Trans-ionospheric propagation measurements using signals from the GPS satellites

The possibility of using 'signals of opportunity' from the GPS constellation of navigation satellites for long-term ionospheric research is discussed. GPS spacecraft broadcast on dual frequencies to permit ionospheric range and range-rate error corrections. The frequencies are modulated at 10.23 MHz and can be analyzed to extract the total electron content (TEC) on the track between transmitter and receiver. Measuring the differential carrier phase (DCP) between the frequencies yields a relative TEC count accurate to within 3 x 10 to the 14th electron/sq m. Tests with the constellation of five prototype GPS satellites revealed the DCP scintillation can be accurately determined. Implementation of directional, instead of normal omni-directional, antennas is expected to increase both TEC and scintillation measurement accuracies. Finally, an operational GPS system will allow probing the ionosphere in four directions simultaneously.