GPS: now in Space!
GPS is widely extended, there are no doubts about it, we use it every time. From all kind of transportation vehicles for positioning and navigating through their way to destination, to smartphones and other personal use devices whose apps rely on location based services; even agriculture, smart electrical grids, atmospheric weather prediction and measurement systems or banks make use of GNSS, which stands for Global Navigation Satellite Systems, the family which GPS belongs to.
However, almost little to slightly nothing is talked about the space applications of GPS. But is it possible to use GPS-provided location out of the Earth? This might be a natural question that everybody who knows how GPS works may wonder, so before proceeding, lets recall how GPS works at this article.
The arising question is natural: satellites send signals towards the Earth, so how is GPS navigation still possible in altitudes higher than their own MEO orbits?
To define GPS applications in space, two spatial regions must be defined: Terrestrial Service Volume and Space Service Volume. It is known that on Earth at least four satellites are needed to get a position fix, therefore, good satellite visibility conditions are required. Regarding this, space is not the exception but the higher you are, the less satellites could get, therefore although GPS use in space exists, its performance is not the same than that which we are used to.
Here is where the two previously mentioned spatial regions come into the equation. TSV (Terrestrial Service Volume) is defined as the region expanding from Earth’s surface to an altitude of 3.000 km, therefore LEO satellites can make use of GPS.
On the other hand, SSV (Space Service Volume) is the region which expands from altitudes of 3.000km to 36.000 km, the GEO orbit. This region can be splitted into two, one for medium altitudes (3.000 km to 8.000 km) and other for high altitudes (8.000 km to 36.000 km).
Check out our post GPS for Meteorology, a technology that makes use of GPS onboard LEO satellites!
However, the performance of GNSS in space is not as that on Earth. While within the TSV it is possible to receive four satellites simultaneously during the majority of the time, this visibility decreases with altitude and therefore, satellite signals coming over the Earth’s limb are extremely important.
This is because GNSS satellites focus the majority if their signal’s energy towards the Earth with a limited antenna beamwidth. It is important to highlight that while both directional and omnidirectional antennas exist, GPS receivers (as those in our smartphones) use the latter while GPS transmitters (as those on satellites) use the former. Coming back to the use of GPS satellites over the Earth’s limb, while navigation on Earth is performed by those satellites above us, in space it is also possible to get signals coming from satellites which are at the opposite side of the Earth, this is like saying below us. This is due to the increase of altitude, which also increases satellites visibility.
When a spacecraft is launched towards the space, with the increasing of altitude the visibility of GPS satellites changes. Within LEO orbit, the spacecraft receives signals almost from the same satellites as we do on Earth’s surface. However, while entering into MEO orbit and continue towards higher altitudes, the spacecraft visibility will change from being mainly the GPS satellites overhead (those also visible by us on Earth) to those in the opposite side of the Earth.
Recall that altitude increases satellite visibility, but also remember that GPS satellites focus their energy towards the Earth in a conical form, so depending where the spacecraft is, it might fall out of the coverage area of some satellites right above it. Furthermore, spacecraft could surpass the GPS satellite orbit altitude and their signals will be no longer received, therefore satellites from the opposite part of the Earth are needed. Here is where the side lobe signals play an important role, since these contain the energy which is not focused into the main lobe and whose use can be exploited by the spacecraft in those regions where either Earth could be shadowing satellites in its opposite side or the spacecraft is out of the range of these satellite’s main lobe.
The Space Service Volume performance definition is still under development by the United Nation’s International Committee on GNSS (ICG) along with international collaboration. In 2018 the United Nations Office of Outer Space Affairs released “The Interoperable GNSS Space Service Volume” which defines the coordinate definition and baseline performance of the SSV internationally with all six GNSS providers: USA (GPS), Europe (Galileo), Russia (GLONASS), China (BeiDou), Japan (QZSS) and India (NavIC).
Undoublty GNSS is an extremely important sensor also in space, and several missions have been making use of it for multiple purposes. From navigation (such as International Space Station and precise orbit determination missions), to science (such as GOES-R, NASA’s Magnetospheric Multiscale (MMS) and CYGNSS and other meteorology missions presented in out previous post mentioned above).
It must be also taken into account that the precision at very high altitudes might lead to bad satellite geometry, therefore bad Dilution of Precision which basically tells us that the more equally distributed are the GNSS satellites with respect to us, the better the performance. However, we must keep paying attention to the GNSS continuously growing technology and advancements since its space applications are becoming more and more present in Space Exploration, Communication and Navigation, SPACEWAYFNDER’s main topics!
Thank you for reading!
|Inside GNSS. Navigating in Space: Taking GNSS to New Heights. https://insidegnss.com/wp-content/uploads/2018/01/novdec16-MILLER.pdf|
|NASA. Use of GPS/GNSS for Future NASA Missions. https://www.gps.gov/multimedia/presentations/2010/01/bangkok/miller.pdf|
|Spaceflight101.com. CYGNSS Instrument Overview. http://spaceflight101.com/cygnss/cygnss-instrument-overview/|
|Europapress.es Consiguen medir el nivel del mar mediante reflexiones de GPS. https://amp.europapress.es/ciencia/misiones-espaciales/noticia-consiguen-medir-nivel-mar-reflexiones-gps-20160222185714.html|
|NASA. The Interoperable Space Service Volume. https://www.nasa.gov/directorates/heo/scan/communications/policy/ssv/|