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How GPS Works

Real Time Kinematic (RTK)

The Relative Positioning procedure whereby carrier phase measurements (or corrections) are transmitted in real-time from a Reference or Base Station to the user's roving receiver. Centimetre accuracy is achieved without the need to record and post-process double-differenced carrier phase observables.

Real-Time DGPS

A Base Station computes, formats, and transmits pseudo-range corrections via some sort of data communication link (e.g., VHF or UHF radio, cellular telephone, FM radio sub-carrier or satellite com link). The roving receiver requires some sort of data link receiving equipment to receive the transmitted DGPS corrections so that they can be applied to its current observations. Most GPS receivers are so-called "RTCM-capable", which means that they can accept industry standard DGPS correction messages if the real-time data link is provided.

Receiver Autonomous Integrity Monitoring (RAIM)

A form of receiver self-checking in which redundant pseudo-range observations are used to detect if there is a problem or "failure" with any of the measurements -- only four measurements are needed to derive 3-D coordinates and the receiver clock error, hence any extra measurements can be used for checking. Once the failed measurements have been identified they may be eliminated from the navigation fix. RAIM is a concept that has been introduced by aviation users who are concerned that GPS does not have the level of Integrity necessary for non-precision airport approaches or GPS-aided landing.

Relative Positioning

The determination of relative positions between two or more receivers which are simultaneously tracking the same GPS signals. One receiver is generally referred to as the Reference or Base Station, whose coordinates are known in the satellite datum. The second receiver may be stationary or moving. However its coordinates are determined relative to the Base Station. In carrier phase-based positioning this results from the determination of the baseline vector, which when added to the Base Stations coordinates generates the User's coordinates. In pseudo-range-based GPS positioning, the coordinates are derived from the User receiver's observations after they have had the differential corrections applied (either in the real-time or post-processed mode).

 

A strip or ridge of rocks, sand, or coral that rises to or near the surface of a body of water.

 

Changes in terrain; elevations or depressions in the land.

 

The use of light and dark tones to depict the form of the land; makes the map appear as if illuminated by the sun, creating highlights and shadows.

 

 

Radio Frequency Identification - A method of identification using radio frequency. For objects, animals, or people and does not require direct contact or line-of-sight scanning.

 

 

A line of the surface of the earth that crosses all meridians at a constant angle.

RINEX

Receiver INdependent EXchange format. A set of standard definitions and formats to promote the free exchange of GPS data and facilities the use of data from any GPS receiver with any post-processing software package. The format includes definitions for three fundamental GPS observables: time, phase, and range.

 

 

Relative Kinematic Positioning - Improves GPS accuracy by using a combination of differential GPS (DGPS) correction data, transmitting GPS signal phase information and ambiguity resolution techniques via statistical tests—possibly with processing in real-time (real-time kinematic positioning, RTK).

Root Mean Square (RMS)

The square root of the average of the squared errors.

 

 

A set of waypoints that define the intended path of travel.

Rover

Any mobile GPS receiver collecting data during a field session. The receiver's position may be computed relative to another, stationary GPS receiver at a Base Station. May also be referred to as the Mobile Receiver.

Radio Technical Committee for Maritime Applications (RTCM)

RTCM Special Committee 104 has developed standard message types for use by differential GPS transmitting stations. The message content has been defined and hence when the RTCM-104 standard (version 2.2 is the latest) is implemented within a user receiver, it is able to decode and apply the DGPS corrections to its raw data in order to generate a DGPS-corrected coordinate.


R95

A position accuracy measure. The R95 value is defined as a circle's radius, when centred at the true position, encloses 95% of the data points in a horizontal scatter plot.

 

 

Real Time Kinematic - Land survey based on a differential use of carrier phase measurements of the GPS, Glonass and/or Galileo signals where a single reference station provides the real-time corrections of even to a centimetre level of accuracy.

- S -

 

See selective availability.

 

A dip along the top of a ridge.

Satellite Constellation

See Constellation, or Space Segment.

 

 

Space Based Augmentation System - Any system that uses 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.

 

The distance on a map that corresponds to the same points on the earth.

 

See secure digital.

 

A unit of measurement; one minute contains 60 seconds; one degree contains 3600 seconds.

 

Complex fluid motion in the outer core of the Earth that causes the magnetic field to change slowly with time.

 

A type of technology for mass storage devices in the form of a type of memory media with a compact design and is compatable with different digital camera brands and MP3 devices.

Selective Availability (SA)

Intentional degradation of the Absolute Positioning performance capabilities of the NAVSTAR satellite system for civilian use (the Standard Positioning Service) by the U.S. military, accomplished by artificially "dithering" the clock error in the satellites. Has generally been mitigated through the use of Relative Positioning techniques. SA was activated on 25 March 1990, and was removed on the 1st May 2000 (midnight Washington D.C. time).

 

 

Spherical Error Probable - The radius of a sphere within which there is a 50 percent probability of locating a point or being located.

 

Surface Elevation Table - A portable mechanical leveling device for measuring the relative elevation of wetland sediments.

 

A small stream.

 

A pipe connecting two canals.

SINEX

Solution Independent Exchange format. A solution output format recently developed by geodesists to permit the exchange of solution information between organisations, from which the original normal equation systems for precise GPS adjustments can be reconstructed. These reconstructed equation systems can be combined with other normal equation systems to create new GPS baseline solutions.

 

 

An attachment for a PDA which allows you to use items like network or modem cards, PCMCIA or CF cards, digital cameras, a GPS/GPRS phone, or an extra battery pack with a PDA.

 

1) A depression or hollow, usually filled with deep mud or mire. 2) A stagnant swamp, marsh, bog, or pond, especially as part of a bayou, inlet, or backwater.

 

Speed Made Good - Required speed to destination given the current course.

 

Short Message Service (also called text messaging) - A function available on digital GSM networks allowing text messages to be sent and received via the network operator to a mobile phone, or from the Internet, using an "SMS gateway" website.

 

see Speed Over Ground

 

A long, relatively wide body of water, larger than a strait or a channel, connecting larger bodies of water.

Space Segment

The space-based component of the GPS system (i.e., the orbiting satellites and their signals). The satellites may be differentiated into various groups. e.g. the Block II, Block IIA, Block IIR, and Block IIF satellites.

 

The speed at which the GPS unit and the person operating it are moving with respect to the earth's surface (ignoring tidal and current activity).

 

A body that is shaped like a sphere but is not perfectly round.

Spherical Error Probable (SEP)

A statistical measure of the 3-D positioning precision. The SEP value is defined as a sphere's radius, when centred at the true position, encloses 50% of the data points in a 3-D scatter plot. Thus, half the data points are within a 3-D SEP sphere and half are outside the sphere.

Standard Positioning Service (SPS)

The civilian Absolute Positioning accuracy obtained by using the pseudo-range data obtained with the aid of a standard single-frequency C/A-Code GPS receiver. Under "Selective Availability" the horizontal accuracy is stated to be 100m 2drms (or 95% of the time).

 

 

A small ridge.

Static Positioning

Location determination when the receiver's antenna is presumed to be stationary on the earth. In the case of pseudo-range-based techniques this allows the use of various averaging techniques to improve the accuracy. Static Positioning is usually associated with GPS Surveying techniques, where the two GPS receivers are static for some observation period which may range from minutes to hours (and even in the case of GPS geodesy, several days).

 

 

A unit of length equal to 5280 feet (1609 meters).

 

 

A vast semi-arrid grassy plain.

Stop-and-Go Positioning

This is a GPS Surveying "high productivity" technique which is used to determine centimetre accuracy baselines to static points, using site observation times of the order of 1 minute. Only carrier phase that has been converted into unambiguous "carrier pseudo-range" is used, necessitating that the ambiguities be resolved BEFORE the survey starts (and again at any time the satellite tracking is cut, e.g. due to signal obstructions). It is known as the "stop & go" technique because the coordinates of the receiver are only of interest when it is stationary (the "stop" part), but the receiver continues to function while it is being moved (the "go" part) from one stationary setup to the next. As the receiver must track the satellite signals at all times, hence the transport of the receiver from one static point to another must be done carefully.

 

A wide river valley.

 

The region of the atmosphere above the troposphere and below the mesosphere.

 

See SET.

 

Scalable Vector Graphics - An XML markup language for describing two-dimensional vector graphics, both static and animated. It is an open standard created by the World Wide Web Consortium, which is also responsible for standards like HTML and XHTML.

 

1. A low tract of land, especially when moist or marshy. 2. A long, narrow usually shallow trough between ridges on a beach, running parallel to the coastline. 3. A shallow troughlike depression that carries water mainly during rainstorms or snow melts.

- T -

 

Rock debris at the base of a mountain or cliff.

 

A usually artificial pool, pond, reservoir, or cistern, especiallly one used to hold water for drinking or for irrigation.

 

A small mountain lake, especially one formed by glaciers.

 

An optical instrument consisting of a small mounted telescope rotatable in horizontal and vertical planes, used to measure angles in surveying, meteorology, and navigation.

 

Topologically Integrated Geographic Encoding & Referencing - A U.S. Census Bureau mapping system.

 

See TTFF.

 

TOPography EXperiment for Ocean Circulation; unflown NASA precursor mission to TOPEX/Poseidon.

 

The graphic representation of the landform on a map by the use of contour lines.

 

A unit of survey of the public lands of the United States, normally a quadrangle approximately 6 miles on a side with boundaries conforming to meridians and parallels within established limits, containing 36 sections.

Track (TRK)

The direction of movement relative to a ground position. Commonly associated with navigation applications.

 

A collection of points that show the path travelled.

 

A single point on the path travelled.

 

A precision surveying instrument; a theodolite in which the telescope can be reversed in direction by rotation about its horizontal axis.

 

The method of determining a location on a map by using two or more points through which lines of known directions are drawn; the intersection of these lines is the desired location; can use distance and/or angles.

 

The same method as triangulation, except trilateration uses only distances for locating.

Triple-Difference

A linear combination of Double-Difference carrier phase observables by which the cycle ambiguity parameters can be eliminated and which is less affected by unrepaired cycle slips than Double-Differences. A Triple-Differenced observable is created by differencing two consecutive Double-Differences (the same pair of receivers and the same pair of satellites, but separated in time). A useful observable for obtaining approximate baseline solutions or for detecting cycle slips in the Double-Differenced observables.

Trivial Baseline

Trivial Baselines are those baselines formed when more than two GPS receivers are used simultaneously in the field to perform static GPS surveys. For example, when 3 receivers at points A, B, C are deployed only 2 baselines are independent (either A-B & A-C, AB & B-C, or AC & C-B), with the other one being trivial. This trivial baseline may be processed, but because the data used for this baseline has already been used to process the independent baselines, the baseline results should not be used for Network Adjustment or for quality control purposes unless the statistics (and variance-covariance matrix) are appropriately downweighted.

Troposphere, Tropospheric Delay

The Troposphere is the neutral atmosphere comprising the lower 8km of the atmosphere. The Tropospheric Delay on GPS signals is of the non-dispersive variety because it is not frequency-dependent and hence impacts on both the L1 and L2 signals by the same amount (unlike that within the Ionosphere). The wet and dry components of the Troposphere cause the Delay to the signals, with the wet component be responsible for approximately 10% of the total delay. Various Tropospheric Delay models have been developed to estimate the Delay as a function of the satellite elevation angle, receiver height, and meteorological parameters such as temperature, pressure and humidity. The Delay in the Zenith direction is approximately 2.5m, increasing as the elevation angle of the satellite signal reduces. (This behaviour is described by the so-called Mapping Function, so that the Delay near the horizon is 3-5 times higher than in the Zenith direction.) The Delay is largely eliminated in Relative or Differential Positioning, however the residual Tropospheric Delay increases as the baseline length increases and may be a significant source of error (especially in the height component) for very high precision GPS Geodesy.

 

 

The geographical north pole.

 

 

Time To First Fix - The time it takes for a GPS receiver to lock onto the satellite signals and determine the initial position.

2drms

A position accuracy measure defined as twice the RMS of the horizontal error. This approximately corresponds to the 95% confidence interval, or "two sigma" value. For example, under the policy of "Selective Availability" GPS Absolute Positioning accuracy is claimed to be 100m 2drms, which means that approximately 95% of the horizontal position solutions will be within 100m of the correct value.

- U -

 

Universal Mobile Telecommunications System - A transmission system for text, digitized voice, video, and multimedia for mobile computer and phone users no matter where they are located in the world.

UTC (Coordinated Universal Time)

Formerly referred to as GMT or Greenwich Mean Time. This is the basis of "civilian time".

Universal Transverse Mercator (UTM)

A grid coordinate system that projects global sections onto a flat surface to measure position in specific zones. These zones are 6° wide and are stepped along the equator such that each zone corresponds to a north-south strip of the earth.

 

 

Universal Polar Stereographic - An azimuthal projection that is used with Universal Transverse Mercator (UTM) projection systems to represent polar regions.

User Segment

That component of the GPS system that includes the user equipment, applications and operational procedures.

 

 

United States Geological Survey.

 

 

U.S. National Grid - A standard to create a more favorable environment for developing location-based services within the United States and to increase the interoperability of location services appliances with printed map products by establishing a nationally consistent grid reference system as the preferred grid for National Spatial Data Infrastructure (NSDI) applications. The U.S. National Grid is based on universally defined coordinate and grid systems and can, therefore, be easily extended for use world-wide as a universal grid reference system.

 

 

Vertical Angle Benchmark - A bench mark with elevation established by vertical angle methods.

 

Maps that use lines and polygons as opposed to pixels to display an image. Image resolution is sharp even when the image is displayed or printed at an enlarged perspective.

 

A grassland of southern Africa.

Velocity-Made-Good (VMG)

The speed you are closing in on a destination along a desired course. A navigation term.

 

 

Velocity Over Ground - The speed at which the GPS unit and the person operating it are moving with respect to the earth's surface (ignoring tidal and current activity). Same as Speed Over Ground (SOG) and ground speed.

Waypoint

A (usually two-dimensional) coordinate that is input into a navigation device, such as a GPS receiver, representing a position that a vessel, aircarft, vehicle or person has to navigate to, with the aid of GPS (and/or any other position fixing device).

Wide Area Augmentation System (WAAS)

WAAS is a US Federal Aviation Authority (FAA) funded system of equipment and software that augments GPS accuracy, availability and integrity. The WAAS provides a satellite signal for WAAS users to support enroute and precision approach aircraft navigation. Similar systems are under development in Europe (where it is known as EGNOS -- European Geostationary Navigation Overlay System), Japan (where it is known as MT-SAT), and Australia.

World Geodetic System 1984 (WGS84)

A global Geodetic Datum defined and maintained by the US Department of Defense. As the Control Segment coordinates and the Broadcast Ephemerides are expressed in this Datum, the GPS positioning results are said to be in the WGS84 Datum. In the case of Point Positioning this is largely true, although the level of accuracy achievable under the policy of Selective Availability is so poor that the link to the WGS84 Datum is very approximate. In the case of Relative Positioning, the baseline vector may be determined to quite high accuracy (at the sub-centimetre level using precise GPS Surveying techniques), however the coordinate (and therefore the Datum) of the unknown point is almost completely defined by the Datum of the Base Station. This may not be coincident with the WGS84 Datum at better than a few tens of metres! If GPS Geodesy techniques are used, with known station coordinates expressed in the ITRS and precise ephemerides obtained from the IGS, it is more correct to state that the subsequent set of coordinates are expressed in one of the ITRS frames (e.g. ITRF92, ITRF94, etc.). The WGS84 and the ITRS are compatible at the one metre level. However, the ITRS is a more precise realisation of an earth-fixed, earth-centred terrestrial reference system.

 

 

World-Wide Media eXchange - A centralized index of digital photos, where photos are tagged by the geographic location where they were shot.

- X -

XTE

See Crosstrack Error.

- Y -

Y-Code

The term used to refer to the encrypted P-Code, generated within the satellites and transmitted on both the L1 and L2 carrier signals under the policy of "Anti-Spoofing". Civilian GPS receivers use proprietary signal processing techniques to make measurements of pseudo-range and carrier phase on both L-Band frequencies.

- Z -

 

The point on the celestial sphere that is directly above the observer.

 

A telescope for observing stars near the zenith.

Zero Baseline

A Zero Baseline test can be used to study the precision of receiver measurements (and hence its correct operation), as well as the data processing software. The experimental setup, as the name implies, involves connecting two GPS receivers to the same antenna. When two receivers share the same antenna, biases such as those which are satellite (clock and ephemeris) and atmospheric path (troposphere and ionosphere) dependent, as well as errors such as multipath CANCEL during data processing. The quality of the resulting "zero baseline" is therefore a function of random observation error (or noise), and the propagation of any receiver biases that do not cancel in double-differencing.