|| 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 constant standard. Radionavigation system accuracy
is usually presented as a statistical measure of system error and
is characterized as follows:
- 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
- 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.
||A type of transmission
characterized by variable waveforms representing information, contrasted
with digital. A standard clock with moving hands is an analog device,
whereas a clock with displayed and changing numbers is a digital
device. The human voice and audible sounds are analog. Modern computers
are invariably digital, but when they communicate over telephone
lines, their signals must be converted to analog using a modem ( a
modulator/demodulator). The analog signal is converted back into
a digital form before delivering it to a destination computer.
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.
Vehicle Location - AVL
||A type of system
using any sort of technology to track or locate a vehicle.
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.
||The range of
frequencies in a signal.
II, IIR, IIF satellites
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
or clear/acquisition code modulated onto the GPS L1 signal. This
code is a sequence of 1023 pseudorandom binary biphase 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."
wave having at least one characteristic, such as frequency, amplitude
or phase, that may be varied from a known reference value by modulation.
||A signal processing
strategy that uses the GPS carrier signal to achieve an exact lock
on the pseudorandom code.
of the unmodulated fundamental output of a radio transmitter. The
GPS L1 carrier frequency is 1575.42 MHz.
measurements based on the L1 or L2 carrier signal.
||- see code
division multiple access
||A channel of
a GPS receiver consists of the circuitry necessary to receive the
signal from a single GPS satellite.
of time to transmit either a "0" or a "1" in a binary pulse code.
Also, an integrated circuit.
||Number of chips
per second. For example, C/A code = 1.023 MHz.
||In a circular
normal distribution, 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.
||- see C/A code.
between the clock's indicated time and true universal time.
in the time reading between two clocks.
multiple access (CDMA)
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.
based on the C/A code.
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.
network of GPS monitor and control stations that ensure the accuracy
of satellite positions and their clocks.
in the measured carrier beat phase resulting from a temporary loss-of-lock
in the carrier tracking loop of a GPS receiver.
||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.
|DGPS - see
positioning - DGPS A technique used to improve positioning or navigation
accuracy by determining the positioning error at a known location
and subsequently incorporating a corrective 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.
information is expressed, stored and transmitted by either analog or digital
means. In a digital form, this information is seen in a binary state
as either a one or a zero, a plus or a minus. The computer uses digital
technology for most actions.
Precision - DOP
of the purely geometrical contribution to the uncertainty in a position
fix. Standard terms for the GPS application are: GDOP: Geometric
( 3 position coordinates plus clock offset in the solution) PDOP: Position
(3 coordinates) HDOP: Horizontal (2 horizontal coordinates) VDOP:
Vertical (height only) TDOP: Time (clock offset only) RDOP: Relative
(normalized to 60 seconds)
mean square (drms)
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.
introduction of digital noise. This is the process the DoD used
to add inaccuracy to GPS signals to induce Selective Availability.
||- see dilution
||A signal processing
strategy that uses a measured doppler shift to help the receiver
smoothly track the GPS signal. Allows more precise velocity and position
change in the frequency of a signal caused by the relative motion
of the transmitter and receiver.
earth-fixed - ECEF
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.
||- see earth-centered
mean sea level. Vertical distance above the geoid.
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 mathematical figure formed by revolving an ellipse about its minor
axis. It is often used interchangeably with spheroid. Two quantities
define an ellipsoid, the length of the semimajor axis, a, and the flattening,
f = (a - b)/a, where b is the length of the semiminor axis. Prolate
and triaxial ellipsoids are always described as such.
of vertical distance above the ellipsoid. Not the same as elevation
above sea level. GPS receivers output position fix height in the WGS-84
of accurate positions or locations of a celestial object as a function
of time. Available as "broadcast ephemeris" or as postprocessed
interval or data frequency, as in making observations every 15
seconds. "Loading data using 30-second epochs" means loading every
||- see Fast-switching
||A single channel
which rapidly samples a number of satellite ranges. "Fast" means
that the switching time is sufficiently fast (2 to 5 milliseconds)
to recover the data message.
range of frequencies.
of signal amplitudes as a function of frequency.
related to the determination of the size and shape of the Earth (geoid)
by direct measurements.
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.
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.
above the geoid is often called elevation above mean sea level.
Dilution of Precision (GDOP)
|GNSS - Global
Navigation Satellite System
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 U.S. Department
of Defense Global Positioning System: A constellation of 24 satellites
orbiting the earth at a very high altitude. GPS satellites transmit
signals that allow one to determine, with great accuracy, the locations
of GPS receivers. The receivers can be fixed on the Earth, in moving
vehicles, aircraft, or in low-Earth orbiting satellites. GPS is used
in air, land and sea navigation, mapping, surveying and other applications
where precise positioning is necessary.
GPS Interface Control Document is a government document that contains
the full technical description of the interface between the satellites
and the user.
word in the GPS message that contains synchronization information
for the transfer of tracking from the C/A to the P -code.
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.
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.
||A shared boundary
between various systems or programs. An interface is also the equipment
or device that makes it possible to interoperate two systems. For
example, it is common to interface the 911 telephone system with a computer-aided
dispatch (CAD) system. Both hardware and software are needed to provide
band of charged particles 80 to 120 miles above the earth's surface,
which represent a nonhomogeneous and dispersive medium for radio
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.
change in the propagation speed of a signal as it passes through
method used to track a time-varying signal in the presence of noise.
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 signal transmitted by each GPS satellite at 1572.42 MHz. The
L1 broadcast is modulated with the C/A and P-codes and with the navigation
L-band signal is centered at 1227.60 MHz and carries the P-code and
|MDT - Mobile
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.
||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.
containing multiple independent channels, each of which tracks one
satellite continuously, so that position solutions are derived from
simultaneous calculations of pseudoranges.
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.
channel through which a series of signals from different satellites
can be sequenced.
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.
Datum, 1983. Based on GRS-80 ellipsoid
Datum, 1927, Based on Clark's Spheroid of 1866
of a second.
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.
||National Marine Electronics
Association interface standard communications protocol that permits different types
of electronic equipment to communicate. more...
of time over which GPS data is collected simultaneously by two or
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.
|PDOP - Position
dilution of precision
figure of merit 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.
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.
position produced from one receiver in a standalone mode.
level of military dynamic positioning accuracy provided by GPS, using
the dual-frequency P-code.
(shortened form of pseudo-satellite)
differential GPS receiver that simulates the signal of a GPS satellite
and can be used for ranging. The data portion of the signal may also
contain differential corrections that can be used by receivers to correct
for GPS errors.
|PRN - Pseudorandom
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.
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
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.
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
similar to relative positioning, except that one or both of the points
may be moving. A data link is used to relay error terms to the moving
vessel or aircraft to improve real-time navigation.
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, propagation delays, and so forth.
of performing a specified function without failure under given conditions
for a specified period of time.
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.
||- see selective
in space of a set of satellites. In the case of GPS, the fully operational
constellation is composed of six orbital planes, each containing
four satellites. GLONASS has three orbital planes containing eight satellites
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.
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,
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.
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.
||- see standard
||A GPS receiver
channel that multiplies the received signal by itself to obtain a
second harmonic of the carriers that does not contain the code modulation.
Used in "codeless" receiver channels.
||A measure of
the dispersion of random errors about the mean value. If a large
numberof 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
civilian positioning accuracy obtained by using the single frequency
C/A code. Under selective availability conditions, guaranteed to be no
worse than 100 meters 95 percent of the time (2 drms).
determination accomplished with a stationary receiver. This allows
the use of various averaging or differential techniques.
vehicle or space vehicle
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 relatable to UTC: UTC-GPS = seconds. (The changing
constant = 5 seconds in 1988.)
||- see user
and operating software by which a receiver operator executes procedures
on equipment (such as a GPS receiver) and the means by which the equipment
conveys information to the person using it: the controls and displays.
Accuracy - URA
to the range-measurement error from an individual error source (apparent
clock and ephemeris prediction accuracies). This is converted into
range units, assuming that the error source is uncorrelated with all
other error sources. Values < 10 are preferred.
||The part of
the whole GPS system that includes the receivers of GPS signals.
||- see universal
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).
Geodetic System 1984)
mathematical ellipsoid used by GPS since January, 1987.
version of the P-code.