The benefits of future GNSS
THE conventional geomatics industry including mapping and surveying applications has been revolutionized with the use of GPS, which is the best known, and currently fully operational satellite based navigation system operated by USA (Parkinson, and Spilker Jr., 1995). In the mean time, Russia also operates its own satellite based navigation system called GLONASS. The USA is modernizing GPS in order to retain its superiority in satellite based navigation technologies (MacDonald, 2002,). In order to keep up with USA’s progress in building next generation system, Russia is taking serious steps to modernize GLONASS as well (Federal Space Agency for the Russian Federation, 2005). The GPS and GLONASS signals are free but its availability is not guaranteed and currently most users are prepared to accept this risk (Parkinson, and Spilker Jr., 1995). However, as satellite navigation becomes a vital technology across a number of critical industrial sectors, the prospect of, for example, a nation’s transport infrastructure becoming dependent on this technology is a strategic risk that most industrial countries are not willing to accept. This argument initiated the Galileo program in Europe. Therefore, those systems form the mainframe of Global Navigation Satellite Systems (GNSS) (MacDonald, 2002,).
Unlike GPS, Galileo will also offer a guaranteed service to users who are willing to pay for it (e.g. commercial service – CS, and Public Regulated Service PRS) in addition to a free signal similar to that of GPS (Open Service – OS and Safety of Life service – SoL). Galileo will be available to the public in 2012 (European Commission, 2003). Despite many technical differences between these three GNSS systems, the commonality of the carrier frequencies they use creates the potential for the future development of an interoperable GNSS receiver.
The vast majority of the world will be users of these existing systems. The fundamental questions then are: “Which system or systems should a country use?”; “How to choose a combination of the systems?”; “What are the benefi ts and respective merits of those systems?” There is no simple answer to these questions, as the best solution will undoubtedly depend on the targeted application, which has its own requirements in terms of accuracy, reliability, robustness, cost and other application-specifi c criteria. What can be provided, however, is a means whereby parameters that describe these performance requirements can be computed.
Comparing those pricy hardware GNSS simulators, high-accuracy software simulations are a cost-effective and precise tool to evaluate the performance characteristics attainable from the future GNSSs, and have been recognized as an appropriate pre-development tool for satellite navigation systems and applications in Canada and European countries. In addition, the entire hardware simulators available on the market can only emulate the signals from a single system at the present time. On the other hand, a dual systems simulator is easier to implement in the software level. The technical benefits of this approach lie in the fact that the software simulations are reproducible and totally controlled, and parameters can be changed individually if necessary for an in-depth understanding of the underlying effects. This paper introduces a qualitative assessment of the performance characteristics of the future GNSS infrastructure around Taiwan area using a multi-system software simulation toolkit being developed; therefore, representative results over Taiwan are demonstrated.
Evolving From GPS to Future GNSS
Motivated by the United States Department of Defense (DoD), the current GPS has experienced three decades’ development. Although the original motivation was only for military purposes, GPS has been widely used in civilian applications during the past few decades. However, the integrity, availability, and accuracy still need further improvement for various applications. For the surveying industry, applications can be classifi ed according to the achievable accuracy:
* Single Point Positioning (SPP) is the technique for which GPS was originally designed and delivers the Standard Positioning Service (SPS) performance mentioned above. 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 reference station). The accuracy achievable from DGPS can range from a few meters down to few decimeters, depending on the quality of the receiver and the DGPS technique used (Parkinson, and Spilker Jr., 1995).
* GPS Surveying also works differentially but can achieve centimeter accuracy using a special measurement technique. A typical receiver, for both SPP and DGPS1, 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) (Lachapelle, 2002). However, receivers used in surveying and geodesy measure the phase of the underlying carrier wave signal (the so-called carrier phase). For baselines between points separated by more than 20km, it is important that such receivers can also correct for the ionosphere (Lachapelle, 2002). For shorter baselines, dual-frequency receivers are necessary for rapid initialization of cmlevel 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 receivers are more expensive than receivers used for SPP and DGPS.
Therefore, a GPS modernization program was initiated in the late 1990’s, in an attempt to upgrade GPS performance for both civilian and military applications. The GPS modernization program started with the cancellation of SA in 2000. It will be followed by the addition of a new a second civil code on L2 (L2C), then a third civil frequency L5. Further modernization consists of the assessment and design of a new generation of satellites to meet military and civil requirements through 2030. Table1 includes a summary of the launch schedule of the modernized GPS satellites according to MacDonald (2002).
GPS Block IIR-M is the second part of Block IIR, with eight modernized satellites being built by Lockheed Martin. The IIRM satellites will have a new civil signal on L2 at higher signal power than normal IIR satellites. The Boeing Company has the contract for GPS Block IIF, with nine satellites in total that are intended to provide improved anti-jam capability, increased accuracy, higher integrity, and secured operational M-codes. Additionally, a third civil code at a new frequency L5 will also be included. The purpose of the GPS III program is to deliver major improvements in accuracy, assured service, integrity, and fl exibility for civil users. Currently led by both Lockheed Martin and Boeing both, the team of GPS III program has proposed the use of the same signal structure as Galileo for its open signals and decided the year 2012 as the target date of the launch of fi rst GPS III satellite.