The University of New South Wales, Sydney, Australia
GGlobal Navigation Satellite Systems involve satellites, ground stations and user equipment to determine positions around the world and are now used across many areas of society.
Among currently used GNSSs, the Global Positioning System (GPS) from the US is the best known, and currently fully operational, GNSS. Russia also operates its own GNSS called GLONASS. Fuelling growth during the next decade will be next generation GNSSs that are 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. Watershed advances in a major technology like GNSS only occur in 20 to 30 year cycles, therefore this is an opportune time to explore potential opportunities and issues for the geospatial industry.
THE GLOBAL POSITIONING SYSTEM
The most widely used current system is the Global Positioning System (GPS). The current constellation of 29 Block IIA/IIR satellites (US Coast Guard Navigation Center, 2006) operate without a hitch and civilian applications of GPS are now considered to be quite mature. The US has just launched one 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 of relevance for later discussions:
- GPS broadcasts two signals in the so-called L1 and L2 frequency bands: L1 at 1575.42MHz and L2 at 1227.60MHz.
- 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.
- GPS provides two levels of service:
- Civilian users have access to the Standard Positioning Service (SPS), the C/A-code allows direct L1 measurements to be made. Specifying the accuracy of the SPS depends on many factors. Recent testing has shown that typically available accuracy from the SPS is often less than 10m. The Precise Positioning Service (PPS) enables enhanced accuracy and availability that is not available to civilian users by permitting the direct measurement of pseudorange on both the L1 and L2 signals using the Y-code.
For the geospatial industry, applications can be classified according to the achievable accuracy:
- Single Point Positioning (SPP) is the technique for which GPS was originally designed and delivers the 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 metres down to few decimetres.
- 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).