GNSS Satellite (GIOVE-A)

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.

Number of Satellites21 + 3 21 + 327 + 330
+ 5 GEO
Number of orbital planes633?
Semi-major axis26600 km25440 km29600 km? 21500 km
Orbital revolution period11:58 H11:15 H14:07 H? 12:35 H
Inclination55 deg64 deg56 deg? 55 deg
Satellite Mass1100 kg (IIR)1400 kg700 kg? 2200 kg
Solar panel area14 m223 m213 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.