Methods of improving on GPS positional accuracy
The basis of the positional calculations are outlined above, whereby the receiver uses the measured ranges from the satellites to fix its position in 3D space. Using a GPS receiver the standalone accuracy by this method is about 3m. However, there are several methods that can be used to improve on this:
1. Wide area differential or “WAAS” corrections. This method utilises corrections either transmitted from beacons or from the GPS satellites to remove some of the error from the satellite orbit, clock, and atmospheric effects. In practice the improvements are quite small. In common with all good quality GPS systems, all Race Technology products have the ability to use WAAS corrections when appropriate.
2. Local differential corrections (single differencing). This uses a local static GPS receiver to correct the received ranges of the moving GPS receiver. In a low multipath environment, single differencing typically reduces errors to the sub meter range.
3. Real Time Kinematic (RTK) calculations. Again this method requires a trackside reference receiver and works by tracking the number of carrier waves between the satellite and the receiver. Because the carrier phase can be tracked far more accurately than the raw range information (about 1mm compared to around 20cm) the result is a positional accuracy of just a few centimetres. The problem with this method is that it is not particularly robust, as both receivers need to have a clear view of all satellites for several minutes before any positional information is available and even trees by the trackside can cause the solution to fail. In practical terms this is satisfactory for surveying applications; however it is not readily suited to automotive testing.
Note that none of these methods directly impact the accuracy of the speed measurements; there is very little that can be done to improve these using differential corrections.