Ionospheric effects on GPS signals in the Arctic region using early GPS data from Thule, Greenland

Thule, Greenland, is near the magnetic North Pole, and therefore any high-speed GPS data recorded there are of great potential import in specifying the severity of ionospheric effects on modern commercial navigation and ranging systems that must transit the polar latitudes. While there have now been many sets of GPS receiver data recorded at high latitudes, including GPS-IGS data routinely taken from Thule, Greenland, virtually all of these data have not been recorded at a high enough rate to measure the actual amplitude and phase scintillation effects. One notable exception is very early data taken from Thule, in the late 1980s and early 1990s, a period of high solar activity where data were taken from a L1/L2 dual-frequency receiver having the capability of tracking only one GPS satellite at a time, on both the L1 and the L2 channels with a data sampling rate of 20 Hz. The receiver phase-locked loop bandwidth was 16 Hz; thus, the sampling rate was not fast enough to insure that all the spectral components up to the receiver bandwidth were measured. We report on a new analysis of these “old” data illustrating that the rapid rates of change in total electron content (TEC) and high values of the amplitude scintillation index, S4, reported from these data are very relevant today with the advent of modern civilian aircraft routinely flying in the polar latitudes. For instance, we see a very high correlation between individual amplitude fades on the GPS L1 and L2 channels, indicating that users who suffer instantaneous signal loss on one frequency may not be able to use the secondary frequency to recover the GPS signal modulation, since it, too, will suffer an almost identical fade. Also, the occurrence of large, rapid changes in TEC, previously reported from these high-speed Thule GPS data, has been confirmed and may limit precise positioning in the polar cap latitudes.