMS range residual results for QZSS. (Chart: Authors)
Figure 7 illustrates the ionspheric residual results for the same satellites as Figure 6. This time, however, the resulting ionospheric residual values are calculated using pairs of data from the same satellite on different carrier frequencies. The range residual results compare the code and carrier from specific satellites and frequencies.
Figure 7(a) shows that the ionospheric residual results are affected by low elevation masks, and that the L1L2CW (L1 CA code and L2 P(Y) code available on all the satellites) combinations are the noisiest, followed by L2L5WX (L2 P(Y) code and L5 code available on Block IIF satellites, PRN 26, PRN01, PRN25), followed by L1L2CX (L1 CA code and L2 C code available on Block IIF and Block IIR-M satellites, PRN31, PRN26, PRN01 and PRN25), followed by L1L5CX (L1 CA code and L5 code, Block IIF satellites, PRN01, PRN25, PRN26) and finally the least noisy were the L2L5XX results (L2 C code and L5 code available on Block IIF satellites, PRN26, PRN25 and PRN01).
Figure 7A. Ionospheric residual results for GPS. (Chart: Authors)
Figure 7(b) illustrates the BeiDou ionospheric residual plots, illustrating that satellite C14 is much noisier for all three combinations of B1B3, BB1B2 and B2B3 in that order. The B1B2 combinations for the satellites are generally the noisiest, and then the B1B3 and B2B3 combinations are intertwined. The Galileo results again illustrate that the E1 combinations are generally noisier, and again we see the effect of low elevation angle masks, Figure 7(c). Generally, however, the Galileo results are less noisy than GPS, as are the BeiDou results.
Figure 7B. Ionospheric residual results for BeiDou. (Chart: Authors)Figure 7C. Ionospheric residual results for Galileo. (Chart: Authors)
The GLONASS results are again generally the noisiest, and again PRN09 is noisier than PRN10, with the L1P combinations being noisier, Figure 7(d). Figure 7(e) for QZSS shows that there are generally two groups of results. The upper set consists of L1L2ZX, L1L5ZX, L1L2XX, L1L5XX, L1L6ZX and L1L6XX from highest to lowest noise respectively. The lower, less noisy, group consists of L1L2CX, L1L5CX, L2L5XX, L2L6XX, L1L6CX and L5L6XX from highest to lowest noise respectively. Further details about the various codes and carrier values can be found in the RINEX 3.02 manual produced by the IGS.
Figure 7D. Ionospheric residual results for GLONASS. (Chart: Authors)Figure 7E. Ionospheric residual results for QZSS.(Chart: Authors)
Conclusions
A surveyor checks an urban construction project. (Photo: Topcon)
These preliminary results illustrate that there are differences in the noise values for various GNSS, frequencies as well as satellite generations and orbit types. It can be seen that generally L1, B1 and E1 have noisier results, and are affected moreso by low elevation mask data, and hence multipath. It can also be seen that newer generations of satellites do indeed produce better quality data.
Some specific satellites produce lower quality data such as GLONASS PRN09 and BeiDou C14. This could be due to multipath produced at the satellite.
Today roughly 100 GNSS transmit data, and typically users can gather data from 30 to 50 at any time. Positioning requires nowhere near this number of satellites, therefore decisions are needed as to which satellites and which data to use in a positioning solution. Our findings imply that our approach could be used in such decision-making in GNSS processing software, helping the software to choose the optimum satellites to draw from in a positioning solution.
Acknowledgments
This work described in this article was first presented at the FIG 2018 conference held in Istanbul, Turkey. The authors acknowledge the use of data supplied from the Curtin University GNSS Centre.
Manufacturers
The GNSS receiver used is a Trimble NET R9, and the antenna is a Trimble TRM 59800.00 SCIS choke ring antenna. A ComNav K508 GNSS receiver supplied some of the BeiDou results.
GETHIN WYN ROBERTS is an associate professor at Fróðskaparsetur, the University of the Faroe Islands. He is past Chairman of the FIG’s Commission 6, Engineering Surveys, and previously held posts at the University of Nottingham both in the UK and in China. He holds a Ph.D. in engineering surveying and geodesy from the University of Nottingham.
CRAIG M. HANCOCK is an associate professor in Geodesy and Surveying Engineering and the head of the Department of Civil Engineering at the University of Nottingham, Ningbo, China as well as the head of the Geospatial and Geohazards Research Group. He holds a PhD from the University of Newcastle Upon Tyne.
XU TANG is a research fellow at the University of Nottingham, Ningbo, China. He holds a PhD from Nanjing University.
Elsewhere in this (January) issue you’ll find the hard facts — basic, but hard — concerning the inaugural launch of the long-awaited GPS III constellation. On pages 10 and 12, with some seasoned leavening between, on page 11.
This column instead waxes briefly on the phenomenon of time, and humankind’s struggle to dominate it, to subject the fourth dimension to its own will.
For GPS III has been, yes, long awaited, long debated, long victim to multiple delays of many colors and causes, scrutable and inscrutable, of technological challenges and institutional barriers, and of that base determinant, money. The Government Accounting Office has issued its fair and due share of reports pointing alarmed fingers at constellation gaps and fulfillment shortfalls and the trials of OCX, the ground control system without which GPS III satellites may some day, soon or not-soon, be capable of broadcasting powerful new signals from space, yet not able to do so because of lagging accomplishment on Earth.
It’s often said that GPS is a victim of its own success, that older satellites living beyond their forecast lifetimes have allowed the Air Force to economize by not replenishing when unnecessary. There’s wisdom in this, of course.
Were my friend Don Jewell still with us, he would be justifiably proud of the Air Force for launching this new golden era of the gold standard in positioning — yet he would have seethed for years over the continued pushes to the right.
This reminds me a good deal of the drama and occasional comedy in the rise of Galileo, observed from afar. Next month I’ll give a talk at the European Space Agency, provisionally titled “An Outside History of Galileo,” the bemused viewpoint of one who only heard and interpreted the news, but did not participate in its forming.
For such complex endeavors do not happen easily or speedily or exactly as planned by mere mortals. Nor should they. Despite much gnashing of teeth, no one — in the civil sphere at least — has suffered unduly from the longish delays in either satnav system’s modernization. Perhaps a few lives could have been saved in the military, or greater strategic advantage gained, with the new capabilities that III will offer warfighters, had same been available on schedule, say, four to six years ago. But even this is mere conjecture.
There is a rhythm and a flow to life, and we are part of it. You can hurry neither sundown nor sunrise. Things happen in their own due course.
When full GPS III capabilities arrive — I don’t believe 2023 — then it will still be in good time. In its own best time, actually, to be here.
