| GPS Q&A: Industry experts answer reader's GPS questions. Q. What's the difference between the signals and/or receivers used by civilians, and the signals/receivers used by the military? - A.M. Melbourne, Fla. A. Henri Ayers, Leica Canada: Civilian GPS signals/receivers are affected by the Selective Availability effect which degrades the GPS orbit messages and satellite clocks on both L1 and L2 frequencies. This phenomenon affects the precision and accuracy of both code and phase measurements on both frequencies, the end result being that real-time positions are only accurate to 110M 2-D RMS.
Civilian GPS signals/receivers are also affected by the Anti-Spoofing effect which prevents one from using the full P-code positioning capability on both 1.1 and L2 frequencies. The Anti-Spoofing effect contains the Y-W code which is subimposed on the nominal P-code.
Military GPS signals/receivers are not affected by either Selective Availability and/or anti-spoofing since they have special decoders built in the receiver hardware in order to fully exploit the real-time positioning capability of GPS by a factor of one order of magnitude (10 times) better than the civilian GPS receivers. Arthur Lange, Trimble Navigation: GPS is provided at two levels of service - Standard Positioning Service (SPS) for general public use and an encoded Precise Positioning Service (PPS) primarily intended for use by the Department of Defense (DoD). A GPS L1 signal has two modulations used in range measurement, the coarse/acquisition (C/A) code used for SPS, and the P(Y) code, used for PPS. The DoD uses two techniques to encode the GPS signals and deny the full accuracy of GPS to unauthorized users: Selective Availability (SA) and Anti-Spoofing (AS). With SA the DoD dithers the time broadcast by each satellite and may introduce intentional errors into the broadcast ephemeris. This will result in a range error measured by the (civilian) GPS receiver, resulting in a position error. When AS is activated, the P-code is encrypted, creating the Y-code which prevents an appropriately designed military GPS receiver from being fooled by an intentionally deceptive fake GPS signal. A military receiver must have a valid cryptographic key to enable the ability to remove the effects of SA and use the Y code. Civilian receivers have no way to directly remove the effects of SA and cannot directly use the Y code. However, civilian differential GPS receivers can remove the effects of SA with information received from a differential GPS base station. Dr. Frank van Diggelen, Ashtech Inc.: There are two differences, SA and AS, both of which are routinely overcome by commercial receivers:
1. All the signals are subject to Selective Availability (SA), which means that the satellites lie about where they are, and the satellite clocks lie about the correct time. This prevents a civilian receiver from doing better than 100m accuracy (95 percent horizontal accuracy) in stand-alone mode. Military receivers have a module which corrects the deliberate errors, so the military receivers can get about 15m accuracy in stand-alone mode. Using differential GPS, the effects of SA can be totally cancelled.
2. The P-code is encrypted, and the encryption code is secret. This encryption is called Anti-Spoofing (AS), and protects the military from having the signal spoofed (replaced by a false satellite signal) by enemy forces.
Commercial dual-frequency receivers can nonetheless derive the P-code and carrier independent of the encryption, although commercial receivers can be spoofed - so don't use them in the middle of a battlefield.
Ashtech has a patented technique called Z-tracking which gives the best available performance for tracking code and carrier on both frequencies independent of encryption. This Z-tracking technique is now used by Ashtech, Geotronics and Sokkia. Military receivers have the encryption code built in, but they still end up tracking the same observables as a commercial receiver using Z-tracking. Q. I've discovered (the hard way) that GPS time and Greenwich Mean Time (GMT) are not the same. Why? -V.A. Fall River, Wis. A. Ayers: GPS time is different than GMT because GMT is continuously adjusted for Earth rotation and translation charges with respect to the sun and other celestial reference bodies. The GPS time is not adjusted for celestial phenomena since it is based on the behavior of atomic clocks monitoring the satellite system.
The two time systems are very precise internally although they are not related to the same reference frame. Difference between the two time systems is likely to change over time although their relationship between each other is monitored regularly and precisely. Lange: Universal Coordinated Time (UTC - the abbreviation by scientific convention comes from its French initials) is the worldwide business and scientific time standard. UTC replaces the name Greenwich Mean Time (GMT) which is no longer used in scientific literature. UTC is based on the Earth's rotation and is not uniform. The rotational period of the Earth is slowing down because of the tidal interactions between the Earth and moon. GPS time, on the other hand, is based on atomic clocks and is perfectly uniform. This difference in the definition of UTC and GPS time has resulted in a small discrepancy that has accumulated since the GPS time was started. The difference between UTC and GPS time will continue to grow about 1 second per year, depending on irregularities in the rotational period of the Earth. One of the data messages sent by the GPS satellites is the offset between GPS and UTC, allowing GPS receivers to display UTC time. van Diggelen: This is a manifestation of a beautiful fact about time-telling: Once upon a time, the best way we had of telling time was to use sundials. A second was defined as 1/24x60x60 of the time taken for the Earth to do one full rotation relative to the sun.
Then, more and more sophisticated clocks were developed. Today, we have atomic clocks which measure time incredibly accurately by measuring the radiation frequency of atoms. Now the definition of a second is: "the duration of 9,192,631,770 periods of radiation ... of the cesium atom 133."
GPS time is measured with atomic clocks. Every day in GPS time is 24x60x60 seconds. An average day in Greenwich Mean Time (GMT) is still the average amount of time the Earth takes to rotate once relative to the sun. (Thanks to this definition, the sun rises in the morning and sets at night, and will keep on doing so as long as it exists.)
So how is GMT maintained? By using an atomic clock, but adding a "leap-second" at the rate of about once every 18 months to compensate for the fact that the Earth does not rotate at exactly the correct SI rate.
So how do astronomers decide when to add the leap-seconds? By looking at the sun's position, just like the good old days.
GPS time was initiated on Jan. 6, 1980, at 00:00 (GMT). Since then there have been 10 leap seconds. Whenever a leap-second happens, GMT "stands still" for a second. So now GPS time is 10 seconds ahead of GMT.
GMT and GPS time are not the only time reference systems. There is also UTC (Universal Coordinated Time). For more information on these (and more) time reference systems, an excellent text is "Bowditch American Practical Navigator," Chapter XVIII.
(Note: A well built GPS receiver will automatically adjust for the leap-seconds and provide GMT (or UTC), so you don't have to worry about it.) About the participants:
Henri Ayers is a GPS product specialist for Leica Canada in Willowdale, Ontario. He may be reached at 416-497-2460 (phone), 416-497-2053 (fax), or e-mail: 74271,[email protected] Arthur Lange is the GIS product manager for Trimble Navigation in Sunnyvale, Calif. He may be reached at 408-481-2994 (phone), 408-481-6074 (fax), or e-mail: [email protected] Dr. Frank van Diggelen is the marketing manager for OEM and navigation at Ashtech Inc. in Sunnyvale, Calif. He may be reached at 408-524-1508 (phone), 408-524-1500 (fax), or e-mail: [email protected] Back |
