Download Digital Systems Software Requirements Guidelines

0012 GPS Clock Problem
Date
10/23/98
Source
Risks 20-07 (Neumann, 2001)
Domain
Aircraft
Function
Time sync
Guidelines
2.2.1-2
Description
GPS internal clock problem
“Bob Nicholson” <[email protected]>
Wed, 11 Nov 1998 08:20:39 +0000
[This has been reported earlier, beginning in RISKS-18.24, but is still a problem. PGN]
As a licensed aircraft engineer, I regularly receive “AIRWORTHINESS NOTICES” from the British
CAA. Here is one (verbatim) that may be of interest.
CIVIL AVIATION AUTHORITY: AIRWORTHINESS NOTICE
No. 7, Issue l, 23 October 1998
“The Potential Resetting Of Global Positioning System (GPS) Receiver Internal Clocks”
The timing mechanism within GPS satellites may cause some GPS equipment to cease to function
1.1
after 22 August 1999 due to a coding problem. The GPS measures time in weekly blocks of seconds
starting from 6 January 1980. For example, at midday on Tuesday 17 September 1996, the system
indicates week 868 and 302,400 seconds. However, the software in the satellites’ clocks has been
configured to deal with 1024 weeks. Consequently on 22 August 1999 (which is week 1025; some GPS
receivers may revert to week 1—i.e., 6 January 1980).
Most airborne GPS equipment manufacturers are aware of the potential problem and either have
1.2
addressed the problem previously or are working to resolve it. However, there may be some GPS
equipment (including portable and hand-held types) currently used in aviation that will be affected by this
potential problem.
Action to be taken by aircraft operators who use GPS equipment (including portable and hand-held
2.0
types) as additional radio equipment to the approved means of navigation should inquire from the GPS.
Discussion
The software requirements did not consider the range of the data to be processed. This cause of failure is
classified as an example of error in data size and precision requirements (Guideline 2.2.1-2).
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