What The Hell Is Differential GPS?

This Friday’s shoot-the-shit topic consists of a quick tutorial on DGPS (Differential Global Positioning System). Why should you care?

Simple, it’s capable of 1 to 2 centimeter accuracy. DGPS is currently being used by surveyors and its is being broadly implemented by federal and local agencies. It has tremendous potential, so it’s important you understand it exists and what it can do for you. DGPS is a technology that should be made available to all users, and it wouldn’t hurt to chat with your local or federal elected officials to make it broadly available for commercial use. At least broach the subject with them.

DGPS uses the civil satellite constellation NOT the military system so there are no issues there. The system requires a base station with a GPS receiver that can interface to an r.f transmitter operating in the mid-frequency (MF) range, which is in the 300Khz range and a handheld or vehicle mounted GPS receiver with a built in receiver that can receive in the 300 Khz range.

Here’s how it all works.

Base Station

GPS positioning requires that the GPS receiver see a minimum of 4 satellites. The receiver then calculates the signals time differences and from that determines your position and elevation. In DGPS, the precise location of the base or reference receiver is already know. Knowing its precise location (usually surveyed), the base or reference receiver calculates what the signal time differences should be. It then compares the calculated time differences with the actual time differences for all the satellites it sees and arrives at a numerical value, or difference, referred to as the error or correction.

The base or reference station then formats a message, which includes the error or correction values for all of the satellites in its sky. That message is transmitted as part of a beacon signal in the 300Khz range.

Rover GPS Receiver

The roving GPS receiver then receives the reference station’s transmission, and from that transmission it extracts all of the corrections, or error values, for the satellites in its horizon. The GPS receiver then applies those corrections in its own calculation to determine position and elevation. Then end result is an extremely precise position.

The effective range of the systems is up to 200 kilometers depending on terrain. That means that any handheld or vehicle mounted GPS receiver, compatible with DGPS, can achieve extreme accuracy using the civil satellite constellation.

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