Definitions of GPS Terms

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Definitions

Anti-Spoofing (A-S)

For the NAVSTAR system, anti-spoofing (A-S) is the process whereby The P code used for the precise positioning service is encrypted. The resulting encrypted code is called the Y code. The encryption data can only be decoded by GPS receivers with special decryption circuitry, guarding against fake transmissions of satellite data.

DGPS

Differential GPS - A technique for improving gps solution accuracy. Error is reduced by determining the error at a known location, then subracting that error from the solution at an unknown location.

Dilution of Precision (DOP)

DOP is a function expressing the mathematical quality of solutions based on the geometry of the satellites. Position dilution of precision (PDOP), the most common such value, has a best case value of 1, higher numbers being worse. The best PDOP would occur with one satellite directly overhead and three others evenly spaced about the horizon. Higher numbers of PDOP are worse. PDOP could theoretically be infinite, if all the satellites were in the same plane.

PDOP has a multiplicative effect on the user range error (URE) value. A URE of 32 meters with a PDOP of one would give a user an assumed best accuracy of 32 meters. A PDOP of 2 would result in an assumed accuracy of 64 meters. Many receivers can be programmed to stop providing position solutions above a specific PDOP level (6 is common).

There are several other DOP factors such as horizontal and vertical DOP, geometric DOP and time DOP.

Full Operational Capability

For NAVSTAR, defined as the capability that will occur when 24 operational GPS satellites (Block II/IIA) are operating in their assigned orbits and are available for navigation use.

Initial Operating Capability

For NAVSTAR, defined as the capability that will occur when 24 GPS satellites (Block I/II/IIA) are operating in their assigned orbits and are available for navigation use.

Kinematic Surveying

A precision DGPS surveying technique. The roving user does not need to stop to collect precision information. Meter - centimeter level accuracy is available using modern dual-frequency carrier-phase measurement techniques.

L1 Frequency Band

One of the two radio frequency bands (1602.5625 MHZ - 1615.5000 MHz) transmitted by the GLONASS satellites. Unlike NAVSTAR satellites, each GLONASS satellite transmits on a separate frequency within this band.

L1 Frequency

One of the two radio frequencies (1575.42 MHz) transmitted by the NAVSTAR satellites. Unlike GLONASS satellites, all NAVSTAR satellites transmit on the same frequencies. This frequency carries the C/A code used for the standard positioning service (SPS) and the P code used for the precise positioning service (PPS).

L2 Frequency Band

One of the two radio frequency bands (1246.4375 MHz - 1256.5000 MHz) transmitted by the GLONASS satellites. Unlike NAVSTAR satellites, each GLONASS satellite transmits on a separate frequency within this band.

L2 Frequency

One of the two radio frequencies (1227.6 MHz) transmitted by the NAVSTAR satellites. This frequency carries only the P code used for the precise positioning service (PPS).

LDGPS

Local area Differential GPS - A real-time DGPS system that is made available over a small area - possibly 150 miles or less.

NAGU

Notice Advisory to GLONASS Users - a periodic bulletin alerting users to changes in system performance.

NANU

Notice Advisory to NAVSTAR Users - a periodic bulletin alerting users to changes in system performance.

Precise Positioning Service (PPS)

PPS is the most accurate positioning, velocity, and timing information continuously available, worldwide, from the basic GPS. This service is limited to authorized U.S. and allied Federal Governments; authorized foreign and military users; and eligible civil users. PPS information is usually (but not always) encrypted to prevent use by unauthorized users. The encryption process is known as Anti-Spoofing. GPS receivers that can use the P code provide a predictable positioning accuracy of at least 22 meters (2 drms) horizontally and 27.7 meters (2 sigma) vertically. They can also provide timing/time interval accuracy within 90 nanoseconds (95 percent probability). This improved accuracy is provided in two ways. First, P-code users are not subjected to Selective Availability. Second, access to the L2 channel allows the user to correct for atmospheric propagation errors. Equipment that uses the P code is available for civilian use; however, The capability is useless when Anti-Spoofing is active. Access to GPS receivers that can decrypt the Y code is tightly controlled and is subject to National Security considerations. If you are a civilian user, you will need a really good excuse to get one.

Psuedoranging

(This definition is subject to argument - even more than most!) A technique where the user uses the time delay of arrival of radio signals from multiple radio stations to solve for position.

Psuedolites

Ground-based transmitters that mimic a GPS satellite. Intended to improve geometric solutions in a local area. Could be used around airports for precision instrument landings.

Radionavigation

The determination of position, or the obtaining of information relating to position, for the purposes of navigation by means of the propagation properties of radio waves.

RAIM

Receiver Autonomous Integrity Monitoring - A GPS receiver system that would allow the receiver to detect incorrect signals being transmitted by the satellites by comparing solutions with different sets of satellites. Would require 6 satellites in view at any one time, which in turn would require 30 total satellites, more than are now available.

Selective Availability (SA)

Selective availability is the name of a process that allows the DoD to limit the accuracy available to users of Navstar's Standard Positioning Service. It is accomplished by manipulating navigation message orbit data (epsilon) and/or the satellite clock frequency (dither) This process is intended to prevent potential adversaries of the United States from using low-cost GPS receivers for certain military functions.

Spherical Error Probable (SEP)

A sphere in which position measurements have a 50% chance of being located.

Spoofing

The deliberate transmission of fake signals to fool a GPS receiver. Spoofer must mimic a GPS satellite, rather like a psuedolite but with disruptive intent.

Standard Error Ellipsoid

An ellipsoid in which position measurements have a 20% chance of being located.

Standard Positioning Service (SPS)

SPS is the standard specified level of positioning and timing accuracy that is available, without qualification or restrictions, to any user on a continuous worldwide basis. The accuracy of this service is established by the U.S. Department of Defense based on U.S. security interests. Navstar currently provides horizontal positioning accuracy within 100 meters (2 drms) and 300 meters with 99.99 percent probability. The signals providing standard positioning service are inherently capable of greater accuracy than this. The accuracy of the system is limited through the application of a process called Selective Availability

Static Surveying

A precision DGPS surveying technique. The roving user is required to sit motionless taking data at a point from fifteen minutes to an hour or more.

Stop-and-Go Surveying

A precision DGPS surveying technique, also called semi-kinematic surveying. The roving user is only required to collect data at a point for a few seconds or minutes.

User Range Error (URE)

Estimated error in range due to factors such as unmodelled atmospheric effects, orbital calculation errors, satellite clock bias, multipath and selective availability. This value is transmitted by the NAVSTAR satellites and may be displayed by some GPS receivers. The number is expressed in meters (32 is common with S/A on. I observed 5 meters URE with S/A off.

WAAS

Wide Area Augmentation Service - A service being developed by the FAA to provide wide area DGPS and integrity information to aviation users.

WDGPS

Wide area Differential GPS - A real-time DGPS system that is made available over a large area - possibly global.