Fastrax GPS Dictionary



GPS and Fastrax Related Acronyms

Acronym

Meaning
A/D Analog to Digital
ADC Analog to Digital Converter
AFC Automatic Frequency Control
API Application Programming Interface (Software)
ASIC Application Specific Integrated Circuit
AVL(N) Automatic Vehicle location (and Navigation)
BIT A digital 0 or 1
BOM Bill of Materials
BPSK Bi Phase Shift Keying
BYTE Sequence of 8 bits
C/A code Coarse/Acquisition Code
CDMA Code Division Multiplex Access
CEP Circular Error Probable
CMOS Complementary Metal Oxide Semiconductor
CNR, C/No Carrier to Noise Ratio, also Signal to noise ratio, SNR, S/No
CPM Carrier phase measurement
CSOC Consolidated Space Operations Center
D/A Digital to Analog
DAC Digital to Analog Converter
dB Decibel
dBm Decibel referenced to mW
dBW Decibel referenced to W
DGPS Differential GPS
DoD
United States Department of Defense
DOP Dilution of Precision
DOPPLER A Doppler Shift. A frequency shift due to radial velocity.
DRMS Distance Root Mean Square
DSP Digital Signal Processor
EGNOS European Geostationary Navigation Overlay Service
Ephemeris Commonly used name for Satellite orbital parameters
FOM Figure Of Merit
Galileo A new European GNSS system
GDOP Geometric Dilution of Precision
GLONASS A Russian GNSS System
GMT Greenwich Mean Time
GNSS Global Navigation Satellite System
GPIO General purpose I/O interface
GPS Global Positioning System
HDOP Horizontal Dilution of Precision
HOW Handover Word
IMU Inertial Measurement Unit
INS Inertial Navigation System
I/O Input/Output
ION Institute of Navigation
IP Intellectual Property (or Internet Protocol)
iSuite
iSuite Software
iTalk Binary message format that is part of iSuite Software.
L1 GPS primary frequency, 1575.42 MHz
L2 GPS secondary frequency, 1227.6 MHz
L5 New GPS carrier frequency
LAT Latitude
LEP Linear Error Probable
LHCP Left Hand Circular Polarization
LNA Low-noise amplifier
LO Local Oscillator
LON Longitude
LOS Line of sight
mB Millibar
MB Megabyte, 1048576 bytes
MBit Megabit, 1048576 bits
M-code New military GPS code
MCS Master Control Station
MHz Megahertz (1000000 Hz)
MIPS Million instructions per second
MOPS Million Operations per second
m/s Metres per Second
ms millisecond
MSL Mean Sea Level
MTBF Mean Time Between Failure
MTBM Mean Time Between Maintenance
NANU Notice Advisory to NAVSTAR Users
NAVSTAR Navigation System with Timing and Ranging
NCO Numerical controlled oscillator
NMEA National Marine Electronics Association
NMEA 0183 Navigation message format developed by NMEA
ns Nanosecond
OS Operating system (Computer)
PC Personal Computer
PCB Printed Circuit Board
P-Code Precise Code
PDOP Position Dilution of Precision
PLL Phase locked loop
PM Pulse measurement
PPB Parts per billion
PPM Parts per million
PPS Pulse per second
PRC Pseudorange correction
PRN Pseudo Random Noise
PVT Position Velocity and Time
P(Y) Code See P-code, Y-code
RAIM Receiver Autonomous Integrity Monitoring
RISC Reduced Instruction Set Computer
RCVR Receiver
RF Radio Frequency
RHCP Right hand circularly polarized 
RINEX Receiver Independent Navigation Exchange
RMS Root Mean Square
RTOS Real time operating system
RSS Root Sum Square
RTCM Ratio Technical Commission for Maritime Services
RX receiver or receiving input
SA, S/A Selective Availability
SBAS Satellites based augmentation system
SDK Software Development Kit
SE Search engine
SEP Spherical Error Probable
SI International System of Units
SNR Signal to Noise Ratio
SPS Standard Positioning Service
SV Space vehicle
TBA
To be added
TBD
To be defined
TBR
To be removed
TDOP Time Dilution of Precision
TLM Telemetry
TOA Time Of Arrival
TTFF Time to First Fix
TX transmitter or transmitting output
UART Universal Asynchronous Receiver / Transmitter
UHF Ultra High Frequency
us, µs Microsecond
UTC Universal Time Coordinated
VCO Voltage controlled oscillator
VDOP Vertical Dilution of Precision
VHSIC Very High Speed Integrated Circuit
VLSIC Very Large Scale Integrated Circuit
W-Code Encryption algorithm for P code
WAAS Wide area augmentation system
WGS World Geodetic System
WGS 84 World Geodetic System 1984
Y Code The encrypted version of the P code.
1 PPS 1 Pulse Per Second
2dRMS 2xDRMS


GPS Terminology Explained


Absolute Positioning

Positioning mode in which a position is identified with respect to a Coordinate system. See also Relative Positioning.

Accuracy

The degree of conformance between the estimated or measured position, time, and/or velocity of a GPS receiver and its true time, position, and/or velocity as compared with a known reference.

Almanac

A data file that contains orbit information on all satellites, clock corrections, and atmospheric delay parameters. It is transmitted by each GPS satellite in order to facilitate rapid satellite vehicle acquisition by GPS receivers.

Ambiguity

The unknown number of code sequences between the between a GPS satellite and the receiver.

Analog

A type of transmission characterized by variable waveforms representing information.  See Digital.

Analog to Digital (A/D)

Process of converting information from analog to digital format.

Anti-Spoofing

Anti-spoofing (A-S) is the process whereby the P code used for the precise positioning service (PPS) is encrypted. The resulting encrypted code is known as the Y code. This encrypted code can be used only by receivers with decryption capability.

Anywhere Fix

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

Apogee

The point in the orbit of a satellite is the greatest distance from the center of the earth.

Application Software

These programs accomplish the specialized tasks of the user, while operating system software allows the computer to work. A computer-aided dispatch system is application software, as is each word processing program.

Application Programming Interface (Software)

A software interface that Application Software can call to use features provided by a software component, library or service.

Application Specific Integrated Circuit (ASIC)

Automatic frequency control

A device and/or software that maintains the frequency of an oscillator within the specified limits with respect to a reference frequency.

Automatic Vehicle Location (AVL)

A type of system using any sort of technology to track or locate a vehicle.

Availability

The percentage of time that the services of a navigation system can be used within a particular coverage area. Signal availability is the percentage of time that navigational signals transmitted from external sources are available for use. Availability is a function of both the physical characteristics of the operational environment and the technical capabilities of the transmitter facilities.

Bandwidth

The range of frequencies in a signal.

Baseline

A baseline consists of the resultant three-dimensional vector between a pair of stations for which simultaneous GPS data is collected.

Base Station

A base station is a GPS receiver set up on a location with known position specifically to collect data for differential corrections.

Bi-Phase Shift Keying (modulation)

Bi-Phase Shift Keying (BPSK) allows the transmission of one bit of digital information in both states 0 and 1 by shifting the phase of the carrier from 0° (reference) to 180°.

Block I, II, IIR, IIF Satellites

Generations of GPS satellites. Block I were prototype satellites that began launch in 1978; Block II satellites were used to populate the 24 operation slots in the NAVSTAR constellation; Block IIR are replacement satellites; and Block IIF are for system modernization.

Coarse/Acquisition Code (C/A Code)

The coarse/acquisition code is modulated onto the GPS L1 signal. This code is a sequence of 1023 pseudorandom binary bi-phase modulations on the GPS carrier at a chipping rate of 1.023 MHz, thus having a code repetition period of 1 millisecond. The code was selected to provide good acquisition properties. Also known as the "civilian code."

Carrier

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

Carrier Frequency

The frequency of the unmodulated fundamental output of a radio transmitter. The GPS L1 carrier frequency  is 1575.42 MHz.

Carrier Phase Measurements

GPS measurements based on the L1 or L2 carrier signal.

Cartesian/Geocentric Coordinates

A system of defining position which has its origin at the center of the earth with the x- and y-axes in the plane of the equator. See ECEF.

Channel

A channel of a GPS receiver consists of the circuitry necessary to receive the signal from a single GPS satellite.

Chip

1) The length of time to transmit either a "0" or a "1" in a binary code.
2) An integrated circuit. 

Chip rate  (Chipping rate)

Number of chips per second. C/A code chip rate = 1.023 MHz.

Circular Error Probable - CEP

The radius of the circle containing 50 percent of the individual measurements being made, or the radius of the circle within which there is a 50 percent probability of being located.

Clock bias

The difference between the clock's indicated time and true time (UTC).

Clock Offset

Time difference between two clocks (systems).

Code Division Multiple Access (CDMA)

A method of frequency reuse whereby many radios use the same frequency but each one has a unique code. For example GPS and CDMA phones. See also TDMA.

Code Phase GPS

GPS measurements based on the pseudo random code (C/A or P) as opposed to the carrier of that code.

Control Segment

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

Coordinate System

One of a number of mapping systems used to represent positions. Examples are geographical coordinates (latitude and longitude) and ECEF.

Data Message

A message included in the GPS signal which reports the satellite's location, clock corrections and health. Included is rough information about the other satellites in the constellation. Ephemeris, Almanac, Ionospheric Correction.

Datum

A  horizontal or vertical reference system for making measurements and computations for position referencing. Horizontal datum is for calculating location coordinates. Vertical datum is the zero surface to which elevations or heights are referred to. Examples of horizontal datums are ellipsoids (WGS-84), the State Plane Coordinate System and the Universal Transverse Mercator System. Vertical datums are typically referred to as the geoid (mean sea level).

Decibel

Decibel is widely used in engineering. A decibel is a ratio between two signal powers and this ratio is expressed as ten times the logarithm of the number, that is X = 10 Log10 X dB.

United States Department of Defense (DoD)

Differential GPS  (DGPS)

A technique used to improve positioning or navigation accuracy by determining the positioning error at a known location and subsequently incorporating a differential correction factor (by real-time transmission of corrections or by postprocessing) into the position calculations of another receiver operating in the same area and simultaneously tracking the same satellites. See Base Station, Differential Correction.

Differential Correction

Requires the use of another GPS receiver set up on a position with known location (Base Station). The receiver on the known location computes its location with the GPS satellite data and compares this position with the known value for its actual, known, position. This difference (hence differential) is the error in the transmitted GPS signal. The differential value is then used for correcting, either in real-time or during post processing, the positions collected by other GPS receivers during the same time period, observing the same satellites.

Digital

Information transmission or storage technology that represents all data as series of 1´s and 0´s. See Analog.

Dilution of Precision - DOP

DOP is a function expressing the mathematical quality of solutions based on the geometry of the satellites.
GDOP: Geometric Dilution of Precision (3 position coordinates plus time offset)
PDOP: Position Dilution of Precision (3 coordinates)
HDOP: Horizontal (2 horizontal coordinates)
VDOP: Vertical Dilution of Precision (height only).
TDOP: Time Dilution of Precision

Distance Root Mean Square (DRMS)

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. 

Dithering

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

Doppler-Aiding

A signal acquisition strategy predicted doppler shift to help the receiver. 

Doppler Shift

The apparent change in the frequency of a signal caused by the relative motion of the transmitter and receiver. 

Earth-Centered Earth-Fixed (ECEF)

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. 

Eccentricity

The ratio of the distance from the center of an ellipse to its foci on the semimajor axis. 

European Geostationary Navigation Overlay Service (EGNOS)

A joint project of the European Space Agency (ESA), the European Commission (EC) and Eurocontrol, the European Organisation for the Safety of Air Navigation. EGNOS satellites will provide additional data to GPS receivers improving accuracy and reliability to make them suitable for mission critical applications. Planned to be operational in 2004. 

Elevation

The height above a point of reference. For Example, height above mean sea level, or vertical distance above the geoid. 

Elevation Mask

User definable angle below which a satellite will be tracked, but not used for navigation. This value is set to avoid problems caused by ionospheric refraction, multipath errors , blockage such as buildings and foliage. 

Ellipsoid

A mathematical figure formed by revolving an ellipse about its minor axis 

Ellipsoid Height

The measure of vertical distance above the ellipsoid. Not the same as elevation above sea level. 

Ephemeris

The predictions of current satellite positions transmitted to the user in the data message. 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." 

Epoch

GPS Time + GPS week. Time of observation. 

Frequency Band

A particular range of frequencies in a region of the electromagnetic spectrum. 

Frequency Spectrum

The distribution of signal amplitudes as a function of frequency of the constituent signal waves. 

Geodetic

Geodesy related. 

Geodesy

The science related to the determination of the size and shape of the Earth (geoid) by direct measurements. 

Geodetic Datum

A mathematical model designed to best fit part or all of the geoid. It is defined by an ellipsoid and the relationship between the ellipsoid and a point on the topographic surface established as the origin of datum. 

Geodetic Surveys

Global surveys done to establish control networks (comprised of reference or control points) as a basis for accurate land mapping. 

Geoid

An equipotential surface approximating earth's surface and corresponding to mean sea level. 

Geoid Height

Height difference between geoid and the reference ellipsoid. 

Global Navigation Satellite System (GLONASS)

The Russian equivalent to the American NAVSTAR GPS. GLONASS was designed to provide worldwide coverage. GLONASS has three orbital planes containing eight satellites each, when all slots are filled. GLONASS satellites are failing at a much higher rate than they are being replenished, leaving the system operating a diminishing capacity. 

Global Navigation Satellite System (GNSS)

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

Global Positioning System - GPS

A DOD conceived, implemented and operated constellation of 28 satellites orbiting the earth at a nominal altitude of 20000 km. GPS satellites transmit signals that allow the accurate determination of GPS receiver locations. Receivers can be located at fixed locations, moving on the earth's surface, in the earth's atmosphere, or in low-Earth orbit. GPS is used in air, land and sea navigation, mapping, surveying and other applications where precise positioning is necessary. The GPS signal is provided free of charge to anyone on or near the planet with a GPS receiver and an unobstructed view of the satellites. 

GPS Core

In iSuite Architecture: Software that is responsible of acquiring the satellite signals, tracking the satellites to produce pseudorange observations. iCore.

Greenwich Mean Time (GMT)

Measured from the Greenwich Meridian Line at the Royal Observatory in Greenwich. The Coordinated Universal Time (UTC) has replaced GMT as a reference international time. GMT time zone is currently defined as UTC+0 hours with no daylight saving adjustment. GMT has only historical interest and UTC should be used instead in all technical documentation. 

Handover Word

Second word of each subframe in the Data Message. Used by P/Y code receivers. It contains synchronization information for the transfer of tracking from the C/A to the P-code. 

Integrity

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.

Ionosphere

The band of charged particles between the stratosphere and the exosphere 50 to 250 miles above the earth's surface which represent a nonhomogeneous and dispersive medium for radio signals. 

Ionospheric Corrections

Corrections calculated based on information contained within Navigation Message to correct Ionospheric Delay. 

Ionospheric Delay

A radio wave propagating through the ionosphere experiences delay that depends on current atmospheric conditions. 

iSuite Software

GPS Application Architecture and Software Platform developed by Fastrax Ltd.

iTalk Protocol

Communication protocol developed for iSuite Software. 

Kalman Filter

A numerical method used to track a time-varying signal in the presence of noise. Developed In 1960 by Rudolph E. Kalman. In his paper he described a recursive solution to the discrete-data linear filtering problem. For his original paper, see for example http://www.cs.unc.edu/~welch/kalman/kalmanPaper.html.

L-Band

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.

L1 Signal

The primary L-band signal transmitted by each GPS satellite at 1575.42 MHz. The L1 broadcast is modulated with the C/A and P-codes and with the navigation message. 

L2 Signal

The second L-band signal is centered at 1227.60 MHz and carries the P-code and navigation message. 

L5 Signal

New civil signal at 1176MHz. The L5 signal will be implemented on the modified Block IIF satellites.. 

Master Control Station

Master station of Control Segment. Se also Monitor Stations. 

Monitor Stations

One of the worldwide group of stations used in the GPS control segment to track satellite clock and orbital parameters. Data collected at monitor stations are linked to a master control station at which corrections are calculated and from which correction data is uploaded to the satellites as needed. 

Multipath

Errors caused by the interference of a signal that has reached the receiver antenna by two or more different paths. Usually caused by one path being bounced or reflected from nearby structures or other reflective surfaces. Signals traveling longer paths produce higher (erroneous) pseudorange estimates and, consequently, positioning errors. 

Navigation Engine

Software that is responsible of calculating receiver position based on pseudorange observations and other GPS information (Navigation Message, Data Message). 

Navigation Message

The 1500-bit navigation message broadcast by each GPS satellite at 50 bps on the L1 and/or L2 signals. This message contains system time, clock correction parameters, ionospheric delay model parameters, and the vehicle's ephemeris and health. The information is used to process GPS signals to give user time, position, and velocity.

Not to be confused with e.g. iTalk navigation message. 

Navigation System with Timing and Ranging (NAVSTAR GPS)

The name given to the American GPS satellite constellation. The fully operational constellation is composed of six orbital planes, each containing four satellites. At present, a total of 27 healthy satellites are aloft in the NAVSTAR system; the full complement of 24 and 3 additional satellites to be used as spares. Follow-on satellites are constructed and awaiting launch as needed or desired. 

Notice Advisory to NAVSTAR Users

A periodic bulletin alerting users to changes in system performance.

Observation

Pseudo range measurement.

P-code

The precise or precision code of the GPS signal, typically used alone by U.S. and allied military receivers. A very long sequence of pseudo-random binary biphase modulations on the GPS carrier at a chip rate of 10.23 MHz which repeats about every 267 days. Each one-week segment of this code is unique to one GPS satellite and is reset each week. P-code is not directly used but is first encrypted into Y-code. Access to P-code is only possible if the secret conversion algorithm is known. See W-Code.

PDOP - Position Dilution of Precision

A figure of merit (used without units) expressing the relationship between the error in user position and the error in satellite position, which is a function of the configuration of satellites from which signals are derived in positioning (see DOP). Geometrically, PDOP is proportional to 1 divided by the volume of the pyramid formed by lines running from the receiver to four observed satellites. Small values, such as "3", are good for positioning while higher values produce less accurate position solutions. Small PDOP is associated with widely separated satellites.

PDOP Mask

Usually a user-definable upper limit for the PDOP you will tolerate during collection of a dataset. If PDOP rises above the pre-set limit, GPS data collection will be suspended until the PDOP decreases below the limit.

Perigee

The point in the orbit of a satellite that is the least distant from the center of the earth.

Phase Lock

The technique whereby the phase of an oscillator signal is made to follow exactly the phase of a reference signal. The receiver first compares the phases of the two signals, then uses the resulting phase difference signal to adjust the reference oscillator frequency. This eliminates phase difference when the two signals are next compared.

Polarization

Polarization is a property of light and other electromagnetic waves like radiowaves. Electromagnetic waves can be linearly, spherically or elipcally polarized.  Circularly polarized wave can be either right or left hand circular polarized (RHCP, LHCP).  Polarization is an important concideration when selecting GPS antennas for a specific application.

Point Positioning

A geographic position produced from one receiver in a standalone mode.

Position

The latitude, longitude, and altitude of a point. An estimate of error is often associated with a position.

Post-Processed Differential GPS

Using post-processed differential GPS data collection techniques, base and roving receivers have no data link between them. Each receiver independently records data that will allow differential correction when processed with data from other receivers collecting during the same time period. During collection, the same set of satellites must be in view of all receivers collecting data for later combined processing. Differential correction software is used to combine and process the data collected. See also DGPS.

Precise (or Protected) Code (P-Code)

A sequence of pseudo-random binary biphase modulations on the GPS carrier at a chip rate of 10.23 MHz which repeats every 267 days. Each 1-week segment of code is unique to a particular satellite and is generally reset each week.

Precise Positioning Service (PPS)

The highest level of military dynamic positioning accuracy provided by GPS, using the dual-frequency P-code.

Proportional error

One means of expressing positional accuracy, expressed as the position error divided by the distance to the origin of the coordinate system used, stated in parts per million (ppm).

Pseudolite

A pseudo satellite. A ground-based transmitter that mimics a GPS satellite. Intended to improve geometric solutions in a local area. The data portion of the signal may also contain differential corrections that can be used by receivers to correct for GPS errors.

PRN - Pseudorandom Noise

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.

Pseudorange

A distance measurement, based on the correlation of a satellite-transmitted code and the local receiver's reference code, that has not been corrected for errors in synchronization between the transmitter's clock and the receiver's clock. Pseudorange observation range from 0...300 m and still contain the the integer ambiguity which has to resolved by the navigation engine.

Range

A fixed distance between two points, such as between a starting and an ending waypoint or a satellite and a GPS receiver. Range corresponds roughly (without error corrections) to N*(C/A Code Lenght) + Pseudorange where N is the integer ambiguity.

Range Rate

The rate of change between a 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.

Radio Technical Commission for Maritime services (RTCM)

RTCM is a standard format for DGPS data transmission.

Real-Time Differential GPS

A data collection process whereby a GPS receiver gets real-time correction data from another source in order to filter out the effects of SA and other sources of error. One way to get correction data is from a GPS receiver located at a known position (known as a base station). The GPS receiver at the known position computes, formats, and transmits corrections usually through a data link (e.g., VHF radio or cellular telephone) with each new GPS observation. The roving unit requires some sort of data link equipment to receive the GPS corrections so that the correction factor can be applied to its current observations. Other sources of correction data are Satellite-based systems (like OMNISTAR), CORS stations, etc.

Relative Positioning

The process of determining the relative difference in position between two locations, in the case of GPS, by placing a receiver over each site and making simultaneous measurements observing the same set of satellites at the same time. This technique allows the receiver to cancel errors that are common to both receivers, such as satellite clock and ephemeris errors and propagation delays.

Receiver Autonomous Integrity Monitoring (RAIM)

A GPS receiver system that allows the receiver to detect incorrect signals being transmitted by the satellites by comparing solutions with different sets of satellites.

Receiver Sensitivity

Lowest signal that can be used by the receiver (Acquisition, tracking and navigation sensitivity)

Reference Ellipsoid

A mathematical approximation of Geoid using an Ellipsoid.

Receiver Independent Exchange format (RINEX)

Receiver Independent Exchange format. A set of standard definitions and formats that permits interchangeable use of GPS data from dissimilar GPS receiver models or post processing software.

Roving receiver

A moving receiver.

Satellite Configuration

The state of the satellite constellation at a specific time, relative to a specific receiver.

Satellite Constellation

Satellite system setup in space. GPS constellation consists currently of 29 satellites at ~20180 km height at six orbital planes with 4 satellites on each. At least 4 satellites are visible at any time anywhere.

Selective Availability - SA

A US Department of Defense program that was used to control the accuracy of pseudorange measurements, degrading the signal available to nonqualified receivers by dithering the time and ephemerides data provided in the navigation message. SA was discontinued May 1st 2000.

Signal to Noise Ratio (SNR,S/No, CNR, C/No)

Also, Carrier to Noise Ratio. A measure of the information content of the signal relative to the signal's noise. Higher SNR means better signal.

Space Segment

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.)

Spread Spectrum

The received GPS signal is wide-bandwidth and low-power (-160 dBW). The L-band signal is modulated with a PRN code to spread the signal energy over a much wider bandwidth than the signal information bandwidth. This provides the ability to receive all satellites unambiguously and to give some resistance to noise and multipath.

Space vehicle

A satellite.

Spherical Error Probable  (SEP)

The radius of a sphere within which there is a 50 percent probability of locating a point or being located. SEP is the three-dimensional analogue of CEP.

Standard Deviation (Sigma)

A measure of the dispersion of random errors about the mean value. If a large number of measurements or observations of the same quantity are made, the standard deviation is the square root of the sum of the squares of deviations from the mean value divided by the number of observations less one.
 

Standard Positioning Service (SPS)

SPS is a positioning and timing service provided on the GPS L1 frequency. The GPS L1 frequency, transmitted by all GPS satellites, contains a coarse/acquisition (C/A) code.

SPS is the positioning service used by commercial GPS receivers.

Static Positioning

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

Time Division Multiple Access (TDMA)

Time division multiple access is digital transmission technology that allows a number of users to access a single radio-frequency (RF) channel without interference by allocating unique time slots to each user within each channel. For example GSM phones. See CDMA.
 

Triangulation

In radio technolgy: Process by witch the location of a radio transmitter can be determined by either measuring the the direction of the received signal from two or more different points.
 

Tropospheric Corrections

Corrections applied to correct Tropospheric Delay.

Tropospheric Delay

Time delay in light propagation introduced by Earths troposphere.

Universal Time Coordinated (UTC)

An international, highly accurate and stable uniform atomic time system kept very close, by offsets, to the universal time corrected for seasonal variations in the earth's rotation rate. Maintained by the U.S. Naval Observatory. GPS time is directly convertible to UTC.

User Segment

The portion of the GPS with major components that can be directly interfaced by the user (E.g., GPS receivers)

W-Code

Code that is used to encrypt Y-Code to produce P-code.

Wide area argumentation system (WAAS)

A service being developed by the Federal Aviation administration (FAA) to provide wide area DGPS and integrity information to aviation users. This data can be used to improve the accuracy and reliability of a GPS receiver. See also EGNOS.

World Geodetic System

System that describes the size and shape of the Earth (reference ellipsoid) together with coordinate system and geodetic datum transformations.

 WGS-84 (World Geodetic System 1984)

World Geodetic system of 1984 reference ellipsoid. Used by GPS since January, 1987.

 Y-code

The encrypted version of the P-code. See also W-Code.

 

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