The Future of Global Navigation Satellite Systems

Transcription

1 The Future of Global Navigation Satellite Systems Chris RIZOS School of Surveying & Spatial Information Systems University of New South Wales Sydney, NSW 2052, AUSTRALIA Abstract Global Navigation Satellite Systems (GNSSs) involve satellites, ground stations and user equipment, and are now used across many areas of society. The Global Positioning System (GPS) from the US is the best known, and only currently fully operational GNSS. Russia also operates its own (not fully deployed) GNSS called GLONASS. Fuelling growth in applications during the next decade will be next generation GNSSs that are currently being deployed and developed. Major components are the US s modernized GPS and planned GPS-III, the revitalised GLONASS, and Europe s planned GALILEO system. Furthermore, a number of Space Based Augmentation Systems (SBASs) and Regional Navigation satellite Systems (RNSSs) will add extra satellites and signals to the GNSS mix. Key words GPS, GNSS, GLONASS, Galileo 1. The Current GPS The most widely used satellite-based Positioning, Navigation and Timing (PNT) system is the Global Positioning System (GPS). The current constellation of 30 Block IIA/IIR satellites (US Coast Guard Navigation Center, 2007) operates without a hitch and civilian applications of GPS are now considered to be quite mature. GPS was declared fully operational in 1995, and has over the last decade or so revolutionised the fields of geodesy, surveying and navigation in general. The GPS datum is WGS84, which for all but the most precise geodetic applications may be considered coincident with the International Terrestrial Reference Frame (ITRF, 2007). The US has just launched a third Block IIR-M satellite as part of its modernization program (see below). For a detailed description of the current GPS see UN Action Team on GNSS (2004). While it is beyond the scope of this paper to provide detailed review material, the following points are useful for later discussions: Most GPS satellites broadcast two signals in the so-called L1 and L2 frequency bands: L1 at MHz and L2 at MHz. Measurements made on two frequencies permits the ionospheric delay to be removed, hence improving positioning accuracy. GPS receivers can make pseudorange or carrier phase measurements, on the tracked L1 or L2 frequencies. Civilians using low-cost receivers currently only have direct access to the L1 signal, using the so-called Course Acquisition Code (C/A-code). This means that such receivers are unable to correct for delays to the signal as it passes through the ionosphere, which is now the dominant cause of error for users. Military receivers can access the ranging code (the Precise or P-code, now encrypted as the Y-code under the policy of Anti- Spoofing) on both the L1 and L2 frequencies, which enable them to correct for ionospheric errors. Positioning applications can be classified according to the required accuracy: Single Point Positioning (SPP) is the technique for which GPS was originally designed and delivers horizontal accuracy performance better than 5m. Vertical accuracy is typically times worse than horizontal accuracy. Differential GPS (DGPS) can overcome some of the limitations of GPS by applying corrections to the basic pseudorange measurements, based on a receiver making measurements at a known point (a base or reference station). The accuracy achievable can range from a few metres down to few decimetres, depending on the quality of the receiver and the particular technique (and DGPS correction generating service) that is used. GPS Surveying also works differentially but can achieve centimetre accuracy using a special measurement technique. A typical receiver, for both SPP and DGPS, measure the ranges to the satellites by timing how long the signal takes to come from the satellite (the pseudorange, referred to as such because this measurement is contaminated by the receiver clock error). However, receivers used in surveying and geodesy measure the phase of the underlying carrier wave signal. For baselines between points separated by more than (say) 20km, it is necessary that such receivers can also correct for the ionosphere. For shorter baselines, dual-frequency receivers are necessary for rapid initialisation of cm-level positioning. Given that civilians users only have access to the SPS, surveying receivers employ sophisticated signal processing techniques to measure the phase of the L2 signal. This level of sophistication is a major reason why surveying and geodesy receivers are more expensive than receivers used for SPP and DGPS. 2. GPS Modernization The US has embarked on a program of GPS Modernization to provide better accuracy and more powerful and secure signals from future GPS satellites. It is not possible to describe here this

2 program in detail, and readers are referred to easily accessible information on websites such as USCG NavCen (2007) and PNT (2007). While there are a range of planned improvements, noteworthy are the extra signals to be broadcast by the modernised GPS satellites: An improved code (instead of the current L1 C/A-code) on the L2 frequency of GPS (the so-called L 2 C ) is being implemented to enable civilian receivers to better account for ionospheric error, as well as to be more immune to RF interference and multipath. The first Block IIR-M satellite to broadcast L2C was launched 26 September 2005, the second was launched on 25 September 2006, and the third on 17 November The launch schedule to replace existing satellites is difficult to predict but full operational capability for L2C will not be declared until all 24 satellites (a combination of 8 Block IIR-M and 16 Block IIF satellites) in the constellation are broadcasting the new signal, and that is not expected to occur until 2013 or beyond. The radio spectrum for the L2 signal is not fully protected through the International Telecommunications Union (ITU), as it does not lie in the ITU s Aeronautical Radio Navigation Services band (the L1 frequency does). This means that L2C cannot be relied upon for so-called safety of life applications such as navigation to aid civil aviation. Therefore a third civil frequency at MHz (the so-called L5) is planned for launch on the Block IIF satellites (Fig.1). The first Block IIF satellite launch is scheduled for 2008, with full operational capability (FOC) of L1-L2-L5 GPS satellites (ie 24 satellites, a combination of 16 Block IIF and 8 Block III satellites) unlikely until 2015 at the earliest. GPS-III (Fig.2) will incorporate the extra L2 and L5 signals of the Block IIR-M and Block IIF satellites, as well as a new code on the L1 frequency (the so-called L1C), which will be compatible with GALILEO s L1 signal. However, to preserve backward compatibility with legacy user equipment, all current and planned GPS Block II signals will also be broadcast. The 30 GPS-III satellites are planned for launch from about 2013 until Figure 2. Artist s impression of the GPS-III satellite ( The implications for GPS user equipment is that low-cost receivers may not just be L1-only, as is currently the case, but they may be L2-only or L5-only, or even dual-frequency. (e.g. L1-L5). However, top-of-the-line user equipment for centimetre-level accuracy will take advantage of triple-carrier ambiguity resolution (TCAR) based on L1-L2-L5 pseudorange and carrier phase observations (Feng & Rizos, 2005). Perhaps the single most important shortcoming of GPS is also its most obvious; there are some places where GPS simply does not work due to a lack of available satellites. Therefore while GPS modernization will have a significant impact, a major influence in the future will be systems offering additional satellites/signals to those offered by GPS alone. We may therefore think in terms of a generic, overall GNSS combining a number of sub-systems. Perhaps it is best to speak of a Global Navigation Satellite System of Systems. 3. Russia s GLONASS Figure 1. Artist s impression of the GPS Block IIF satellite ( GLONASS was originally intended to be the Soviet Union s answer to GPS. The design of GLONASS is similar to GPS except that each satellite broadcasts its own particular frequency with the same codes (this is known as a FDMA, or Frequency Division Multiple Access, scheme), while GPS satellites broadcast the same frequencies and a receiver differentiates between satellites by recognising the unique code broadcast by a given satellite (this is known as a CDMA, or Code Division Multiple Access, scheme). GLONASS can also provide a different level of service to Military users compared to Civilian users. For a detailed description of GLONASS see UN Action Team on GNSS (2004). Current status information is available from the Roscosmos Information Analytical Center web site at GLONASS (2007). Unlike GPS, the GLONASS Control Segment is entirely located in Russia and some of the former territories of the Soviet Union. The GLONASS datum is therefore not WGS/ITRF, though plans are mooted to change to an ITRF-compatiable datum. Since the collapse of the Soviet Union, the Russian Federation has struggled to find sufficient funds to maintain GLONASS and at the time of writing (March 2007) there were 10 satellites functioning (as opposed to the 24 necessary for FOC). However, the Russian Federation has commenced a program to revitalise GLONASS, with planned 24 satellite FOC by the end of 2009 (with 18 satellites to be available by the end of 2007). In addition: Current activity centres on launching GLONASS-M satellites

3 with an improved 7-year design lifetime, which broadcast in the L1 and L2 bands (though not on the same frequencies as GPS). From 2008 it is planned to launch GLONASS-K satellites (Fig.3) with improved performance, and which will also transmit a third civil signal known as L3 in the Aeronautical Radio Navigation Services band near (but not identical) to GPS s L5 frequency. A full constellation broadcasting three sets of civil signals is unlikely before the middle of the next decade. Figure 3. GLONASS-K satellite ( Although the frequencies of GPS and GLONASS are different, a single antenna can track the transmitted signals. The data modelling challenges for integrated GPS/GLONASS processing have already been addressed, and survey-grade receivers capable of tracking both GPS and GLONASS have been available for many years. These combined receivers have demonstrated a marked improvement in reliability and availability in areas where satellite signals can be obstructed, such as in urban areas, under tree canopies or in open-cut mines. All major manufacturers of surveygrade GPS receivers now can supply integrated GPS/GLONASS receivers. However, recently there has been discussion of at least the L1 frequency on the new GLONASS-K satellites being of a CDMA design, in order to make these signals compatible with GPS and GALILEO. Greater efforts are likely to be made in the future to increase the degree of interoperability of GPS and GLONASS (and GALILEO), by having frequency overlaps at the L1 and L5 bands, at the very least. 4. European Union s GALILEO Perhaps the most exciting impact on the future of GNSSs is the decision by the European Union to launch its GALILEO project. For a detailed description of GALILEO see European Commission Directorate General Energy and Transport (2007) and the UN Action Team on GNSS (2004). For the purposes of this paper the following points are relevant: The design calls for a constellation of 30 satellites in a similar medium earth orbit (MEO) configuration to GPS, but at an increased altitude (approximately 3000km higher than GPS) which will enable better signal availability at high latitudes. The exact signal structure has not been made public for all of the trackable signals, but GALILEO satellites (Fig.4) will broadcast signals compatible with the L1 and L5/L3 GPS and GLONASS frequency bands. Those GALILEO signals are designated as L1, E5a and E5b. GALILEO will also broadcast in a third frequency band at E6; which is not at the same frequency as L2/L2C GPS/GLONASS. There will be up to ten trackable signals! GALILEO will offer five levels of service, one of which is fee-based and one of which is restricted: o The Open Service uses the basic L1/L5 frequency band signals, free-to-air to the public with performance similar to single- or dual-frequency GPS and GLONASS. o The Safety of Life Service allows similar accuracy as the Open Service but with increased guarantees of the service, including improved integrity monitoring to warn users of any problems. o The Public Regulated Service is to be available to EU public authorities providing civil protection and security (e.g. police, quasi-military), with encrypted access for users requiring a high level of performance and protection against interference or jamming. o The Search and Rescue Service is designed to enhance current space-based services (such as COSPAS/SARSAT) by improving the time taken to respond to alert messages from distress beacons. o The Commercial Service allows for tailored solutions for specific applications based on supplying better accuracy, improved service guarantees and higher data rates. This is a fee-based service. The design of the GALILEO Open Service signal at the L1 frequency is identical to the GPS-III L1C signal, a significant move to GNSS interoperability. The GALILEO ground control segment has elements similar to the GPS or GLONASS networks of tracking stations and master control stations. It will be deployed globally and the datum will be ITRF. With GPS, under the firm control of the US Military, and GLONASS, under the control of the Russian Military, augmentation systems to improve accuracy or reliability are operated completely external to the GPS and GLONASS system architectures. Such services are available from third parties such as FUGRO s Omnistar and NAVCOM s Starfire, or maritime DGPS beacons operated by national maritime safety authorities. GALILEO, on the other hand, has a much more open architecture, whereby systems to improve service can be brought inside the system through a provision for regional elements and local elements. The GALILEO system architecture allows for regional Up-Link Stations to facilitate those improved services tailored to local applications in different regions of the world. It is planned that GALILEO will be operated by a civilian agency under a Public Private Partnership (PPP) model whereby the European Commission through the Galileo Supervising Authority owns the physical system (satellites, ground stations, etc) as a public asset, but a Concessionaire will be responsible for day-to-day operations. The business model is still a close kept secret, however the Concessionaire will probably seek to cover costs and generate profit through the Commercial Service, Public Regulated Service and receiver design royalties. At the time of writing (March 2007) negotiations with the European Commission are frustratingly slow. GALILEO has moved out of its development phase and into the In Orbit Validation (IOV) phase. The first satellite (the Galileo IOV Experiment - GIOVE-A) was launched on 28 December 2005, and commenced broadcasting signals two weeks later. A second GIOVE satellite is planned to be launched by the end of The full constellation was originally planned to be launched , with FOC by However these dates have slipped and it is unlikely that FOC will be available until , perhaps only a few years before GPS-III s FOC.

4 efficiency, availability and reliability. Extra satellites improve continuity: GPS, GLONASS and GALILEO being independent GNSS means major system problems, unlikely as they are, are a very remote possibility of occurring simultaneously. Figure 4. Artist s impression of the GALILEO satellite ( For users seeking high accuracy and availability it is likely that they will want GNSS receivers that can track all satellites, and as many of the signals as is possible. TCAR techniques will be used for rapid ambiguity resolution (Feng & Rizos, 2005), as in the case of modernized GPS. However, it is inevitable that there will be design tradeoffs, and that there will be many receiver options available. The satellite visibilities for the GPS, GLONASS, GALILEO and a combined GPS/GLONASS/GALILEO receiver with a masking angle of 15 are shown in Fig.5. The combined system indicates a minimum of 17 satellites over the 24 hour period with an average of about 21 visible satellites. At the time of the simulations (in 2005) the GPS constellation consisted of 29 healthy satellites and had an average of about 7 visible satellites. The GALILEO and GLONASS constellations offered on average 8 and 6 visible satellites respectively. The GPS and GLONASS systems both had a minimum of 4 visible satellites, while GALILEO had a minimum of 6. Figure 5. Satellite visibility for Sydney over 24 hours with 15 masking angle (starting 1200h 27 May 2005). 5. The Benefits of More Satellites GPS and GLONASS combined have already demonstrated the benefits of extra satellites, and GALILEO brings all that and more. The benefits of the expected extra satellites and their signals outlined above can be categorised in terms of continuity, accuracy, Extra satellites and signals can improve accuracy: More satellites to observe means a given level of accuracy can be achieved sooner. More signals means more measurements can be processed by the receiver s positioning algorithm. Position accuracy is less susceptible to the influence of satellite geometry. The effects of multipath and interference/jamming are mitigated, meaning the measurement quality is higher. GALILEO also has the ability to deliver improved DGNSS accuracy directly, in the receiver via the RF frontend, through the Commercial Service. Extra satellites and signals can improve efficiency: For carrier phase-based positioning, to centimetre accuracy, the extra satellite signals will significantly reduce the time required to resolve ambiguities. Extra satellites and signals can improve availability (of satellites at a particular location): Improved ability to work in areas where satellite signals can be obscured, such as in urban canyons, under tree canopies, open-cut mines, etc. A hot research topic is indoor GNSS. Some receivers are now capable of measuring GPS signals inside buildings, and an increase in the number of available satellites will make indoor positioning more robust. Extra satellites and signals can improve reliability: With extra measurements the data redundancy is increased, which helps identify any measurement outlier problems. The new measurements will be more independent than the current L1 and L2 measurements, because code-correlation techniques (based on a knowledge of the PRN modulating range codes) will be used, rather than the current codeless/crosscorrelation techniques employed in today s dual-frequency GPS receivers. The current L2 GPS measurements by survey-grade receivers are more noisy and less continuous than those expected to be made on either of the new signals L2C or L5, hence reliable dual-frequency operation will be enhanced. More signals means that service is not as easily denied due to interference or jamming of one frequency, that may prevent the making of critical pseudorange and/or carrier phase measurements. 6. SBAS and RNSS While GPS, GLONASS and GALILEO are the generally accepted GNSSs, a discussion of future satellite navigation systems is incomplete without mentioning Space Based Augmentation Systems (SBASs) and Regional Navigation Satellite Systems (RNSSs). SBASs are essentially extra satellites transmitting signals intended to address shortcomings of GNSS for enhanced accuracy, availability and integrity for civil aviation users. The International Civil Aviation Authority (ICAO) defines in some detail the nature of SBASs and the special type of messages and signals that they should broadcast. The satellites in a SBAS usually number two or three, and are in geostationary earth orbit (GEO). They are

5 supported by a network of ground stations (similar to the GNSS Control Segment) that collect data that is used to generate the DGNSS and integrity messages. There are currently three distinct SBASs in-orbit covering different parts of the world: the US s. Wide Area Augmentation System (WAAS); the EU s European Geostationary Navigation Overlay System (EGNOS); and Japan s MTSAT Satellite Augmentation System (MSAS). It must be emphasised that all SBASs transmit the same signals, so that a WAAAS-capable receiver can also track EGNOS signals. Although it was intended that only GNSS receivers designed for aviation applications could track these signals, the reality is that many lowcost single-frequency GPS receivers are SBAS-capable. The primary advantage of SBAS is the higher SPP accuracy through the implementation of DGPS-capability, through the RF frontend. (The DGPS corrections are modulated on the broadcast L1 signal using special message types.) Both China and India plan to develop and launch their own SBASs. RNSSs, on the other hand, are intended to provide signal coverage over a nation or region. The constellations are much smaller than GNSSs, perhaps only 5-7 satellites (depending upon the specific orbit configuration). They may be considered stepping stones to GNSSs. Certainly the Chinese and Indian RNSSs can be viewed in this context. What other characteristics distinguish RNSSs from SBASs or GNSSs? In general, a RNSS need not transmit at the standard GNSS frequencies. In fact both China and India have filed applications with the ITU to use downlick frequencies other than those at L1, L2/E6 or L5/E5. This is an unfortunate development, as it sacrifices any notion of interoperability with current or planned GNSSs. The following comments may be made with respect to announced RNSS intentions: The Quasi-Zenith Satellite System (QZSS) was originally a multi-satellite augmentation system proposed to the Japanese government by a private sector consortium. It is NOT intended as an SBAS (Japnan s SBAS is the MSAS). In 2006, after industry lost interest in satellite-based navigation, the government of Japan agreed to fund QZSS and to promote it as an R&D project. The current plan is to launch three satellites broadcasting GPS-like (and perhaps GALILEO- and GLONASS-like) signals in an inclined GEO configuration that increases the number of navigation satellites available at high elevation angles over Japan (hence the term quasizenith ). This would benefit modified GNSS receivers operating in areas with significant signal obstructions such as urban canyons. It is expected that a demonstration QZSS satellite will be launched in For a detailed description of QZSS see UN Action Team on GNSS (2004) and Tsujino (2005). The orbital configuration of the QZSS constellation is such that the satellites will also pass over South East Asia and Australia. However, presentations by Japanese government officials make reference to a 7 satellite RNSS known as JRANS (Japanese Regional Navigation Satellite System), which will consist of the 3 QZSS satellites plus the MTSAT satellite in a GEO and 3 additional satellites in high earth orbit (HEO) (Takahashi, 2004). The GPS and Geo Augmented Navigation (GAGAN) system is India s SBAS (and hence will be compatible with GPS, and perhaps GLONASS/GALILEO as well, as required by ICAO). It is intended to support aviation in the Indian subcontinent region. The Indian Regional Navigation Satellite System (IRNSS) on the other hand is intended for use only in India, and little effort has been made to date to ensure even minimum interoperability with other GNSSs. Little is known about the IRNSS except for what appears in documents filed for frequency allocation with the ITU. It is a multi-frequency system, and comprises a 7 satellite constellation. For a number of years China has had its own RNSS, known as BEIDOU (2007). In 2006 China announced it would develop its own GNSS, referred to as COMPASS. Few details are available, but it appears that, like the IRNSS, little thought has been given to enable some level of interoperability with the other GNSSs. It may be that this scheme is intended to send political messages to the other GNSSs, and that when planning proceeds in earnest the advantages of having some signals interoperable with established GNSSs will become obvious and appropriate collaboration will be instigated. The future of GNSS/RNSS is bright. More satellites and more signals will be welcome by many user communities. However, the challenge will be to integrate all these satellite systems into one Global Navigation Satellite System of Systems. Although RNSSs may be justifiable at the narrow national level in terms of providing independent Positioning, Navigation and Timing (PNT) capability, compatibility and interoperability of all GNSSs, SBASs and RNSSs would ensure the benefits of satellite-based PNT are far greater than what any individual system can provide. 8. References BEIDOU (2007), en.wikipedia.org/wiki/beidou_navigation_system, accessed 8 March European Commission (2007), GALILEO - European Satellite Navigation System accessed 5 March Feng, Y., & Rizos, C. (2005), Three carrier approaches for future global, regional and local GNSS positioning services: Concepts and performance perspectives, 18th Int. Tech. Meeting of the Satellite Division of the U.S. Institute of Navigation, Long Beach, California, September, ITRF (2007), International Terrestrial Reference Frame 2005, see accessed 5 March National Space-Based PNT (Positioning, Navigation & Timing) Coordination Office accessed 5 March Rooscosmos Information Analytical Center (2007), General GLONASS, accessed 5 March Takahashi, H. (2004), Japanese Regional Navigation satellite System The JRANS Concept, Journal of Global Positioning Systems, 3(1-2), Tsujino, T. (2005), Effectiveness of the Quasi-Zenith Satellite System in Ubiquitous Positioning, Science & Technology Trends, Quarterly Review No. 16, pdf. Web site accessed 5 March UN Action Team on GNSS (2004), Report of the UNISPACE III Action Team on Global Navigation Satellite System (GNSS), Office for Outer Space Affairs accessed 5 March US Coast Guard Navigation Center (2007), Home page, accessed 5 March US Coast Guard Navigation Center (2007), GPS Modernization, accessed 5 March 2007.

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