As I'm studying, I found that GPS has to have 5 satellites or 4 satellites with "blind encoders" to use RAIM.

What is a blind encoder and what is its purpose?

  • Can you reference the source you are learning from or provide the relevant quotes from it? Because as far as I can tell, RAIM needs 5 satellites for error detection and 6 for correction and there is no way around it. Also, I am not sure what “blind-encorder” is it talking about, but there does not seem to be anything called blind encoder around GPS. – Jan Hudec Nov 18 at 18:32
  • As I know, GPS knows our altitude (via blind encoder which is another point of reference for position information. Therefore we need only 4 satellite to trilaterate our position. – Mun Park Nov 18 at 18:50
  • Closely related. I think you may be mixing some different things here: a blind encoder is usually part of an aircraft's altimeter, which has nothing to do with RAIM. – Pondlife Nov 18 at 19:08
  • Sorry for that. So could you let me know what blind encoder is how it is used in A/C? – Mun Park Nov 18 at 19:51
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    @MunPark GPS doesn't know your altitude. It knows the altitude of its satellites, but it has no idea what your altitude (or your position in any dimension) is. Your flight computers may know because of data from your altimeter(s), but the Global Positioning System doesn't know, as it's a transmit-only system that doesn't even know who is receiving its signals, let alone their altitudes. – reirab Nov 19 at 10:46

A blind encoder is an altimeter that encodes pressure altitude and outputs it on a databus. They are "blind" in that they do not display data to the pilot. Most general aviation units use a parallel bus with Gillham encoding. More expensive units and air data computers use serial bus output, commonly ARINC 429.

Historically, the blind encoder existed to provide pressure altitude data to the transponder to enable Mode C reporting.

When it was proposed to use GPS as a primary means of navigation in the early 1990's, the FAA deemed that a means of ensuring the integrity of the signal was essential. The longer term solution was to establish WAAS which provided both augmentation and an integrity channel. The short term solution was to have the receivers include an internal monitor, which took on the name RAIM for Receiver Autonomous Integrity Monitor.

RAIM algorithms have evolved several times over the years. Since early GPS receivers were often single or 3 channel receivers, the use of altitude as a substitute for an additional satellite was common. (Altitude is just another range in the rho-rho computation.)

Later GPS receivers moved to 6 and 12 channel receivers, thus enabling RAIM algorithms that don't need altitude data except in extreme cases.

So yes, the number of satellites needed for a RAIM calculation can be reduced by one when pressure altitude is available from a blind encoder.

  • +1, I had no idea that there's a link between altitude encoding and RAIM. I'm not doubting your answer, but do you have any sources on the algorithms you mentioned? – Pondlife Nov 18 at 21:49
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    The information is in the GPS MOPS, RTCA DO-229. The basics are covered in Appendix G—REQUIREMENTS FOR BAROMETRIC ALTIMETER AIDING in DO-229C. I'm not sure if the reference is still valid in the current version DO-229E. I do know that the newer version does require the use of FDE RAIM as the algorithms have evolved. – Gerry Nov 18 at 23:43
  • thanks for that! – Pondlife Nov 19 at 0:08
  • Of course the barometric altimeter reading depends on the weather, so the position obtained with use of barometric altitude has lower accuracy. Good enough for en-route navigation, but not for approaches or anything needing RNP. – Jan Hudec Nov 20 at 22:21
  • @JanHudec Certainly not CAT I or APV approaches, but a TSO C129 GPS with RAIM is capable of RNP 0.3 non-precision approaches. When combined with an FMS the same GPS sensors support LNAV/VNAV approaches. They did require pilot input of the local altimeter setting. Most of those approaches had MDAs in the 250 to 400 foot range. – Gerry Nov 20 at 23:36

Receiver autonomous integrity monitoring Receiver autonomous integrity monitoring (RAIM) is a technology developed to assess the integrity of global positioning system (GPS) signals in a GPS receiver system. It is of special importance in safety-critical GPS applications, such as in aviation or marine navigation

Check out the GPS article in this FAA newsletter to start with:

RAIM prediction is what is of primary concern for Non-WAAS equipped GPSs. If you are WAAS equipped then you are good to go without doing prediction.

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