New GNSS Developments And Their Impact On The Geospatial Industry
GPS Modernization
The US has embarked on a program of GPS Modernization to provide accuracy and more powerful signals from future GPS satellites. While there are various improvements planned, the important issues revolve around extra signals to be broadcast by future GPS satellites:
- An improved code (instead of the current C/A-code) on the L2 frequency of GPS (the so-called L2C) 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 launch schedule to replace existing satellites is difficult to predict but operational capability for L2C will not be 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, 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 in civil aviation and emergency service operations. Therefore, a third civil frequency at 1176.45 MHz (called L5) is planned for launch on the Block IIF satellites. The first Block IIF launch is scheduled for 2008, with full operational capability 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, which 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 Block II signals will also be broadcast. The 30 GPS-III satellites are planned for launch from about 2013 until 2017.
The implications for GPS receivers is 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.
FROM GPS TO GNSS
There are places where GPS 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.
GLONASS from Russia
GLONASS was originally designed as 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.
Since the collapse of the USSR, the Russian Federation has struggled to find sufficient funds to maintain GLONASS and at the time of writing (May 2006) there are 15 satellites functioning (as opposed to the 24 necessary for full operational capability). However, the Russian Federation has commenced a program to revitalise GLONASS, with planned full operational capability as early as 2008. In addition:
- Current activity centres on launching GLONASS-M satellites with an improved 7-year design lifetime, which will broadcast in the L1 and L2 bands (though not on the same frequencies as GPS).
- From 2007 to 2008 it is planned to launch GLONASS-K satellites with improved performance, and which will also transmit a third civil signal in the Aeronautical Radio Navigation Services band near (but not identical) to GPS's L5 frequency.
- The full constellation is planned to be broadcasting three sets of civil signals by 2012. It is worth noting that at the end of 2004 the Indian Government announced it would be contributing funds to assist Russia revitalise GLONASS.
Although the frequencies of GPS and GLONASS are different, a single antenna can track all 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.
Galileo from the EU
Perhaps the most exciting impact on the future of GNSSs is the decision by the European Union to launch its Galileo project. The following points are relevant:
- The design calls for a constellation of 30 satellites in a similar orbital 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 finalised but Galileo satellites will broadcast signals compatible with the L1 and L5 GPS/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.
- Galileo will offer five levels of service, two of which are fee-based and one of which is restricted.
- The Galileo ground segment has elements similar to the GPS or GLONASS global networks of tracking stations and master control stations.
- 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 architectures. 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.
- Galileo is to be operated by a Civilian Agency and the business-operating model is more open than in the case of GPS/GLONASS. Galileo uses a Public Private Partnership (PPP) whereby the European Commission owns the physical system (satellites, ground stations, etc) as a public asset, but a Concessionaire will be responsible for the day-to-day operation. The business model is a secret, however the Concessionaire will probably seek to generate profit through the Commercial Service and Safety of Life Service, while also delivering agreed service levels for the other services.
- Galileo has moved out of its development phase into 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. GIOVE-B will be launched in late 2006.
- The full constellation was planned to be launched 2007-08, with full operational capability by 2009, however these dates will almost certainly slip a year or two.
For users seeking accuracy and availability it is likely that they will want GNSS receivers that track all satellites, and as many of the signals as is possible. However, it is inevitable that there will be many tradeoffs made, and that there will be many receiver options available.