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Spirent KB Article
Doc ID: FAQ18234
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Positioning Application: Is the software compliant with the change to the Issue of Data, Clock (IODC) value selection process in the GPS control segment software?
The U.S. Air Force released a memorandum in December 2017 which informs the GPS community of an IS-GPS-200H compliant change to the Issue of Data, Clock (IODC) value selection process in the GPS control segment software.
The IODC values will now be any numbers in the range 0 to 1023 to support requirements in IS-GPS-200H paragraph 188.8.131.52.
Historically, the IODC values were typically chosen from the range 0 to 255.
This IS-GPS-200H-compliant change to the IODC selection process was trialled in September 2017, however the software update was removed after numerous reports of GPS receivers experiencing problems with IODC values in the range 256 to 1023.
The update will be reinstalled in a later update to the GPS software no earlier than 01 Mar 2018.
Positioning Application - ICD compliance:
As per the above memorandum, there has been no change to the ICD (IS-GPS-200H), but rather a change in policy regarding the selection process for the IODC values by the GPS control segment software.
The Positioning Application software was already fully compliant with the ICD before this memorandum was released, thus users of Spirent GNSS simulators will not be affected by this change.
IODC value selection change explained:
Issue of Data, Clock (IODC) is a 10-bit binary word used to identify the period of validity of the navigation data; Issue of Data, Ephemeris (IODE) is a related parameter and is equal to the eight least-significant bits of the IODC word
According to IS-GPS-200H, values of the IODC that correspond to IODE values in the range 240 to 255 are not allowed in normal operation.
This corresponds to the 8-bit binary words
The Air Force memorandum states that the two most-significant bits of the 10-bit IODC word were not historically employed in setting the IODC but will now be used to extend the range of possible IODC values
The consequence of the memorandum is that broadcast IODC values may now include those in the range 01 0000 0000 (decimal 256) through 11 1110 1111 (decimal 1007) – utilising those extra two bits.
However, the same restriction on IODE applies, thus values in the following ranges are invalid in normal operation:
(decimal 496) to
(decimal 752) to
(decimal 1008) to
Users attempting to enter a value xxxx of IODC in the Positioning Application in an invalid range will encounter a warning message. For example: "
Invalid IODC, lower 8 bits must not be in the range 240 to 255. For value of xxxx illegal range is 1008 to 1023.
Users testing the 'extended operation' mode should refer to
Product : Positioning,SimGEN,PosApp,Scenario,Navigation Data,GPS