GPS Glossary


SA (Selective Availability)

Selective Availability (SA) is the intentional degrading of GPS accuracy by altering the satellite clock at random and by broadcasting inaccurate orbit parameters to degrade the positioning accuracy by 100m or greater. Therefore, it is needed to apply the DGPS technology to improve the accuracy problems.

DGPS (Differential Global Positioning System)

The DGPS is a technology applied to enhance the positioning accuracy of the GPS by deducting the difference of the detected measurement of an unknown location with the measured difference of a known location in order to find out the more accurate location.

PPS (Precise Positioning System)

The PPS is available only to users who have the decoding system, the access authority and a specific receiver. In general, only the US military and government agencies and some other US-government-approved people can have the access to the PPS. The accuracy of PPS is 17.8m horizontally and 27.7m vertically. The time accuracy is 100 n-second.

SPS (Standard Positioning System)

The SPS is the GPS opened for civilian use free of charge. Most GPS receivers support the SPS because the SA has been added to the system. The accuracy of SPS is 100m horizontally and 156m vertically. The time accuracy is 167 n-second.

A-S (Anti-Spoofing)

After the US Department of Defense activated the AS, the P-code is locked and becomes the Y-code. General users are unable to decode the Y-code though some GPS receiver makers can obtain more accurate code measurement with specific circuit technologies (e.g. Ashtech Z-tracking). Yet receivers without similar technologies will be unable to make the measurement, such as Leica Wild GPS-System 200. Before AS was activated on 31 January 1994, the GPS could execute distance measurement with P-code and full wavelength L1 and L2 carrier phase measruement. After AS was activated, users can obtained only half wavelength data on L2, and the measurement noise is also big.

MOB (Man Over Board)

MOB refers to warning of man over board. It is often used for sailing. When there is someone over board, other crews on board, such as the captain, can use the MOB function to locate the position where the man is over board. After activating the function, the GPS will enter the navigation mode and point to the location where the man is over board. It is very useful for rescuing an overboard person in unfavorable weather.


It is a navigation term referring to making a mark at a particular location. Users can set a waypoint in any location on a GPS receiver and save it in the memory as a future reference.

Simulator Mode

It is a unique feature on Garmin45. It simulates the satellite signal in receiving condition. It helps users to practice using the GPS in an indoor environment or at places where satellite signals are not received. Users can use the normal mode indoor, when a Garmin45 cannot receive satellite signals in normal mode for 15 minutes, it will shut down automatically for the reason of saving energy.

Single Band Vs Dual Band

NAVSTAR positioning satellites send signals in 2 frequencies: L1 and L2. L1 sends the C/A code for civilian use and the P1 code for military use; and L2 sends only the P2 code for military use. Therefore, general GPS for civilian use can only receive L1 signals and unlocked codes for positioning purpose. There are 2 methods to calculate a location. The commonest way is to calculate data regarding time and satellite positions from L1 codes received from any 4 satellites (at least 3) by means of complex matrix computing. The accuracy is 15m on average.

The carrier phase measurement can locate an object more accurately. It is a technology calculating distance according to the characteristics of radio signals. Theoretically, it is assumed that signals from the satellite to the receiver travels on a straight line. In reality, signals are deviated due the atmospheric influences, especially the ionosphere effect. The ionosphere allows us to receive contents broadcast from a radio station located hundreds of miles away. 

Yet, the radio frequencies that used by the GPS will not be deviated much, and the error is often within 1m. The use of dual band can help to eliminate errors occur in the ionosphere effect because they are correlated. Therefore, if a GPS receiver can receive both L1 (civilian band) and L2 (military band) signals at the same time, it will be easier to justify the errors that caused in the ionosphere and eliminate them before calculating the location of an object.