GPS Glossary

Absolute Positioning

Positioning mode in which a position is identified with respect to a well-defined coordinate system, commonly a geocentric system (i.e., a system whose point of origin coincides with the center of mass of the earth).

Almanac

A data file that contains orbit information on all satellites, clock corrections, and atmospheric delay parameters. It is transmitted by a GPS satellite to a GPS receiver, where it facilitates rapid satellite vehicle acquisition within GPS recievers.

Anywhere fix

The ability of a receiver to start position calculations without being given an approximate location and time.

Attribute

A value that describes a feature. Features may have zero to many attributes. The attributes for features are described in the data dictionary. Values for attributes are entered while collecting the data. An example of an attribute would be the name of a building, or the height of a tree.

Availability

The number of hours per day that a particular location has sufficient satellites (above the specified elevation angle and less than the specified PDOP value) to make a GPS position fix.

Baseline

A baseline consists of a pair of stations for which simultaneous GPS data has been collected.

Base station

Also called a reference station. A receiver that is set up on a known location specifically to collect data for differentially correcting rover files. The base station calculates the error for each satellite and, through differential correction, improves the accuracy of GPS positions collected at unknown locations by a roving GPS receiver.

Base station file

The data file created by the base station. Base station files follow a naming pattern: A7-Month-Day-Hour.SSF, such as A7110413.SSF for a file created on November 4th during the hour of 13:00 (GMT). The base station will write a new file every time the hour changes. If there is an error in a file or the station has an error writing the file, the new file for that hour will have extension .001.

C/A code

The standard (Clear/Acquisition) GPS code; also known as the "civilian code" or S-code.

Carrier

A radio wave having at least one characterstic (e.g. frequency, amplitude, phase) that can be varied from a known reference value 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. More accurate than standard approach.

Carrier beat phase

The phase of the signal which remains when the incoming Doppler-shifted satellite carrier signal is beat (the difference frequency signal is generated) with the nominally-constant reference frequency generated by the receiver.

Carrier frequency

The frequency of the unmodulated fundamental output of a radio transistor.

Channel

A channel of a GPS receiver consists of the radio frequency, circuitry, and software necessary to tune the signal from a signal GPS satellite.

Clock bias

The difference the clock's indicated time and true universal time.

Code phase GPS

GPS measurements based on the C/A code.

Constant offset

In some cases you will be unable to receive GPS signals while mapping a desired feature. A constant offset can be configured that will allow you to stand a distance away from the feature, yet record the feature's position.

Constellation

Refers to either the specific set of satellites used in calculating positions or all the satellites visible to a GPS receiver at one time.

Control segment

A world-wide network of GPS monitoring and control stations that ensure the accuracy of satellite positions and their clocks.

Coordinate System

What mapping system is used to represent postions. Some examples are latitude/longitude and state plane. The datalogger by default uses latitude/longitude. You can convert your data into the desired coordinate system using PFinder.

Cycle slip

A discontinuity of an interger number of cycles in the measured carrier beat phase resulting from a temporary loss-of-lock in the carrier tracking loop of a GPS receiver.

Data Dictionary

Defines the fields you will fill during data collection using the GPS equipment. Very similar to describing the fields in a record for a database program.

Datalogger

Also called a data recorder. A handheld, lightweight data entry computer. It can be used to store additional data obtained by a GPS reciever.

Data message

A 1500 bit message included in the GPS signal which reports the satellite's location, clock corrections, and health.

Datum

Changing technology has led to different values for the same geographic points over time. Datum refers to which standard you are using for known points. If you are going to use your collected data with prexisting data, you need to match up the datum and coordinate systems. Some examples of datum are NAD-27 and WGS-84 (North American Datum 1927, World Geodetic System 1984).

Differential Correction

Using either GPS positions collected from a base station (located on a known position) simultaneously or RTCM broadcasts to increase the accuracy of your position information.

Dilution of Precision

The multiplicative factor that modifies range error. It is caused solely by the geometry between the user and their set of satellites; known as DOP or GDOP.

Dithering

The introduction of digital noise. This is the process the Department of Defense (DoD) uses to add inaccuracy to GPS signals to induce Selective Availability.

Dongle

A security device that fits into the 25-pin port on the back of the laptop. Not a GPS term, but you need to know it to use the PFinder and Community Base Station software.

Elevation mask

see Satellite elevation mask.

Emphemris

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" implies that the switching time is sufficiently short (2 to 5 milliseconds) to recover the data message.

Feature

Any item about which you want position information. Your data dictionary will describe the features you wish to collect. A feature may be a single point, a line, or an area.

Geodetic surveys

Global surveys done to establish control networks (comprised of reference or control points) as a basis for accurate land mapping.

Geometric Dilution of Precision

See Dilution of Precision.

Global Navigation Satellite System (GLONASS)

This is the Russian counterpart to GPS. GLONASS provides worldwide coverage, however, its accuracy performance os optimized for the northern latitudes. and is specificed as identical to that of GPS SPS.

Ionospheric refraction

The change in the propagation speed of a signal as it passes through the ionosphere.

Multipath error

Errors caused by the interference of a signal that has reached the receiver antenna by two or more different paths. This is 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.

NAVSTAR

The name given to GPS satellites. NAVSTAR is an acronym for NAVigation Satellite Timing and Ranging.

NAD 27

North American Datum of 1927. Older and obsolete horzontal datum of North America. NAD 27 depends upon an early approximation of the shape of the earth, known as the clarke Spheriod of 1866, designed to fit only the shape of the conterminous United States, and utilizing a specific Earth surface coordinate pair as its center of reference.

NAD 83

North American Datum of 1983. Official horizontal datum of North America. NAD 83 relies on the more precise Geodetic Reference System of 1980 (GRS 80).

NAVD 88

North American Vertical Datum of 1988. Effort underway by the National Geodetic Survey (NGS) to readjust the North American Vertical Datum. The NAVD 88 readjustment will remove distortions from the continent-wide vertical geodetic (height) reference system.

P-code

The Precise or Protected 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 1-week segment of this code is unique to one GPS satellite and is reset each week.

Point positioning

See Absolute positioning.

Position

The latitude, longitude, and altitude of a point. An estimate of error is often associated with a position.

PDOP

Percent Dilution of Position. Measure of the geometrical strength of the GPS satellite configuration. The amount of error in your position. PDOP less than 4 gives the best accuracy (under 1 meter). Between 4 and 8 gives acceptable accuracy. Greater than 8 gives poor accuracy.

PDOP Mask

The upper limit for PDOP that you wish to collect data. If PDOP goes above the PDOP mask, GPS data will no longer be collected (until the PDOP decreases). The default is 6.

Post-processed differential GPS

In post-processed differential GPS the base and roving recievers have no active data link between them. Instead, each records the satellite observations that will allow differential correction at a later time. Differential correction software is used to combine and process the data collected from these receivers.

Precise Positioning Service (PPS)

The most accurate dynamic positioning possible with GPS, based on the dual frequency P-code.

Proportional error

One means of expressing positional accuracy, expressed as the position error divided by the distance to the origin of the coordinate system used, stated in parts per million (ppm).

Pseudo-lite

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 noise (PRN) code

A signal with random noise-like properties. It is very complicated but repeated pattern of 1's and 0's.

Pseudo-range

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.

Range

A fixed distance between two points, such as between a starting and an ending waypoint or a satellite and a GPS reciever.

Real-time differential GPS

A base station which computes, formats, and transmits corrections usually through some sort of data link (e.g. VHF radio or cellular telephone) with each new GPS observation. The roving unit requires some sort of data link receiving equipment to receive the transmitted GPS corrections and get them into the GPS receiver so they can be applied to its current observations.

Relative positioning

The determination of relative positions between two or more receivers which are simultaneously tracking the same GPS signals.

RINEX

Receiver INdependent EXchange format. A set of standard definitions and formats to promote the free exchange of GPS data and facilities the use of data from any GPS receiver with any software package. The format includes definitions for three fundamental GOS observables: time, phase, and range.

Rover

Any mobile GPS receiver collecting data during a field session. The receiver's position can be computed relative to another, stationary GPS receiver.

Rover file

The data file created by the rover. After being transferred to PFinder, the rover file has extension .SSF. File names follow a naming pattern: R-Month-Date-Hour-Sequence, for example, file R110413A.SSF was created on November 4th, during the hour of 13:00 (GMT), and is the first file created in that hour. A second file created in this hour would be named R110413B.SSF

RTCM

Real Time Correction Measure. RTCM is a format for using Differential GPS broadcasts over radio to provide real-time correction of the error in the calculated positions. Trimble ProBeacon is the Differential GPS receiver we have.

Satellite elevation mask

Required elevation of the SV above the horizon. SVs with elevation below the mask will not be used to compute positions. This is done because SVs near the horizion have more error due to atmosphere.

S-code

See C/A-code.

Satellite configuration

The state of the satellite constellation at a specific time, relative to a specific user or set of users.

Satellite constellation

The arrangement in space of a set of satellites.

Selective Availibility

SA. Errors in data and satellite-clock dithering deliberately induced by the Dept. of Defense to restrict full GPS accuracy to authorized users, typically the U.S. military.

SNR

Signal to Noise Ratio. A measure of the information content of the signal relative to the signal's noise. A higher number is desirable.

Slow switching channel

A sequencing GPS receiver channel that switches too slowly to allow the continuous recovery of the data message.

Space segment

The space-based component of the GPS system (i.e. the satellites).

Space vehicle (SV)

A GPS satellite.

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 in the earth. This allows the use of various averaging techiques that improve the accuracy by factors of over 1000.

Tropospheric correction

The correction applied to the measurement to account for tropospheric delay.

User segment

The component of the GPS system that includes the receivers.

Y-code

Classified PRN code, similar to the P-code, though restricted to use by the military.