Most of the error in an SPS measurement comes from the fact the receiver has no way of actually knowing how the speed of the satellite signal has been affected as it passed through the ionosphere. It gets a small clue as there are some estimates included in the navigation message, which allow it to make a guess at the likely effects, but these are only estimates. In practice it means the speed by which the delay is multiplied is not quite correct—that, in turn, means the calculated pseudo-range is also incorrect.
The only way to allow for this is to measure, rather than estimate, how the signal speed is being affected as it makes its way down through the atmosphere. This is where differential GPS come in. DGPS can be achieved in a number of ways, but they essentially use the same technique.
A separate GPS receiver (called a base-station) is placed at a pre-surveyed point and turned on. Because the location of the base station is already known with great accuracy, the base station is able to compare the position measurement generated by its own GPS receiver with the known co-ordinates. Any difference means the signal from one or more satellites is being delayed. All the system has to do then is work out how much correction should be applied to each satellite in order to correct the GPS position measurement.
Once the base station knows how the signal from each satellite needs to be corrected, it can share this information with any other GPS receivers (often called rovers) in the area. This is often achieved using radio modems. The roving receiver checks if corrections are available for the satellites it is seeing and applies them where applicable—improving the accuracy of its pseudo-range measurements even though it is not at a known location.
There are also other ways to achieve DGPS. Rather than using your own local base-station (that you set-up on your own site), it is possible to receive differential corrections via a web-based service and data connection. In this case, your receiver (the rover) works out where it is in the normal way. It then transmits its location to a central server that works out where the nearest reference station is to you, before sending back the differential corrections from that location. The beauty of systems such as these is that you could drive from one end of a country to the other and receive valid corrections for the entire journey.
Another method is to use satellite-based correction services called SBAS (Satellite Based Augmentation System). These work in a slightly different way. Ground stations on earth at known locations work out the differential corrections in different geographic locations. These corrections are then uploaded to the satellite where they can be broadcast down to any receiver that is enabled to receive them. However, as well as needing a receiver with the correct hardware, you often have to pay a subscription fee in order to unlock the corrections.
Using SBAS services improves the accuracy of receivers, but not to the same degree as a local base station. When using DGPS, the accuracy of position measurements often improves to tens of centimetres, say around 40 cm. DGPS that is achieved through SABS is often sub-metre accurate, somewhere in the range of 50–60 cm.
Another technique that improves the accuracy of GPS position measurements is Real-Time Kinematic (RTK). Read on to learn why it’s one of the hardest techniques to understand and the most intensive to implement.