AEP 5.5c Update Update: Bugs Identified, Being Fixed

Discussion in 'Global Navigation Satellite Systems' started by Sam Wormley, Feb 17, 2010.

  1. Sam Wormley

    Sam Wormley Guest

    AEP 5.5c Update Update: Bugs Identified, Being Fixed
    February 11, 2010

    Here is the latest on the AEP 5.5 update and and another error that
    actually occurred two weeks later.

    On January 11, 2010 when the GPW Wing and the 2SOPS loaded the updated
    AEP 5.5C software to the "B" side of the ground control segment, it
    immediately caused a problem with a specific subset of GPS
    SAASM receivers.

    First of all, readers need to understand that one of the purposes of the
    5.5C AEP update is to enable selective availability anti-spoofing module
    (SAASM) functionality in coded receivers. The software for the SAASM
    functionality has been resident in various certified SAASM receivers for
    sometime, but was never implemented in the ground control segment of the
    GPS. The AEP 5.5C update alleviates that problem for the majority of
    SAASM receivers, but for one manufacturer it has caused problems. The
    updated control segment software sends a specific code to SAASM
    receivers that allows them to authenticate the message and ensure that
    the code is correct, and is being sent from the GPS and not some other
    source. For most receivers this worked without a hitch, but for one
    manufacturer there is a software bug or glitch that must be corrected
    before the receiver can authenticate. This fix is in progress and will
    most likely be implemented as a software or firmware update to the

    Another problem with a different set of receivers manifested itself
    exactly two weeks after the AEP 5.5C update was made on January 11.
    Those that have researched this problem in some depth feel that the
    problem is totally unrelated to the AEP update and would have occurred

    On January 25, another set of GPS receivers started experiencing a major
    issue that has essentially rendered them unusable until the problem is

    Reportedly, the Federal Aviation Administration identified a nuisance
    alarm (once per 12.5 minutes) with a civil timing receiver they use.

    To understand this problem, recall that GPS keeps track of time in
    weekly segments that are represented by a number expressed in binary
    terms as part of a 32-bit word. This particular receiver, for some
    reason, stores the GPS 32-bit word with the timing information in the
    same general area where the error codes for the receiver are stored. You
    can probably see where this is going. Incredibly, the current weekly
    code in binary in the current navigation message perfectly emulates one
    of the receivers error codes, also in binary for this particular
    receiver and you guessed it: the receiver sees the timing portion of the
    navigation message and interprets it as an error message and immediately
    resets. This reset behavior becomes a continuous looping process that
    continues until the receiver resets a certain number of times, and as a
    planned safety feature virtually locks the receiver and it will no
    longer receive and integrate the navigation message. In other words, the
    receiver refuses to synchronize with the navigation message.

    Again this is considered by all concerned to be another receiver
    software bug that must be dealt with by the manufacturer, and that
    process is ongoing.

    Of course software bugs, or glitches, are a fact of life for our digital
    software-controlled world today, and although it is more of a rarity in
    the GPS world than in the PC world, it does happen.

    Conspiracy theorists who have been active over the last month can relax
    now. There is no grand assault on the GPS by terrorists or others, it is
    just a couple of software bugs that have indeed wreaked some havoc with
    certain receivers. But the solutions are being implemented as you read this.

    Yes, as it turns out, strict compliance with the receiver interface
    control document (ICD) may have prevented the first issue, but the
    second is just one of those serendipitous events that will occur from
    time to time no matter how careful we are.
    Sam Wormley, Feb 17, 2010
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