Approach Basics (GX50/60)

navigation as long as you verify the accuracy of your position by other means at least every 15 minutes until RAIM becomes available again. This can be accomplished by cross-checking your GPS position against that of other navigation instruments. Even during times of no RAIM, if your Apollo GPS determines that it cannot provide an accurate position due to poor satellite availability or geometry, it will flag the navigation display as invalid. If the Apollo GX is providing valid navigation data, then it is still capable of providing good IFR navigation as long as the satellite data is good.

What Does A RAIM Alarm Mean?

A RAIM alarm means that the navigation data cannot be trusted to be within the alarm limit. A RAIM alarm means that the Apollo GX has determined that there is something wrong with the data it is receiving from one or more satellites. The data may be corrupted by a satellite failure, such as a clock failure, or it may have been corrupted by some form of signal interference. A RAIM alarm will also be accompanied by flagging all navigational displays as invalid. If you should ever see a RAIM alarm, do not trust the GPS position, and revert to other means of navigation as appropriate.

How Often Do Satellite Failures Occur?

There is not enough data to provide a good answer; however, it is known that they can and do happen. Satellite clock failures, one of the more probable failures, is more likely to occur on the older Block-I satellites, but this does not mean that it can’t happen on the newer ones. When a satellite failure does happen, it can result in undetected error of miles in the position if a receiver is using the failed satellite in its navigation solution.

When a satellite fails, even if that failure is detected by an earth-based monitoring station, it can be some time before an uplink to that satellite can occur. During that time, there is no way for a GPS receiver to know that the information from that satellite is bad

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