Developments in Global Navigation Satellite Systems
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GPS modernised, Galileo launched

Christian Tiberius and Kees de Jong
Department of Mathematical Geodesy and Positioning
Delft University of Technology
The Netherlands

Abstract

This paper provides an overview of the technical developments of Global Navigation Satellite Systems (GNSS), such as the US Global Positioning System (GPS) and the proposed European Galileo system. After a brief historical overview, the modernisation of GPS will be discussed; in particular the signals to be made available in the coming years to civil users of the system. Although Galileo is still under development, it is demonstrated that high-precision applications will benefit significantly from an integrated GPS/Galileo positioning system.

Introduction

Since the mid-nineties, volume and diversity of GPS applications have been rapidly increasing. GPS positioning has become popular for personal use in hiking and recreational sailing. GPS receiver chips are embedded in wristwatches and mobile phones. For a long time, GPS has played a significant role in the area of professional positioning as surveying, geodesy and navigation.

Both mass-market and high-end professional applications will benefit from developments to take place in worldwide satellite navigation in the near future. Plans for the next ten years include considerable modernisation of the US GPS and the deployment of a brand new European system, named Galileo. This article will discuss the technical developments, from the user point of view, thereby briefly addressing mass-market applications, and in detail, high-end professional applications.

Fig. 1: Ten GPS block I satellites were launched during the years 1978-1985. In 1989, the first block II satellite was launched and within five years, blending into block IIA satellites, the system built up to 24 satellites in orbit. At present block IIR satellites are launched. The graph is based on launch and decommission dates from http://listserv.unb.ca/archives/canspace.html.

Global Positioning System

The first GPS satellite was put into orbit in 1978. Up until 1985 a total of 10 experimental block I satellites were launched. In 1989 the first operational block II satellite was launched and the system reached full operational capability in 1994. Figure 1 gives an overview of the number of GPS satellites available since 1978. Currently there are 28 satellites - four more than the nominal constellation of 24 satellites. However, seven of them are on single string, which means there is no redundancy to perform one or more critical tasks. Fortunately, 16 new satellite-launches have already been scheduled for the period 2002-2006 to maintain constellation health.

Over the years, the mean mission duration (MMD) of the satellites has increased considerably. From a contractual MMD of six years and a design life of 7.5 years, the MMD has gone up to 9.6 years for the block II satellites to over 10 years for the block IIA satellites. For the latest block IIR and the new block IIR-M satellites, the design life is 10 years and the current MMD is nearly eight years. The future block IIF satellites, to be launched from 2005 onwards, will have a design life of 12 years and an MMD of 10 years. Originally these values were higher, but with the increased operational life span of the current satellites this would delay further modernisation to the completely redesigned GPSIII, which should provide users with the best possible satellite navigation system for the next 30 years.

Fig. 2: GPS single point positioning performance at a stationary reference station (Kootwijk in the Netherlands) over 24 hours at a 30 second interval, with a satellite elevation angle cut-off of 10 degrees, on 12th January, 2002. Left the spread in horizontal position; Right the vertical position versus time. Ionospheric delays have been accounted for using dual frequency data. Standard deviation of the East and North components are 6.3 and 5.7m respectively, for a Height of 11.3m.

On 2nd May 2000, Selective Availability (SA), the deliberate degradation of the GPS signals by the US Department of Defense, was switched off. This greatly enhanced single point positioning performance (see Figure 2). The horizontal position error is currently at the 10-20 metres level, with a standard deviation of about 6 metres in either component, and the vertical position error is at the few tens of metres level, with a standard deviation of slightly over 10 metres.

Selective Availability may not be switched on again, although this is not being guaranteed. In the near future the US government will have the option to deny access to the civil GPS signals on a regional basis once the new military GPS signals have been implemented on the modernised block IIR-M satellites.

Mass-market applications, as mentioned in the introduction, commonly rely on L1-only CA-code receivers. For precision applications (centimetre or better), high-end dual-frequency receivers are in use. Over the years about 50,000 of these receivers have been sold. Prior to 1994 the P-code on the L2 carrier was freely available. After the encryption of the P-code into the secret Y-code, referred to as Anti-Spoofing (AS), manufacturers of civil receivers developed (semi-) codeless techniques to acquire and track the L2 signals regardless at the expense of a decrease in signal to noise ratio. Unlike SA, AS will remain switched on. However, in addition to the current L1 signal, two new signals will become available for civil users.

GPS modernisation

The last twelve block IIR satellites will be modified into IIR-M ones. The first one of this new series is already scheduled for launch next year. In addition to the usual broadcasting scheme, these satellites will transmit a civil signal also on the L2-frequency. This signal is not the anticipated CA-code (as on L1), but a new, different code, designated the L2-CS (Civil Signal). This new code has the potential for signal tracking even in adverse conditions in which CA-code tracking would not be possible. One could consider tracking satellites at low satellite elevation, under foliage, and to some extent even of indoor positioning.

In the long run, the L2 CS-code might replace the L1 CA-code in cheap single frequency (mass-market) applications. Dual frequency range measurements are essential in high-precision applications to account for ionospheric delays and to enhance carrier phase ambiguity resolution (see Table 1). High-end civil dual frequency systems will be based on L1 CA-code and the newly designed L2 CS-code. Purchase of new hardware is likely to be required to exploit the latter capability. Whether, as a result of this, the dual-frequency market will expand and give rise to price drop is not yet clear.

Details of the new signal can be found in Fontana et al (2001). Compared with present semi-codeless techniques to get around Anti-Spoofing, measurement performance will improve considerably. Less cycle slips should be experienced on L2 and satellites will be tracked further down to the horizon. A level of measurement precision similar to the present L1 CA-code performance is anticipated.

Initial Operational Capability of the L2 CS-code is expected for about 2008. As transmission of the encrypted P(Y)-code on the L2-frequency will be continued, a word of caution is in order for differential (and relative) positioning applications during the period of transition; receivers based on both semi-codeless techniques and on the new Civil Signal will be operated.

A similar problem has occurred already before. Mixing receivers with different semi-codeless reconstruction techniques to overcome Anti-Spoofing can yield biases in the pseudo-range solution of up to 0.6 metres, due to differential hardware or equipment delays in the satellites (the P(Y)-code and CA-code are broadcast as two different components of the L1-signal). Again, mixing measurement techniques, ‘old’ equipment with semi-codeless tracking of the encrypted P(Y)-code on one site and new equipment with direct CS-code tracking (the same satellite) on the other, might give rise to decimetre biases in the pseudo-range solution and millimetre biases in the carrier phase solution. The surveying and navigation community is asked to be prepared and to start coordinating the transition in relation to active reference networks and user equipment. In practice, it should at least be clearly indicated by which measurement technique each satellite has been observed on L2.

To take early advantage of the new CS-code, receiver complexity has to increase in order to allow tracking of ‘old’ and ‘new’ satellites together on L2.  ‘Old’ satellites will be around until completion of the transition, so when dual frequency observations are required, the only option on offer will be semi-codeless tracking. In this respect users will of course depend on what equipment manufacturers offer.

As an alternative, in the interim, the new satellites might be set to broadcast the CA-code on L2 (according to the original plans) and switched to CS-code at a later date, when perhaps a sufficient number of satellites with this capability is available. Tracking CA-code on L2 can, for most recent high-end dual frequency receivers, be realised through firmware modifications.

The first block IIF satellite is to be launched in 2005. In addition to full civil dual frequency capability as discussed above, these satellites will offer a third frequency - the L5 at 1176.45MHz - with a P-code like signal. This high chipping rate code might not come into view for mass-market applications, but will offer high performance ranging capabilities (such as more robust carrier tracking) to the top end of the market. It is anticipated that the L5 code measurement precision will be a few times better than the present L1 CA-code precision, see van Dierendonck and Hegarty (2000). Obviously, new user equipment will be needed for L5 capability.

Within 10 years, GPS will turn into a civil triple frequency system. It is expected that high-end receivers, used for differential and relative positioning in precise geodetic and navigational applications, eventually will track pseudo-range (code) and (carrier) phase on all three frequencies L1, L2 and L5.

In addition to new satellites, more frequencies and new signals, the system’s infrastructure is to be upgraded on the control segment under the GPS Accuracy Improvement Initiative. Now Selective Availability is off, GPS (single point) positioning and timing accuracy can be further improved by providing higher quality broadcast clock and ephemerides. An additional new monitor station and six NIMA stations will be added, the data processing at Master Control Station (MCS) will be improved and the satellites will be uploaded more frequently.

Galileo

A few years ago, the European Union initiated an ambitious project to develop a civil satellite navigation system of its own, named Galileo, to be operational by 2008. The system will ensure independence from the US (military), but next to this strategic interest, there is clearly an economic aspect. Principal applications of the system will lie in navigation for transportation and fleet management; in particular for (road) vehicles, airplanes and marine vessels.

Political decisions on development of the system have been postponed as disagreement between EU member states arose over financial issues. At the time of writing (February 2002) the European Council is expected to take a (final) decision on funding the development phase of the system (including in-orbit validation) by 25th March, 2002. A sequel, will focus on the technical aspects of the proposed system.

On the technical side, Galileo will be not too different from (modernised) GPS. According to Salgado et al., (2001) the constellation features 27 satellites, distributed evenly and regularly over three orbit planes – where GPS has 6 orbit planes with (nominally) 4 satellites. The constellation is potentially enhanced by three active spare satellites. The projected altitude is slightly larger than for GPS.

The frequency and signal plan of Galileo are not yet definitive, but the proposal described in Hein et al (2001) is believed to serve as the current baseline. Like modernised GPS, Galileo will employ three bands in the frequency spectrum (see Figure 3).

Fig. 3: Overview of GNSS signals in the frequency spectrum. Indicated are civil GPS and Galileo signals; non-civil signals are left grey. The CA-code on L1 and the CS-code on L2 cover only small 2MHz bands. The precision code on the L5-frequency involves a wide 20MHz band. Moreover the signal is broadcast in two components, one with navigation data message and one without; the latter allows for a (more) robust carrier tracking. The Galileo signals in the upper L-band part overlay the GPS L1-signal. In the lower L-band part, both options are given for Galileo: overlaying the GPS L5-signal (E5a) and a separate (similar) signal right next to GPS L5 (E5b). The Galileo E6-band starts 20MHz right of the GPS L2-band. The Galileo signals discussed are envisaged to be broadcast in two components. Signal power levels have not been indicated in this overview.

At the upper end of the L-band spectrum, a small band is available on either side of the GPS L1-band. It is proposed to overlay the GPS L1-signal by using the very same central frequency of 1575.42MHz. It is planned that an overlay signal, with most power concentrated in the aforementioned side bands (E2 and E1), will be reserved for Public Regulated Service. An Open Service signal, also overlaid, would occupy a smaller band in the GPS L1-signal band (see Figure 3). A type of ranging code is proposed for the latter that is different from present GPS signal modulation, yielding a so-called split-spectrum signal. The resulting measurement precision for the pseudo-range is expected to lie between present GPS L1 CA-code precision and future GPS P-code like precision.

At the lower end of the L-band spectrum, directly adjacent to the GPS L5-band, the E5-band has been reserved for Galileo. Different options are still open, such as operating apart from GPS or creating an overlay here as well. Considering the wide-band available, a high performance signal is expected, with a GPS P-code like signal, as planned on L5. High-end market and critical applications are expected to rely on the E5-signal.

Finally, a reservation exists for Galileo in the mid-range of the L-band spectrum, as indicated in Figure 3. The E6-band is a 40MHz wide band, and is to be shared with radar applications. The signal in the E6-band is reserved for commercial services, as opposed to the aforementioned signals that offer Open Services: that is freely accessible to anybody. Content, such as integrity aspects, implementation and operation of the commercial service are not yet clear. Charging user-fees may prohibit wide-spread use, but a guarantee of service and related liability might, on the other hand, be appreciated by certain user-groups. Measurement performance of the signal is expected to be in between the present GPS L1 CA-code and future P-code like performance.

Mass-market applications of Galileo are likely to rely on single frequency positioning, just as with GPS. More demanding applications, such as precise navigation and geodesy, representing only a few percent of the market, may employ dual frequency Open Service signals (E2-L1-E1- and E5-band). Also adding the commercial service signal in the E6-band would result in a triple frequency system. Possible radar interference might prohibit use of the E6-signal for (safety) critical applications. Eventual availability of Galileo equipment and (commercial) services will of course be related to the economic interests of manufacturing companies and providers.

Integrity

According to the US Federal Radionavigation Plan (FRP), integrity is defined as ‘the ability of a system to provide timely warnings to users when the system should not be used for navigation’, see FRP (1999). GPS itself cannot provide such warnings. This can only be accomplished by augmentation systems such as DGPS reference networks. Galileo, on the other hand, will be able to provide integrity information to its users, provided they are willing to pay for it. For example, information from up to six regional integrity-monitoring networks may be included in the navigation messages from the satellites. This should guarantee an alert time to Galileo users of only six seconds. For local areas and safety-critical applications, such satellite-based integrity monitoring may not meet the requirements. Further improvement can be achieved by using ground-based signals. In this case the integrity message can be included in the differential correction data stream. This way users will be warned within one second. However, such local components are not part of the Galileo system.

A number of autonomous regional and global DGPS reference networks already exist. The US Wide Area Augmentation System (WAAS), the European Geostationary Navigation Overlay System (EGNOS) and the Japanese Multi-functional Transport Satellite Augmentation System (MSAS) are examples of regional satellite-based systems that are currently being developed, see for example WAAS (2002) and ESA (2002). EGNOS will cease to exist as an independent system in 2015, when it will become part of Galileo. The only networks that are operational on a regional and global scale are deployed by the private industry. The International Association of Lighthouse Authorities (IALA) operates a number of DGPS reference stations. These are examples of Local Area Augmentation Systems (LAAS). Although the coverage area of each station is limited, the reference stations are valuable sources of integrity for maritime users.

A recent report of the US Department of Transportation, highlights the fact that GPS and Galileo are vulnerable to interference, jamming, spoofing, shadowing and meaconing (Volpe, 2001). For safety-critical applications it is therefore important to have an independent system, such as Loran-C, as a backup.

GPS - Galileo integration

Once Galileo is operational, civil users will have access to a hybrid GNSS consisting of more than 50 satellites. From a technical point of view, it should be no problem to develop single- or multi-frequency receivers that are capable of receiving both GPS and Galileo signals. Similar receivers were developed in the recent past for the combined use of GPS and the Russian Glonass. When processing the data from such integrated receivers, care should be taken to account for differences in coordinate and clock systems. For example, rather than estimating one clock bias as is the case with the current GPS-only receivers, two of these biases should be estimated, one for each system.

Fig. 4: Major applications of Galileo will be road, air and marine navigation.
Through positioning and consequent fleet-management, maritime transport may
become increasingly more efficient.

Conclusion

This article has offered an overview of on-going and future developments in Global Navigation Satellite Systems with emphasis on the US GPS and European Galileo system. The future of the Russian Glonass system is unclear; although recent announcements indicate full operational capability should again be achieved in 2007. The current constellation consists of seven operational satellites. China seems to be working on its own satellite system, known as the Beidou Experimental Satellite Navigation System. Two satellites were launched in 2000. However, no further information is available on this system.

References

ESA (2002)              www.esa.int/export/esaSA/navigation.html

Fontana, R D, Cheung, W and Stansell, T (2001). The modernised L2 Civil Signal. GPS World, Volume 12, Number 9, September, pp 28-34.

FRP (1999)              www.navcen.uscg.gov/pubs/frp1999/

Hein, G W, Godet, J, Issler, J-L, Martin, J-C, Lucas-Rodriguez, R and Pratt, T (2001). The Galileo frequency structure and signal design. Proceedings of ION GPS 2001, Salt Lake City, UT, 11-14 September 2001.

Salgado, S, Abbondanza, S, Blondel, R and Lannelongue, S (2001). Constellation availability concepts for Galileo. Proceedings of ION NTM 2001, Long Beach, CA, 22-24 January 2001, pp 778-786.

van Dierendonck, A J and Hegarty, C (2000). The new L5 Civil GPS Signal. GPS World, Volume 11, Number 9, September, pp 64-71.

Volpe (2001)          www.navcen.uscg.gov/gps/geninfo/FinalReport-v4.6.pdf

WAAS (2002)          gps.faa.gov/Programs/WAAS/waas.htm

In addition some primary internet information sources concerning GNSS are:

• The US Coast Guard site on GPS at www.navcen.uscg.gov/
• The EC DG for Energy and Transport site on Galileo at europa.eu.int/comm/energy_transport/en/gal_en.html
• The EC/ESA Galileo Program Office at www.galileo-pgm.org/
• The newsletters on Galileo issued by the Genesis Office at www.genesis-office.org/

Christian Tiberius
obtained his PhD, in recursive data processing for kinematic GPS surveying, from the Delft University of Technology in 1998. His research interests included the implementation of Kalman filtering algorithms, data quality control and fast and efficient carrier phase ambiguity resolution with the LAMBDA method. He is currently an assistant professor at the Department of Mathematical Geodesy and Positioning and is responsible for several courses in the Geodesy curriculum. Recent research activities include detailed statistical analyses of high-end GPS receivers’ measurements and GPS-Galileo ambiguity resolution.

Kees de Jong
is an assistant professor at the Department of Mathematical Geodesy and Positioning of Delft University of Technology (DUT). He holds an MSc and a PhD in Geodesy. Prior to joining DUT in 1998, he worked for a number of companies in The Netherlands and Japan as a senior software engineer and consultant in GPS positioning and telecommunications. His main research interests are in the fields of real-time GNSS positioning and quality control and marine geodesy.