TRENDS IN GPS TECHNOLOGY |
Predicting the future is always a risky enterprise! GPS is undergoing tremendous
growth, with many product innovations being made. Furthermore, technology
advances are leading resulting in organisational and institutional changes
which will have a profound impact on the "positioning professions",
as well as on the community in general. Perhaps the most obvious trend
therefore is the ever expanding range of GPS applications. In the following
sections, summary remarks will be made with regards to:
The issue of GPS "modernization" has recently attracted significant attention because of the dramatic improvements that would be conferred to GPS users. Furthermore, the implementation of these improvements would have far-reaching effects on GPS users in the coming decades, as well as impact on the plans for the development of the European Union's own satellite-based navigation system known as 'Galileo' (expected to become operational by the year 2008). The modernization of GPS is a difficult and complex matter involving not only the civil and military users (and their requirements), but such further issues as costs (for the modernization), spectrum definition, security, institutional concerns about management, and the future operation of GPS as a national and international resource. By 1999 most of the elements of the modernization plans had been identified, however, at the time of writing (2000) the commitment of the U.S. Congress to fund these GPS improvements was not forthcoming. Curiously, the only conclusion that can be drawn is that the U.S. is not yet prepared to put aside parochial interests in favour of world leadership in satellite-based positioning, and in so doing simply fueling criticism (mainly in Europe) that no one country should have total control over such a critical utility as a global satellite-based navigation system. Much of the following status report on GPS modernization plans is drawn from the excellent articles written by Keith McDonald (readers should access the U.S. Institute of Navigation Newsletter, Spring 1999, for the most up to date information). The GPS improvements identified for implementation in the next decade are:
The Presidential Decision Document (PDD) on GPS (of 29 March 1996) stated that both the L! and L2 frequency bands would be available for civilian use, and that a third civil frequency signal would be authorised. The former implies that the policy of 'Anti-Spoofing" would be changed. The latter set off an intensive search for new frequencies that would satisfy a range of requirements, including the military and the Federal Aviation Authority. Vice President Al Gore announced on 25 January 1999 that the third frequency had been identified at 1176.45MHz (just below the GPS L2 frequency). (This third frequency has been variously referred to as 'L3', 'L3c' or 'L5', and such designations will force a revision of the dual-frequency combinations L3 and L5 defined in section 6.4.2 and 6.4.4.)
For backward compatibility purposes, the existing C/A codes on L1 and the P/Y codes on L1 and L2 will be retained until modernized GPS satellites are deployed and new GPS user equipment is produced that operates with the modernized signals, unlikely before 2020. Current plans are to transmit C/A codes on L2, and plans for the third frequency are for higher bit rate, longer codes than the current C/A codes (10.23Mbps, with code length between 4000 and 10230 bits). (Higher rate code sequences would provide improved ranging accuracy, lower code noise, better cross-correlation properties, acceptable acquisition times, better isolation between codes, and reduced multipath interference.) The military dependence on the C/A codes to provide the timing information necessary to acquire the P/Y codes will be eliminated with the transmission of a new set of military codes, called the 'M codes' (these codes would be encrypted). Present indications are that these M codes will be transmitted on "split spectrum" scheme, offset about 6-9MHz above and below the centres of the GPS L1 and L2 frequencies.
Of immediate interest to the civilian user community is that the PDD on GPS stated that the continuance of "Selective Availability" would be reviewed on an annual basis after 2000, with a view to discontinue SA no later than 2006. The removal of SA combined with the availability of a second civilian frequency (the L2 signal) would improve GPS single point positioning to the sub10 metre range (the dual-frequency signal tracking would allow the ionospheric delay bias to be determined and removed from the GPS pseudo-range and carrier phase measurements). A significant impact on the demand of DGPS services such as that provided by the U.S. Coast Guard National DGPS Network, and other private service providers, is expected.
Improvements will also be made to the GPS Control Segment. Recent improvements in the CS software used to estimate the GPS satellite orbits (and to generate the Navigation Message) have demonstrated that braodcast ephemeris accuracy is at the 1-2 metre level (mostly). The planned addition of six ground stations of the National Imagery and Mapping Agency (NIMA) to the GPS tracking network will also improve the quality of the GPS tracking measurements and the computed orbit and satellite clock error parameters. More frequent uploads to the satellites is also planned. It is expected that during the next decade, the combined effect of these CS improvements would lead to sub-metre (and possibly decimetre) ephemeris accuracy. Table 12.3-1 summarises the anticipated system performance (in terms of accuracy of positioning, velocity and time determination) for different GPS user modes, if the planned modernization of GPS is implemented.
Table 12.3-1. Predicted GPS Performance in Position, Velocity,
Attitude and Time
due to GPS Modernization Proposals (from K. McDonald, U.S. ION Newsletter,
Spring 1999).
Some GPS modernization proposals will result in improved signal availability (which impacts on system integrity and accuracy, through the greater number of satellite signals or 'redundancy' that can be tracked by GPS receivers). One suggestion is to increase the number of satellites by 6 to 12, to provide for a GPS constellation of 30 to 36 spacecraft. Apart from the obvious cost implications associated with extra satellites, the critical issue is (paradoxically) the significantly improved life span of the Block IIA and IIR satellites, which threatens to push back the launch dates for the Block IIF satellites (which are expected to carry the new GPS signals referred to earlier). In addition, the transmitted power levels on the second and third GPS frequencies will be raised which will mean that signals are more likely to be tracked during periods of signal interference of shadowing than currently possible on the L2 frequency.
The modernization of GPS will provide many new capabilities, such as two new civilian bands (L2 and 'L5'), new civil and military signals, higher signal power levels, more extensive ground tracking and more frequent and accurate ephemeris updates. All of these improve accuracy, integrity and other aspects of system performance. Additionally, the management of GPS has changed, and now ushers in the possibility of coordinated civil and military funding and oversight. These plans are welcomed, and have the potential of cementing the dominant role of GPS as the de facto standard for satellite system performance, and a crucial component of any international Global Navigation Satellite System (GNSS). Nevertheless there are long-standing concerns internationally because of the U.S. military origin and control of the system, as well as doubts about the ability of the executive and legislative parts of the U.S. government to recognise the importance associated with GPS modernization, as well as the risks involved in delaying or discarding various aspects of the program.
An example of this concern is the current planned schedule for GPS spacecraft
launches. At the current planned launch rates for GPS Block IIR and IIF
satellites, the new modernized capabilities will not be available to most
GPS users until about 2020! Significant modifications to the next GPS satellites
will need to be made VERY soon in order to move the date at which modernized
capabilities are available to 2010. This concern has been a driving factor
behind the European Union's own plans to develop and deploy their own satellite-based
positioning system by 2008. It remains to be seem if such an initiative
will involve the 'merger' with the GLONASS system and, more importantly,
will be 'compatible' with GPS.
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© Chris Rizos, SNAP-UNSW, 1999