Space Applications

Applications
Title	Space Applications
Author(s)	GMV.
Level	Medium
Year of Publication	2011

GNSS systems were originally designed for earth-based positioning and navigation. Despite this, real-time spacecraft navigation based on space borne GNSS receivers is becoming a common technique for low-Earth orbits and geostationary orbits, allowing satellites to self-determine their position using GNSS, reducing dependence on ground-based stations.

The space community started experimenting with spaceborne receivers very early in the deployment of the GPS network.The first spaceborne GNSS receiver was deployed in Landsat 4 in July 16^th 1982. The GPSPAC receiver deployed with Landsat 4 was also deployed with Landsat 5 and 2 other US Department of Defense missions and despite the few number of GPS satellites deployed (at that time only 6 Block I satellites were deployed), the GPSPAC was able was able to demonstrate the feasibility of using GNSS for space navigation.

Operational Contraints of Space-borne Receivers

The space environment presents differences from the terrestrial environment that don't allow us to take the assumption that a receiver working flawlessly on the ground will work properly in space.

Velocity

The first big difference is the spacecraft velocity and specially the bigger relative velocity between the GNSS receiver and the GNSS satellites. Bigger velocity induce bigger Doppler shifts and the receiver will have to scan a larger range of frequencies to acquire the GNSS signal. Also at this velocity the receiver will track each satellite for a shorter period and visible satellites will change faster. This might require a different selection logic and tracking loop design in order to cope with this more dynamic environment. In general this increases the time required for the signal acquisition and some terrestrial receivers are limited by design to work below certain velocities since it is not expected that a terrestrial receiver can achieve the velocities achieved by a spacecraft.

Orbit

Orbit altitude and geometry are also a constraining factor for space-borne receivers. GPS satellites orbit the earth at an altitude of roughly 20 200 km and the GNSS signal is transmitted by directional antennas pointed to earth and taking into account the earth diameter. For Low Earth Orbit spacecrafts the signal reception conditions are roughly the same as in terrestrial applications. When progressing in Medium Earth Orbit until near the altitude of the GPS satellites signal acquisition becomes increasingly more difficult since it is more likely that the spacecraft will fall outside the coverage cone of the GPS satellite and might require to track weaker signals from the sidelobes of the satellite and to track satellites on the opposite side of the earth not being obstructed by it. For Medium Earth Orbit above the GPS satellites orbit, High Earth Orbit or High Elliptical Orbits the problem becomes even more harsh since only the weaker signals from the sidelobes and the signal from satellites in the opposite side of earth are available.

Antena

Usually terrestrial receivers antennas

Attitude Control

Detailed information about Attitude Control can be found here.

Precise Orbit Determination

Detailed information about Precise Orbit Determination can be found here.

Satellite Realtime Navigation

Detailed information about Satellite Realtime Navigation can be found here.

Low-Orbit Satellite Positioning

Detailed information about Low-Orbit Satellite Positioning can be found here.

Satellite Formation Flying

Detailed information about Satellite Formation Flying can be found here.