After the Air Force made a minor software update to the Global Position System on the 21st of September 2017, it received reports that “several thousand receivers were impacted.”

In a 7 December memo to GPS users and equipment manufacturers the Air Force said “Due to the nature of the impacted receivers, 2 SOPS subsequently removed the software update.” The RNT Foundation has since learned that most of the receivers impacted were in first responder systems used in Japan and Korea.

The Air Force had been transmitting a GPS time-related value (called Issue of Data, Clock, or IODC) as a number with from one to three digits. Beginning on September 21st that changed to a number with from one to four digits. This new scheme complied with long standing published specifications. We understand that the impacted receivers began failing about three days later.

GPS receiver manufacturers do not have to build equipment to a common standard, nor is there a uniform testing, ranking or certification program for most GPS equipment.

The Air Force provides manufacturers and users a regularly updated GPS “Interface Specification.” It describes current and potential characteristics of GPS signals so that manufacturers can design equipment to receive and use them.

However, manufacturers are free to design and build their equipment as they wish. If a design considers only the characteristics of the current signal, future changes to the signal, even though they are in accordance with the published specification, can result in the receivers malfunctioning.

We understand that the receivers that failed in September, and many of those that had problems during the 13.7 microsecond GPS-wide timing error in January of 2016, were not fully compliant with the interface specifications issued by the Air Force.

The Air Force memo says that the software update will be reinstalled “no earlier that 1 March 2018.” This is to allow users and manufacturers to upgrade their equipment so as to be able to use the new one to four digit format for IODC.

GPS/GNSS receiver resilience has been a topic of discussion within the technical community recently. A workshop is planned in Washington, DC for April. The workshop will be the subject of an open meeting Tuesday the 30th of January 2018 at the PTTI Conference in Reston, VA.