Ionosphere Gradient Detection for Cat III GBAS

A new approach for the GBAS Approach Service Type D (GAST D) standard is to share mitigation of ionosphere errors between the ground and airborne subsystems. This lead to the need for an ionosphere gradient monitor located in the ground subsystem in order to ensure the integrity of the system is not compromised in the presence of an ionosphere gradient. Initially, several airborne monitors were identified, however, after extensive simulation during the ICAO SARPS technical validation effort, an additional need for a ground-based integrity monitor was also realized in order to mitigate the threat. Specifically, an ionosphere gradient threat is defined as a spatial ionosphere delay gradient acting between the ground station and the airplane on approach that far exceeds the gradients occurring under normal conditions. When the differential delay in the GPS signal between the ground station and approaching aircraft is large enough, the result can be undetected errors in the corrected pseudorange which exceed acceptable levels and therefore a loss of system integrity. At mid latitudes, ionosphere gradients which could result in unacceptably large errors are rare, but may exist during ionosphere storms. The objective of this paper is to describe a ground-based absolute monitor used to support mitigation of ionosphere gradient threats during CAT III operations and demonstrate how the monitor meets the requirements for GAST D. The Ionosphere Gradient Monitor (IGM) described in this paper is required to detect any spatial ionosphere delay gradient with a magnitude greater than 1.5 m at D, where D is the distance between the reference point of the ground station and the threshold of the runway. The IGM utilizes carrier phase double difference measurements over multiple sets of reference antenna pairs spaced over short baselines (approximately 200 meters). Because this method requires highly accurate phase measurements, the paper also addresses calibration of antenna pairs over both azimuth and elevation to minimize antenna phase variations. Test results included in this paper demonstrate the IGM’s capability to detect anomalous iono gradients affecting satellites which were acquired prior to an iono gradient’s presence as well as satellites which are acquired within the iono gradient region. This paper includes test results from simulation along with real recorded site data from a fielded CAT III prototype located at the FAA William J. Hughes Technical Center at the Atlantic City airport and another located at the Rio de Janeiro, Brazil Galeao International Airport. This paper also outlines the specific ground (GPS) receiver antenna siting constraints required for implementation of the monitor.