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AEP 5.5c Update Update: Bugs Identified, Being Fixed

Sam Wormley
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      02-17-2010, 02:17 AM
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.

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