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3 Answers

Trying to understand RAIM better (or at all)

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Aircraft Systems

I am trying to do a better job of explaining RAIM to my students.  Despite reading several articles, include one from Garmin, about what it is and what it does, I get a lot of contradictory and misleading information.  Can you help with the following?

  1. Once a GPS receiver is turned on and selects a set of satellites to use, a piece of software called RAIM checks the selected satellites for accuracy and a good geometry to resolve an accurate location.  The software may reject one or more satellites and chose other that give more accurate results and a good geometry.  Does this sound right?  (RAIM is software, it works in the background)
  2. If the receiver has RAIM sofware, it can't be turned off by the user?
  3. If the receiver is equipped with WAAS (software and hardware) does the manufacturer equip the receiver with RAIM software anyway? (since WAAS can be turned off (or at least I think it can)
  4. I have read "RAIM must be available at your location to fly an IFR approach" on several web sites.  Since RAIM is built into the receiver, it is always available, so what does this mean?  I have read that if the receiver has WAAS capability it doesn't need RAIM. 
  5. How often does RAIM software recheck the satellite status or is it running all the time?
    That is enough for now!  

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3 Answers

  1. Best Answer


    John D Collins on Oct 04, 2021

    RAIM (Receiver Autonomous Integrity Monitoring) is an algorithm used to determine the integrity of a GPS position. Integrity is the measure of trustworthiness. GPS positions are determined by the GPS satellites broadcasting the time they are at a position in their orbit. A GPS receiver determines the time that the receiver actually receives the signal and uses the difference in time to calculate how far the GPS satellite is from the receiver. Radio signals travel at the speed of light, so D (distance) = C (Speed of light) X Time. By receiving from four satellites, and using a form of triangulation, the receiver can calculate mathematically its position in time and space (latitude, longitude, altitude, and a precise time.) The geometry of the satellites matter, with best results obtained with the GPS satellites spread far apart and both high and low on the horizon, This is similar to using two VOR’s to determine your location. For example, If the two VORs are 90 degrees apart to your location, one along your course and the other 90 degrees to your course, then you get a better result than when they are almost in line with each other. The closer the satellites in view are to each other, the lower the quality of the position that can be determined by the receiver.

    RAIM uses a fifth satellite to check the position of the other 4. The fifth satellite is substituted for one of the other four and a position is calculated. This process is repeated for the other three satellites by substituting the fifth satellite one at a time to generate a total of 5 positions, the original one, and the four substitute ones. So with 5 independent positions calculated, if they all are close to each other, then the conclusion is that the position can be trusted. How far apart the 5 positions are determines the RAIM calculation.

    What if there won’t be 5 satellites in view when you arrive at the destination? Then the receiver won’t be able to perform the calculations. Since the positions of the satellites is known, this can be predicted beforehand. This is the pre-flight check to determine if RAIM is going to be available.

    So there are two related issues, first are there going to be sufficient satellites with good enough geometry to provide the ability for the RAIM algorithm to work when the aircraft gets to the point where an approach is going to be made. Second, assuming there are sufficient satellites to perform the algorithm in the receiver, is the RAIM result satisfactory or are the RAIM positions too far apart to trust the result.

    FDE (Fault Determination and Exclusion) is an advanced feature of RAIM found in most modern GPS receivers. This allows the algorithm, to determine if a particular satellite generates bad positions, it may be tagged as such and not used in the position calculations or excluded.

    With WAAS, there are ground stations that receive and monitor all the GPS satellites. These ground stations know their precise location and precisely where the GPS satellites are, so they can compare the distance to the satellite with what it is supposed to be and any differences can be used by the WAAS system to generate corrections. These corrections are up linked to the WAAS Geostationary satellites for broadcast to a WAAS GPS receiver. They also broadcast integrity data for each satellite, so there is no need for a WAAS GPS receiver to use RAIM to determine integrity. As long as the WAAS GPS receiver is receiving both the corrections and integrity data from the WAAS Geostationary satellite, RAIM is not used in the receiver. But if the aircraft flies outside of the WAAS coverage area or there is a system wide WAAS outage, the RAIM algorithm is also built into the WAAS GPS receiver and it gets used to determine the integrity.

    So to answer your specific questions.

    1. Is true
    2. RAIM runs continuously and in most GPS receivers will alert the pilot if there is an integrity issue for the operational mode the aircraft is currently in, Enroute, Terminal, or Approach mode. Each has its own separate integrity limits. It is not pilot selectable.
    3.Yes, both are available, but WAAS integrity is used anytime the aircraft is in the WAAS service volume and receiving the WAAS signals.
    4. The RAIM available indicates that there are sufficient satellites in view with adequate geometry to perform the RAIM calculations. If not, then RAIM has no way of being determined. Assuming that there are sufficient satellites in view with acceptable geometry, then RAIM calculations may be performed. If the calculations exceed limits, then the integrity is not good enough for the mode the aircraft is currently in (enroute, terminal, approach) and other means of navigation must be used. If RAIM is available just before the FAF and the result of the RAIM calculation is acceptable for the approach (integrity better than +/- 0.3 NM), then the approach may be continued even if RAIM becomes unavailable. That is because the accuracy most likely hasn’t changed, but the anility to confirm the integrity of the position has been lost. A WAAS GPS does not use RAIM if inside the WAAS service volume.

    5. It is running all the time, but the criteria for the integrity changes with the mode of the operation being performed (Enroute 2NM, Terminal 1 NM, Approach 0.3 NM).

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  2. Grayson on Oct 04, 2021

    Hi John,
    Thank you very much for the very detailed answer! My understanding increased a bunch.
    Where did you learn this? Can you point me to a book or web site that explains RAIM in detail?
    You said \”What if there won’t be 5 satellites in view when you arrive at the destination? Then the receiver won’t be able to perform the calculations. Since the positions of the satellites is known, this can be predicted beforehand. This is the pre-flight check to determine if RAIM is going to be available.\”
    How the heck does RAIM know where I\’m going! (Monitoring my web searches?). I can only guess that this calculation is done only if I input a flight plan that has an approach selected? I also assume that if I don\’t enter a flight plan at all, it can only base calculations on ENROUTE mode.?

    THanks again!
    Grayson

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  3. John D Collins on Oct 04, 2021

    The FAA has a RAIM prediction tool, see https://sapt.faa.gov/default.php

    Most online briefing systems will determine RAIM for the route you intend to file and fly. You provide the ETD and route and the software will check RAIM along your route and at your destination. Within 30 NM of the departure and destination airports it assumed you need Terminal mode, at the destination it assumes you need a non precision approach mode and the rest of the route it assumes uses enroute mode. So you say when and where and it does the calculations. Most GPS have a function to check RAIM at a point, usually at the destination. You specify where and when you expect to arrive, and it does the check for availability.

    Modern GPS have a Baro-enhancement that allows the algorithm to check for RAIM using the pressure altitude as if it is one satellite, thus allowing a Baro-Aided system to compute RAIM with only 4 satellites in view.

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