Service Information Letter Briefing

Honeywell has issued 2 Service Information Letters (SILs) concerning the “GPS week rollover” that will occur at approximately midnight UTC on April 6, 2019. The SIL’s were written to inform operators of the results of GPS week rollover testing on various Honeywell GNSSU models used in aviation applications.

What is GPS Week Rollover?

The Global Positioning System currently consists of 24 satellite vehicles (SVs) plus spares, evenly spaced around the Earth in six high orbital planes. The current position of a GPS receiver can be determined by calculating the exact distance from the receiver to the SVs. Typically four SVs are required for position calculation.

Information about the orbits of the SVs is contained in the GPS almanac, which is date-coded by GPS week. The GPS receiver uses the almanac data and the time entered during initialization to estimate the approximate position of each SV during the acquisition mode. Upon acquiring the first SV, the GPS receiver checks whether its stored almanac is current and, if not, proceeds to download the current almanac from the SV. The GPS receiver also begins to use time and data information from the SV in acquiring additional SVs.

GPS week is coded using 10 bits. At approximately midnight UTC on April 6, 2019, the GPS week will change from 1023 to 0 (1024). After this transition, devices need to correctly interpret “GPS week = GPS week number + 1024.” Failure to accurately interpret the GPS week date-code will prevent the GPS receiver from recognizing and downloading more current almanacs after the GPS week rollover. Failure to use a current almanac during acquisition may result in an extended acquisition time.

GPS Week Testing

Testing and analysis has shown that the GPS sensors will function as defined by the equipment’s specifications during the GPS week rollover event in April 2019, and all subsequent events thereafter. The SILs provide more details about the test results for specific units.

SIL D201809000008 Equipment Effectivity covers the following GNSSU devices: