| The Sustainable Development Reference Link (Revised 21 April, 1999) |
GPS Bug: Introduction The topics on this page are shown immediately below. [ GPS Bug | How GPS Works | Time Keeping | Impact of Bug | Links] GPS Bug The atomic clocks in the GPS satellites were set with base dates in January 1980 and the indicator for the week was set at a maximum of 1024. This means that on August 22, 1999 the atomic clocks will reset themselves to week 0. GPS receivers, navigation equipment and time-keeping equipment which has not been properly programmed to deal with this change will experience problems when the week counter is reset -- but it is not clear how serious these problems will be. The bug is expected to be more of a problem with older equipment (pre-1994) than more recent equipment. This problem has received much less publicity than the Y2K bug in computers and as a result there has been little done to set criteria for GPS EOW rollover compliance at a national or international level. (Click here for a letter from Professor R. Langley that describes the GPS atomic clock in more detail.) Diagnosing the problem is difficult. The GPS receivers require extremely sophisticated electronic equipment for testing. All but the most sophisticated users will have to rely on information from the manufacturers of their equipment to determine in advance how their equipment will deal with the rollover. This site provides links to the web sites of the main GPS receiver manufacturers (many of which have now provided compliance information) as well as links to two sites which have partial lists of compliant and non-compliant equipment. A page of links to manufacturers of GPS timekeeping equipment is also provided. Go to top The GPS system was developed by the US Department of Defense for the purposes of accurate navigation and positioning. It consists of both satellites and ground receivers, each of which perform well defined functions. Four satellites must be used in order to determine the position of one ground receiver. The first three satellites are used to determine the position relatively accurately, and the fourth is used to synchronize the time clock of the receiver with the extremely accurate atomic clocks of the satellites. The method by which three satellites are able to determine the position of a receiver is referred to as "triangulation". The following is a simplified explanation of the process. One satellite is initially located and the distance from this satellite to the ground receiver is calculated. (The method of calculating distance involves complicated codes and measurements of the speed of radio signals.) Another satellite is then located, and the distance to the receiver is likewise calculated. Positioning is determined by use of simple geometry. If the distance to a satellite is assumed to be the radius of a sphere (of which the satellite is the centre), then the area bounded by the two spheres represents the relative location of the ground receiver. The addition of a third satellite can provide improved accuracy. The point where all three spheres intersect identifies the position of the receiver with some small level of error. The error occurs because, among other factors, the receivers’ clocks are not as accurate as the atomic clocks of the satellites. It would be far too expensive to install atomic clocks on the receivers. Since there are only 24 GPS satellites, the installation of atomic clocks on them was feasible. The accuracy of an atomic clock can be achieved, however, with use of a fourth satellite. When the distance to this satellite is measured, the fourth sphere will not intersect with the first three due to inaccurate timing of the receiver. The receiver takes this discrepancy into account and adjusts its clock to be synchronized with those of the satellites. There are other measurement errors that come into play, but all may be quite accurately corrected for using various methods. In fact, the US Department of Defense purposely incorporates some "noise" into the system (which it is later able to decrypt) to prevent potential enemies from using GPS to develop their own weapons. There are many uses of the GPS system. The most traditional is that of locating positions on the earth’s surface. Similarly, GPS is used for navigation in the air, on water, and on land, and can also be used for tracking a moving object. Precise maps are made by way of GPS. GPS Time Keeping Potential Impact of the GPS End-of-Week Rollover Most of the GPS manufacturers are not explicit about what kind of problems non-compliant GPS receivers will experience at the time of the roll over. However, by collecting information from a number of sources, it has been possible to identify some of the potential problems:
There have also been suggestions that particular problems will exist for some receivers during week 0, but that following week 0 they will operate normally. The GPS clocks will face the same kinds of problems with respect to dates and time. Only the manufacturer is in a position to advise on whether equipment is compliant, and if it is not compliant, what problems will occur and how they might be solved. GPS Information -- links to various web pages that describe GPS or deal specifically with the Rollover issue. GPS Receiver Manufacturers -- links to web sites of
manufacturers of GPS receivers and equipment. The list is subdivided into
manufacturers that have information on compliance on their web sites and those that do
not. This page also provides links to two sites that have partial lists of compliant and
non compliant equipment..
[ For further information or to provide suggestions contact GRI Copyright©1998 GRI |