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1 Answers

Raim Failure on Garmin 430

Asked by: 8561 views Instrument Rating

According to FAR/AIM.

There are generally 2 types of RAIM failure.

1. when satellites are not enough

2. accuracy exeeds the limit

*If those cases happen in flight. What will be the indication in the GPS? (Garmin 430 or other GPS too)

* what if the airplane is equipped with WAAS? is the indication will be the same?

Thanks in advance

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1 Answers

  1. John D Collins on Jan 30, 2014

    RAIM is a calculation that a GPS receiver makes to determine if the position information is suitable for the navigation mode the receiver is using. It works on the principal that a position takes 4 satellites in view. If more satellites are in view, then one of the four satellites being used for the position can be substituted one at a time with one of the extra unused satellites. This generates 5 possible positions or more. The difference in the positions can be used to estimate the integrity of the position. This calculated value can be compared to the required integrity for the phase of flight and if it is not acceptable, warn the pilot. For enroute mode the RAIM value should be under 2 NM, terminal mode under 1 NM, and approach mode 0.3 NM.

    Not all geometries of satellites provide the same accuracy of position. This is similar to determining a position with two intersecting VOR radials. If they cross at 90 degrees and are close to the station, the position error is small, whereas if they cross at 20 degrees and are far from both stations, the position error is large. The GPS satellites transmit data necessary to determine the locations of all satellites. This enables the receiver to predict at a specific location and time how many satellites will be in view. This is called RAIM prediction and is used to determine in advance if there will be sufficient satellites in view to permit the receiver to perform the RAIM check.

    With insufficient satellites or poor geometry, a suitable RAIM check can’t be determined. This does not mean that if the GPS receiver has sufficient satellites to determine a position that the position is inaccurate, it just means there is no way to estimate its accuracy. This is indicated on the GNS430 by a message “RAIM Not Available” and an annunciation of INTEG in an amber color on the GPS status line. One can continue to use the GPS under these circumstances but cross check with other navigation means.

    If there are sufficient satellites in view and the RAIM calculation can be accomplished, it may determine that the calculated RAIM value is greater than the permitted value for the mode of operation of the GPS. In this case, the navigation information can’t be trusted and the GNS430 will output a message: “RAIM Position Warning”. The GPS status will indicate an annunciation of WARN and all CDI information will be flagged. At this point, the pilot must revert to other means of navigation,

    WAAS does not use RAIM as long as the WAAS system is up and running. It gets the integrity information directly from the Geostationary WAAS satellites and therefore does not need to estimate it thru using on board calculations. Only if the entire WAAS system goes down or the aircraft is operating outside of the WAAS service volume will the WAAS GPS use RAIM. In order to use the WAAS approaches (LPV or LP) or obtain vertical guidance for LNAV/VNAV or LNAV+V, the WAAS system must be available and the integrity must meet more stringent criteria. If the WAAS system goes unavailable, then the receiver reverts to a standard GPS receiver and may only fly LNAV procedures without any vertical guidance and based on RAIM being satisfactory.

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