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| GPS has changed the way people navigate the oceans, the skies and land, as well as touch our lives any many other ways, from agriculture, to the construction and maintenance of our infrastructure. Here, you can find an introduction to global positioning and sources for more information. | |||||||||||||||||||||||||||
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Global Positioning Information on the Web , Global Positioning Glossary GPS-The Global Positioning System, under development by the Department of Defense for over 20 years, consists of 21 satellites, plus three back-up satellites in predictable orbits around the earth. The system provides 24-hour positioning information regardless of weather. Launched aboard Delta rockets and tracked under Air force administration, GPS consists of the Space Segment (the satellites) and the Control Segment (the network of tracking stations which monitor and control the GPS satellites in orbit). Ashtech has recently enhanced the performance of its GPS positioning systems by combining GPS receivers, with GLONASS receivers. GLONASS is the Russian equivalent of GPS--by combining GPS and GLONASS, you have a combined satellite constellation with many more than the standard 24-satellite constellation of GPS alone, which offers much better system availability and integrity. To see how this helps in tricky positioning applications, check out this Quicktime movie that shows how Ashtech GPS+GLONASS technology can help. GPS has wide applicability.
Linked to a vehicle, it becomes a tool of navigation. Within the
context of a coordinate system, it is an instrument of surveying. With
a cellular phone or transceiver, it becomes a method of GPS works on the principle of triangulation. By knowing its distance from three or more satellites, the receiver can calculate its position by solving a set of equations. Information from three satellites is needed to calculate longitude and latitude at a known elevation; four satellites are needed to include altitude as well. Satellites orbit the earth twice a day at an altitude of 10,900 miles, repeatedly broadcasting their position and the time. The atomic clocks aboard each satellite keeps time by the vibration of atoms and are accurate to one second in 30 years! In theory, the distance from satellite to receiver can be calculated by multiplying the time it takes for the signal to arrive by the speed at which it travels -- the speed of light. In practice, more sophisticated calculations are required to account for the fact that receiver clocks are not as accurate as satellite clocks. Upper atmospheric conditions and solar disturbances can also interfere with signal reception. All GPS receivers need direct and unobstructed line of sight access to each satellite. While a single receiver can provide accurate positioning to about 100 feet, accuracy to a fraction of an inch is possible by using two receivers. One is fixed to a spot whose coordinates are already known. The other, whose location is sought, logs the same satellite data and the errors are resolved, in real-time for pin-point navigation or by post-processing for precision geodetic surveying. Because GPS was designed for military use, it contains a number of features to limit its use to national defense. Each satellite broadcasts two signals, one for commercial use (C/A-Code) and a more accurate one for military use (P-Code). The Pentagon can encrypt the P-Code signal ("anti-spoof" mode or AS) so that unauthorized receivers cannot understand the information, however more advanced commercial receivers such as the Ashtech Z-12 can compensate by correlating the components of the P-Code for continued use in high resolution positioning. Another restriction is "selective availability", or SA, in which the data transmitted by the satellites contain deliberate errors for all but military receivers. If SA is turned on, the accuracy of commercial receivers droops from 100 feet to 300 feet. (However, using two receivers together in a differential mode can correct for this misinformation). Commercial use of GPS has proven invaluable in many fields. It has revolutionized surveying. It can be used to track everything from migrating animal herds to the creep of the earth's crust. Using GPS provides an entirely new way of navigating and piloting, on land, sea or in the air. It is one of our best space adventures yet. Global Positioning Links on the WebGPS Information
Other GPS-Related Sites (Non-commercial)
Commercial Vendors
Global Positioning GlossaryGPS, GIS and LIS Technologies, plus Aerial and Orbital Remote Sensing, have developed technical terms peculiar to their own usages, and for the uninitiated these terms can be confusing. Following is a glossary of the more common definitions/descriptions in use within these disciplines. Many of the cited terms either do not apply to, or have not been used, in describing various Ashtech products. However, once a potential user inquires about the various usages, these definitions should prove valuable.
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A
- aerotriangulation (phototriangulation)
- a complex process vital to aerial photogrammetry that involves extending vertical and/or horizontal control so that the measurements of angles and/or distances on overlapping photographs are related to a spatial solution using the perspective principles of the photographs. 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 such a way that the three-point terrain triangle and the camera's eye three-point triangle (within the photographic frame) are analogous.
- almanac
- set of parameters used by a GPS
receiver to predict the approximate locations of a GPS satellite and the
expected satellite clock offset. Each GPS satellite contains and transmits
the almanac data for all GPS satellites.
(See ephemeris).
- ambiguity
- the initial bias in a carrier-phase observation of an arbitrary number of cycles; the uncertainty of the number of cycles a receiver is attempting to count. If wavelength is known, the distance to a satellite can be computed once the number of cycles is established via carrier-phase processing.
- antenna
- a variety of GPS antennas ranging from simpler microstrip devices to complex choke ring antennas that mitigate the effects of multipath scattering.
- Anti-Spoofing (AS)
- the process of encrypting the P-Code modulation sequence so that the code cannot be replicated by hostile forces. When encrypted, the P-Code is referred to as the Y-Code (see Y-Code & Spoofing).
- atomic clock
- a clock whose frequency is maintained using electromagnetic waves that are emitted or absorbed in the transition of atomic particles between energy states. The frequency of an atomic transition is very precise, resulting in very stable clocks. A cesium clock has an error of about one second in one million years. For redundancy purposes, GPS satellites carry multiple atomic clocks. GPS satellites have used rubidium clocks as well as cesium clocks. The GPS Master Control Station uses cesium clocks and a hydrogen maser clock.
B
- baseline
- the measured distance between two receivers or two antennas.
- bipolar biphase shift key (BPSK)
- the modulation technique used on GPS satellites. In this method, a binary bit transition results in a 180-degree shift of the carrier.
C
- cadastral survey
- a survey that defines boundaries, property lines, etc., and pertains to cadastre, an official register of ownership, the extent and value of real property. Cadastral surveys usually determine taxation.
- carrier frequency
- the basic frequency of an unmodulated radio signal. GPS satellite navigation signals are broadcast on two L-band frequencies, L1 and L2. L1 is at 1575.42 Mhz, and L2 is at 1227.6 Mhz.
- carrier phase
- 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. In a survey-grade GPS receiver, the receiver can lock on to a satellite and, keeping track of the number of whole cycles of the carrier, creates a cumulative phase of the signal which is often referred to as integrated Doppler.
- C/A (clear acquisition) 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.
- carrier phase
- the cumulative phase of either the L1 or L2 carrier of a GPS signal, measured by a receiver while locked-on to the signal (also known as integrated Doppler).
- channel
- refers to the hardware in a receiver that allows the receiver to detect, lock-on and continuously track the signal from a single satellite. The more receiver channels available, the greater number of satellite signals a receiver can simultaneously lock-on and track.
- Circular Error Probable (CEP)
- the radius of a circle, centered at the true location, within which 50% of position solutions fall. CEP is used for horizontal accuracy (see SEP).
- constellation
- refers to the collection of orbiting GPS satellites. The GPS constellation consists of 24 satellites in 12-hour circular orbits at an altitude of 20,200 kilometers. In the nominal constellation, four satellites are spaced in each of six orbital planes. The constellation was selected to provoke a very high probability of satellite coverage even in the event of satellite outages.
- Conventional Terrestrial System (CTS)
- a standardized reference system, originating at the planet's center of mass, that is designed to allow uniformity in geodetic measurements and computations.
- cycle slip
- a loss of count of carrier cycles as they are being measured by a GPS receiver. Loss of signal, ionospheric interference and other forms of interference cause cycle slips to occur (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 (see RTCM SC-104 format) 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 than a normal SPS solution.
- Dilution of Precision (DOP)
- a measure of the receiver-satellite(s) geometry. DOP relates the statistical accuracy of the GPS measurements to the statistical accuracy of the solution. Geometric Dilution of Precision (GDOP) is composed of Time Dilution of Precision (TDOP) & Position Dilution of Precision (PDOP), which are composed of Horizontal Dilution of Precision (HDOP) & Vertical Dilution of Precision (VDOP).
- Doppler shift
- a shift similar to that experienced by audio phenomena, except 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
- (see single difference) the arithmetic differencing of carrier phases measured simultaneously by a pair of receivers tracking the same pair of satellites. Single differences are obtained by each receiver from each satellite; these differences are then differenced in turn, which essentially deletes all satellite and receiver clock errors.
E
- Earth Centered, Earth Fixed (ECEF)
- a Cartesian coordinate system centered 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
- an adjustable feature of GPS receivers that specifies that a satellite must be at least a specified number of degrees above the horizon before the signals from the satellite are to be used. 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)
- a mathematical representation of the earth that is 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 (plural: ephemeredes)
- a set of parameters used by a GPS receiver to predict the location of a GPS satellite and its clock behavior. Each GPS satellite contains and transmits ephemeris data its own orbit and clock. Ephemeris data is more accurate than the almanac data but is applicable over a short time frame (four to six hours). Ephemeris data is transmitted b the satellite every 30 seconds. (See almanac).
F
- firmware
- 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
- the number of times that a periodic event occurs per unit of time. For GPS, frequency usually refers to the radio frequency, in Hz, of either of two basic carriers transmitted by each satellite (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).
- 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 specify its orientation in relation to the Earth's average spin axis and Greenwich reference meridian.
- geodetic height (ellipsoidal height)
- the height of a point above an ellipsoidal surface. The difference between a point's geodetic height and its orthometric height equals the geoidal 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 Orbiting Navigation Satellite System (GLONASS)
- the Russian version of GPS.
- 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
- a force that is the vector sum of gravitational attraction of the various masses within the planet (gravitation) plus the centrifugal force caused by the rotation of the Earth. Unit of measurement: the gal = 1 cm per m/sec2.
H
- hydrographic and bathymetric surveying
- surveying or mapping of harbors, inlets or 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
- refers to 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. GPS signals penetrate the ionosphere but are delayed. The ionospheric delays can be either predicted using models, though with relatively poor accuracy, or measured 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 UTC, Saturday/Sunday, 6 January 1980 at Greenwich. The Julian date for GPS time zero is 2,444,244.5.
K
- kinematic surveying
- a method which initially solves wavelength ambiguities and retains the resulting measurements by maintaining a lock on a specific number of satellites throughout the entire surveying period.
L
- L1 & L2
- designations of the two basic carrier frequencies transmitted by GPS satellites that contain the navigation signals. L1 is 1,575.42 Mhz and L2 is 1,227.60 Mhz.
- L-band
- a nominal portion of the microwave electromagnetic spectrum ranging from 1 to 2 Ghz.
M
- multipath
- the reception of a signal both along a direct path and along one or more reflected paths. The resulting signal results in an incorrect paseudorange measurement. The classical example of multipath is the "ghosting" that appears on television when an airplane passes overhead.
- multiplexing
- a technique used in some GPS 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 modulated onto the satellite's signals. The navigation 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 NAVSTAR 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)
- Another term for GPS or sometimes used in conjunction with GPS as in "NAVSTAR GPS.".
O
- On-the-Fly (OTF)
- a term used to describe the technique of resolving differential carrier-phase integer ambiguities without requiring a GPS receiver to remain stationary.
- orbit
- the path a satellite takes in space.
- orthometric height (orthometric elevation)
- the height of a point above the geoid.
P
- P-Code
- "precise" or "protected" code which 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 the satellite from all other GPS satellites.
- photogrammetry
- an aerial remote sensing technique whose latest innovations employ a high-resolution aerial camera with forward motion compensation and uses GPS technology for pilot guidance over the designated photo block(s). Photogrammetry forms the baseline of many Geographic Information Systems (GIS) and Land Information System (LIS) studies and endeavors.
- post-processing -
- the reduction and processing of GPS data after the data was actually 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.
- pseudorandom noise (PRN)
- the P(Y) and C/A codes are pseudo-random noise sequences which modulate the navigation signals. The modulation appears to be random noise but is, in fact, predictable hence the term "pseudo"random. Use of 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 GPS receiver antenna and the GPS satellite. The pseudorange is approximately the geometric range biased by the offset of the receiver clock from the satellite clock. The receiver actually measures a time difference which is related to distance (range) by the speed of propagation.
Q
- quartz oscillator
- the timing device within a receiver that synchronizes the receiver's operation and maintains time for the receiver.
R
- ratio
- a measure of the precision of observations that takes into account the resolution of ambiguities and arrives at an RMS value during the processing computations.
- real-time
- refers to immediate, "on the spot," GPS data collection, processing and position determination (usually) within a receiver's firmware, rather than post-processing "after the fact" via a computer in an office environment.
- 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" receiver(s).
- Real-Time Z(tm)
- Ashtech's proprietary technique that includes Carrier Phase Differential (CPD) processing. Real-Time Z features "on the fly" (OTF) ranging data acquisition and differential processing.
- Reference Network
- a series of monuments or reference points with accurately measured mutual vectors/distances that is used as a reference basis for cadastral and other types of survey.
- Reference Station
- a point (site) where crustal 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.
- RINEX
- the Receiver-INdependent EXchange format for GPS data, which includes provisions for pseudorange, carrier-phase, and Doppler observations.
- root mean squared (RMS)
- a statistical measure of the scatter of computed positions about a "best fit" 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 Image Mapping (SIM)
- a product of remote sensing where discrete blocks of orbital photography are "mosaicked" into a comprehensive whole, then "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)
- the process whereby DoD "dithers" the satellite clock and/or broadcasts erroneous orbital ephemeris data to create a pseudorange error (see Standard Positioning System).
- Spherical Error Probable (SEP)
- a navigational measure of accuracy equaling the radius of a sphere, centered on the true location, inside which 50% of the computed solutions lie. (See CEP.)
- 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 vary due to fluctuations caused by the Earth's polar moment of inertia as exerted through tidal deformation and other mass transports.
- single difference
- the arithmetic "differencing" of carrier phases simultaneously measured by a pair of receivers tracking the same satellite (between-receivers and satellite), or by a single receiver tracking two satellites (between-satellite and receivers); the former essentially deletes all satellite clock errors, while the latter essentially deletes all receiver errors.
- software
- usually refers to a set of advanced modules, such as Ashtech's PRISM II Package, that allows the user to plan efficient surveys, organize and acquire GPS data, verify and download GPS data into a computer, process and analyze the measurements, perform a network adjustment, and report/archive the final results.
- Spoofing
- the process of replicating the GPS code in such a way that the user computes incorrect position solutions.
- Standard Positioning System
- the less accurate GPS capability which is available to all. (See Anti-Spoofing and Selective Availability).
- static observations
- a GPS survey technique that requires roughly one hour of observation, with two or more receivers observing simultaneously, and results in high accuracy's and vector measurements.
T
- triple difference
- the arithmetic difference of sequential, doubly-differenced carrier-phase observations that are free of integer ambiguities, and therefore useful for determining initial, approximate coordinates of a site in relative GPS positioning, and for detecting cycle slips in carrier-phase data. (See single difference & double difference)
U
- Universal Time Coordinated (UTC)
- 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 11 second offset between GPS time and UTC at the start of 1996. 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
W
- World Geodetic System 1984 (WGS 84)
- a set of U.S. Defense Mapping Agency 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 DoD.
- 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 (ranging) impossible. Carrier phase measurements can still be accomplished, although doubling the carrier frequency halves the wavelength, further weakening an already weak signal. This method required collecting data over a much longer period.
2. Cross correlation, where no local (receiver) code is generated to match the L1 & L2 encrypted Y-codes. The ionosphere "slows" the L2 Y-code slightly in respect to the L1 Y-code, hence the difference between these distances can be measured and, once known, matched and multiplied to remove the codes and leave pure carrier frequencies for measurement. This does away with the half-wavelength problem, but again results in a weakened signal that necessitates longer observation periods.
3. Code correlation & squaring. Here, the L1 & L2 Y-Codes are compared against a locally generated P-Code; the difference (the encrypting Y-code signal) 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. Ashtech's "Z-Technique" (see Z-Tracking(tm)).
Z
- Z count
- a 29-bit binary number consisting of the fundamental GPS time unit. The (10) most significant bits carry the GPS week number, and the (19) least significant bits give the time of week (TOW) count in units of 1.5 seconds.
- Z-Tracking(tm)
- Ashtech's proprietary (patented) process for mitigating or eliminating the effects of DoD Anti-Spoofing (AS) and thereby retaining receiver lock and tracking capability at all times on the satellites in view. 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, hence with sufficient signal integration the encrypting signal bit is estimated for L1 and L2 and each is fed to the other frequency, thus removing the encrypting code from each carrier frequency, which can then be measured.
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