The ionosphere is a significant source of error in satellite navigation systems, such as GPS. The position of a receiver is calculated by measuring the time delay between a radio signal emitted from each satellite and the signals reception at the receiver. Assuming a constant speed of light this time delay can be converted to a receiver-satellite distance. By comparing the distance to multiple satellites a GPS receiver can determine its three dimensional position.

The ionosphere disrupts this approach since the GPS radio signal is slowed by the presence of free electrons, causing an additional time delay and hence an error in the distance to each satellite. The greater the total number of electrons (TEC) on the signal path the greater the time delay. The GPS system broadcasts on two frequencies and since the ionosphere is a dispersive medium the time delay for a given TEC will depend on the frequency of the signal. This allows the TEC to be measured by examining the differential time delay between the two frequencies.

The IPS near real time ionospheric TEC map uses dual frequency GPS data from a range of receivers across the Australasian region including IPSnet, Geoscience Australian and SunPoz instruments. Data is obtained on an hourly basis. The ionosphere is modelled as a thin shell at a fixed height of 400km. The point directly below where the line of sight between a GPS reciever and satellite passes throught this shell is known as the ionospheric pierce point (IPP). The TEC map is composed of a series of spherical capharmonic (SCH) basis functions. These are related to spherical harmonics, but defined over a region rather than the entire globe.

The TEC measured at each IPP is used to determine the optimal co-efficients of the SCH basis functions at each point in time using a Kalman Filter, a common technique for real time state estimation. The resulting map can be used to correct for the effects of the ionosphere in real time for improved GPS positioning. The results can also be used for post processing of data. If you are interested in this service please contact IPS.