Raim & Rmp
Receiver Autominous Integrity Monitoring is an airborne technique for evaluating how good the received data is. GPS needs a minimum of four satellites in electronic view at any time in order to obtain a unique position solution ( three position coordinates plus time). More than four in view, and theoretically you can tell that something is amiss if you don't get the same position solution from all possible four- satellite solutions. Trouble is that the accuracy of predicting the errors depends on the geometry of the satellites and more importantly on the number of satellites in the total constellation, and with GPS at about26 satellites, at times it is not much use. According to the experts, results would be somewhat improved with even the few GLONASS satellites, and greatly with a fullset of Galileo (if and when it comes to pass).
The maths is complex and there are different ways of implementing it. Stanford University are the prime Geeks in this area. There are several paers on the web. Iam reasonably technical, but I have difficulty understanding the technicality fine print.