Introduction.

On January 26, 2016, a temporary error in the data upload system for the US Air Force Global Positioning Satellite System (GPS) caused incorrect data to be transmitted from the satellites on the commercial L1 band channel used by most commercial GPS receivers. A statement released by the US Air Force is shown below:

50th Space Wing Public Affairs press release on the 26 Jan 2016 event follows:

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On 26 January at 12:49 a.m. MST, the 2nd Space Operations Squadron at the 50th Space Wing, Schriever Air Force Base, Colo., verified users were experiencing GPS timing issues. Further investigation revealed an issue in the Global Positioning System ground software which only affected the time on legacy L-band signals. This change occurred when the oldest vehicle, SVN 23, was removed from the constellation. While the core navigation systems were working normally, the coordinated universal time timing signal was off by 13 microseconds which exceeded the design specifications. The issue was resolved at 6:10 a.m. MST, however global users may have experienced GPS timing issues for several hours. U.S. Strategic Command’s Commercial Integration Cell, operating out of the Joint Space Operations Center, effectively served as the portal to determine the scope of commercial user impacts. Additionally, the Joint Space Operations Center at Vandenberg AFB has not received any reports of issues with GPS-aided munitions, and has determined that the timing error is not attributable to any type of outside interference such as jamming or spoofing. Operator procedures were modified to preclude a repeat of this issue until the ground system software is corrected, and the 50th Space Wing will conduct an Operational Review Board to review procedures and impacts on users. Commercial and Civil users who experienced impacts can contact the U.S. Coast Guard Navigation Center at (703) 313-5900.
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GPS Operations Center, GPS User Operations

There are 24 satellites in the “baseline” GPS constellation, although the total number of satellites in the constellation varies from time to time (actually as of today there are 32 satellites total in the constellation).

Each satellite orbits the earth twice per day in a medium earth orbit (MEO) at an altitude of approximately 20,200 kilometers (12,550 miles) and independently broadcasts navigation and timing information. The main control center for the satellites is located at Schriever Air Force Base in Colorado and it is from here that corrections are uploaded to each satellite nominally twice per day.

Impact

The reported issue related to the UTC timing portion of the GPS L band message, and to definitively determine the exact impact that the stated issue had on individual receivers would require an analysis of which satellites had corrections uploaded at exactly what time between 12: 49 a.m. and 6:10 a.m. on January 26th, and which of those satellites were in turn in use by a specific GPS receiver to calculate timing corrections at a particular location around the globe.

While this is technically feasible, the task is probably a fruitless one, however, a generalized analysis of the impact may be useful.

As the anomaly was in place for around 6 hours, and each satellite orbits once every twelve hours, and on the assumption the spacing of the satellites in orbit is fairly even, a rough estimate of the number of satellites affected would yield :

(6 / 12) x 24 i.e. 12 satellites affected.

First, the observed impact would almost certainly vary from receiver to receiver, dependent upon the location and particular receiver type. Many GPS timing receivers like the ptf 3203A, utilize a receiver engine that incorporates a TRAIM (Time Range Autonomous Integrity Monitoring) algorithm. This algorithm is designed to take the data received from all “tracked” satellites, calculate a statistical mean value, and compare this mean to the data received from each individual satellite to insure that none of the data received from individual satellites varies from the mean by more than a set limit.

If a satellite’s data is in variance by more than the set limit, that data is rejected and the mean is re-calculated using just the good data. This technique works well in rejecting a “rogue” satellite, however when a number (or all!) of the satellites included in the solution are transmitting bad data, this technique clearly is not sufficient to detect/correct the error.

In this case, where at its peak twelve satellites were affected, a TRAIM algorithm would have soon become ineffective, albeit providing at least some small mitigation of the problem for the first and last approximately 45 minutes of the anomaly.

After this, the receivers would have exhibited a jump of approximately 13 micro seconds on the position of the 1PPS output. The error between the received 1PPS output, and a 1PPS output generated from the receiver’s internal RF reference ( usually 10MHz) is typically used to control the frequency of the receiver, by driving the error to zero, and maintaining it there.

A step change would have the impact of increasing this error by the amount of the step (13 micro seconds in this case) with the effect of causing the receiver control loop to change the frequency of the receiver in an effort to drive the error back down to zero.

The rate of change of the frequency is a function of the loop time constant of the control loop which is in turn dependent upon the type of internal oscillator used (usually a TCXO, OCXO, or rubidium based oscillator) and the particular manufacturers’ control implementation.

For the ptf 3203A instruments, most of which utilize a high quality OCXO, the loop time constant is around 159 seconds, and so the instrument would have exhibited a frequency offset of around 1E-10 for several hours as it endeavored to drive the phase error back down to zero. Once the anomaly was removed, the effect would have been observed in the opposite direction as the instrument endeavored to remove the inadvertently applied offset.

Conclusions

An issue of this type is very unusual and is difficult to predict, but when it arises the impact can be substantial as the use of GPS disciplined timing receivers around the world is prolific. The latest addition to the ptf family of GNS receivers, the ptf 3207A has the potential to provide a degree of mitigation to this type of problem by monitoring additional satellite systems and comparing solutions provided by each.