Glossary

Base Station – the stationary receiver used in DGPS to measure errors in GPS signals for use in differential correction.

Clock Bias – difference between true universal time and your clock.

Control Segment – the group of monitoring and command stations that constantly check and correct the space segment (satellites) of the GPS system.

Data Logger – hardware the interprets and stores information collected during fieldwork with any GPS receiver.

Differential Correction – the process of comparing two GPS files in order to eliminate errors from the second (rover) file.

Differential Global Positioning System (DGPS) – using a mobile (rover) and a stationary (base station) receiver to provide raw data for differential correction.

Dilution of Precision (DOP) – measure of uncertainty in satellite signals, the lower the reading the better.

Dithering – the introduction of noise into a signal, used by the control segment to induce selective availability.

Doppler Shift – apparent change in frequency due to motion in either transmitter and/or receiver.

Elevation – height, either measured as position above mean sea level (average height of sea level) or above ellipsoid.

Ellipsoid – interchangeable with spheroid, a symmetrical representation of the Earth’s surface.

Ephemeris – in GPS, refers to the positions of the satellites in their orbits.

Frequency – number of sine waves that pass a single point in one unit of time.

Geodetic Datum – model used to best represent a portion of the Earth’s surface.

Geodosy – the science of determining the actual shape of the Earth’s surface, vital to using GPS accurately.

GPS Addressing – the set of coordinates measured with a GPS unit used to measure the position on the Earth.

Horizontal Dilution of Precision (HDOP) – measure of uncertainty in readings relating to only the horizontal (x and y coordinates) measurements, usually more accurate than readings relating to three dimensions (x, y, & z coordinates).

Intentional Errors – errors induced by the control segment such as Selective Availability.

Kinematic Positioning – determining a position while in motion.

LORAN – long range ground-based navigation system with accuracy of .25 nautical miles at best.

Mask Angle – the use-defined angle below which satellite signals are ignored.

Mission Planning – first step in GPS project, involves creating a data dictionary, checking satellite availability and receivers (rover and base station).

Multi-Channel Receiver – a GPS unit which can track more than one satellite.

Multi-Path Errors – errors caused when satellite signals are bounced off objects on the landscape (such as buildings) and interpreted by a receiver as real signals.

Nanosecond (ns) – one billionth of a second.

Nautical Mile – equal to 1.156 land-miles or 1.853 km.

NAVSTAR – the constellation of satellites that make-up the GPS space segment.

OMEGA – ground-based navigation system using low-frequency signals, accuracy of 3-6 miles only.

Phases of a GPS Project – logical order of steps to begin and successfully complete any GPS project; Mission Planning, Data Collection, Data Transfer, Data Correction and Application.

Position Dilution of Precision (PDOP) – the most commonly used DOP value, measures uncertainty in three dimensions.

Psuedo-Random Code – a signal that appears as random noise, key component in satellite ranging.

Random Errors – errors created at irregular intervals, types include errors from satellites, multiple paths and the atmosphere.

Real-Time Correction – the process by which a differential signals are applied in real-time while taking measurement.

Rover – the mobile receiver unit used to collect measurements for differential correction.

Satellite Constellation – the arrangement of satellites in space.

Satellite Ranging – the process of using radio waves, highly accurate atomic clocks and trigonometry to determine the distance between receiver and satellites.

Selective Availability – introduced noise in the satellite signals intended to disrupt accuracy in order to control who successfully uses GPS signals, controlled by the US military.

Space Segment – the network of satellites that make up the GPS system, responsible for broadcasting low power signals for use by receivers on the ground to calculate positions.

TRANSIT – first space-based navigation system, initially used for shipping, not as accurate as GPS.

User Segment – the group of individuals and groups that make use of GPS for measuring features on the surface of the planet (include recreational users, surveyors, military, etc.).

Vertical Dilution of Precision (VDOP) – the measurement of uncertainty in the vertical (z-axis) dimension.