International Committee on GNSS-13 Focuses on PNT in High Earth Orbit and Beyond Since last reported in the November/December 2016 issue of Inside GNSS, signicant progress has been made to extend the use of Global Navigation Satellite Systems (GNSS) for Positioning, Navigation, and Timing (PNT) in High Earth Orbit (HEO). This update describes the results of international eorts that are enabling mission planners to condently Artist’s rendering employ GNSS signals in HEO and how researchers are of Orion docked to the lunar-orbiting extending the use of GNSS out to lunar distances. Gateway. Image courtesy of NASA tarting with nascent GPS space to existing ones, multi-GNSS signal Moving from Dream to Reality flight experiments in Low- availability in HEO is set to improve Within the National Aeronautics and SEarth Orbit (LEO) in the 1980s significantly. Users could soon Space Administration (NASA), the and 1990s, space-borne GPS is now employ four operational global con- Space Communications and Naviga- commonplace. Researchers continue stellations and two regional space- tion (SCaN) Program Office leads expanding GPS and GNSS use into— based navigation and augmenta- eorts in PNT development and policy. and beyond—the Space Service Vol- tion systems, respectively: the U.S. Numerous missions have been own in ume (SSV), which is the volume of GPS, Russia’s GLONASS, Europe’s the SSV, dating back to the rst ight space surrounding the Earth between Galileo, China’s BeiDou (BDS), experiments in 1997. NASA, through 3,000 kilometers and Geosynchronous Japan’s Quasi-Zenith Satellite Sys- SCaN and its predecessors, has sup- (GEO) altitudes. This has spawned tem (QZSS), and India’s Navigation ported high altitude GNSS develop- exciting space missions through the with Indian Constellation (NavIC). ment for more than 20 years. Others promise of improved PNT perfor- When combined, such “super-con- around the globe, including the Euro- mance, quick trajectory maneuver stellation” of 100+ GNSS satellites pean Space Agency (ESA), have also recovery, and enhanced on-board greatly improves signal coverage and performed similar ight experiments autonomy. However, there are limita- also increases the diversity of system to demonstrate feasibility of employ- tions that need to be addressed when architectures, frequencies and geom- ing GNSS at high altitudes. However, to using GNSS signals in HEO because etry. is, in turn, improves overall fully exploit all the GNSS signals, par- of their weak signal strength and the PNT performance and resiliency. ticularly given the signal-sparse envi- limited number of over-the-limb sig- With such new PNT capabilities ronment at high altitudes, the dream nals available from any one GNSS underway, mission planners are set- of ubiquitous PNT in HEO and beyond constellation. ting their sights well beyond the SSV, will only be realized if GNSS constel- With the recent development and even employing GNSS on upcoming lations are designed to be interoper- deployment of multiple GNSS con- missions such as the lunar-orbiting able for SSV use and their capabilities stellations, and ongoing upgrades Gateway. clearly documented and supported. 32 InsideGNSS JANUARY/FEBRUARY 2019 www.insidegnss.com Recently there has been a transition ous other specific improvements and JAMES J. MILLER from experimentation to operational new capabilities. It is due to this dem- NASA use of GNSS in high altitudes, includ- onstrated on-orbit performance that BENJAMIN W. ASHMAN ing the Magnetospheric Multiscale the GOES-R team has recommended NASA GODDARD SPACE FLIGHT CENTER Mission (MMS) and the Geostation- that future GEO satellites consider use FRANK H. BAUER ary Operational Environmental Satel- of GNSS for navigation to take advan- NASA lite (GOES-16). MMS is a NASA Solar tage of their available capabilities. ese DANIEL BLONSKI Terrestrial Probe launched in 2015 and examples highlight the importance in currently in a 1.2 Earth Radii (RE) x developing interoperable multi-GNSS EUROPEAN SPACE RESEARCH AND TECHNOLOGY CENTRE OF THE EUROPEAN SPACE AGENCY 25 RE orbit, which is more than 40% capabilities to further improve exist- of average lunar distance. MMS navi- ing GNSS-based capabilities available MADELEINE BRONSTEIN gation performance has significantly to users in HEO and beyond. For spe- OVERLOOK SYSTEMS TECHNOLOGIES, INC. exceeded mission requirements thanks cic information on MMS and GOES-16 JENNIFER E. DONALDSON to the availability of GPS signals. GOES- navigation performance see 3, 4 and 5 NASA GODDARD SPACE FLIGHT CENTER 16 is the on-orbit designation of the rst under Additional Resources. WERNER ENDERLE of four NASA / National Oceanic and ESA’S EUROPEAN SPACE OPERATIONS CENTER Atmospheric Administration (NOAA) Multilateral Interoperability through A. J. ORIA GOES-R series of weather satellites International Engagement OVERLOOK SYSTEMS TECHNOLOGIES, INC. being launched between 2016 and 2024. In the early 2000s, bilateral meet- JOEL J. K. PARKER GOES-R represents a major leap forward ings were held to discuss GNSS signal NASA GODDARD SPACE FLIGHT CENTER in capability and technology, with GPS a interoperability. ese included, among O. SCOTT SANDS critical element in making this happen, others, bilateral meetings with the U.S. NASA, GLENN RESEARCH CENTER and improves weather forecasts to 5–7 and Russia on GPS/GLONASS interop- days, a full two days greater than the erability and the U.S.-European Union current capability, along with numer- on GPS/Galileo interoperability. ese www.insidegnss.com JANUARY/FEBRUARY 2019 InsideGNSS 33 SSV and featured on television news cover- age. e booklet is available in printed form at the UN Headquarters in New York, U.S., and online (for link see 8 under Additional Resources). ICG Denition of the Space Service Volume e multi-GNSS SSV has been dened as the volume of space between 3,000-kilo- meter and 36,000-kilometer altitudes, FIGURE 1 Left) SSV Information Booklet, (Right) Formal presentation at ICG-13 by ESA (Werner where terrestrial GNSS performance Enderle) and NASA (James. J. Miller) standards may not be applicable. GNSS service in the SSV is dened by three key discussions ultimately led to a multi- a key role in the pursuit of interoper- parameters: pseudorange accuracy, min- lateral dialog in 2005 when the United able multi-GNSS capabilities to support imum received power, and signal avail- Nations (UN) formed the International space users. ability. Signal availability is the percent Committee on GNSS (ICG). To maximize the utility of GNSS of time that GNSS signals are available, e ICG is a forum to facilitate dis- for high-altitude space users, coordina- and is calculated both as the availability cussion on GNSS benefits to people tion among all GNSS service providers of one or four GNSS signals (depending around the world by promoting the use is taking place within two ICG work- on the metric being used). When there of GNSS and its integration into infra- ing groups: Working Group B (WG-B) are less than four signals available then structures (particularly in developing which supports “Enhancement of GNSS the employment of on-board naviga- countries) and encouraging compatibili- Performance, New Services and Capa- tion lters enable users to obtain PNT ty and interoperability among GNSS and bilities” and leads SSV development, updates even with as little as one GNSS regional navigation satellite systems. It coordination, and outreach; and Work- signal. A sub-metric is the Maximum was formally established in 2005 by the ing Group S (WG-S) which supports Outage Duration (MOD), defined as UN General Assembly 61/111 resolution, “Systems, Signals and Services” and the maximum duration when a space and the 1st ICG meeting was held in coordinates the underlying GNSS sig- user in a particular orbit cannot obtain 2006. Membership includes core GNSS nal interoperability. As part of the WG-B availability of at least one single signal and regional space-based navigation work plan, three years ago an SSV Team or at least four signals. e SSV covers system/augmentation provider nations, undertook a series of initiatives co- a large range of altitudes and the GNSS as well as nations with active programs chaired by ESA and NASA to continue performance degrades with increasing to implement or promote GNSS services the implementation of an interoperable altitude. erefore, to allow for a more and applications. e ICG also recog- GNSS SSV and provide recommenda- accurate reection of these performance nizes associate members and observers tions to GNSS Service. Such efforts variations, the SSV is subdivided into from organizations and associations included an independent assessment of two distinct regions: that focus on GNSS services and appli- navigation performance in HEO using • Lower SSV for Medium Earth Orbits cations. Also, a GNSS Provider’s Forum, combined GNSS signals. Preliminary (MEO): Covers 3,000–8,000 km alti- which typically meets concurrently with results of these efforts were presented tude. It is characterized by reduced the ICG Plenary Meetings, was estab- in the November 13, 2016 Inside GNSS signal availability from a zenith- lished at the 2nd ICG meeting with the article “Navigating in Space.” Since then facing antenna alone, but increased aim to promote greater compatibility an authoritative publicly-available SSV availability if both a zenith and and interoperability among current and Booklet has been developed to detail nadir-facing antennas are used. future providers of the GNSS. Its mem- the multi-GNSS SSV, including coordi- • Upper SSV for GEO and HEO bers work to address key issues such as nated denitions, benets and use cases, Orbits: Covers 8,000–36,000 km ensuring protection of GNSS spectrum constellation-specic SSV signal perfor- altitude. It is characterized by signi- and matters related to orbital debris and mance data, and example technical data. cantly reduced signal received power orbit de-coniction. At the 3rd meeting The first edition of the SSV Booklet, and availability because most signals of the Provider’s Forum, held in 2008, titled “e Interoperable Global Navi- travel across the limb of the Earth.