GPS Glossary

Anywhere fix: The ability of a receiver to start position calculations without being given an approximate location and approximate time.
Bandwidth: The range of frequencies in a signal.
C/A code: The standard (Coarse/Acquisition) GPS code. A sequence of 1023 pseudo-random, binary, biphase modulations on the GPS carrier at a chip rate of 1.023 MHz. Also known as the "civilian code."
Carrier: A signal that can be varied from a known reference by modulation.
Carrier-aided tracking: a signal processing strategy that uses the GPS carrier signal to achieve an exact lock on the pseudo random code.
Carrier frequency: The frequency of the unmodulated fundamental output of a radio transmitter.
Carrier phase GPS: GPS measurements based on the L1 or L2 carrier signal.
Channel: A channel of a GPS receiver consists of the circuitry necessary to receive the signal from a single GPS satellite.
Chip: The transition time for individual bits in the pseudo-random sequence. Also, an integrated circuit. Also a snack food. Also a betting marker.
Clock bias: The difference between the clock's indicated time and true universal time.
Code phase GPS: GPS measurements based on the pseudo random code (C/A or P) as opposed to the carrier of that code.
Control segment: A world-wide network of GPS monitor and control stations that ensure the accuracy of satellite positions and their clocks.
Cycle slip: A discontinuity in the measured carrier beat phase resulting from a temporary loss of lock in the carrier tracking loop of a GPS receiver.
Data message: A message included in the GPS signal which reports the satellite's location, clock corrections and health. Included is rough information on the other satellites in the constellation.
Differential positioning: Accurate measurement of the relative positions of two receivers tracking the same GPS signals.
Dilution of Precision: The multiplicative factor that modifies ranging error. It is caused solely by the geometry between the user and his set of satellites. Known as DOP or GDOP
Dithering: The introduction of digital noise. This is the process the DoD uses to add inaccuracy to GPS signals to induce Selective Availability.
Doppler-aiding: A signal processing strategy that uses a measured doppler shift to help the receiver smoothly track the GPS signal. Allows more precise velocity and position measurement.
Doppler shift: The apparent change in the frequency of a signal caused by the relative motion of the transmitter and receiver.
Ephemeris: The predictions of current satellite position that are transmitted to the user in the data message.
Fast switching channel: A single channel which rapidly samples a number of satellite ranges. "Fast" means that the switching time is sufficiently fast (2 to 5 milliseconds) to recover the data message.
Frequency band: A particular range of frequencies.
Frequency spectrum: The distribution of signal amplitudes as a function of frequency.
Geometric Dilution of Precision (GDOP): See Dilution of Precision.
Hardover word: The word in the GPS message that contains synchronization information for the transfer of tracking from the C/A to P code.
Ionosphere: The band of charged particles 80 to 120 miles above the earth's surface.
Ionospheric refraction: The change in the propagation speed of a signal as it passes through the ionosphere.
L-band: The group of radio frequencies extending from 390 MHz to 1550 MHz. The GPS carrier frequencies (1227.6 MHz and 1575.42 MHz) are in the L band.
Multipath error: Errors caused by the interference of a signal that has reached the receiver antenna by two or more different paths. Usually caused by one path being bounced or reflected.
Multi-channel receiver: A GPS receiver that can simultaneously track more than one satellite signal.
Multiplexing channel: A channel of a GPS receiver that can be sequenced through a number of satellite signals.
P-code: The Precise code. A very long sequence of pseudo random binary biphase modulations on the GPS carrier at a chip rate of 10.23 MHz which repeats about every 267 days. Each one week segment of this code is unique to one GPS satellite and is reset each week.
Precise Positioning Service (PPS): The most accurate dynamic positioning possible with standard GPS, based on the dual frequency P-code and no SA.
Pseudolite: A ground-based differential GPS receiver which transmits a signal like that of an actual GPS satellite, and can be used for ranging.
Pseudo random code: A signal with random noise-like properties. It is a very complicated but repeating pattern of 1's and O's.
Pseudorange: A distance measurement based on the correlation of a satellite transmitted code and the local receiver's reference code, that has not been corrected for errors in synchronization between the transmitter's clock and the receiver's clock.
Satellite constellation: The arrangement in space of a set of satellites.
Selective Availability (SA): A policy adopted by the Department of Defense to introduce some intentional clock noise into the GPS satellite signals thereby degrading their accuracy for civilian users.
Slow switching channel: A sequencing GPS receiver channel that switches too slowly to allow the continuous recovery of the data message.
Space segment: The part of the whole GPS system that is in space, i.e. the satellites.
Spread spectrum: A system in which the transmitted signal is spread over a frequency band much wider than the minimum bandwidth needed to transmit the information being sent. This is done by modulating with a pseudo random code, for GPS.
Standard Positioning Service (SPS): The normal civilian positioning accuracy obtained by using the single frequency C/A code.
Static positioning: Location determination when the receiver's antenna is presumed to be stationary on the earth. This allows the use of various averaging techniques that improve accuracy by factors of over 1000.
User interface: The way a receiver conveys information to the person using it. The controls and displays.
User segment: The part of the whole GPS system that includes the receivers of GPS signals.