Trilateration

A receiver’s position on the earth’s surface is determined by a process called trilateration, which involves measuring the distance from the receiver to 3 satellites. Since we know the signal is travelling at the speed of light (Approximately 300,000000 m/s), distance is measured by measuring the time of travel for the signal sent from the satellite, to reach the receiver.

Taking the measurement to a third satellite, will determine whether the receiver’s position is at position 1 or 2. To determine a fix in 3 dimensions a fourth satellite is needed, so the minimum number of viewable satellites for a full GNSS fix is 4.

The time it takes a GNSS signal to reach the receiver is the way that distance from the satellite is measured. Clearly, accurate measurement of time is fundamental to accurate measurement of distance. At the speed of light, just 1 microsecond equates to 300m. User’s receivers on the ground cannot afford the highly accurate rubidium clocks housed in the satellites and most use quartz. This causes a fix inaccuracy at the user’s receiver on the ground which is removed by successive adjustment of the receiver clock to match that of the satellite, which then achieves convergence of fix. This is shown in the following diagram:

This shows that in 2D a fix can be obtained with 3 satellites. When this is extended to true 3D space 4 satellites are needed.

Blackroc manufacture a range of professional grade GNSS solutions which enables satellite fix accuracy in real time from submetre to 1cm (RTK)

Procyon GNSS range

Procyon

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