GPS Caution Flag

April 8, 1999

GPS Caution Flag

Y2K gets all the attention, but a similar anomoly with satellite navigation will occur four months earlier

BY William Keller, Manager, East-West Gateway Coordinating council

May 1999

Many people have thrown up their hands in exasperation wondering why the switch to GPS as a primary means of air navigation has taken so long. But there are valid reasons to approach GPS cautiously; and, there is a relatively unmentioned anomaly in the Global Positioning System that is approaching our industry in August.

Despite reams of technical data and reports that exist on this topic, the 1999 GPS anomaly is not a common topic of discussion in flight schools, among airport managers, or among pilots. But each needs to have a realistic understanding and appreciation for the challenge, know where to find relevant material, and be able to take steps to assist others in understanding this issue.

A glitch in the system: the 1999 rollover
Heralded as a replacement for all surface-based navigation devices, GPS was a creation of the U.S. Department of Defense in the late 1970s. GPS relies on a continuous and accurate signal from a network of two dozen military Navstar satellites stationed in a geosynchronous orbit around the globe. The satellite system is the orbital portion of the GPS operational triangle. Ground stations, consisting of six monitoring stations, four ground antennas, and a master control station, are another portion of the triangle.

Completing the triangle are the handheld and panel-mounted GPS receiver units used by pilots, sailors, vehicles, trains, trucks, bicyclists, hikers, and the occasional thermonuclear tipped missile. The primary function of these units is to compare the incoming satellite time signals to a chip-based clock in the handheld unit. Because receiver units rely on this comparison to perform accurately, they are the weak link. An improperly programmed receiver may be fooled when the satellite clocks reset themselves and think a satellite is billions of miles away, rather than the actual 10-12,000 miles that should be realized. This incorrect comparison is the unintended consequence referred to as the 1999 GPS Rollover.

Technically speaking
The civilian component of GPS is accurate to within 100 meters of a known position, and the Defense mode is accurate to within less than 3 meters (the exact number is classified). Computer chips in GPS receivers derive position and velocity information by correlating data uploaded to the satellites by ground stations. The primary measurement interpreted by these receivers is the passage of time.

Each satellite has an atomic clock set with a "zeroed" time of 00:00:00 UTC January 6, 1980. Due to limits of emerging computer technology in the late ’70s, the maximum number of weeks the clock will register is 1,024 (or 7,168 days). The more technical angle relies on understanding the Julian Calendar and an exact computation of days. What this means for all GPS users is that at the very last second of week 1,023, the atomic clock resets itself to 00:00:00 UTC January 6, 1980. The specific time this will occur is at 00:00:00 UTC August 22, 1999, or midnight (UTC) between Saturday the 21st and Sunday the 22nd . There is no doubt the atomic clocks will roll over — this is the known element. What is unknown is how well GPS receivers have been equipped to handle this anomaly.

Among his last remarks before leaving his position as Assistant Secretary of Defense for 3CI (Command, Control, Communications, and Intelligence), Emmet Paige, Jr., stated, "The most significant system today that is not [future compliant] is GPS, which would have more impact than anything else." The DoD and civilian GPS industry have been making weak efforts to correct the problem over the past two decades. The main source of the weakness is that the 1999 Rollover was never part of the systematic testing protocols until early 1994. Receivers made before December, 1994, are in the highest failure risk category. It may be old, and it may be reliable, but a GPS unit from this era should be certificated compliant or upgraded by the manufacturer. Despite the implementation of testing protocols, some companies have acknowledged that units shipped after this date do not account for the 1999 Roll-over in a satisfactory manner, and they should be contacted for a certificate of compliance.

The universal question centers on the reliability of GPS navigation and the potential need for operable backup systems. If there are GPS errors, does the aviation community then back away from the rush to discard surface-based navigation?

The level of concern toward the GPS Rollover varies by industry segment. Unfortunately, industry experts surveyed about the GPS Rollover are aware of the potential legalities of voicing an opinion on the issue, resulting in an anonymous aggregate of informal findings. Airport consultants regard the GPS Rollover as a non-issue for planning purposes, but a priority issue for engineering departments using GPS survey equipment.

Airport managers at various size facilities respond with a vague understanding of the topic and voice no plans to address the issue with their tenants.

In the most affected industry segment, the pilots, there exists a strong need to obtain clear guidance and knowledge. Pilots that participate in professional training on a regular basis have, for the most part, been briefed on the GPS Rollover. However, the larger general aviation pilot community is increasingly reliant on GPS for point to point navigation. Most pilots in this realm are uninformed on the 1999 Rollover; once informed, most react to the issue with the same skepticism given the Y2K issue.

It's worth noting that GPS is vulnerable to the Y2K anomaly as well. When you imagine the GPS triangle, the Y2K vulnerability is in the ground stations and their pervasive reliance on a two digit date code. Programmers, in a move to save valuable space (valuable at the time, at least), dropped the digits 1 and 9 from their lines as a year reference — not a problem until we move to a 2 and 0 for date reference. Steps have been taken to address the ground station compliance issue, with a repair project awarded to Boeing a few years ago.

So, with the realization that the rollover is going to occur and an understanding of how GPS works, what can be done about it? Airports and tenants can begin an effort to inform pilots at their airport via letter to tenants, a flyer, or at least a neighborly posting on a bulletin board. Knowledge and resources need to be shared to address the issue.

Manufacturers' phone numbers, website addresses, and other items are readily available online and from aviation trade groups. Any user of a GPS unit should, at a minimum, contact the receiver manufacturer for a statement of compliance.