Chapter 2
Differential GPS

What is DGPS? 

            Armed with our knowledge of what GPS is, and how critical it is to control error, we can look at how DGPS helps to make GPS even more accurate. That’s right, more accurate! DGPS can correct measurements taken from a moving vehicle to within a couple of meters, and offers an even higher degree of precision for measurements taken at a stationary point. 

            DGPS has one purpose, to correct the errors we encounter while in the field. How? Simple, by increasing the number of receivers we are using to two. Okay, so its not that simple, but that is the first step!

            Before we begin looking at how these two receivers correct our measurement, we need to understand why every GPS reading isn’t perfect. Thus, we need to talk about errors. 

Random Errors 

Satellite Errors

            The first type of satellite error concerns timing. Yes, each satellite has four atomic clocks, so they are pretty good, but they aren’t perfect, and as we saw in the last chapter, even small clock errors in the satellites can translate into huge errors for us on the ground.

            The second type centers around the satellites position in space. The satellites are in very high orbits, and this keeps them virtually free from perturbations due to drift, however, it not perfect, and as with timing, small errors in position data can spell trouble for us on the ground.          

The Atmosphere

            Since GPS uses radio waves to transmit data to us, the atmosphere each signal has to pass through causes small errors. This error results when our receiver calculates distance by using the signal time, because we assume that signals travel at the speed of light, which it only does in a vacuum. Some high-end receivers do have an added function to help compensate for atmospheric disturbance, but the atmosphere is not a standard thickness nor is moisture equally distributed throughout it. All of these factors add errors. 

Multi-Path Error

            This is perhaps the easiest type of error to understand. The radio waves that are transmitted by the satellites are sometimes bounced off stationary objects, and create a ghost signal, think of when your TV’s reception is poor and you see double pictures, that is because one signal is a reflection and reaches your TV a fraction of a second later than the original signal. The most common source of Multi-Path error is reflections caused by buildings.

Intentional Error 

Selective Availability

             This is the single worst type of error we encounter while using GPS for measurements. Basically, the US Department of Defense introduces noise into the signal to corrupt the accuracy, and it does a great job of doing so, introducing an error of up to 100 meters while in the field.

            Luckily for us, this is the exact reason DGPS was invented, and can correct the measurements to within a few meters.

ALERT! By the time you read this, chances are you won’t have to worry about Selective Availability at all, plans by the US have all but phased this out. The only time Selective Availability (also known as SA) is turned on is during wartime, and then it is usually only over the countries involved in the conflict. The US government has plans to stop SA from being activated completed by the year 2006.

            Even though SA is usually turned off, and even if you know that SA is turned off, it is always important to correct your data to compensate for the other types of errors discussed.

How DGPS Works 

            Above we mentioned that we needed two receivers for DGPS, well, its actually that second receiver that makes all the difference. The first receiver is called a rover, and that is the unit that you carry around with you and take measurements with, the second receiver is called the base station, and it sits still on a point taking measurements.

            So, the problem, in a nutshell, is the fact that our receiver uses timing of signals from at least three different satellites to determine its position, and each of those signals has its own set of errors depending on its trip to the receiver.

            The solution hinges on the enormous distances between us and the satellites in space. Since we are so far away from those satellites, two receivers on the ground, even if they are a few hundred kilometers apart, receive basically the same signals, with the same errors. Therefore, that second receiver, the base station, sits on a known point and collects data, as it does this, it references the signals it receives with the known point it is on, and determines the error.

            Next, it is a relatively simple process of comparing the base station data (stored in a file) with the rover data (also stored in a file). There are two main ways to do this, one is real-time correction and the other is called post processing. The first way, as the name suggests, involves correcting data as it is recorded, this is accomplished by using radio transmitters and receivers attached to the two GPS receivers. The second is accomplished by using software to compare the two files after the data has been gathered. We will discuss each method in more detail later in this manual.