In last chapter we learnt how satellites help us get our position in 3D space. Now lets see how many satellites are there serving this purpose.
GPS
NAVSTAR Global Positioning System also called NAVSTAR , GPS is hosted by United States of America operated by US Space Force. The Project was started by Defense Department in 1973. First transponder placed in orbit in 1978 and full constellation(24 as per initial plan) placed in orbit by the year 1993.
After KAL0007 incident in 1980 GPS made available to public which was only for military before that. In 1990 GPS signals degraded for public use which was changed to accuracy of 50-100 meter instead of 6-12 meter, termed as SA or selective Availability. Select Availability add up two two noises as delta error and epsilon error, one is in the pulse data and other in satellite position data. Selective availability removed in year 2000 by Clinton.
| Band | Frequency in MHz | Phase | Original usage | Modernized usage |
| L1 | 1575.42 (10.23 × 154) | I,Q | I for precision (encrypted), Q for coarse acquisition Code | I for precision (encrypted), Q for L1C civilian use |
| L2 | 1227.60 (10.23 × 120) | I,Q | I for precision code (encrypted), | I for precision code (encrypted), Q for L2C civilian use |
| L3 | 1381.05 (10.23 × 135) | For nuclear test detection Link | ||
| L4 | 1379.9133… (10.23 × 1214/9) | for additional ionospheric corrections | ||
| L5 | 1176.45 (10.23 × 115) | I,Q | Safety of life data signal |
GLONASS
Global Navigation satelite Systems GLONASS is owned by Russia. The project started in 1976 with first launch in 1982 last in 1995 completing full constellation
First launch year: 1982
The formerly Soviet, and now Russian, Global’naya Navigatsionnaya Sputnikovaya Sistema, (GLObal NAvigation Satellite System or GLONASS), is a space-based satellite navigation system that provides a civilian radionavigation-satellite service and is also used by the Russian Aerospace Defence Forces. GLONASS has full global coverage since 1995 and with 24 satellites.
Galileo
This constelllation maintained by European Space Agency (ESA) live from 2016 named after the Italian astronomer Galileo Galilei. european countries need a positioning system independent of USA or Russia which can be downgraded whenever they want. Galileo has better accuracy (1meter) compared to gps which has 3 meter accuracy.
BeiDou
The first saltelite under beidou launched in 2000, BeiDou started as the now-decommissioned Beidou-1, an Asia-Pacific local network on the geostationary orbits. The second generation of the system BeiDou-2 became operational in China in December 2011.[11] The BeiDou-3 system is proposed to consist of 30 MEO satellites and five geostationary satellites (IGSO). A 16-satellite regional version (covering Asia and Pacific area) was completed by December 2012. Global service was completed by December 2018.[12] On 23 June 2020, the BDS-3 constellation deployment is fully completed after the last satellite was successfully launched at the Xichang Satellite Launch Center.