Positional confidence with differential navigation using NAVSTAR GPS code phase

NAVSTAR GPS is a satellite navigation system developed by the United States primarily for military use. A civil spin-off using the lower precision C/A code offers real time position and timing information with wide ranging applications. However, under conditions of reduced performance, because of deliberate degradation or inherent system limitations, the application of differencing techniques offers the capability to reduce and in some cases remove a substantial proportion of the systematic errors. The authors summarise the basic differential operation together with the sources of error that affect its performance. Results from a measurement campaign are included with differences between receiver types discussed