A Glossary of GPS Terms

A B C [D - H] [I - Q] [R - Z]

- A -

Absolute Positioning

Mode in which a position is determined, using a single receiver, with respect to a well-defined coordinate system, typically a Geocentric system (i.e., a system whose point of origin coincides with the centre of mass of the earth). Also referred to as Point Positioning, or Single Receiver Positioning.

Almanac

A data file that contains the approximate orbit information of all satellites, which is transmitted by each satellite within its Navigation Message. It is transmitted by a GPS satellite to a GPS receiver, where it facilitates rapid satellite signal acquisition within GPS receivers. Almanac data is kept current within a GPS receiver to facilitate "hot starts" by permitting the Doppler Shift of each satellite signal to be determined and configuring each tracking channel for this Doppler-shifted carrier frequency.

Antenna

That part of the GPS receiver hardware which receives (and sometimes amplifies) the incoming L-Band signal. Antennas come in all shapes and sizes, but most these days use so-called "microstrip" or "patch" antenna elements. The geodetic antennas, on the other hand, may use a "choke-ring" to mitigate any multipath signals.

Antenna Splitter

An attachment which can be used to split the antenna signal into two, so that it may be fed to two GPS receivers. Such a configuration forms the basis of a Zero Baseline test.

Anti-Spoofing (AS)

Is a policy of the U.S. Department of Defense by which the P-Code is encrypted (by the additional modulation of a so-called W-Code to generate a new "Y-Code"), to protect the militarily important P-Code signals from being "spoofed" through the transmission of false GPS signals by an adversary during times of war. Hence civilian GPS receivers are unable to make direct P-Code pseudo-range measurements and must use proprietary (indirect) signal tracking techniques to make measurements on the L2 carrier wave (for both pseudo-range and carrier phase). All dual-frequency instrumentation must therefore overcome AS using these special signal tracking and measurement techniques.

Anywhere Fix

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

Ambiguity

Carrier phase measurements can only be made in relation to a cycle or wavelength of the L1 or L2 carrier waves because it is impossible to discriminate different carrier cycles (they are all "sine waves" if one ignores the modulated messages and PRN codes). Integrated carrier phase measurements may be made by those receivers intended for carrier phase-based positioning. In this case the change in receiver-satellite distance can be measured by counting the number of whole wavelengths since initial signal lock-on and adding the instantaneous fractional phase measurement. However, such a measurement is a biased range or distance measurement because the initial number of whole (integer) wavelengths in the receiver-satellite distance is unknown. This unknown value is referred to as the "ambiguity". It is different for the different satellites, and different for the L1 and L2 measurements. It is, however, a constant if signal tracking continues uninterrupted through an observation session. If there is signal blockage, then a "cycle slip" occurs, causing the new ambiguity after the cycle slip to be different from the value before. Cycle slip repair therefore restores the continuity of carrier cycle counts and ensures that there is only one ambiguity for each satellite-receiver pair.

Ambiguity Resolution

If the initial integer ambiguity value for each satellite-receiver pair could be determined, then the ambiguous integrated carrier phase measurement can be corrected to create an unambiguous, but very precise (millimetre observation accuracy), receiver-satellite distance measurement. A solution using the corrected carrier phase observations is known as an "ambiguity-fixed" or "bias-fixed" solution. The mathematical process or algorithm for determining the value for the ambiguities is Ambiguity Resolution. Tremendous progress has been made in AR techniques, making today's carrier phase-based GPS systems very efficient by cutting down the length of observation data needed (resulting in so-called "rapid static surveying" techniques) and even allowing this process to occur while the receiver is itself in motion (in so-called "on-the-fly" AR techniques). (In practice, the AR process and the ambiguity-fixed solutions are carried out on the double-differenced carrier phase observables, not on the one-way satellite-receiver measurements.)

Attribute

A characteristic which describes a Feature. Attributes can be thought of as questions which are asked about the Feature. Typically associated with geospatial data gathering for inclusion within Geographic Information Systems (GIS).

Availability

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

- B -

Baseline

A Baseline consists of a pair of stations for which simultaneous GPS data have been collected. Mathematically expressed as a vector of coordinate differences between the two stations, or an expression of the coordinates of one station with respect to the other (whose coordinates are assumed known, and is typically referred to as a "Base" or "Reference" Station).

Base Station

Also called a Reference Station. In GPS navigation, this is a receiver that is set up on a known location specifically to collect data for differentially correcting data files of another receiver (which may be referred to as the "mobile" or "rover" receiver). In the case of pseudo-range-based Differential GPS (DGPS) the base station calculates the error for each satellite and, through differential correction, improves the accuracy of GPS positions collected at unknown locations by another (roving) GPS receiver. For GPS Surveying techniques, the receiver data from the base station is combined with the data from the other receiver to form double-differenced observations, from which the baseline vector is determined.

Bearing

Also referred to as the Azimuth. The compass direction from a position to a destination. The "north" direction is "zero bearing", and the angle is measured clockwise through 360°. May be referred to a number of "north" directions, including magnetic north, (projection) grid north, or geographic north.

Bias

All GPS measurements are affected by biases and errors. Their combined magnitudes will affect the accuracy of the positioning results (they will bias the position or baseline solution). Biases may be defined as being those systematic errors that cause the true measurements to be different from observed measurements by a "constant, predictable or systematic amount", such as, for example, all distances being measured too short, or too long. Biases must somehow be accounted for in the measurement model used for data processing if high accuracy is sought. There are several sources of biases with varying characteristics, such as magnitude, periodicity, satellite or receiver dependency, etc. Biases may have physical bases, such as the atmosphere effects on signal propagation or ambiguities in the carrier phase measurements, but may also enter at the data processing stage through imperfect knowledge of constants, for example any "fixed" parameters such as the satellite ephemeris information, station coordinates, velocity of light, antenna height errors, etc. Random errors will not bias a solution. However, outlier measurements, or measurements significantly affected by multipath disturbance (which may be considered a transient, unmodelled bias), will bias a solution if the proportion of affected measurements is relatively high compared to the number of unaffected measurements. For this reason, long period static GPS Surveying is more accurate (less likely to be biased) than "rapid static surveying" or kinematic (single-epoch) positioning.

Binary Shift-Key (BSK) Modulation

BSK is a modulation technique by which a binary message, such the Navigation Message or the PRN codes (consisting of 0's and 1's), is imprinted on the carrier wave. Unlike Amplitude Modulation (AM) and Frequency Modulation (FM), BSK Modulation does not alter the signal level (the "amplitude") or the carrier wavelength (the "frequency"). At a change in value of the message from 0 or 1, or from 1 to 0, the carrier wave is reversed (the phase is "flipped" by 180°). All reversals take place at the zero-crossings of the carrier (sine) wave (i.e., where the phase is zero).

- C -

C/A-Code

The standard (Clear/Acquisition) GPS PRN code, also known as the Civilian Code or S-Code. Only modulated on the L1 carrier. Used by the GPS receiver to acquire and decode the L1 satellite signal, and from which the L1 pseudo-range measurement is made.

Carrier

A radio wave having at least one characteristic (e.g., frequency, amplitude, phase) that can be varied from a known reference value by modulation. In the case of GPS there are two transmitted carrier waves: (a) L1 at 1575.42MHz, (b) L2 at 1227.60MHz, modulated by the Navigation Message (both L1 and L2), the P-Code (both L1 and L2) and the C/A-Code (L1).

Carrier Phase

GPS measurements made on the L1 or L2 carrier signal. May refer to the fractional part of the L1 or L2 carrier wavelength (approximately 19cm for L1, 24cm for L2), expressed in units of metres, cycles, fraction of a wavelength or angle. (One cycle of L1 is equivalent to one wavelength, and similarly for L2.) In carrier phase-based positioning, such as employed in GPS Surveying techniques, carrier phase may also refer to the accumulated or integrated measurement which consists of the fractional part plus the whole number of wavelengths (or cycles) since signal lock-on.

Carrier-Aided Tracking

A signal processing strategy that uses the GPS carrier signal to achieve an exact lock-on the PRN code. More efficient and accurate than the standard approach.

Circular Error Probable (CEP)

A statistical measure of the horizontal precision. The CEP value is defined as a circle's radius, when centred at the true position, encloses 50% of the data points in a horizontal scatter plot. Thus, half the data points are within a 2-D CEP circle and half are outside the circle.

Class of Survey

Class of Survey is a means of categorising the internal quality, or precision of a survey. The number of categories, the notation applied, and the accuracy tolerances defining the transition from one class to another are defined by individual nations. Typically they are based on traditional geodetic surveying categories, supplemented by several extra categories of higher precision applicable to GPS Surveying and GPS Geodesy techniques, and may be different for horizontal surveys and vertical surveys. The attachment of a particular Class "label" (e.g. A, B, etc.) to a survey, comprising a few or many points within a "network", carried out using GPS or any other technique, is performed as part of the process of "network adjustment" in which the relative error ellipses (in the horizontal case) between coordinated stations are computed and compared with the accuracy standards that must be met for various categories of Class. See Minimally Constrained.

Clock Bias

The difference between the receiver or satellite clock's indicated time and a well-defined time scale reference such as UTC (Coordinated Universal Time), TAI (International Atomic Time) or GPST (GPS Time).

Coarse Acquisition (C/A)

See also C/A-Code. A spread spectrum direct sequence code that is used primarily by commercial GPS receivers to determine the pseudo-range to a transmitting GPS satellite, modulated on the L1 carrier.

Code Phase

GPS measurements based on the C/A-Code. The term is sometimes restricted to the C/A- or P-Code pseudo-range measurement when expressed in units of cycles.

Constellation

Refers to either the specific set of satellites used in calculating a position, or all the satellites visible to a GPS receiver at one time, or the entire ensemble of GPS satellites comprising the Space Segment.

Control Point

Also called a Control Station or Geodetic Control Station. A monumented point to which coordinates have been assigned by the use of terrestrial or satellite surveying techniques. The coordinates may be expressed in terms of a satellite reference coordinate system (such as with respect to WGS84, or to ITRS), or a local geodetic datum.

Control Segment

A world-wide network of GPS monitoring and upload telemetry stations operated by, or on behalf of, the US Department of Defense. The tracking data is used by the Master Control Station at Colorado Springs to calculate the satellites' positions (or "broadcast ephemerides") and their clock biases. These are formatted into the Navigation Message which is uploaded on a daily (perhaps more frequently) basis by the Control Segment stations.

Correlator

The GPS receiver "software" or electronic means, implemented in some fashion (either analogue or digital) within a Tracking Channel, used to shift or compare the incoming signal with an internally generated signal. This operation is performed on the PRN codes, but may be used for more "exotic" mixed signals in the case of L2 measurements, where under the policy of Anti-Spoofing (AS) the L2 PRN code is not known. Correlator design may be influenced such that it is optimised for accuracy, mitigation of multipath, acquisition of signal under foliage, etc.

Course-Made-Good (CMG)

The bearing from your starting point to your present position. Commonly used in marine or air navigation.

Crosstrack Error (XTE)

The distance you are off a desired course in either direction. Commonly used in marine or air navigation.

Cutoff Angle

The minimum acceptable satellite elevation angle (above the horizon) to avoid blockage of line-of-sight, multipath errors or too high Tropospheric or Ionospheric Delay values. May be preset in the receiver, or applied during data post-processing. For navigation receivers may be set as low as 5°, while for GPS Surveying typically a cutoff angle of 15° is used.

Cycle Slip

A discontinuity of an integer number of cycles in the measured (integrated) carrier phase resulting from a temporary loss-of-lock in the carrier tracking loop of a GPS receiver. This corrupts the carrier phase measurement, causing the unknown Ambiguity value to be different after the cycle slip compared with its value before the slip. It must be "repaired" (the unknown number of "missing" cycles determined and the carrier observation subsequent to the cycle slip all corrected by this amount) before the phase data is processed in double-differenced observables for GPS Surveying techniques.