Monday, June 16, 2014
Wow, I can't believe I haven't written this post yet. I have mentioned RAIM in other posts, but I haven't explicitly explained RAIM. RAIM used to mean Redundant Autonomous Integrity Monitoring it was a technology built into many GPS receivers. TSO-C129 required GPS receivers to have RAIM built in. It would monitor the quality of the GPS signal, and if things were bad enough, the RAIM system would alert the pilot that things aren't working.
If you go back to the NextGen post I did a little more than a year ago, I mentioned RAIM, and the FAA's RAIM prediction tool: http://www.raimprediction.net this tool will predict places in the CONUS where the RAIM alert will go off in the future.
The GPS signal is just telling you what the time was when the satellite sent the signal. The satellites also send location information along with the time. The almanac is the location of all the satellites in the constellation. Knowing the time the satellite sent it's signal, and knowing where the satellite was when the signal was sent, the GPS receivers are able to triangulate (sphere-iate?) their location.
The satellite time message is sent every few minutes, and is susceptible to all kinds of problems. The message may bounce off buildings, mountains or other vehicles causing a wrong distance to be calculated. Other times the satellites will be down for maintenance, and testing, so it won't be available for measurements. Knowing the current status of the constellation is critical to make a valid prediction. Knowing local terrain will help make predictions more accurate.
With GPS Helpers, RAIM prediction is not needed. Using satellite or ground based augmentation systems (IE WAAS, LAAS) the number of satellites isn't as critical. Knowing the WAAS system health is required, and the FAA will issue NOTAMs if the WAAS system isn't up to snuff.
As you can see RAIM has its use, and can make older GPSs more usable.