Scintillation on global navigation satellite signals and its mitigation

Abstract

The scintillation effects on the Global Positioning system (GPS) or other GNSS (global navigation satellite system) receivers have been investigated by many researchers and several mitigation strategies have been proposed in this regard but the problem is not yet fully solved. This thesis covers the investigation of scintillation effects on GPS receivers and developing a mitigation approach which can play an important role in mitigating the effects of scintillation on these and other GNSS receivers. Firstly, a new GPS signal acquisition method known as the repetitive block acquisition (RBA) is presented which can be used to speed up the GPS signal acquisition in case fast acquisition is required. This acquisition method is implemented using coarse-acquisition (C/A) codes and tested by collecting real GPS data. The RBA method can also be used for other codes as well. It is rather difficult to show that how scintillation affects the acquisition process in a GPS receiver because mostly it results in tracking loop loss of lock due to cycle slip. However, during strong amplitude scintillation which is usually most important at low or near-equatorial latitudes, deep power fades resulting from amplitude scintillation result in the selection of long data records which leads to slow acquisition due to long acquisition times. It is shown in this thesis that, by using the RBA method, the acquisition time can be reduced to a fairly low level by reducing the number of computations involved in acquisition compared to other well-known methods such as the parallel FFT-based method and zero padding method (ZP). Secondly, the scintillation effects on the GPS tracking loop have also been investigated in this thesis and, based on this investigation, a new improved analogous phase scintillation index, σw φa, has been designed to more accurately represent the phase scintillation intensity at European high latitudes. This is then also validated using the real GPS data from Trondheim, Norway (63.41o N, 10.4o E). The σw φa uses dual frequency (L1 & L2) based vi time and spatial variations of total electron contents (TEC) at 1 Hz for estimating the phase scintillation values. For deriving the σw φa, the low frequency TEC fluctuations due to Doppler shift of the satellite/receiver motion and also due to the slowly varying background ionosphere need to be removed in order to consider only the high frequency TEC fluctuations which are responsible for scintillation due to the fast moving electron density irregularities which is done by using the wavelet transform. The σw φa is really an improved version of σφa where, rather than using time-invariant digital high pass filters (HPF), which according to several researchers are in-appropriate for filtering the non-stationary raw GPS signals affected by the ionospheric scintillation, a wavelet-based filtering technique is used. Although, the wavelet transform has been used previously in detrending raw amplitude and phase observations at 50 Hz for estimating the scintillation indices (amplitude and phase), due to the high sample data rate it may not be desirable to use this transform due to its very high computational cost. Since, σw φa uses TEC data at 1 Hz so this problem has been overcome. The performance of the new improved index (σw φa) is investigated and is also compared with the previously proposed σφa and σφ indices using one whole year of data from a GPS receiver at Trondheim, Norway (63.41o N, 10.4o E). The raw TEC observations and the σw φa index are then used in estimating the tracking phase jitter using two different methods. The phase jitter helps in defining the tracking thresholds for the tracking loops in a receiver which is useful in updating the tracking loop parameters during scintillation conditions as required in robust GPS/GNSS receiver designs because the phase jitter decides how wide the tracking (and thus the noise) bandwidth should be allowed in the tracking loop for the tracking to remain efficient. It is shown that if the phase jitter is estimated using the new proposed methods, generally a better estimate can be obtained compared to the previously proposed phase jitter estimation methods which employs σφa and σφ indices. These new phase jitter estimation methods can further be used in GPS/GNSS receivers for updating the tracking loop parameters during scintillation conditions and hence can serve as a good alternative for mitigating the effects of scintillation on GPS/GNSS receivers.