• Predictable - The accuracy of a radionavigation system's position solution with respect to the charted solution. Both the position solution and the chart must be based upon the same geodetic datum.
• Repeatable - The accuracy with which a user can return to a position whose coordinates have been measured at a previous time with the same navigation system.
• Relative - The accuracy with which a user can measure position relative to that of another user of the same navigation system at the same time.

The various generations of GPS satellites: Block I were prototype satellites that began being launched in 1978; 24 Block II satellites made up the fully operational GPS constellation declared in 1995; Block IIR are replenishment satellites; and Block IIF refers to the follow-on generation.

C/A code

A radio wave having at least one characteristic, such as frequency, amplitude or phase, that may be varied from a known reference value by modulation.

GPS measurements based on the L1 or L2 carrier signal.

A method of frequency reuse whereby many radios use the same frequency but each one has a unique code. GPS uses CDMA techniques with Gold's codes for their unique cross-correlation properties.

An automated system for processing dispatch business and automating many of the tasks typically performed by a dispatcher. Abbreviated CAD (not to be confused with computer-aided design which is also known as CAD) is application software with numerous features and functions. A basic CAD system provides the integrated capability to process calls for service, fleet management and geographical referencing.

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

The root-mean-square value of the distances from the true location point of the position fixes in a collection of measurements. As typically used in GPS positioning, 2 drms is the radius of a circle that contains at least 95 percent of all possible fixes that can be obtained with a system at any one place.

The introduction of digital noise. This is the process the DoD used to add inaccuracy to GPS signals to induce Selective Availability.

Cartesian coordinate system where the X direction is the intersection of the prime meridian (Greenwich) with the equator. The vectors rotate with the earth. Z is the direction of the spin axis.

That angle below satellites should not be tracked. Normally set to 15 degrees to avoid interference problems caused by buildings and trees and multipath errors.

A list of accurate positions or locations of a celestial object as a function of time. Available as "broadcast ephemeris" or as postprocessed "precise ephemeris."

Measurement interval or data frequency, as in making observations every 15 seconds. "Loading data using 30-second epochs" means loading every other measurement.

The particular equipotential surface that coincides with mean sea level and that may be imagined to extend through the continents. This surface is everywhere perpendicular to the force of gravity.

Geometric Dilution of Precision (GDOP)

Organizing concept of a European system that would incorporate GPS, GLONASS, and other space-based and ground-based segments to support all forms of navigation.

The GPS Interface Control Document is a government document that contains the full technical description of the interface between the satellites and the user.

The word in the GPS message that contains synchronization information for the transfer of tracking from the C/A to the P -code.

The physical components of a computer system. Reference is often made to "hardware" and "software"; in that context, "hardware" consists of the computer, input and output devices and other peripheral equipment.

The ability of a system to provide timely warnings to users when the system should not be used for navigation as a result of errors or failures in the system.

The band of charged particles 80 to 120 miles above the earth's surface, which represent a nonhomogeneous and dispersive medium for radio signals.

A wave propagating through the ionosphere experiences delay. Phase delay depends on electron content and affects carrier signals. Group delay depends on dispersion in the ionosphere as well and affects signal modulation (codes). The phase and group delay are of the same magnitude but opposite sign.

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

The group of radio frequencies extending from 390 MHz to 1550 MHz. The GPS carrier frequencies (1227.6 MHz and 1575.42 MHz) are in the L-band.

A device, typically installed in a vehicle, that consists of a small screen, a keyboard or other operator interface, and various amounts of memory and processing capabilities.

Interference caused by reflected GPS signals arriving at the receiver, typically as a result of nearby structures or other reflective surfaces. Signals traveling longer paths produce higher (erroneous) pseudorange estimates and, consequently, positioning errors.

A modulator/demodulator. When two computers communicate over telephone lines and similar media, digital signals must be converted to analog during transmission, then back again to digital at the destination. Modems are always used in pairs, one at each end. They are rated according to the speed, typically in "bits per second," at which the information can pass through the transmission medium.

A sequence of digital 1's and 0's that appear to be randomly distributed like noise but that can be reproduced exactly. Their most important property is a low autocorrelation value for all delays or lags except when they coincide exactly. Each GPS satellite has unique C/A and P pseudorandom-noise codes.

The determination of position, or the obtaining of information relative to position, for the purpose of navigation by means of the propagation properties of radio waves. GPS is a method of radionavigation.

The rate of change between the satellite and receiver. The range to a satellite changes due to satellite and observer motions. Range rate is determined by measuring the Doppler shift of the satellite beacon carrier.

Receiver INdependent EXchange format A set of standard definitions and formats that permits interchangeable use of GPS data from dissimilar GPS receiver models or postprocessing software. The format includes definitions for time, phase, and range.

A DoD program that controls the accuracy of pseudorange measurements, degrading the signal available to nonqualified receivers by dithering the time and ephemerides data provided in the navigation message.

The portion of the GPS system that is located in space, that is, the GPS satellites and any ancillary spacecraft that provide GPS augmentation information (i.e., differential corrections, integrity messages, etc.)

Location determination accomplished with a stationary receiver. This allows the use of various averaging or differential techniques.

Satellite vehicle or space vehicle

A consistent set of parameters describing the size and shape of the Earth, the positions of a network of points with respect to the center of mass of the Earth, transformations from major geodetic datums, and the potential of the Earth (usually in terms of harmonic coefficients).

The mathematical ellipsoid used by GPS since January, 1987.