A Systems Approach to Ionospheric Delay Compensation

Abstract

Dual-frequency and single frequency ionospheric delay compensation algorithms are fundamental GPS receiver methods that attempt to remove the signal delay that occurs when NAVSTAR satellites' transmissions pass through the ionosphere. Dual-frequency algorithms normally provide higher accuracy navigation solutions than single-frequency compensation algorithms, but environmental factors and signal conditions may adversely impact the performance of the dual- frequency algorithms. Receiver noise and frequency-dependent multi-path are two terms that often lead to dual-frequency ionospheric delay compensation errors. An alternative ionospheric delay compensation algorithm has been designed, implemented, and tested that moves the dual- frequency compensation algorithm from the GPS receiver into a Kalman filter that serves as the fundamental integration component in a tightly-coupled or ultra-tightly-coupled GPS- aided Inertial Navigation System (INS) integration. This ionospheric delay algorithm adds a number of dedicated states to the Kalman filter. One of these states represents a normalized global ionospheric delay that models the ionospheric delay that is common to all satellites. An additional Kalman filter state is allocated to each tracked satellite to model the normalized component of ionospheric delay error unique to that satellite. An obliquity factor, that describes the distance that the GPS signal passes through ionosphere, scales the normalized delay estimated by the filter to attain the actual estimate of ionospheric delay for each satellite. Using a Kalman filter to estimate the ionospheric delays is particularly beneficial when the received GPS signal is attenuated by environmental factors such as foliage, when receiver noise is high, or when simultaneous track of L1 and L2 frequencies is not possible. This Kalman filter design has become a common component of Boeing's GPS-aided INS design. Significant laboratory and flight test data have demonstrated the benefits of this design and have substantiated inertial navigation simulation predictions.