- A -
Acquisition Time
see TTFF
Active antenna
An amplified antenna
Active Leg
The segment of the route currently being traveled.
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.
An imaginary line on the earth's surface connecting points where the magnetic declination is zero.
AGPS
Assisted Global Positioning System - A land station assists GPS in acquiring position.
Almanac Data
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.
Atimeter
An instrument that measures altitude or elevation with respect to a reference level, usually mean sea level, by means of air pressure.
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.)
APRS
Automatic Position Reporting System - An amateur radio based automatic position reporting system
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.
Atimeter
An instrument that measures altitude or elevation with respect to a reference level, usually mean sea level, by means of air pressure.
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.)
APRS
Automatic Position Reporting System - An amateur radio based automatic position reporting system
for tracking and digital communications.
arcGIS
A family of software products that form a complete GIS (Geographic Information System).
Archipelago
A large group of islands.
Comprehensive software in the arcGIS family that has advanced geoprocessing and data conversion capabilities.
Arete
A sharp, narrow mountain ridge or spur.
ARGOS
Advanced Research and Global Observation Satellite - An advanced research and development
satellite launched by the United States Air Force. Carries various space and atmospheric experiments.
A ringlike coral island and reef that nearly or entirely encloses a lagoon.
AttributeA 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).
Automatic Vehicle Location
A system of providing real-time location information for emergency vehicles, delivery trucks, service vehicles, etc.
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.
Azimuth
The horizontal angle (0 - 360 degrees) from a reference point, usually true north or true south. Also known as a bearing.
Azimuth Ring
The dial on a compass, marked with zero to 360 degree markings.
- 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.
Bench mark
A material object, natural or man-made, with a known elevation or horizontal location. Bench marks can be used as reference points when traveling a route or in determining the elevation of nearby land features.
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).
Bitmap image
An image with 1 bit of color information per pixel, also known as a bitmapped image.
The dial on a compass, marked with zero to 360 degree markings.
- 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.
Bench mark
A material object, natural or man-made, with a known elevation or horizontal location. Bench marks can be used as reference points when traveling a route or in determining the elevation of nearby land features.
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).
Bitmap image
An image with 1 bit of color information per pixel, also known as a bitmapped image.
Bluetooth
A standard of radio technology that makes it possible to transmit signals over short distances between telephones, computers and other devices.
Bosk
A small, wooded area.
Bourn
A small stream.
Bourne
Same as bourn.
Bluetooth GPS - See Bluetooth.