Comment by jhayward | Hacker News Reader

Comment by jhayward | Hacker News Reader

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There are some misunderstandings in the above.

1) AGPS is not local positioning. It is a "speed-up" assist from cellular networks to provide the Almanac (e.g., satellite orbit info) quicker than a GPS receiver can get it from the satellites. All AGPS receivers use the actual GPS satellite signals to calculate the receiver's position. No sats, no position.

2) RTK does not provide a local positioning system. RTK allows a base station receiver, which is in a stable/not-moving site, to observe GPS position calculations over a long period of time and 'averages' the positions to arrive at a highly precise position for itself. It then monitors the real-time positions it calculates and broadcasts a list of error corrections to be applied to the satellite signals to reduce the real-time error, which is constantly changing. Again, no sats, no pos.

There are a couple of things I wonder about RTK that maybe you or somebody else in this thread can help me with?

1. Where does the kinematics in the name come from? The base station is stationary, and kinematics is about the movement of objects.

2. How does the correction signals work? Is it as simple as "now the position is 3.56 m to the left of the true position, correct your position accordingly", or is it more advanced with detailed timing corrections for each satellite's signal or similar?

I'm not an expert either, but I do know it's per satellite, and uses several data points per satellite.

How I understand it, RTK comes into play you want to get very precise location on a moving vehicle. To make the location reading more precise you need to take it over some time. To average it while moving you need to know _how_ you're moving. Therefore the need for RTK.

The RTK position is then combined with the measured error (measured near that location), because GPS is precise but not accurate enough on it's own. Without correction a stationary point slowly drifts around a bit (accuracy: roughly 0.1-1 meters). The correction is called differential gps, and you can provide it yourself with a base station, or use a service that does just gives it to you (from their network of error measuring base stations), see NTRIP. The accuracy with correction can be greater than 0.01m

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