Carrier Phase and Code Based Absolute Slant Ionosphere Gradient Monitor for GBAS

Abstract

Single frequency Ground Based Augmentation System for a Global Navigation Satellite System (GNSS) like GPS can correct a large amount of errors by applying differential corrections. However effects like ionosphere errors can have a large spatial decorrelation and even after differential correction, large residual ionosphere biases can occur and can drive to an unacceptable large position error of the aircraft during an automatic landing. A network of multiple pairs of single frequency receivers is suggested that detects ionosphere gradients in the range of 300 to 2000 mm/km. These pairs of single frequency receivers is using a double difference carrier phase technique and baselines optimally chosen to allow the highest level of residual carrier phase error. In order to reduce the number of monitors, another variant based on smoothed code based absolute ionosphere gradient monitors is proposed. This type of monitor suggest an upper bound for the range of gradients to be detected. Hence using 3 aligned receivers and combining carrier phase and code based ionosphere gradient monitor, it is possible to detect any gradient above 300 mm/km. The condition for that is to use a very large baseline (2 km) for the code based monitor to detect all gradients above 500 mm/km. The other baseline (221 m) is used by a carrier phase based monitor to detect from 300 to 500 mm/km. If it is possible to reduce the code smoothed double difference residual error, the minimum detectable gradient can even be reduced, making the range to be detected by a carrier based monitor smaller.