Fig. 20 satellites 'a* and 'b'

Fig. 21 SATELLITES 'A', <B* AND ⁴C'

If we also know that we are, for example, 25,000 miles from satellite 'C', our position is further restricted to the area where the three spheres interact.

Theoretically 3 measurements are all we need to find the correct position.

GPS calculates how long a radio signal from a satellite takes to reach us and then calculates the distance to the satellite based on that time by using the equation Distance = Velocity x Time.

If it can be determined exactly when the satellite started sending its radio signal, and exactly when it was received, it can be calculated how long the signal took to reach us.

These measurements must be very precise, down to nanoseconds or better. The GPS satellites achieve this accuracy by using atomic clocks, but GPS receivers are equipped with very precise electronic clocks, which are not always precise enough and can introduce a "clock error".

By using trigonometry and three perfect measurements a position can be located in three-dimensional space. But by using a forth measurement the clock error can be eliminated.

System Operation

The GPS receiver processor unit has in its memory the location of the satellites in their orbits at any specific time, because they follow a known orbit. By knowing the exact location of a satellite and the time taken for the signal to arrive from the satellite, the distance to the satellite can be calculated.

Fig. 22 four satellite ranging

Using four range measurements at the same time gives an accurate aircraft position.

Each satellite transmits two L - Band signals. LI 1-57542 GHz and L2 1-2276 GHz for navigational use. A third satellite status signal is transmitted on 1-78374 GHz to the ground monitor stations.

There are some minor errors in the system. Tiny variations can occur in the altitude, speed and positioning of a satellite. Any changes are monitored by a GPS master control station and the five monitor stations which continuously track the satellites and send corrections back to the satellites.

All GPS satellites synchronise to Universal Time Co-ordinated (UTC) and the precise timing of the transmissions from the satellite are controlled by on­board atomic clocks. These clocks are very accurate such that they would gain or lose only a second every 1,000 years. However this is still not accurate enough so the satellite clocks are compared with Caesium clocks maintained by the master control station and any corrections are transmitted to the satellite.

Other small variations can be caused by ionospheric and atmospheric delays.

GPS provides two positioning services.

The Standard Positioning Service (SPS) is for civilian access and has an accuracy of 15-25 meters in 95% of the position fixes. For security reasons, this can be downgraded to 100 meters in 95% of the fixes.

The Precision Positioning Service (PPS) is for military users and has an accuracy of 18 meters or less in 95% of the fixes.

15.10. GPS В737.

Подробно рассматривается в [4] рекомендуемой литературы.