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How does GPS positioning work?


Bringing satellite positioning down to earth (or down to dirt)







GPS stands for Global Positioning System and is a satellite navigation system owned and operated by the US Department of Defense.

Its proper name is NAVSTAR GPS: Navigation Signal Timing And Ranging Global Positioning System.

It consists of a constellation of 24 satellites plus 3 spares distributed between 6 orbital planes. By receiving the transmitted signals from several of these satellites, a GPS receiver on earth can accurately determine its position.

How does GPS work in a 3D grade
                              control system?

If you have used a recreational Garmin GPS receiver for your outdoors activities or if you have navigated the streets of an unfamiliar town using a Magellan in-car navigation system, you are no doubt impressed by the capabilities of these little devices, many of which now retail for well under US$ 100.

However convenient these may be, though, they can not determine their position accurately enough to be used for earthworks.

In order to utilize GPS for any kind of precise measurements, we need to add a second receiver, -a base station.
Trimble base station and GCS900
                              grader
The base station must be set up on a known point, meaning  a point which location on the job site coordinate system is known to the 3D machine control system.

When the computer can get positioning data simultaneously from the base station and the rover (the GPS receiver on the blade of the machine) is can calculate the position of the rover with much greater accuracy.

We use a radio to transmit the position measured by the base station to the 3D machine control computer in the cab.
A Trimble base station and GPS machine control system


The radio may be one of several different types, but FM-modulated UHF and 900MHz spread-spectrum are the two most common technologies.

The radio is one-way, -from the base to the machine only. The positioning data can be sent once or several times per second. (The on-board GPS receiver operates at a higher rate of measurement, though).

Now that the 3D machine control computer has positioning data from both receivers, it can calculate the exact position of the cutting edge of the blade and commence generating cut/fill information. You can read more about how it does that HERE.

A base station may be all-in-one or modular, meaning the GPS antenna, receiver and radio may be contained in a single box or each in its own housing.

Leica's GS15, Trimble's SPS882, Topcon's GR3 and HIPER-series receivers are self-contained base stations while Leica's GS10, Trimble's SPS852 and Topcon's GB-series receiver are all modular.

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