1. Technical aspects
a. Characteristics
GPS gives at the same time:
- geographic position
- altitude
- time
- speed of displacement
4 satellites are necessary to:
- measure
- the length of the path taken by the radio waves => altitude
- the distance between the satellite and the user
- compare the atomic clock of the satellite with the reciever's clock => time
- calculate the position
b. Borders
- Spring of mistakes:
- clock mistakes during a fast displacement
- difficulty for the waves to spread through the different coasts
of the atmosphere
- reflection due to obstacles rising off the ground
- Accuracy:
For a standard GPS equiped with PPS:
- horizontal: 22 meters
- vertical: 27.7 meters
- time: 200 nanoseconds
- Different ways to refine accuracy
- do the average of several results concerning the same point
(problem: needs to measure on a very long time)
- differential GPS : correction of the measures thanks to a network
of reference points with a position, jour and altitude defined
-> accuracy < 1 m
- Geodesics recievers : immediate correction thanks to the recording
and processing of woth signals produced by the satellite
-> accuracy < 1 cm
