Global Satellite Navigation Glossary

Development of Global Navigation Satellite Systems (GNSS) technologies has spawned its own set of technical terminology, particular to its many uses and applications. The following is a glossary of the more common definitions and descriptions used within these various disciplines. As you investigate Magellan technologies and how they have been applied their professional and Magellan^®-branded consumer products and solutions, you may find this glossary quite valuable.

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

A

Aerotriangulation (phototriangulation)
Aerotriangulation, also known as phototriangulation, is a complex process vital to aerial photogrammetry. It involves extending vertical and/or horizontal point control so that the measurements of angles and/or distances displayed on an overlaid photograph are made relative to a spatial solution using photographic perspective principles, correcting for photographic distortions. Aerotriangulation consists of mathematically extending the vectors/angles of a triangular pattern of known reference points on or near the designated photo-block terrain upward through a rectangle representing the area of the photo-block as seen by the camera's optical center. In this way, the three-point terrain triangle and the three-point triangle of the camera's eye within the photographic frame are made analogous.

Almanac
The almanac is a set of parameters used by a Global Positioning System (GPS) receiver to predict the approximate location of a navigation satellite and the expected offset of the satellite's clock. Each GPS satellite contains and transmits the almanac data for the entire satellite network. See: Ephemeris.

Ambiguity
In global satellite navigation technology, the ambiguity refers to the initial bias in the carrier-phase observation of an arbitrary number of wave cycles. Ambiguity is the uncertainty of the number of cycles a receiver is attempting to count. If the wavelength is known, the distance to a satellite can be computed once the number of cycles is established by processing the global navigation satellite signal's carrier phase.

Antenna
Magellan offers a variety of global navigation satellite antennas designed to pick up navigation satellite signals. Antennas range from simpler microstrip devices to complex choke ring antennas that mitigate the effects of multipath scattering sometimes caused when signals are reflected off tall buildings, cliffs, mountains or other localized terrain.

Anti-Spoofing (AS)
Anti-Spoofing is a process by which the P-Code modulation sequence of satellite navigation signals is encrypted to prevent replication by potentially hostile forces. When encrypted, the P-Code is referred to as the Y-Code. See: Y-Code, Spoofing.

Atomic Clock
An atomic clock is an extremely accurate timekeeping device regulated by the natural regular oscillations of an atom or molecule. An atomic clock powered by a hydrogen atom (maser) is accurate to 1 part in 2 quadrillion. A cesium atom powered clock has an error of about one second in one million years. GPS satellites carry multiple atomic clocks, regulated by these precise atomic vibrations, to ensure accuracy. The GPS Master Control Station uses cesium clocks and a hydrogen maser clock.

B

Baseline
The baseline is the measured distance between two satellite navigation receivers or antennas.

Bipolar Biphase Shift Key (BPSK)
BPSK is the modulation technique for carrier phase encoding on global navigation satellites using binary bit transition that results in a 180-degree shift of the carrier.

C

Cadastral survey
Cadastral survey is a land survey that defines boundaries, property lines, and other measurements pertaining to an official register of ownership, known as a cadastre. A cadastral survey helps determine the expanse and value of real property for resale and taxation.

Carrier frequency
The basic frequency of an unmodulated radio signal is known as the carrier frequency. GPS satellite navigation signals are broadcast on two L-band frequencies, L1 and L2. L1 broadcasts at 1575.42 Mhz, while L2 broadcasts at 1227.6 Mhz.

Carrier phase
Carrier phase is the fraction of a cycle often expressed in degrees, where 360 degrees equals a complete cycle. Carrier phase can also mean the number of complete cycles plus a fractional cycle. A survey-grade GPS receiver can lock-on to a satellite and count the number of whole cycles of the carrier frequency, measuring the cumulative phase of the signal. This is often referred to as integrated Doppler.

Clear Acquisition (C/A) Code
C/A-Code consists of a sequence of 1023 bits (0 or 1) that repeats every millisecond. Each satellite broadcasts a unique 1023-bit sequence that allows a receiver to distinguish between various satellites. The C/A-Code modulates only the L1 carrier frequency on GPS satellites. The C/A-Code allows a receiver to quickly lock-on to a satellite.

Channel
Channel refers to a set of hardware in a receiver that detects, locks on and continuously tracks the signal from a single navigation satellite. The more receiver channels available, the greater number of satellite signals a receiver can simultaneously lock-on and track.

Circular Error Probable (CEP)
CEP is the radius of a circle, centered at a true location, within which fifty percent of positioning solutions fall. CEP is used to achieve horizontal accuracy. See: SEP.

Constellation
Constellation refers to the network of orbiting global navigation satellites, with deference to their relative positions. The pattern created by the relative positioning of a navigation satellite network is designed to achieve a very high probability of global satellite coverage even in the event of satellite outages.

Conventional Terrestrial System (CTS)
A standardized reference system, originating at the Earth's center of mass, designed to allow uniformity in geodetic measurements and computations.

Cycle slip
As a receiver measures the carrier frequency cycles of a global navigation satellite signal there may be periodic loss of signal due to atmospheric and other interferences. This is known as cycle slip. See: Carrier phase.

D

Differential GPS (DGPS)
A technique whereby data from a receiver at a known location is used to correct the data from a receiver at an unknown location. Differential corrections can be applied in either real time or by post processing. Since most of the errors in GPS are common to users in a wide area, the DGPS-corrected solution is significantly more accurate. See: RTCM SC-104.

Dilution Of Precision (DOP)
DOP relates the statistical accuracy of the global navigation satellite measurements to the statistical accuracy of the solution. Geometric Dilution of Precision (GDOP) is composed of Time Dilution of Precision (TDOP) and Position Dilution of Precision (PDOP), which are composed of Horizontal Dilution of Precision (HDOP) and Vertical Dilution of Precision (VDOP).

Doppler shift
A shift occurring in the electromagnetic spectrum, where an apparent change in signal frequency occurs as the transmitter and receiver move toward or away from one another.

Double difference
Double difference is the mathematic difference between the measurements of a carrier phase from two receivers tracking the same pair of satellites simultaneously. Each receiver obtains a single difference from each satellite; these differences are compared in turn, essentially eliminating all satellite and receiver clock errors. See: Single difference.

E

Earth Centered, Earth Fixed (ECEF)
A Cartesian coordinate system beginning at the Earth's center of mass. The Z-axis is aligned with the Earth's mean spin axis. The X-axis is aligned with the zero meridian. The Y-axis is 90 degrees west of the X-axis, forming a right-handed coordinate system.

Elevation mask
Elevation mask is an adjustable feature of GPS receivers that specifies a satellite must be a certain number of degrees above the horizon before its signals are used for positioning. Satellites at low elevation angles (five degrees or less) have lower signal strengths and are more prone to loss of lock thus causing noisy solutions.

Ellipsoid of revolution
Often referred to simply as ellipsoid, ellipsoid of revolution is a mathematical representation of the Earth as an ellipse that is rotated about its minor axis. An ellipsoid is an equipotential surface of a rotating, homogeneous body. Various ellipsoid models have been determined to approximate the geoid in local areas and in a global sense. GPS uses the WGS84 earth model which is based on the GRS80 ellipsoid.

Ephemeris
Ephemeris is a set of parameters used by a global navigation satellite receiver to predict the location of a satellite and its clock behavior. Each satellite contains and transmits ephemeris data about its own orbit and clock. Ephemeris data is more accurate than the almanac data but is applicable over a short time frame from four to six hours. Ephemeris data is transmitted by the satellite every 30 seconds. See: Almanac.

F

Firmware
Firmware is the electronic heart of a receiver, where coded instructions relating to receiver function, and sometimes data processing algorithms, are embedded as integral portions of the internal circuitry.

Frequency
Frequency refers to the number of times a radio wave cycle repeats during a particular unit of time, usually one second. See: L1 & L2.

G

Geodetic coordinates
A coordinate system whose elements are latitude, longitude and geodetic height. The latitude is an angle based on the perpendicular to the ellipsoid. Longitude is the angle measured in the XY plane. See: ECEF, Ellipsoid.

Geodetic datum (horizontal datum)
A specifically oriented ellipsoid typically defined by eight parameters which establish its dimensions, define its center with respect to Earth's center of mass and its orientation in relation to the Earth's average spin axis and Greenwich reference meridian. See: Ellipsoid.

Geodetic height (ellipsoidal height)
The geodetic height is the distance of a point above an ellipsoidal surface. The difference between a point's geodetic height and its orthometric height equals the geoidal height. See: Geodal height, Othometric height.

Geoid
The equipotential surface of the Earth's gravity field which best fits mean sea level. Geoids currently in use are GEOID84 and GEOID90.

Geoidal height (geoidal separation; undulation)
The height of a point on the geoid above the ellipsoid measured along a perpendicular to the ellipsoid.

GLObal NAvigation Satellite System (GLONASS)
GLONASS is the Russian Global Navigation Satellite System (GNSS). See: GNSS.

GNSS (Global Navigation Satellite System)
A global navigation satellite system is a network of satellites that transmit ranging signals used for positioning and navigation anywhere around the globe; on land, in the air or at sea. The US Global Positioning System (GPS), the Russian GLObal NAvigation Satellite System (GLONASS) and the upcoming European GALILEO system are examples of GNSS.

GPS week
GPS time started at Saturday/Sunday midnight, January 6, 1980. The GPS week is the number of whole weeks since GPS time zero.

Gravity
Gravity is the mutual force of attraction between all particles or bodies that have mass. Gravity often refers to the vector sum of the gravitational attraction of various masses within the planet plus the centrifugal force caused by the rotation of the Earth.

H

Hydrographic and bathymetric surveying
Hydrographic and bathymetric surveying is the surveying or mapping of harbors, inlets and deep water locations. Hydrography is the study of the physical characteristics of oceans, lakes and rivers as well as the elements affecting safe navigation. Bathymetry is the measurement and study of water depths.

I

Ionosphere
The ionosphere consists of the layers of ionized air in the atmosphere extending from 70 kilometers to 700 kilometers and higher. Depending on frequency, the ionosphere can either block radio signals completely or change the propagation speed. Global navigation satellite signals are delayed as they penetrate the ionosphere. This ionospheric delay can be either predicted using models, though with relatively poor accuracy, or corrected using two frequency receivers.

J

Julian date
The number of days that have elapsed since 1 January 4713 B.C. in the Julian calendar. GPS time zero is defined to be midnight, Universal Time Coordinated (UTC), Saturday/Sunday, 6 January 1980 at the 0-degree meridian at Greenwich, London, England. The Julian date for GPS time zero is 2,444,244.5.

K

Kinematic surveying
Kinematic surveying is a method that initially solves wavelength ambiguities and retains the resulting measurements by maintaining a lock on a specific number of satellites throughout an entire surveying period.

L

L1 & L2
L1 and L2 are designations of the two basic carrier frequencies transmitted by GPS satellites that contain the navigation signals. The L1 frequency is 1,575.42 Mhz, while the L2 frequency is 1,227.60 Mhz.

L-band
A nominal portion of the microwave electromagnetic spectrum ranging from 1 to 2 Ghz.

M

Multipath
Multipath is the reception of a signal both along a direct path and along one or more reflected paths. Multipath signals result in an incorrect pseudorange measurement.

Multiplexing
Multiplexing is a technique used in some global navigation satellite receivers to sequence the signals of two or more satellites through a single hardware channel. Multiplexing allows a receiver to track more satellites than the number of hardware channels at the cost of lower effective signal strength.

N

Navigation messages
Data that is modulated onto a navigation satellite signal is known as a navigation message. The data is transmitted at 50 bits per second and contains ephemeris and clock data for that particular satellite, other data required by a receiver to compute position velocity and time and almanac data for all GPS satellites. The data is transmitted in 1500 bit frames, each requiring 30 seconds to transmit. A complete set of data to include all almanacs, timing information, ionospheric information and other data requires 12-1/2 minutes to transmit.

NAVigation Satellite for Timing And Ranging (NAVSTAR)
NAVSTAR is the original designation given to the Global Positioning System (GPS) by the U.S. Department of Defense. NAVSTAR is occasionally used today, either by itself or in conjunction with GPS, as in NAVSTAR GPS.

O

On The Fly (OTF)
OTF is a common term used to describe the technique of resolving differential carrier-phase integer ambiguities without requiring a global navigation satellite receiver to remain stationary.

Orbit
Orbit is the path a satellite takes as it travels around the planet.

Orthometric height (orthometric elevation)
Orthometric height is the distance of a point above the geoid. See: Geoid.

P

P-code
The P in P-code stands for "precise" or "protected." The P-code is bi-phase shift modulated on both the L1 and L2 carrier frequencies. P-code has a 10.23MHz bit rate and, as implemented in GPS, has a period of one week. Each satellite has a unique P-code that is used to distinguish a satellite from all other satellites.

Photogrammetry
Photogrammetry is an aerial remote sensing technique, the latest innovations of which employ a high-resolution aerial camera with forward motion compensation using global satellite navigation technology for pilot guidance over the designated photo block. Photogrammetry forms the baseline of many Geographic Information Systems (GIS) and Land Information System (LIS) studies and endeavors.

Post processing
Post processing is the reduction, calculation and processing of global satellite data after the data has been collected in the field. Post processing is usually accomplished on a computer in an office environment where appropriate software is employed to achieve optimum position solutions.

Precise Positioning System (PPS)
The more accurate GPS capability that is restricted to authorized, typically military, users.

Pseudo-kinematic surveying
A variation of the kinematic method where roughly five-minute site occupations are repeated at a minimum of once each hour.

Pseudo-Random Noise (PRN)
The P(Y) and C/A codes are pseudo-random noise sequences, which modulate navigation signals. The modulation appears to be random noise but is, in fact, predictable; hence the term pseudorandom. This technique allows the use of a single frequency by all GPS satellites and also permits the satellites to broadcast a low power signal.

Pseudorange
The measured distance between the global navigation satellite and the receiver antenna is known as the pseudorange. The pseudorange is approximately the geometric distance biased by the offset of the receiver clock from the satellite clock. The receiver calculates the difference between the time a signal was transmitted from the satellite and the time it is received. This calculation is affected by the speed of the signal, thus creating the pseudorange measurement.

Q

Quartz oscillator
Quartz oscillator is the timing device within a receiver that synchronizes the operation and maintains time for the receiver.

R

Real time
Real time refers to immediate global navigation satellite data collection, processing and positioning in the field.

Real-time kinematic (RTK)
A DGPS process where carrier-phase corrections are transmitted in real-time from a reference receiver at a known location to one or more remote rover receivers.

Real-Time Z^TM
Magellan's unique technology that includes Carrier Phase Differential (CPD) processing to provide On The Fly (OTF) ranging data acquisition and differential processing in its professional GPS and GNSS solutions.

Reference network
A reference network is a series of monuments or reference points with accurately measured mutual vectors/distances used as a reference basis for cadastral and other types of survey.

Reference station
A reference station is a point or site where surface stability, or tidal current constants, have been determined through accurate observations, and which is then used as a standard for the comparison of simultaneous observations at one or more subordinate stations. Certain of these are known as Continuous Operating Reference Stations (CORS), and transmit reference data on a 24-hour basis.

Receiver-Independent Exchange (RINEX) format
The Receiver-INdependent EXchange is a GPS data format, which includes provisions for pseudorange, carrier-phase, and Doppler observations.

Root mean squared (RMS)
RMS is a statistical measurement of the scatter of computed positions to determine the most likely position solution. RMS can be applied to any random variable.

RTCM SC-104 format
A standard format used in the transmission of differential corrections.

S

Satellite-Based Augmentation System (SBAS)
A Satellite-Based Augmentation System is any system that employs a network of geostationary satellites and ground stations to enhance the performance of a Global Navigation Satellite System (GNSS) by providing signal corrections and additional satellites with known and constant positions. Three SBAS are currently in operation around the world, covering specific geographic regions; North America's Wide Area Augmentation System (WAAS), Europe's European Global Navigation Overlay System (EGNOS) and Asia's Multi-transportation Satellite Augmentation System (MSAS).

Satellite Image Mapping (SIM)
SIM is a product of remote sensing where discrete blocks of orbital photography are combined into a comprehensive whole and geocoded, or computer-linked to specific Mercator, Lambert Conformal, or other types of projections that include a scale factor and reference geoid, with each pixel related to a specific latitude and longitude.

Selective Availability (SA)
Selective Availability is a process whereby the U.S. Department of Defense dithers the satellite clock and/or broadcasts erroneous orbital ephemeris data to create a pseudorange error to prevent adversaries from using the extremely accurate GPS positioning data. See: Standard Positioning System.

Spherical Error Probable (SEP)
SEP is a navigational measurement of accuracy equaling the radius of a sphere, centered on a true position, inside which 50% of the computed solutions lie. See: Circular Error Probable.

Sidereal Time
Sidereal time is defined by the hour angle of the vernal equinox. Taking the mean equinox as the reference yields true or apparent Sidereal Time. Neither Solar nor Sidereal Time are constant, since angular velocity varies due to fluctuations caused by the Earth's polar moment of inertia as exerted through tidal deformation and other mass movements.

Single difference
Single difference is the mathematic difference between the measurements of a carrier phase from two receivers simultaneously tracking the same satellite to eliminate any satellite clock errors, or one receiver simultaneously tracking two satellites to eliminate any receiver errors. See: Double difference.

Software
Software usually refers to a set of advanced computer modules that allow the user to plan efficient surveys, organize and acquire satellite navigation data, verify and download data, process and analyze measurements, perform network adjustments, and report and archive the final results.

Spoofing
Spoofing is the process of replicating the global navigation satellite code in such a way that a receiver calculatesan incorrect position solution.

Standard Positioning System (SPS)
SPS is the less accurate GPS capability which is freely available to anyone using a GPS receiver. See: Anti-Spoofing, Selective Availability.

Static observations
A navigation satellite survey technique that requires roughly one hour of observation, with two or more receivers observing simultaneously, and results in high accuracy and vector measurements.

T

Triple difference
The mathematic difference of sequential, doubly-differenced carrier-phase observations that are free of integer ambiguities, and useful for determining initial, approximate coordinates of a site in relative global navigation satellite positioning, and for detecting cycle slips in carrier-phase data. See: Double difference, Single difference.

U

Universal Time Coordinated (UTC)
UTC is the time as maintained by the U.S. Naval Observatory. Because of variations in the Earth's rotation, UTC is sometimes adjusted by an integer second. The accumulation of these adjustments compared to GPS time, which runs continuously, has resulted in an offset between GPS time and UTC. After accounting for leap seconds and using adjustments contained in the navigation message, GPS time can be related to UTC within 20 nanoseconds or better.

V

Vehicle Independent Positioning (VIP)
Magellan's VIP technology makes installation of the Magellan 750NAV, vehicle navigation GPS receiver, simple. The compact, transportable navigation computer and docking station offer a variety of mounting positions in most vehicles. Other systems require extensive wiring, which could critically reduce the vehicle's reliability. With VIP, the vehicle's position is computed by using feedback from 750NAV internal sensors to sense turns.

W

Wide Area Augmentation System (WAAS)
Developed by the United States government, WAAS is a Satellite-Based Augmentation System (SBAS) that calculates the errors in the GPS signal at several monitoring stations around the country, then transmits error correction messages from geostationary satellites to GPS receivers.

World Geodetic System 1984 (WGS 84)
WGS 84 is a set of U.S. Defense Mapping Agency (DMA) parameters for determining global geometric and physical geodetic relationships. Parameters include a geocentric reference ellipsoid; a coordinate system; and a gravity field model. GPS satellite orbital information in the navigation message is referenced to WGS 84.

X

Y

Y-code
The designation for the end result of P-code during Anti-Spoofing (AS) activation by the U.S. Department of Defense. See: Anti-Spoofing, P-code.

Y-Code tracking, civilian
Several methods of obtaining valid data from encrypted Y-code are available:

1. Signal squaring (now obsolete) multiplies the signal by itself, thus deleting the carrier's code information and making distance measurement impossible. Carrier phase measurements can still be accomplished, although doubling the carrier frequency halves the wavelength, further weakening an already weak signal. This method requires collecting data over an extended period of time.
2. Cross correlation is used where no local satellite navigation correction code is generated to match the L1 and L2 encrypted Y-codes. The ionosphere slows the L2 Y-code slightly in respect to the L1 Y-code creating a difference between the measurement of these distances. Once this difference is known, the frequencies can be matched and multiplied to leave pure carrier frequencies for measurement. This method removes the half-wavelength problem, but also results in a weakened signal that necessitates longer observation periods.
3. Code correlation and squaring is a process wherein the L1 and L2 Y-codes are compared against a locally generated P-code. The difference in the signals is thus revealed, measured and squared so that pure carrier frequencies can be measured. Squaring once again weakens the resulting half-wavelengths of both carrier frequencies, and once again requires longer observation periods.
4. Magellan's Z-Tracking is a unique method for retaining receiver lock and tracking capability. This technique separately matches the Y-code on L1 and L2 against a different, locally generated P-code; a correlation that exposes the encrypting code on each frequency. Both carriers also contain the encrypting code, and so with sufficient signal integration, the encrypting signal is estimated for L1 and L2. Each calculation is fed to the other frequency, removing the encrypting code from each carrier frequency, which can then be purely measured.

Z

Z count
Z-count is a 29-bit binary number consisting of the fundamental GPS time unit. The 10 most significant bits carry the GPS week number, while the 19 least significant bits give the Time Of Week (TOW) in units of 1.5 seconds.

Z-Tracking^TM
Magellan's Z-Tracking process for mitigating or eliminating the effects of the U.S. Department of Defense Anti-Spoofing (AS) is a unique method for retaining receiver lock and tracking capability. This technique separately matches the Y-code on L1 and L2 against a different, locally generated P-code, a correlation that exposes the encrypting code on each frequency. Both carriers also contain the encrypting code, and so with sufficient signal integration, the encrypting signal bit is estimated for L1 and L2. Each calculation is fed to the other frequency, removing the encrypting code from each carrier frequency, which can then be purely measured.