Shortly after GPS World’s 2017 Leadership Awards ceremony during ION GNSS+ week, the Institute of Navigation rolled out its own distinguished panel of award recipients at a conference luncheon.
ION’s Satellite Division presented Terry Moore with the Johannes Kepler Award, its highest honor. It is perhaps a bit of editorial license to call Terry Moore “one of our own,” but he has been an advisor to the magazine for lo, these 17 years or more. During that time his technical papers have formed the basis for several feature articles, and he has guided many of his students and colleagues to authorship in these pages.
Director of the Nottingham Geospatial Institute (NGI) at the University of Nottingham, where he has long served as professor and dean, he is also a consultant and advisor to European and UK government organizations and industry. He did extensive work on the introduction and implementation of WGS 84 as the standard reference system for air and marine navigation, developed software tools for coordinate transformations and map projections, and pioneered the use of raw GPS code- and carrier-phase data from low-cost receivers.
He is the founding director of the GNSS Research and Applications Centre of Excellence, which targets knowledge transfer between the NGI and business. He has a long career of volunteer service for both ION and the Royal Institute of Navigation. In this as in other things he exemplifies the best of the scientific community, or of any community for that matter.
Among his articles for the magazine are “Not Just a Fairy Tale: A Hansel and Gretel Approach to Cooperative Vehicle Positioning,” 2014; “Network RTK for Intelligent Vehicles,” 2013; “Aiding Indoor Pedestrian Navigation with Building Heading,” 2011; “Integrating Computer Vision and Inertial Navigation for Pedestrian Navigation,” 2011; “Assessing Network RTK Wireless Delivery,” 2009; “Ubiquitous Positioning: Anyone, Anything: Anytime, Anywhere,” 2007; and “Simulation GPS in Urban Traffic Environments,” 2005.
I was privileged to serve as in-house editor for many if not all of these articles. A learning experience that could have been more so had I applied myself harder. Story of my life.
Nowhere to be found in the curriculum vitae of this Ph.D. in space geodesy are his performance as Commander Bond in “GNSS Murder, Mystery and Mayhem at the Mansion,” where he drank a mean martini, shaken not stirred, nor his regular appearances as vocalist at the NavtechGPS Open Mic Night, most recently dueting on “Paradise by the Dashboard Lights.”
All of us at the magazine join in congratulating Terry on this well-deserved honor!
The GPS Operational Control System’s launch and checkout system will control launch and early orbit operations and the on-orbit checkout of all GPS III satellites. (Image: Raytheon)
The Space and Missile Systems Center announced that the United States Air Force has accepted delivery of the GPS Next Generation Operational Control System (GPS OCX) Launch and Checkout System (LCS) baseline from Raytheon Intelligence and Information Systems.
Also known as Block 0, LCS demonstrated conformance through test and analysis with all contractual requirements. OCX Block 0 is the foundation for Raytheon’s future Block 1 and 2 delivery, slated for delivery in 2022.
LCS is a fully modernized cyber-secure ground system complete with the computing hardware, operations center workstations, and mission application software necessary to launch the first GPS III satellite into orbit and perform initial on-orbit testing.
LCS forms the basis for the full system delivery, referred to as Block 1, which will provide higher accuracy and globally deployed modernized receivers, to ensure anti-jam capability for military users. It will also provide control of both legacy and modernized satellites and signals, including the new international L1C and modernized Military Code.
Currently, mission operators are utilizing LCS as part of the GPS III Mission Readiness Campaign. The ground system is performing as expected during the rehearsals and space vehicle checkout, giving the Air Force confidence in its readiness to support launch and on-orbit operations.
OCX has had numerous challenges delaying the delivery of this critical capability, and this delivery marks a significant program milestone providing the Air Force with a cyber-hardened ground system to support the launch and on-orbit checkout of the GPS III satellites.
“This is a major milestone for the program, and it keeps the U.S. Air Force on track to launch the first modernized GPS satellite into space next year,” said Dave Wajsgras, president of Raytheon Intelligence, Information and Services. “We have strong forward momentum on the program, and we will deliver the full capability in 2021.”
The first launch of a GPS III satellite is scheduled for 2018.
A review of the history of the GLONASS program, its current status and an overview of the plans for the immediate future of the satellite constellation, its navigation signals and the ground support network.
English versions of the GLONASS CDMA interface control documents are now available. See Further Reading.
Richard Langley
On Oct. 12, 1982, the Soviet Union launched the first GLONASS satellite. Whether in reaction to the development of GPS or simply to fulfill the requirement for a system with similar capabilities for its armed forces, the Soviet Union began the development of the Global’naya Navigatsionnaya Sputnikovaya Sistema or Global Navigation Satellite System in 1976 just three years after the start of the GPS program. The first test satellite, code-named Kosmos 1413, was accompanied by two dummy or ballast satellites with the same approximate mass since the Soviet Union was already planning to launch three GLONASS satellites at a time with its powerful rockets to save on launch costs.
But because of launch failures and the characteristically brief lives of the satellites, a further 70 satellites were launched before a fully populated constellation of 24 functioning satellites (providing full operational capability or FOC) was achieved in early 1996. Unfortunately, the full constellation was short-lived. Russia’s economic difficulties following the dismantling of the Soviet Union hurt GLONASS. Funds were not available, and by 2002 the constellation had dropped to as few as seven satellites, with only six available during maintenance operations! But Russia’s fortunes turned around, and with support from the Russian hierarchy, GLONASS was reborn. Longer-lived satellites were launched, as many as six per year, and slowly but surely a full constellation of 24 satellites returned. And on Dec. 8, 2011, FOC was again achieved and has been subsequently more or less maintained — the system has even operated sometimes with in-orbit spares.
While GLONASS-only and survey-grade dual-system GPS/GLONASS receivers have been around for more than a decade, manufacturers took notice of GLONASS’s rebirth and began producing chips and receivers with GLONASS capability for the consumer market. In 2011, Garmin released handheld receivers supporting both GPS and GLONASS. In the same year, various cell-phone manufacturers started offering GLONASS capability with their embedded positioning modules. The early GPS/GLONASS receivers paved the way for the multi-GNSS receivers we have today, with their capability to track not just GPS and GLONASS satellites but those of the European Galileo and Chinese BeiDou systems, as well as those of the Japanese Quasi-Zenith Satellite System (not to mention the satellites of the satellite-based augmentation systems).
I documented the development of GLONASS in this column back in July 1997, and a team of authors from the Joint Stock Company Russian Space Systems discussed the plans for modernizing GLONASS in an April 2011 article. An update is overdue. So, in this article, I will briefly review the history of the GLONASS program, discuss its current status, and overview the plans for the immediate future of the satellite constellation, its navigation signals and the ground support network.
EARLY YEARS, PRESENT DAY
During the Cold War, information about GLONASS was scarce. Apart from the general characteristics of the satellite orbits and the frequencies used for transmitting the navigation signals, the Ministry of Defence of the Soviet Union revealed little else. However, sleuthing by Professor Peter Daly and his students at the University of Leeds provided some details about the signals’ structure. With the advent of glasnost and perestroika, and the eventual demise of the Soviet Union, information about GLONASS became more readily available. Eventually, the Russians released the Interface Control Document (ICD). This document, similar in structure to the Navstar GPS Space Segment/Navigation User Interfaces ICD-GPS-200, describes the system, its components, and the structure of the signal and the navigation message intended for civil use. Its latest version was published in 2016, but so far this version is only publicly available in Russian.
Satellites and Signals. Six models of GLONASS satellites (also known as Uragan, Russian for Hurricane) have been launched so far. Russia (actually the former Soviet Union) launched the first 10 satellites, called Block I, between October 1982 and May 1985. It sent up six Block IIa satellites between May 1985 and September 1986 and 12 Block IIb satellites between Apri1 1987 and May 1988, of which six were lost because of launch-vehicle-related failures. The fourth model was the Block IIv (v is the English transliteration of the Russian alphabet’s third letter). By the end of 2005, the Russians had deployed 60 Block IIvs. Each subsequent satellite generation contained equipment enhancements and also achieved longer lifetimes.
A prototype GLONASS-M (for Modernized) satellite was launched on Dec. 1, 2001, along with two Block IIvs with the first two production GLONASS-M satellites included in the triplet launches of Dec. 10, 2003, and Dec. 26, 2004. Two GLONASS-M satellites were included in the triplet launch of Dec. 25, 2005. The new design offered many improvements, including better onboard electronics, a longer lifetime, an L2 civil signal, and an improved navigation message. Like earlier versions, the GLONASS-M spacecraft still used a pressurized, hermetically sealed cylinder for the electronics.
FIGURE 1. Image from Reshetnev Information Satellite Systems, manufacturer of the GLONASS satellites, celebrating the 35th anniversary of the launch of the first GLONASS satellite in 1982 (“35 years of service to the world”).
All GLONASS satellites launched since December 2005 have been GLONASS-M satellites with the exception of two GLONASS-K1 (sometimes referred to as just GLONASS-K) satellites, launched on Feb. 26, 2011, and Nov. 30, 2014. GLONASS-K1 satellites are markedly different from their predecessors. They are lighter, use an unpressurized housing (similar to that of GPS satellites), have improved clock stability and a longer, 10-year design life. They also include, for the first time, code-division-multiple-access (CDMA) signals on a third frequency accompanying the legacy frequency-division-multiple-access signals (I’ll discuss these shortly). All of the GLONASS satellites have been manufactured by the Joint Stock Company Reshetnev Information Satellite Systems, located in Zheleznogorsk near Krasnoyarsk in central Siberia, and named after Mikhail Fyodorovich Reshetnev, the founding general director and chief designer. The Reshetnev company was formerly known as the Scientific Production Association of Applied Mechanics (Nauchno Proizvodstvennoe Ob”edinenie Prikladnoi Mekaniki or NPO PM). The Roscosmos State Corporation for Space Activities (formerly the Federal Space Agency), commonly known as Roscosmos, is the governmental body responsible for GLONASS.
FIGURE 1 includes artist’s images of the initial GLONASS, GLONASS-M and GLONASS-K1 satellites.
GLONASS satellite orbits are arrayed in three planes, separated from one another in right ascension of ascending node by 120 degrees, with eight satellites in each plane. The satellites within a plane are equally spaced, separated in argument of latitude by 45 degrees. Satellites in adjoining planes are shifted in argument of latitude by 15 degrees. The satellites are placed into nominally circular orbits with a target inclination of 64.8 degrees and semimajor axis of approximately 25,510 kilometers, giving them an orbital period of about 675.8 minutes. These satellites have ground tracks that repeat every 17 orbits or eight sidereal days. The GLONASS orbit planes are numbered 1–3 and contain orbital slots 1–8, 9–16 and 17–24, respectively.
FIGURE 2 shows the status of the constellation on Oct. 17, 2017. The orbital slot number (also called almanac slot) and frequency channel (discussed below) are given in parentheses. The recently launched GLONASS 752 was set healthy on Oct. 16, 2017, resulting in a fully operational 24-satellite constellation. All of the satellites are standard GLONASS-M satellites except for GLONASS 755, which includes a transmitter for the new third frequency, and GLONASS 701K and 702K. These last two are GLONASS-K1 satellites, with 702K operational while 701K is undergoing flight tests. The “K” isn’t part of the official GLONASS number but has been added to avoid ambiguity. A GLONASS-M satellite launched on Dec. 10, 2003, was also called GLONASS 701. Similarly, the International GNSS Service (IGS) refers to GLONASS 701K and 702K as 801 and 802, respectively. IGS also designates GLONASS 751 as GLONASS 851 to prevent confusion with Kosmos 2080, a GLONASS-IIv satellite launched on May 19, 1990, and also called GLONASS 751. And it designates GLONASS 753 as GLONASS 853 to prevent confusion with Kosmos 2140, a GLONASS-IIv satellite launched on April 14, 1991, and also called GLONASS 751.
FIGURE 2. Status of GLONASS constellation on Oct. 17, 2017. A green square identifies the location of a healthy satellite and orange, a test satellite. Orbital slot numbers and frequency channels are given in parentheses.
The satellites have been traditionally launched three at a time by Proton boosters from the Baikonur Cosmodrome near Leninsk in Kazakhstan. However, starting with the launch of the first GLONASS-K1 satellite, several GLONASS satellites have been launched singly on Soyuz rockets from the Plesetsk Cosmodrome north of Moscow.
Unlike GPS and the other GNSSs, GLONASS uses FDMA rather than CDMA for its legacy signals. Originally, the system transmitted the signals within two bands: Ll, 1602.0–1615.5 MHz, and L2, 1246.0–1256.5 MHz, at frequencies spaced by 0.5625 MHz at L1 and by 0.4375 MHz at L2:
L1k = 1602. + 0.5625k (MHz)
L2k = 1246. + 0.4375k (MHz)
This arrangement provided 25 channels, so that each satellite in the full 24-satellite constellation could be assigned a unique frequency (with the remaining channel reserved for testing). Some of the GLONASS transmissions initially caused interference to radio astronomers, who study very weak natural radio emissions in the vicinity of the GLONASS frequencies. Radio astronomers use the frequency bands of 1610.6–1613.8 and 1660–1670 MHz to observe the spectral emissions from hydroxyl radical clouds in interstellar space, and the International Telecommunication Union (ITU) has afforded them primary user status for this spectrum space. Also, ITU has allocated the 1610–1626.5-MHz band to operators of low-Earth-orbiting mobile communications satellites. As a result, the GLONASS authorities decided to reduce the number of frequencies used by the satellites and shift the bands to slightly lower frequencies.
The system now uses only 14 primary frequency channels with k values ranging from –7 to +6, including two channels for testing purposes (currently –5 and –6). (The +7 channel has also been used in the past for testing purposes.) How can 24 satellites get by with only 14 channels? The solution is for antipodal satellites — satellites in the same orbit plane separated by 180 degrees in argument of latitude — to share the same channel. This approach is quite feasible because a user at any location on Earth will never simultaneously receive the signals from such a pair of satellites. The move to the new frequency assignments started in September 1993.
Like the legacy GPS signals, the GLONASS signals include two pseudorandom noise (PRN) ranging codes: ST (for Standartnaya Tochnost or Standard Precision) and VT (for Visokaya Tochnost or High Precision) similar to the GPS C/A- and P-codes, respectively (but at half the chipping rates), modulated onto the L1 and L2 carriers.
As with GPS, GLONASS transmits the high-precision code on both L1 and L2. But, unlike the GPS satellites, the GLONASS standard-precision code has also been transmitted on the L2 frequencies beginning with the GLONASS-M satellites. (A separate civil code, L2C, has been added to the GPS L2 signal transmitted by Block IIR-M and subsequent satellites.) The GLONASS ST code is 511 chips long with a rate of 511 kilochips per second, giving a repetition interval of 1 millisecond. The VT-code is 33,554,432 chips long with a rate of 5.11 megachips per second. The code sequence is truncated to give a repetition interval of 1 second. Unlike GPS satellites, all GLONASS satellites transmit the same codes. They derive signal timing and frequencies from one of the onboard atomic frequency standards (AFSs) operating at 5 MHz. The various GLONASS satellite series since Block II through to the GLONASS-M series have three cesium AFSs on each satellite. The transmitted signals are right-hand circularly polarized, like GPS signals, and have comparable signal strengths.
Navigation Message. Like GPS and the other GNSSs, the GLONASS signals also contain navigation messages providing satellite orbit, clock and other information. Separate 50-bits-per-second navigation messages are modulo-2 added to the ST- and VT-codes. The ST-code message includes satellite clock epoch and rate offsets from GLONASS System Time; the satellite ephemeris given in terms of the satellite position, velocity and acceleration vectors at a reference epoch; and additional information such as synchronization bits, data age, satellite health, offset of GLONASS System Time from Coordinated Universal Time (UTC) as maintained by the National Metrology Institute of the Russian Federation UTC(SU) as part of the State Time and Frequency Service, and almanacs (approximate ephemerides) of all other GLONASS satellites. Note that, unlike GPS System Time, for example, GLONASS System Time has no integer offset from UTC and so leap-second jumps are added to GLONASS System Time simultaneously with those added to UTC. Note, however, that GLONASS System Time is offset by a constant three hours to match Moscow Standard Time (MSK, an abbreviation for Moscow).
The full message lasts 2.5 minutes, and is continuously repeated between ephemeris updates (nominally once every 30 minutes), but the ephemeris and clock information is repeated every 30 seconds.
The GLONASS authorities have not published, at least publicly, details of the VT-code navigation message. It is known, however, that the full message takes 12 minutes and that the ephemeris and clock information are repeated every 10 seconds.
Geodetic System. GLONASS ephemerides are referenced to the Parametry Zemli 1990 (PZ-90 or, in English translation, Parameters of the Earth 1990, PE-90) geodetic system. PZ-90 replaced the Soviet Geodetic System 1985, SGS 85, used by GLONASS until 1993. PZ-90 is a terrestrial reference system with its coordinate frame defined in the same way as that of the International Terrestrial Reference Frame (ITRF). The initial realization of PZ-90 had an accuracy of one or two meters.
However, in an effort to bring the system closer to the ITRF (and GPS’s WGS 84 geodetic reference system), two updates to PZ-90 were carried out. The first update, resulting in PZ-90.02 (referring to 2002), was adopted for GLONASS operations on Sept. 20, 2007, and brought the frame of the broadcast orbits (and hence derived receiver coordinates) closer to ITRF and WGS 84. Another realization, PZ-90.11, adopted on Dec. 31, 2013, reportedly reduced the differences to the sub-centimeter level.
TABLE 1 lists the defining constants and parameters of PZ-90.
TABLE 1. Fundamental geodetic constants and some of the parameters of the PZ-90 geodetic system as used by GLONASS.
The new GLONASS-K satellites transmit additional signals. GLONASS-K1 transmit a CDMA signal on a new L3 frequency (1202.025 MHz), and GLONASS-K2, in addition, will feature CDMA signals on the L1 and L2 frequencies.
FIGURE 3. Circular reflector array on a GLONASS-K1 satellite, surrounding navigation signal inner antenna elements. Photo from Reshetnev Information Satellite Systems.
Control Segment. Similar to GPS and other GNSSs, GLONASS requires a network of ground stations for monitoring and maintaining the satellite constellation and for determining the orbits of the satellites and behavior of their operating AFSs. The tracking network uses stations only within the territory of the former Soviet Union, supplemented with satellite laser ranging stations to help with orbit determination since all GLONASS satellites contain laser reflectors (see FIGURE 3).
Having a non-global network of tracking stations for determining the satellite orbits and AFS behavior results in slightly degraded GLONASS signal-in-space range error (SISRE). Recently, a number of tracking stations overseas have been established in conjunction with the development of the Russian satellite-based augmentation system (SBAS), the System for Differential Correction and Monitoring (SDCM). SDCM will function in a similar fashion to the Wide Area Augmentation System or WAAS, the U.S. SBAS, and the other SBASs in operation. The addition to the tracking network of the overseas SDCM stations, which already includes stations in Antarctica and South America with more stations coming, could help improve SISRE. Roscosmos also uses a global network of IGS and other tracking stations to monitor the health of the GLONASS constellation (see FIGURE 4).
FIGURE 4. Roscosmos global GLONASS satellite health monitoring network with 22 reporting stations on Oct. 18, 2017, between 13:00 and 14:00 MSK.
Performance. SISRE has improved over the years and is currently at the level of about 1 to 2 meters. In part, this is due to the better performance of the on-board AFSs carried by the latest GLONASS-M satellites compared to the first GLONASS-M satellites. Their relative one-day stability has improved from 10-13 to 2.4 × 10-14. FIGURE 5 shows a time series of recent values of SISRE determined by the Information and Analysis Center for Positioning, Navigation and Timing. These error levels can result in pseudorange-based positioning errors using GLONASS broadcast orbits and clocks about a factor of two worse than those provided by GPS — although, at any given instant, positioning accuracy will also be impacted by atmospheric effects and multipath and these could dominate the signal-in-space errors.
FIGURE 5. GLONASS daily root-mean-square signal-in-space range error in meters as determined by the Information and Analysis Center for Positioning, Navigation and Timing.
Much higher positioning accuracies can be obtained using GLONASS orbits and clocks provided by the IGS and its participating analysis centers. This is particularly true if carrier-phase measurements are used instead of or as a supplement to pseudorange measurements. A combination of appropriately weighted GPS and GLONASS measurements has shown to be beneficial in terms of availability, accuracy and efficiency, especially for high-accuracy positioning carried out using the real-time kinematic or RTK approach. Furthermore, the precise point positioning (PPP) technique, based on real-time or post-processing of dual-frequency carrier-phase measurements with precise satellite ephemeris and clock data, has demonstrated that kinematic decimeter-level accuracy is possible using GLONASS data or GLONASS data in combination with GPS data. GLONASS-only static PPP solutions over 24 hours have achieved accuracies at the millimeter level.
Users. The initial uptake of GLONASS by both civil and military users in the former Soviet Union and subsequently in Russia, not to mention outside Russia, was minimal. Prototype GLONASS-only receivers were developed for the military, and foreign GPS/GLONASS receivers were developed by several manufacturers for scientific and other advanced applications. The IGS added a set of GLONASS-tracking receivers to its network in 1998 and has continuously increased the number of such receivers since then. However, consumer use of GLONASS both within and outside Russia has only recently taken off with the development of GLONASS-only and combined GPS/GLONASS chipsets. Such chipsets are now featured in many mobile phones and in handheld GNSS receiver and vehicle navigation units.
NEW AND IMPROVED
As previously mentioned, the GLONASS-K1 satellites include a CDMA signal accompanying the legacy FDMA signals on a new L3 frequency of 1202.025 MHz. The ranging-code chipping rate for the CDMA signal is 10.23 megachips per second with a period of 1 milliseconds. It is modulated onto the carrier using quadrature phase-shift keying (QPSK), with an in-phase data channel and a quadrature pilot channel. The set of possible ranging codes consists of 31 truncated Kasami sequences. (Kasami sequences, introduced by Tadao Kasami, a noted Japanese information theorist, are binary sequences of length 2m – 1 where m is an even integer. These sequences have good cross-correlation values approaching a theoretical lower bound. The Gold codes used in GPS are a special case of Kasami codes.) The full length of these sequences is 214 – 1 = 16,383 symbols, but the ranging code is truncated to a length of N = 10,230 with a period of 1 milliseconds.
The associated navigation message symbols are transmitted at a rate of 100 bits per second with half-rate convolution coding. The so-called navigation message superframe (2 minutes long) will consist of 8 navigation frames (NFs) for 24 regular satellites in the GLONASS first modernization stage and 10 NFs (lasting 2.5 minutes) for 30 satellites in the future. Each NF (15 seconds long) includes 5 strings (3 seconds each). Every NF has a full set of ephemerides for the current satellite and part of the system almanac for three satellites. The full system almanac is broadcast in one superframe.
The lighter, unpressurized K1 satellites feature two cesium and two rubidium AFSs. The relative daily stability of one of the rubidium AFSs on a K1 satellite is reported to be 4 ×10-14. As a result, the SISRE for this satellite is about 1 meter. Plans call for adding a CDMA signal to L2 on future versions of the K1 satellites, dubbed K1+ (see below).
GLONASS-K2 Satellites. These satellites will be heavier than the K1 and K1+ satellites with greater capabilities including a CDMA signal at the GPS/Galileo L1/E1 frequency. Reshetnev ISS will initially build two K2 satellites before going into mass production. It had been planned to transition to the K2 satellites much sooner, only launching the two K1 satellites now in orbit. But apparently plans changed because of the sanctions restricting the delivery of radiation-resistant electronic components from the West.
Now, Reshetnev ISS will build an additional nine GLONASS-K1 satellites. It’s not clear how many of these might be of the K1+ variety. The GLONASS-K1 satellites will now be transition satellites between the existing GLONASS-M satellites (including the half-dozen or so that have been manufactured and stored on the ground for future launch as needed) and the future GLONASS-K2 satellites.
One of the first K2 satellites will host a passive hydrogen maser (PHM) AFS. The PHM has been under development for about a decade, and multiyear ground tests displayed a reliability and one-day stability of 5 × 10-15. It is expected to contribute to future 0.3-meter SISRE.
According to a recent report, GLONASS-K2 satellites will begin flight tests in 2018, with mass production of GLONASS-K2 satellites to begin in the 2019–2020 time frame.
Improved Tracking Networks. The development of the SDCM and its associated tracking network has already been mentioned. The SDCM network stations are equipped with combined GPS/GLONASS dual-frequency receivers, hydrogen maser atomic clocks and direct communication links for real-time data transfer. As mentioned earlier, GLONASS authorities are looking at whether additionally using the SDCM stations for GLONASS orbit and clock determination would significantly enhance the accuracy of the broadcast data.
CONCLUSION
GPS, the oldest GNSS, is continuing to modernize and will soon launch the first Block III or GPS III satellite. Already, GPS Block IIR-M and Block IIF satellites are transmitting new signals. Galileo is fielding modern satellites right from the get go, and BeiDou is about to start launching the operational version of its BeiDou-3 satellites. GLONASS is not to be outdone. It has provided useful positioning, navigation and timing services since at least 1996. While at times the service level has dropped below acceptable levels, it is now a dependable system and, with announced improvements, will be a contender in the future world of multi-GNSS.
FURTHER READING
Official GLONASS Update
“GLONASS Programme Update” by I. Revnivykh presented at the 11th Meeting of the International Committee on Global Navigation Satellite Systems, Sochi, Russia, Nov. 6–11, 2016.
In-depth Description of GLONASS
“GLONASS” by S. Revnivykh, A. Bolkunov, A. Serdyukov and O. Montenbruck, Chapter 8 in Springer Handbook of Global Navigation Satellite Systems, edited by P.J.G. Teunissen and O. Montenbruck, published by Springer International Publishing AG, Cham, Switzerland, 2017.
“GLONASS: Developing Strategies for the Future” by Y. Urlichich, V. Subbotin, G. Stupak, V. Dvorkin, A. Povalyaev and S. Karutin in GPS World, Vol. 22, No. 4, April 2011, pp. 42–49.
“GLONASS: Review and Update” by R.B. Langley in GPS World, Vol. 8, No. 7, July 1997, pp. 46–50. Correction: GPS World, Vol. 8, No. 9, Sept. 1997, p. 71. Available on line:
“GLONASS Spacecraft” by N.L. Johnson in GPS World, Vol. 5, No 11, Nov. 1994, pp. 51–58.
SimActive Inc., a developer of photogrammetry software, has launched an automated solution for direct georeferencing from real-time kinematic (RTK) positioning.
Within the new workflow feature, users can achieve get high accuracy in projects without the use of ground control points (GCP), saving time in collecting and processing data.
Martin Instrument, a reseller of SimActive and surveying equipment, is benefitting from the automation. “Direct georeferencing greatly helps reducing cost for applications like corridor mapping,” said Mike Minick, vice president of sales at Martin Instrument. “The new automated option within SimActive software for direct georeferencing greatly facilitates the user workflow.”
“With RTK GPS available on drones, the use of direct georeferencing is growing within the industry,” said Louis Simard, CTO of SimActive. “Correlator3D allows users to maximize their hardware and software investment.”
For a live demonstration at the Commercial UAV Show (Nov. 15-16, London, United Kingdom), visit SimActive’s booth or send an email to [email protected].
Ligado Networks will appear and present at the National Space-Based Positioning, Navigation and Timing Advisory Board’s (PNTAB) meeting on Nov. 15 in Southern California.
Ligado and its predecessors have sought to install high-powered ground transmitters that have been shown to harm and overwhelm GPS signals and receivers in their general vicinity. The controversy has simmered for at least eight years without resolution.
That final resolution will ultimately be taken by the Federal Communications Commission (FCC), although congressional participation is also conceivable, since national infrastructure security is involved.
Meeting Locale. The PNTAB meeting will take place Wednesday, November 15, 2017, 9:00 a.m. to 5:00 p.m.; and Thursday, Nov. 16, 9 a.m. to 1 p.m., at the Crowne Plaza Redondo Beach & Marina Hotel, 300 North Harbor Drive, Redondo Beach, California, approximately a half hour’s drive south of Los Angeles International Airport. The meeting will be open to the public up to the seating capacity of the room. Visitors will be requested to sign a visitor’s register.
From June 28, 2017, PNTAB presentation by Brad Parkinson.
The central issue in this long-running fight is the as-yet unknown — though uniformly predicted by the various rounds of testing over the last eight years — effects of Ligado signals on a huge installed industrial and governmental base of GPS receivers, some of which are essential to the nation’s critical infrastructure.
Ligado Networks, the current-day incarnation of once-bankrupt LightSquared, seeks FCC permission to apply the satellite-based frequency licenses it owns to be broadcast from a ground-based network. This would put a powerful nearby signal immediately adjacent to the much weaker, more distantly emanating GPS signals, and by the way, those from other GNSS as well. Tests in 2011 and further testing in 2016 demonstrated these powerful signals interfering with GPS receivers.
Brad Parkinson
The Ligado appearance comes in response to an open letter, posted on Oct. 10 by PNTAB First Vice-Chair Brad Parkinson, inviting Ligado CEO Doug Smith to speak to the Advisory Board. That invitation itself emerged after a season of what have been termed “attack” statements issued in various forums by Ligado, which were in turn stimulated by two early-summer letters:
1. A June 27 letter from the American Geophysical Union, Aerospace Industries Association, American Meteorological Society, Aircraft Owners and Pilot s Association, Airlines for America, General Aviation Manufacturers Association, International Air Transport Association, Iridium Communications, Thales USA and other organizations (totaling 22) to the FCC opposing Ligado’s request.
“The undersigned organizations . . . write to reiterate that the threat of harmful interference from Ligado’s proposed ancillary terrestrial component (“ATC”) service remain real and persistent. Contra ry to the assertions in Ligado’s FCC advocacy and recent media blitz, its proposed terrestrial operations continue to pose a significant interference risk to numerous parties . . . . The risks to these critical services are very real and, consistent with the public interest, cannot be brushed aside.
That letter further notes that “Ligado seeks the ability to sell its spectrum to the highest bidder, underscoring the uncertainty of any prospective value of the services it has on previous occasions suggested it may provide. There is a clear effort by Ligado to downplay the significance of the technical concerns it continues to receive from numerous directions.”
2. A July 5 letter from the PNT Advisory Board to Deputy Secretary of Defense Robert O. Work and Deputy Secretary of Transportation Jeffrey A. Rosen, the co-chairs of the National Executive Committee for Space‐based Positioning, strongly opposing the Ligado proposal.
From June 28, 2017, PNTAB presentation by Brad Parkinson.
“The revised [Ligado] proposal to the FCC is fundamentally unchanged from a previous proposal reviewed in 2011. Extensive government testing in 2011 and in 2016, clearly shows that both proposals cause definitive harmful interference to many classes of GPS receivers.”
“All GPS stakeholders should be wary of any incremental approaches to deploying mobile broadband services in the mobile satellite systems (MSS) band. For example, initial services could operate at reduced power levels on a temporary basis to protect only a subset of GPS users, before moving to full — power levels that will cause widespread interference to many other classes of GPS users. Regulatory decisions must be based on the ultimate end-state of any systems proposed for operation in the bands adjacent to GPS, and must protect all classes of GPS users. Unfortunately, the latest industry proposal does not acknowledge the legitimacy of, and the need to protect, dozens of precise applications of great national importance.”
From June 28, 2017, PNTAB presentation by Brad Parkinson.
Round Two. The struggle has been a prolonged one, with many twists and turns, however coalescing into two main periods of activity:
2011-12, when the first round of tests showed then-LightSquared’s proposed network would overload the vast majority of GPS receivers. The Federal Communications Commission (FCC) tabled the proposal, and the company, holding spectrum licenses whose value could range far into the billions of dollars, filed for bankruptcy.
2016–18. LightSquared emerged from Chapter 11 in 2015 as Ligado Networks, positing a modified network plan, but one whose organizing concept remains unchanged, causing deep and continued alarm over GPS interference. 2017 tests, conducted by a firm and a government organization hired by Ligado, essentially reconfirmed the 2011 results. The tests found that the proposed ground towers would significantly interfere with GPS receivers as far away as 4 to 5 kilometers, “killing them dead” in the words of one expert who reviewed the test data.
Parkinson’s October 10 letter invites Ligado CEO Doug Scott “to provide the committee with clear up-to-date design information. . . . How might the system as now envisioned be deployed? How many ground terminals are needed, for example, and where would they be?”
Previous LightSquared and Ligado presentations have been long on promise but short on details. In fact, sound technical underpinning has not been communicated.
From June 28, 2017, PNTAB presentation by Brad Parkinson.
Parkinson writes “we would therefore encourage you to specifically describe your implementation plan , with a corresponding test plan address ing the issues we have openly raised . We request you specifically focus on those regarding the potential for interfering with any GPS /GNSS services that operate in the protected Space – to – Earth L band (1559 – 1610 MHz) . Included should be all modes of operation and the use of all current and future GNSS sign als. Without these specific technical details and corresponding evaluations, we can only conjecture as to what you are really proposing .”
Later, he affirms “our focus is to provide advice based on deep engineering and related expertise . As you know, interference to GPS/GNSS can adversely affect numerous safety – of – life systems , other vital national assets, and applications comprising over $60 billion of annual U.S. productivity benefits .”
Parkinson and the PNTAB have had better luck securing a Ligado appearance than did GPS World magazine. In August of this year, Ligado’s senior vice president and chief engineer for radio access technologies thrice declined an invitation to give a brief Expert Opinion for the September issue on the question: How can the safety, security, and full utility of GNSS applications be ensured while evolving best, most efficient use of limited, very valuable electromagnetic spectrum?
Just a Refresher. The PNTAB meeting will be held Wednesday, November 15, 2017, 9:00 a.m. to 5:00 p.m.; and Thursday, November 16, 2017, 9:00 a.m. to 1:00 p.m., at the Crowne Plaza Redondo Beach & Marina Hotel, 300 North Harbor Drive, Redondo Beach, CA, approximately a half hour’s drive south of Los Angeles International Airport. The meeting will be open to the public up to the seating capacity of the room. Visitors will be requested to sign a visitor’s register.
Ligado is by no means the only item on the Committee’s docket, but is very likely to be the pièce de résistance. The full agenda for the meeting includes:
Update on U.S. Space-Based Positioning, Navigation and Timing (PNT) Policy and Global Positioning System (GPS) modernization.
Prioritize current and planned GPS capabilities and services while assessing future PNT architecture alternatives with a focus on affordability.
Examine methods in which to Protect, Toughen, and Augment (PTA) access to GPS/Global Navigation Satellite Systems (GNSS) services in key domains for multiple user sectors.
Assess economic impacts of GPS/GNSS on the United States and in select international regions, with a consideration towards effects of potential PNT service disruptions if radio spectrum interference is introduced.
Review the potential benefits, perceived vulnerabilities, and any proposed regulatory constraints to accessing foreign Radio Navigation Satellite Service (RNSS) signals in the United States and subsequent impacts on multi-GNSS receiver markets.
Explore opportunities for enhancing the interoperability of GPS with other emerging international GNSS.
Examine emerging trends and requirements for PNT services in U.S. and international fora through PNT Board technical assessments, including back-up services for terrestrial, maritime, aviation, and space users.
Spurious signals in the Black Sea have repeatedly placed seagoing vessels, according to their navigation systems, on the site of an airport hundreds of miles from their true positions.
The incidents were reported in the August and October issues of this magazine, and in Mike Jones’ Defense PNT e-newsletter column for October. Experts initially concluded the problems probably indicated a spoofing attack in the area.
Satellite image of the Black Sea.
A reader of the Defense PNT e-newsletter commented, “We have been following this case for quite some time now. We track all merchant vessels worldwide on the basis of Automatic Identification System (AIS), 24/7. The AIS transponder uses the GPS receiver for its position report.”
Our correspondent is the director of a company that offers server- and web-based tools that can be incorporated in GIS and asset tracking and tracing systems.
“The ‘spoofing’ is still going on,” he continued. “Even today ships were placed on the airport runway. In total, over 600 vessels were placed on the runway since early June. Our preliminary conclusion is that the ‘spoofing’ is probably not done on purpose. The most likely cause of this spoofing is a GPS re-radiator transmitter located in the hanger close to the end of the runway. This device is used for testing GPS when planes are placed inside the hanger. So, line-of-sight interference?”
The comment drew the immediate interest of security consultants who continue their investigations.
Baltic Incidents. Meanwhile, the Washington Post reported that a disruption of Latvia’s cellular network and emergency-services hotline may have resulted from a test of Russia’s electronic-warfare capabilities.
A 16-hour outage in October occurred at the time of major Russian military exercises. If substantiated, this could reveal electronic-warfare assets with capacity to disrupt civilian communications remotely. Such a tool could severely hamper authorities’ ability to organize a quick civilian response in case of war.
“Because of maneuver warfare’s reliance on communication, Russia has invested heavily in electronic warfare systems which are capable of shutting down communications and signals across a broad spectrum,” stated a December 2016 publication by the U.S. Army’s Asymmetric Warfare Group. “The Russians layer these systems to shut down FM, SATCOM [satellite communication], cellular, GPS and other signals.”
Organized by Professor Zak Kassas from the University of California, Riverside, the workshop will feature presentations and panels by experts and leaders from government (National Science Foundation, Office of Naval Research, Air Force Research Laboratory, Department of Transportation), industry (Google, Daimler, and Ford) and academia (The Ohio State University, UC San Diego, University of Southern California).
The workshop will discuss opportunities and challenges (technical, commercial, ethical, and legal) associated with developing fully autonomous systems that are cognizant and trustworthy for safety-critical applications. Examples include unmanned aerial vehicles (UAVs), self-driving cars and unmanned underwater and surface vehicles.
Kassas, director of the Autonomous Systems Perception, Intelligence, & Navigation Laboratory (ASPIN), leads a team of researchers developing reliable and accurate navigation that exploits existing signals of opportunity, rather than GPS, to meet the stringent requirements of fully-autonomous systems, such as UAVs and self-driving cars.
Two more Galileo satellites have reached Europe’s Spaceport in French Guiana, joining the first pair of navigation satellites and the Ariane 5 rocket due to haul the quartet to orbit this December.
Inside the 747. (Photo: ESA)
Galileos 21 and 22 left Luxembourg Airport on a Boeing 747 cargo jet on the morning of Oct. 17, arriving at Cayenne-Félix Eboué Airport in French Guiana on the same day.
Resting within distinctive white air-conditioned containers, the satellites were driven to the cleanroom environment of the preparation building within the space centre.
Waiting for them there were Galileos 19 and 20, which arrived in September.
The four satellites will be launched together in mid-December by a customised Ariane 5, the elements of which reached French Guiana last month by sea.
Galileos 21 and 22 being unloaded from their 747 cargo aircraft at Cayenne – Félix Eboué Airport in French Guiana on Oct. 17. (Photo: ESA)
Galileo is Europe’s own satellite navigation system, providing an array of positioning, navigation and timing services to Europe and the world.
A further eight Galileo Batch 3 satellites were ordered last June, to supplement the 26 built so far.
With 18 satellites now in orbit, Galileo began initial services on Dec. 15, 2016, the first step towards full operations.
Further launches will continue to build the constellation, which will gradually improve performance and availability worldwide.
We examine the performance of EGNOS in Finland, which lies near the northeast periphery of the coverage area, and how this performance can be improved now and in the future.
By Mohammad Zahidul H. Bhuiyan, Heidi Kuusniemi, Auryn Soderini, Salomon Honkala and Simo Marila
INNOVATION INSIGHTS with Richard Langley
“[O]NE ORBIT, WITH A RADIUS OF 42,000 KM, has a period of exactly 24 hours. A body in such an orbit, if its plane coincided with that of the earth’s equator, would revolve with the earth and would thus be stationary above the same spot on the planet. … [A] transmission received from any point on the hemisphere could be broadcast to the whole of the visible face of the globe, and thus the requirements of all possible services would be met.” So wrote writer and futurist Arthur C. Clarke in his October 1945 Wireless World article “Extra-terrestrial Relays: Can Rocket Stations Give World-wide Radio Coverage?,” envisaging the geostationary orbit (GEO) communication satellite.
The first GEO satellite was Syncom III, orbited by the United States in August 1964. Since then, more than 1,000 satellites have been launched into what is known as the Clarke Belt and around 450 are presently active. Most of them are used for civil or military communication. Some are used for direct-to-user TV and radio. Some are used for weather monitoring and other kinds of surveillance. And some are used for augmenting GPS.
While GPS is a remarkable positioning system, its real-time accuracy using L1-frequency pseudorange measurements and its instantaneous integrity are not sufficient for some applications such as aircraft navigation. That is why the U.S. Federal Aviation Administration developed the Wide Area Augmentation System (WAAS), the first satellite-based augmentation system (SBAS). WAAS provides differential correction data and integrity information to GPS users in real time throughout most of North America using a “bent pipe” from a ground station through the GEO satellite to a user’s equipment. It uses a state-space-domain correction approach, which provides corrections for the satellite orbit and clock data transmitted by GPS satellites along with ionospheric propagation delays, all computed from measurements collected by a continent-wide tracking network.
The WAAS concept has been duplicated for other regions. Three other SBASs are in full operation: the European Geostationary Navigation Overlay Service (EGNOS), Japan’s Multifunctional Transport Satellite Satellite-based Augmentation System, and India’s GPS-aided GEO Augmented Navigation System. Russia’s System for Differential Correction and Monitoring is currently in development.
One hitch with GEO satellites whatever their function is their inability to service high latitudes well. At a latitude of 65°, a GEO satellite has an elevation angle of only around 17° at most and at 75°, it’s about 6° or less. Even if a GEO satellite is above the local horizon, communication might be difficult due to the longer signal path length between the satellite and the user.
And so it is with GEO satellites used for SBAS at high latitudes. And there is an additional problem that even if the signals from an SBAS satellite can be received, corrections for some GPS satellites will not be received if they are outside the coverage area of the SBAS tracking network. In this month’s column, we examine the performance of EGNOS in Finland, which lies near the northeast periphery of the EGNOS coverage area, and how this performance can be improved now and in the future.
FIGURE 1. Finnish national GNSS network, FinnRef. The three stations highlighted in red had the worst positioning accuracy in our analyses.
The European Geostationary Navigation Overlay Service (EGNOS) is the first European-operated satellite navigation system and is a precursor to Galileo, Europe’s independent global navigation satellite system (GNSS), now being deployed. EGNOS, as a satellite-based augmentation system (SBAS) similar to the U.S. Wide Area Augmentation System (WAAS), was developed with the vision to improve the performance of GNSSs, such as GPS and Galileo. At the moment, EGNOS only augments GPS, making it suitable for safety-critical applications such as flying aircraft or navigating ships through narrow channels.
Additionally, EGNOS also supports new applications in many different sectors, such as agriculture (for high-precision spraying of fertilizers), transport (enabling automatic road-tolling or pay-per-use insurance schemes) or even precise personal navigation services for general and specific use.
At present, there are two operational geostationary Earth orbiting (GEO) satellites and until March 2017, these satellites had pseudorandom noise code (PRN) numbers 120 and 136 that simultaneously broadcast EGNOS correction messages to European GPS users. The PRN satellites 120 and 136 are located at 15.5°W and 5.0°E. (Since March, PRN 123 has replaced PRN 136 as one of the operational EGNOS satellites.) The use of EGNOS in the northern Europe is much more challenging than elsewhere in Europe due to the relatively low-elevation angle of some EGNOS satellites as seen from there of about 14° or less.
To improve our understanding of the true performance of EGNOS in Finnish territory, we recently carried out a project entitled “Finland’s EGNOS Monitoring and Performance Evaluation (FEGNOS).” At the northeastern edge of the EGNOS coverage area, the availability of the EGNOS geostationary satellites is compromised due to their low-elevation angles. The Finnish Geospatial Research Institute (FGI) at the National Land Survey of Finland (NLS) maintains a network of 20 permanent GNSS reference stations (FinnRef) all over Finland. The core objective of the FEGNOS project is to evaluate the performance of EGNOS at all of those reference stations to determine if the EGNOS system performance reaches its target in Finland.
Building on our initial research, in this article we report on the analysis of EGNOS performance at all 20 FinnRef stations for a year-long time-frame from November 2015 until October 2016. As it is of importance to compare the performance of EGNOS in a geographic region where EGNOS satellite visibility can be poor due to low-elevation angle, we assessed the performance of EGNOS by comparing the receivers’ own decoded SBAS messages against the SBAS messages provided by the EGNOS Data Access Service (EDAS). The daily EDAS SBAS messages can be freely downloaded from the EDAS server with prior authentication from EDAS. The performance analysis has been carried out for the following three cases:
Applying EGNOS corrections obtained from the EDAS server
Applying EGNOS corrections obtained from the receiver-decoded (Rx-decoded) EGNOS messages
GPS stand-alone solution without any EGNOS corrections.
We carried out the data analysis using the EGNOS analyzing tool called PEGASUS (which originally stood for Prototype EGNOS Analysis Using SAPPHIRE, where SAPPHIRE stands for Satellite and Aircraft Database Programme for System Integrity Research) from Eurocontrol. The results show that the Rx-decoded EGNOS performance is not as good as the performance obtained from the EDAS-offered message corrections. The ongoing experience and knowledge learned from the project has helped to identify weaknesses of the EGNOS system at high northern latitudes.
FINNISH NATIONAL GNSS NETWORK, FINNREF
The Finnish National GNSS network, FinnRef, was established on the initiative of the Nordic Geodetic Commission and the director generals of the Nordic Mapping Authorities in the 1990s. FinnRef is part of the Nordic GNSS network, and some stations of the FinnRef network also contribute to the global International GNSS Service (IGS) network and to the European Permanent Network (EPN). The primary function of FinnRef is to offer geodetic-grade GNSS measurements, which have been continuously used for forming and maintaining the national coordinate system (EUREF-FIN). In addition, the FinnRef network is used for many GNSS-related research activities. For example, it is now possible to analyze the positioning performance of different augmentation services via the FinnRef network. Currently, FinnRef also offers an open positioning service based on the differential GNSS (DGNSS) corrections for GPS and GLONASS.
The FinnRef network was renewed during the 2012–2013 timeframe. The renewed FinnRef network now consists of 20 GNSS reference stations, as shown in FIGURE 1. The raw GNSS data from all 20 reference stations is used in the FEGNOS project for EGNOS performance monitoring and analysis.
DATA COLLECTION
EGNOS signal monitoring at all FinnRef stations was carried out for one year from Nov. 4, 2015, until Oct. 31, 2016. There are in total about 360 days of data from the 20 stations out of a possible 366 days (2016 was a leap year). The day-of-year (DOY) information for the collected data set is detailed in TABLE 1. No data was available during DOY 233 and 234 of 2016 due to a technical fault at the FinnRef stations. There are 57 days of data from the year 2015 and 303 days of data from 2016.
Table 1. DOY information for the year-long data set.
Each FinnRef station is equipped with a dual-frequency geodetic-grade receiver. Each receiver generates 1-hour binary proprietary data files with a 1-Hz data rate. Data is pushed to the network server and saved at the conclusion of each hour. This means that there are in total 24 data sets for each single day for one single station. All the stations’ binary data files are then organized under one directory, which is named after DOY for that particular year. The FEGNOS data Collection Tool (FEGCoT) was developed in Matlab to collect data every day automatically from all 20 FinnRef stations.
These three steps are followed for automatic data collection:
Collect: 1-Hz hourly data is collected from the FinnRef server, and then saved to the local hard disk for further processing.
Convert: The saved raw binary-formatted hourly data files from the receivers are converted to RINEX observation, navigation and SBAS data files via the receiver manufacturer’s converter.
Combine: In this step, all 24 one-hour data sets from each station are combined into one single 24-hour data set for every RINEX file type (that is, observation, navigation and SBAS files).
The combined 24-hour RINEX data file for each station is then processed using the PEGASUS software. The key configuration parameters used in the data analysis are listed in TABLE 2. (Note that airborne accuracy designator refers to specifications in the WAAS Minimum Operational Performance Standards, MOPS.)
TABLE 2. PEGASUS configuration parameters.
Two PEGASUS modules are used for data analysis:
Convertor module: The Convertor module translates the RINEX observation, navigation and SBAS data into a generic format, which can then be used by the GNSS_Solution module for detailed analysis. Convertor can also use input from different GNSS/SBAS receivers and then transform the recorded binary data into readable ASCII data.
GNSS_Solution module: The GNSS_Solution module is used to compute a position solution in conformance with the MOPS for GNSS receivers used in avionics (GPS, SBAS or ground-based augmentation systems). In other words, the GNSS_Solution module can be considered as a post-processing MOPS-compliant GNSS receiver. It interfaces with other PEGASUS components, notably the Convertor module.
The elevation cut-off angle and the minimum accepted signal-to-noise ratio are kept low so as to have more satellites available for user-position computation. (The European Global Navigation Satellite Systems Agency (GSA) advises that range measurements from EGNOS satellites not be used for position computation.)
A Matlab-script was written to download EDAS-provided daily SBAS messages automatically from the EDAS server. All the PEGASUS-related processing was also executed by a Matlab-based script.
ANALYSIS OF RESULTS
We analyzed the EGNOS/GPS performance for the above-mentioned cases with the collected year-long data set from the 20 FinnRef stations. The operational time or uptime of each FinnRef station was monitored throughout the FinnRef network nodes on a daily basis. The average uptime of each station for the one-year data set is shown in FIGURE 2. The “b” in station names indicates one of the two data streams available from each station. The figure shows that most of the stations were up for more than 98% of the time, while only few have uptimes close to 95%.
FIGURE 2. Station uptime for all FinnRef stations for the year-long data set.
According to EGNOS Open Service (OS) horizontal and vertical accuracy requirements, the 95% Horizontal Navigation System Error (HNSE) should be less than 3 meters, and the 95% Vertical Navigation System Error (VNSE) should be less than 4 meters in the EGNOS service provision area. The horizontal and vertical position errors at a defined time epoch are computed as the difference between the estimated navigation position and the actual position in horizontal and vertical planes, respectively. The HNSE (95%) and VNSE (95%) were computed for all FinnRef stations with the year-long data set.
The yearly EGNOS performance in terms of HNSE (95%) and VNSE (95%) are shown in FIGURES 3 and 4, respectively. It can be observed that GPS+EGNOS offers significant accuracy improvement compared to GPS stand-alone solutions for all of the stations. Vertical accuracy improvement for EGNOS is greater than the horizontal improvement, mostly due to the better mitigation of ionospheric error compared to stand-alone GPS. We also observed that the Rx-decoded EGNOS performance is not as good as the performance when corrections are obtained from the EDAS server. This might be due to the poor visibility of the EGNOS satellites at northeastern latitudes, which resulted in data aging or partial data loss of EGNOS messages.
FIGURE 3. HNSE (95%) for all FinnRef stations.FIGURE 4. VNSE (95%) for all FinnRef stations.
In FIGURES 5 and 6, the daily EGNOS performance in terms of VNSE (95%) are shown for the two cases: 1) applying EGNOS corrections from EDAS-provided EGNOS messages, and 2) applying EGNOS corrections from Rx-decoded EGNOS messages, respectively.
FIGURE 5. VNSE (95%) performance over time with GPS+EGNOS (EDAS) corrections.FIGURE 6. VNSE (95%) performance over time with GPS+EGNOS (Rx-decoded) corrections.
For a better understanding, the percentage of EGNOS OS requirement failure when analyzed on a daily basis with EDAS offered corrections is presented in FIGURE 7.
FIGURE 7. Percent of EGNOS OS requirement failure with EDAS-provided EGNOS correction messages.
The percentage of EGNOS OS requirement failure was computed from the number of days where the HNSE (95%) ≥3 meters in the case of horizontal navigation solution error and VNSE (95%) ≥ 4 meters in the case of vertical navigation solution error. As observed from Figures 5 and 7, the EDAS offered EGNOS corrections fail to meet the OS requirement only in a few instances. Similarly, the percentage of EGNOS OS requirement failure when analyzed on a daily basis with Rx-decoded corrections is presented in FIGURE 8. It can be easily seen from Figures 6 and 8 that the Rx-decoded EGNOS performance fails to meet the OS requirement in many instances. However, the daily fluctuations are averaged out when the year-long data is taken into account, providing satisfactory performance on the whole.
FIGURE 8. Percent of EGNOS OS requirement failure with Rx-Decoded EGNOS correction messages.
The yearly EGNOS performance in terms of VNSE (99%) is shown in FIGURE 9.
FIGURE 9. Sorted VNSE (99%) performance with GPS+EGNOS (EDAS) corrections for all FinnRef stations.
The three stations with the worst accuracy are highlighted in red in Figure 1. These stations are located on the northeastern border of the EGNOS coverage area. The EGNOS User Differential Range Error Indicator (UDREI) figure for three stations (FINb, VIRb, and SAVb) is shown in FIGURE 10(a), 10(b) and 10(c), respectively.
FIGURE 10. EGNOS UDREI as seen at (a) FINb, (b) VIRb and (c) SAVb.
The stations were chosen so that they represent a wide geographical spread over Finland. According to Figure 10, the satellite UDREI values are in the range of 14 and 15 (marked as blue) at the northeastern edge of the sky plot. A UDREI of 14 indicates “not monitored” and 15 indicates “do not use” for a particular satellite. Even though the satellites had a moderate elevation angle with respect to the user, the EGNOS system was unable to offer corrections to those satellites in the northeastern sky. Relatively lower availability of GPS satellites coupled with the lower number of EGNOS Ranging and Integrity Monitoring Stations (RIMS) at northeastern latitudes contributed to the poorer than expected positioning performance in the northeastern coverage area of EGNOS.
CONCLUSIONS
In this article, we presented a summary of an analysis of EGNOS in Finland for a year-long period, and we explained our automated data collection and data analysis procedure. The following key observations can be made based on the analysis of the year-long data set:
The use of EGNOS significantly improves the positioning performance compared to GPS stand-alone operation.
The vertical accuracy improvement for EGNOS is higher than the horizontal improvement compared to GPS stand-alone performance.
The performance of EGNOS with the receivers’ own decoded message corrections is not as good as the performance obtained through EDAS-provided EGNOS corrections.
EGNOS does not offer corrections for those GPS satellites that are setting in the northeastern sky of the EGNOS coverage area.
The percentage of EGNOS OS requirement failure when analyzed on a daily basis with Rx-decoded corrections is significant. This is mostly due to the poor visibility of GEO satellites from northeastern latitudes.
These findings emphasize the fact that there is a great need at northeastern latitudes for an alternative solution to the GEO satellites broadcasting EGNOS corrections. The existing alternative solution is to download the corrections from the Internet through EDAS at the cost of an additional communication link. The other possible alternative could be to broadcast corrections via inclined geosynchronous orbit satellites, or by some other means.
ACKNOWLEDGMENTS
This article is based on the paper “Performance of EGNOS in North-East European Latitudes” presented at the 2017 International Technical Meeting of The Institute of Navigation held Jan. 30–Feb. 1, 2017, in Monterey, California. The research was conducted within the FEGNOS project, funded by the Finnish Transport Agency and the Finnish Geospatial Research Institute at the National Land Survey of Finland. More information about the FEGNOS project can be found at www.fegnos.net.
MANUFACTURER
The receivers in the FinnRef network are JAVAD GNSS Inc. Delta-G3Ts and the antennas are JAVAD RingAnt_DMs with SCIS radomes.
MOHAMMAD ZAHIDUL H. BHUIYAN received his Ph.D. degree in 2011 from the Department of Electronics and Communications Engineering, Tampere University of Technology, Finland. He is a research manager in the Department of Navigation and Positioning at the Finnish Geospatial Research Institute (FGI) of the National Land Survey of Finland in Kirkkonummi. He is also the acting deputy head of the institute’s Satellite and Radio Navigation Research Group.
HEIDI KUUSNIEMI is the director of FGI’s Department of Navigation and Positioning. She is also an adjunct professor in the Department of Built Environment at Aalto University in Espoo and in the Department of Electronics and Communications Engineering at Tampere University of Technology. She is also the current president of the Nordic Institute of Navigation. She received her M.Sc. and D.Sc.(Tech.) degrees from Tampere University of Technology in 2002 and 2005, respectively.
AURYN SODERINI is an M.Sc. student in the Department of Electronics and Communication Engineering at Tampere University of Technology. He received his B.Sc. in 2012 from the Department of Electronics Engineering at The Third University of Rome.
SALOMON HONKALA is a researcher at FGI. He holds an M.Sc. (Tech.) degree in electrical engineering from Aalto University.
SIMO MARILA is a research scientist in FGI’s Department of Geodesy and Geodynamics. He received an M.Sc. degree in 2011 from Aalto University.
FURTHER READING
• Authors’ Conference Paper
“Performance of EGNOS in North-East European Latitudes” by M.Z.H. Bhuiyan, H. Kuusniemi, A. Soderini, S. Honkala and S. Marila in Proceedings of the 2017 International Technical Meeting of The Institute of Navigation, Monterey, California, Jan. 30–Feb. 1, 2017, pp. 627–636.
• Authors’ Related Work
“Performance Comparison of Differential GNSS, EGNOS and SDCM in Different User Scenarios in Finland” by S. Marila, M.Z.H. Bhuiyan, J. Kuokkanen, H. Koivula and H. Kuusniemi in Proceedings of ENC 2016, European Navigation Conference 2016, Helsinki, Finland, May 30–June 2, 2016, doi: 10.1109/EURONAV.2016.7530550.
“Low-Cost Precise Positioning Using a National GNSS Network” by M. Kirkko-Jaakkola, S. Söderholm, S. Honkala, H. Koivula, S. Nyberg and H. Kuusniemi 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. 2570-2577.
“Finnish Permanent GNSS Network: From Dual-frequency GPS to Multi-satellite GNSS” by H. Koivula, J. Kuokkanen, S. Marila, T. Tenhunen, P. Häkli, U. Kallio, S. Nyberg and M. Poutanen, in Proceedings of UPINLBS 2012, the 2nd International Conference and Exhibition on Ubiquitous Positioning, Indoor Navigation and Location-Based Service, Helsinki, Finland, Oct. 3–4, 2012, doi: 10.1109/UPINLBS.2012.6409771.
• European Geostationary Navigation Overlay Service
EGNOS – the European Geostationary Navigation Overlay System – A Cornerstone of Galileo, edited by J. Ventura-Traveset and D. Flament, ESA SP-1303, European Space Agency, Noordwijk, The Netherlands, 2006.
• EGNOS Data Access Service
“EDAS (EGNOS Data Access Service): Differential GNSS Corrections for Land Applications” by J. Vázquez, E. Lacarra, M.A. Sánchez and Pedro Gómez 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. 3550–3561.
“Satellite Based Augmentation Systems” by T. Walter, Chapter 12 in Springer Handbook of Global Navigation Satellite Systems, edited by P.J.G. Teunissen and O. Montenbruck, published by Springer International Publishing AG, Cham, Switzerland, 2017.
Minimum Operational Performance Standards for Global Positioning/Satellite-Based Augmentation System Airborne Equipment, RTCA/DO-229E, prepared by SC-159, RTCA Inc., Washington, D.C., Dec. 15, 2016.
The Jawaharlal Nehru Technological University-Hyderabad (JNTU-H) and Hexagon Capability Centre India (HCCI) have established a GNSS laboratory at the Centre for Spatial Information Technology, JNTU-H, reports Telangana Today.
The university is located in Kukatpally, Hyderabad, in the Indian state of Telangana.
The lab is equipped with NovAtel GNSS receivers, antenna, systems, cables and other hardware components. The equipment enables reception, processing, analysis and development of navigational data and applications to augment curriculum for JNTU-H students for research and education.
The establishment of the GNSS lab will also provide an opportunity to the students, scholars and faculty members to carry out research in satellite-based navigation and to develop advanced applications.
HCCI will provide internship to the students with financial support and job opportunities. This provision will not only be for CSIT students, but also for students with geo-informatics background from other constituent units of JNTU-H.
After opening the lab, Michael Kinahan, the software director of Hexagon Positioning Intelligence (NovAtel products division of Hexagon group) discussed various technical aspects of the NovAtel products with the potential of applying high-precision positioning capabilities to solve real-world challenges.
Q: What is the GNSS/PNT industry “Issue of the Year”?
Jose Angel Avila Rodriguez, signal and security implementation engineer, European Space Agency
A: The growth of PNT applications has been impressive and will continue. Assurance of PNT will thus gain an ever-increasing role, in both the security and the civil domains.
For GNSS, the key PNT contributor, there is in addition another challenge: its piece in the PNT cake will be contested by newcomers, such as telecom networks. Whether we will continue talking about A-GNSS or instead talk about Assisted 5G, with GNSS in that case taking on the role of signal of opportunity — that will depend on today’s decisions about future GNSS upgrades, the modernized versions of Galileo second generation, GPS III, and Beidou/Compass III, that will be flying around 2040.
Gyles Panther, president and CTO, Tallysman Wireless, Inc.
A: The key issues for PNT going forward, and into the indefinite future, are simply stated: availability and accuracy. Re-deployment of the eLoran infrastructure is a no-brainer. A potentially highly negative step would be the introduction of communication services within the mobile satellite L-band downlink frequency band (1525 MHz to 1559 MHz). Multi-constellational receivers track a much larger number of satellites and better disposed SVs (space vehicles) provide a lower horizontal DOP and hence greater accuracy.
Finally, GNSS needs to be defended against interference both intentional and accidental. Why on earth would we want to damage something that is providing so much utility to mankind?
