Global Navigation Satellite Systems (GNSS)
GNSS Overview
There are currently four main GNSS systems in different states of development. The United States NAVSTAR Global Positioning System (GPS) is the only fully operational GNSS. The Russian GLONASS is a GNSS in the process of being restored to full operation which should be reached by 2009. The European Union's Galileo positioning system is a next generation GNSS in the initial deployment phase. The In Orbit Validation (IOV) phase should take place in 2010. Full Orbit Constellation (FOC) should be reached in 2015 (but this schedule is very "flexible"). China is building up a global system called COMPASS but also referred to as Beidou-2. Beidou-1 is the a regional augmentation system.On these navigation system pages we give an overview of the details of these different systems: GPS, GLONASS, Galileo, and Compass.
GNSS Comparison
The table below shows a comparision of some of the key features of these different GNSS systems.
| GPS | GLONASS | GALILEO | COMPASS | |
| Number of Satellites | 21 + 3 | 21 + 3 | 27 + 3 | 30 + 5 GEO |
| Number of orbital planes | 6 | 3 | 3 | ? |
| Semi-major axis | 26600 km | 25440 km | 29600 km | ? 21500 km |
| Orbital revolution period | 11:58 H | 11:15 H | 14:07 H | ? 12:35 H |
| Inclination | 55 deg | 64 deg | 56 deg | ? 55 deg |
| Satellite Mass | 1100 kg (IIR) | 1400 kg | 700 kg | ? 2200 kg |
| Solar panel area | 14 m2 | 23 m2 | 13 m2 | ??? |
The
number of satellites notation "21 + 3" for GPS and GLONASS
mean
that the minimal system constellation consitst out of 21
satellites with 3 active in orbit spares. So there are a total of 24
satellites in orbit sending signals. Galileo will consist out of a
minimum of 27 satellites with 3 active in orbit spares. So there will
be 10 satellite in each orbital plane. One of these satellites is the
active in orbit spare. The higher number of satellites in the case of
Galileo is caused by the fact that the Galileo satelites fly "above"
the GPS and GLONASS satellites.
Orbit repeat periods
GNSS satelites have to maintain their relative positions. So the orbital planes have to keep the same seperations as well as that all the satellites in an orbital plane have to keep the same separation (distance) with respect to each other. In GPS this is resolved by giving the constellation a repeat cycle of one day. This means that every sidereal day the satellite passes over the same place on Earth. This causes the satellites are in what is called "deep resonance" with the Earths gravity field. Due to this resonance some satellites in the GPS constellation experience significant orbit perturbations. These satellites have to be manoeuvred regularly to keep them close to their nominal orbit position.
In GLONASS the satellites repeat by taking over each others position. Within an orbital plane the satellites are seperated by 15 minutes. Together with the orbital period of 11:15 hours this means that after one day a satellite in an orbital plane passes over the same spot as the "previous" satellite in the same orbital plane did the day before. This ensures that the constellation remains fixed but avoids the effects of resonance. The repeat cycle of the GLONASS satellites is 8 days.
In Galileo one of the key design criterea was that each satellite should only need a maximum of one manoeuvre per lifetime for keeping it in near its nominal position in the orbital plane. Thus the "deep resonance" as experienced by the GPS satellites was avoided. The orbit constellation selected for Galileo results in a a 10 day repeat cycle for the Galileo satellite orbits. After 10 days the orbit of any Galileo satellite repeast itself. The Galileo orbit selection ensures that the whole Galileo constellation remains in "perfect" shape for its whole lifetime with a maximum of one manoeuvre per Galileo satellite.
GNSS Augmentation Systems
Besides completely new GNSS systems there are also GNSS augmentations systems. Most of them are aimed to improve the performance from GPS but some also include(d) GLONASS. In the USA the GPS augmentation system is called WAAS which stand for Wide Area Augmentation System. In Europe the similar system is referred to as EGNOS which stands for European Geostationary Navigation Overlay System. The Japanese system is called MSAS which stands for MTSAT Satellite Augmentation System in which MTSAT stands for Multifunctional Transport Satellite System. India is developing GAGAN which stands for GPS Aided Geo Augmented Navigation.