GNSS Chapter-1: How GPS works ?

GPS is the popular term for satellite navigation, even in our smart phones we always call it as GPS although its a not only GPS but GLONASS GaliLEO QZSS beidou. Android use the term location as its coordinates are actually calculated from fusion of multiple data. So the technology we are discussing here is about satellite navigation system(SATNAV).

How satellite Navigation Works ?

To understand Satellite navigation, we need to first know about the parent technology i.e. Radio navigation system. Satellite navigation is one of radio navigation method but the older one is Ground based Radio Navigation System (Terrestrial navigation system) e.g. LOARAN, DECCA, GEE, OMEGA. Acoustic location(Artillery Sound ranging is opposite where there is one beacon and multiple receivers) is too similar to this where a sound wave sent instead of RF. Basically they all use Time of flight(TOF) and triangulation.

All these techniques are similar, they all Produce a wave(acoustic or electromagnetic) from multiple beacons at fixed Known locations, which emit waves with a time synchronization one after one or simultaneously at different frequencies.

the receiver gets the waves at different times, as the receiver knows the time at which these wave/signals are emitted so it can calculate the distance between itself and the beacons. Once we get the distance we calculate it with the known coordinates of the beacons to get our own coordinates.

GSM Localisation

Now incase of satelite navigation what we have different is

  1. satellites up in sky, easy to get line of sight
  2. Much higher frequency bands
  3. More than 3 satelites
  4. Cordinate in 3D space

Here we have spheres instead of circles to show probable position

Satellite Navigation

To calculate users cordinate in 3D space we have to calculate Translateration in 3D.

3D space Translateration as in Satellite Navigation

The satellite coordinates assuming T1, T2, T3 respectively, if we got the distance from TOF is r1, r2, r3 then we can calculate our receiver position using the cosine laws.

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