GPS Terms Glossary
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).
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.
The ability of a receiver to start position calculations without being given an approximate location and time.
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.
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.
A baseline consists of a pair of stations for which simultaneous GPS data has been collected.
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.
The standard (Clear/Acquisition) GPS code; also known as the "civilian code" or S-code.
A radio wave having at least one characterstic (e.g. frequency, amplitude, phase) that can be varied from a known reference value by modulation.
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.
The frequency of the unmodulated fundamental output of a radio transistor.
A channel of a GPS receiver consists of the radio frequency, circuitry, and software necessary to tune the signal from a signal GPS satellite.
The difference the clock's indicated time and true universal time.
Code phase GPS
GPS measurements based on the C/A code.
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.
Refers to either the specific set of satellites used in calculating positions or all the satellites visible to a GPS receiver at one time.
A world-wide network of GPS monitoring and control stations that ensure the accuracy of satellite positions and their clocks.
What mapping system is used to represent postions. Some examples are latitude/longitude and state plane. The data logger by default uses latitude/longitude. You can convert your data into the desired coordinate system using PFinder.
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.
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.
A 1500 bit message included in the GPS signal which reports the satellite's location, clock corrections, and health.
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).
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.
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.
see Satellite elevation mask.
The predictions of current satellite position that are transmitted to the user in the data message.
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.
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.
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.