# 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