Future Architectures to Provide Aviation Integrity

Ionospheric delay uncertainty creates the largest restriction to the availability of high integrity satellite navigation for today’s single frequency systems. LAAS, WAAS, and the other SBAS providers are limited in their coverage and service levels by the variability of the ionosphere. With the arrival of the new civil signals at L5, comes the ability to directly estimate and remove the ionospheric delay at any point on the Earth. This allows for new architectures exploiting L1 and L5 to bring airplanes within two hundred feet of the ground anywhere on the globe. The FAA has initiated a study panel, called the GPS Evolutionary Architectural Study (GEAS) to look into future architectures to provide this global service. The GEAS has determined that Time-to-Alert (TTA) will be one of the more difficult challenges for any global monitoring approach. To address this problem, the GEAS is looking at two methods, each of which transfers some of the TTA responsibility onto the aircraft. The first method is called Relative RAIM (RRAIM). It uses precise carrier phase measurements to propagate older code based position solutions forward in time. The veracity of the propagation is checked using RAIM on the very low noise carrier phase measurements. In this way, the overall TTA can be less than a second, but the ground is given tens of seconds to minutes to identify a fault. The second method is Absolute RAIM (ARAIM). This is more similar to existing FDE techniques except that the requirements must be made much more precise in order to support smaller alert limits. Again, the aircraft is able to raise a flag within seconds of receiving faulty data. The ground is allowed to take an hour or longer to identify the fault and remove it from future consideration. The protection level equations for both methods will be evaluated in this paper. In addition to the errors considered in today’s equations, the two new methods will include explicit bias terms to improve the handling of nominal biases and non-gaussian error sources. A critical parameter in the performance of these approaches is the strength of the constellation. The performance of each is evaluated for constellations optimized for 24, 27, and 30 satellites. Further, their performance is evaluated under conditions of satellite outages. RRAIM can perform very well with fewer satellites. ARAIM on the other hand is ideal for integrating in Galileo or other satellite constellations. Both of the methods show great promise for global provision of vertical guidance.