A Glossary of GPS Terms

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I/O

Abbreviation for Input/Output.

Ionosphere, Ionospheric Delay

The Ionosphere is that band of atmosphere extending from about 50 to 1000 kilometres above the earth's surface in which the sun's ultraviolet radiation ionises gas molecules which then lose an electron. These free electrons influence the propagation of microwave signals (speed, direction and polarisation) as they pass through the layer. The Ionospheric Delay on GPS signals is frequency-dependent and hence impacts on the L1 and L2 signals by a different amount (unlike that within the Troposphere). A linear combination of pseudo-range or carrier phase observations on the L1 and L2 carrier waves can be created to almost entirely eliminate the Ionospheric Delay. The resulting observable is known as the Ionosphere-Free carrier phase (or pseudo-range). For single-frequency receivers it is not possible to account for this signal bias in this way. A broadcast model is contained within the transmitted Navigation Message, however, it is a relatively poor model (unlikely to account for more than 50% of the effect) as the Delay is very difficult to predict. The magnitude of the Ionospheric Delay is a function of the latitude of the receiver, the season, the time of day, and the level of solar activity. The Delay in the Zenith direction can be several tens of metres, increasing as the elevation angle of the satellite signal reduces (being 3-5 times greater than in the Zenith direction). The Delay is largely eliminated in Relative or Differential Positioning, however, the residual Ionospheric 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. Even when using dual-frequency instrumentation, the Ionospheric Delay can still cause problems during the process of rapid Ambiguity Resolution when phase and range combinations other than the Ionosphere-Free one are used.

Ionosphere-Free Combination

This is a particular linear combination of the observations made on the L1 and L2 carrier waves that eliminates (to the first order) the ionospheric delay on the GPS observables. The ionosphere-free L1 carrier phase combination (in units of L1 wavelengths) is:
f(L1)ion-free = a1.f(L1) + a2.f(L2)

with a1 = f12f12 - f22 and a2 = - f1f2f12 - f22 , f1 and f2 are the frequencies of the L1 and L2 carrier waves respectively. (A similar expression can be developed for the ionosphere-free L2 carrier phase.) The ionosphere-free pseudo-range combination (in metric units) is:
Pion-free = b1.P(L1) + b2.P(L2)

with b1 = f12f12 - f22 and b2 = - f22f12 - f22 .

Independent Baseline

These are baselines observed using GPS Relative Positioning techniques which are the minimum necessary to transfer the Datum from one Base Station to all other stations within a ground network. For example, if there are M stations, there will be M-1 independent baselines linking all the stations. Any extra baselines that are measured are "redundant" baselines which may improve the quality and reliability of the station coordinates after Network Adjustment.

Integrity

A quality measure of GPS performance for critical applications such as civilian aviation. A high level of integrity is sought for such applications.

International GPS Service (IGS)

An initiative of the International Association of Geodesy, as well as several other scientific organisations, that was established as a service at the beginning of 1994. The IGS comprises of many component civilian agencies working cooperatively to operate a permanent global GPS tracking network, to analyse the recorded data and to disseminate the results to users via the Internet. The range of "products" of the IGS include precise post-mission GPS satellite ephemerides, tracking station coordinates, earth orientation parameters, satellite clock corrections, tropospheric and ionospheric models. Although these were originally intended for the geodetic community as an aid to carrying out precise surveys for monitoring crustal motion, the range of users has since expanded dramatically, and the utility of the IGS is such that it is vital to the definition and maintenance of the International Terrestrial Reference System (and its various "frame realisations" ITRF92, ITRF94, ITRF96, etc.).

International Terrestrial Reference System (ITRS)

The most precise, geocentric, globally-defined coordinate system or datum on the earth's surface. It is a more accurate and more convenient a Satellite-Based Datum than the WGS84 Datum. The various "frames" (such as ITRF96, etc.) are realisations of the ITRS for a particular epoch in time, consisting of a set of 3-D coordinates and velocities for hundreds of geodetic stations around the world (all coordinates of fixed stations on the earth change with time due to "continental drift"). Although some of the stations are Satellite Laser Ranging (SLR) stations, or Very Long Baseline Interferometry (VLBI) stations, the vast majority are GPS tracking stations of the IGS network.

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JPO (Joint Program Office)

That part of the U.S. Department of Defense responsible for managing the GPS development, deployment and operation of the GPS system (in particular the Control Segment and the Space Segment, as well as the military User Segment).

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Kinematic Positioning

Kinematic Positioning refers to applications in which the position of a non-stationary object (vehicle, ship, aircraft) is determined.

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L1 Frequency

1575.42MHz GPS carrier frequency which contains the C/A-Code, the encrypted P-Code (or Y-Code) and the Navigation Message. Commercial GPS navigation receivers can track only the L1 carrier to make pseudo-range (and sometime carrier phase and Doppler frequency) measurements.

L2 Frequency

1227.60MHz GPS carrier frequency which contains only the encrypted P-Code (or Y-Code) and the Navigation Message. Military Y-Code capable receivers can, in addition to making L1 measurements, make pseudo-range measurements on the L2 carrier. The combination of the two measurements (on L1 and L2) permits the Ionospheric Delay to be corrected for. Dual-frequency GPS receivers intended for Surveying applications can make L2 measurements using proprietary signal processing techniques. Such measurements are essential if the Ionospheric Delay on carrier phase is to be corrected for (especially on baselines of length greater than about 20-30km) and/or where fast Ambiguity Resolution is needed.

Local Area Augmentation System (LAAS)

Plan by which Local Area Differential GPS (LADGPS), which generates and transmits differential corrections to appropriately equipped aircraft users, is augmented with integrity messages transmitted from the ground and additional ranging signals. LAAS is set up near a major airport, and consists of the DGPS reference station, the integrity monitoring receiver and a pseudolite which transmits "satellite-like" PRN-coded signals to incoming aircraft.

Latitude

A north/south angular measurement of position relative to the equator, in the meridian plane which contains the earth's rotation axis.

L-Band

The group of radio frequencies extending from 390MHz to 1550MHz. The GPS carrier frequencies L1 and L2 are in the L-Band.

Longitude

An east/west angular measurement of position in relation to the Prime Meridian. The angle between the two great circles, one being the Prime (or Greenwich) Meridian and the other a meridian passing through the point of interest. (A great circle that passes through the north and south poles, and hence contains the earth's rotation axis.)

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Mask Angle

See Cutoff Angle

Minimally Constrained

A form of least squares solution in which the observed baseline vectors are treated as "observations" in a secondary network adjustment (see Network Adjustment), and only one coordinate must be held fixed to its known value while all others are allowed to adjust. Typically GPS surveys measure more baselines than the minimum needed to coordinate all the points in the network. These extra "observations" are redundant information that a minimally constrained network adjustment uses to derive optimum estimates of the coordinate parameters, as well as valuable quality information in the form of parameter standard deviations and error ellipses (or ellipsoids).

Multi-Channel Receiver

A GPS receiver that can simultaneously track more than one satellite signal using a dedicated signal electronics channel for each satellite. High quality receivers may have 12 channels for L1, and another 12 channels for L2 signals. Lower quality GPS navigation receivers may have only 6 or 8 channels. In contrast to a Multiplexing Channel Receiver.

Multipath

Interference caused by reflected GPS signals arriving at the receiver, typically as a result of nearby structures or other reflective surfaces. May be mitigated to some extent through appropriate antenna design, antenna placement and special filtering algorithms within GPS receivers.

Multipath Error

Errors caused by the interference of a signal that has reached the receiver antenna by two or more different paths. This is usually caused by one path being bounced or reflected. The impact on a pseudo-range measurement may be up to a few metres. In the case of carrier phase, this is of the order of a few centimetres.

Multiplexing Channel

A channel of a GPS receiver that can be sequenced through a number of satellite signals. In contrast to a Multi-Channel Receiver in which one channel is dedicated to each satellite signal.

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Navigation Message

Also known as the Data Message, containing the satellite's broadcast ephemeris, satellite clock (bias) correction parameters, constellation almanac information and satellite health.

NAVSTAR

The name sometimes given to the GPS satellite system. NAVSTAR is an acronym for NAVigation Satellite Timing and Ranging.

Network Adjustment

A form of least squares solution in which the observed baseline vectors are treated as "observations" in a secondary adjustment (see Minimally Constrained). It may be a minimally constrained network adjustment with only one station coordinate held fixed, or it may be constrained by more than one fixed (known) coordinates. The latter is typical of a GPS survey carried out to densify or connect some newly coordinated points to a previously established control or geodetic framework (see Datum).

NMEA

National Marine Electronics Association, a U.S. standards body that defines message structure, content and protocols to allow electronic equipment installed within ships and boats to communicate with each other. GPS receivers can be configured to output various types of messages in the "NMEA format".

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OEM

Original Equipment Manufacturer. Typically GPS receiver "boardsets" or "engines" that a product developer can embed within some application or hardware package.

On-The-Fly (OTF)

This is a form of Ambiguity Resolution (AR) which does not require that the receivers remain stationary for any length of time. Hence this AR technique is suitable for initialising carrier phase-based Kinematic Positioning. For many applications this introduces considerable flexibility. For example, aircraft do not have to be parked on the ground in order to resolve the carrier cycle ambiguities, and then require that signal lock-on be maintained throughout the kinematic survey. However, dual-frequency instrumentation capable of making both carrier phase and precise (P-Code level) pseudo-range measurements is required.

Order of Survey

In an analogous manner to "Class of Survey", Order of Survey is a means of categorising the quality, or precision, of a static survey. However, it relates to the external quality, and is influenced by the quality of the "external" network information. The number of categories, the notation applied, and the accuracy tolerances defining the transition from one order to another are defined by individual nations. Typically they mirror the categories of Class of Survey, hence an A Class survey may correspond to a 1st Order survey. The labeling of a particular Order (e.g. 1st, 2nd, etc.) to a survey points within a "network" (whether it is carried out using GPS or any other technique) is performed as part of the process of Network Adjustment in which the relative error ellipses (in the horizontal case) between coordinated stations are computed and compared with the accuracy standards that must be met for various categories of Order. However, unlike the Minimally Constrained Network Adjustment that is a prerequisite to establishing the Class of Survey, the Network Adjustment must be constrained to the surrounding geodetic control. Hence a very high quality GPS network (therefore a high Class survey) may be distorted to "fit" the existing control which may have been determined using a lower Class survey. The resulting Order of the Survey would have to match the lower of either the Class of the GPS survey or the Class of the existing geodetic control. If the existing geodetic control is of a lower quality to what can be achieved using modern GPS Surveying techniques, then the geodetic control network must be upgraded or "renovated" using more precise GPS Geodesy techniques.

Outage

Defined as a loss of Availability, due to either there not being enough satellites visible to calculate a position, or the value of the DOP indicator is greater than some specified value (implying that the accuracy of the position is unreliable).

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P-Code

The Precise or Protected code. A very long sequence of PRN binary biphase modulations on the GPS L1 and L2 carrier at a chip rate of 10.23MHz, which repeats about every 267 days. Each one week segment of this code is unique to a GPS satellite and is reset each week. Under the policy of "Anti-Spoofing" the US Dept. of Defense has encrypted the P-Code (replacing it with a so-called Y-Code). Only US military and other authorised users are able to overcome AS using special receivers.

Phase-Smoothed Pseudo-Range

The pseudo-range measurement which has had its "noise" level (random errors) reduced by being combined with the high precision carrier phase. It is still an unambiguous "range" measurement which can be processed using the standard algorithms of Point Positioning or Relative Positioning.

Point Positioning

See Absolute Positioning

Position

The 3-D coordinates of a point, usually given in the form of Latitude, Longitude, and Altitude (or Ellipsoidal Height), though it may be provided in the 3-D Cartesian form, or any other transformed map or geodetic reference system. An estimate of error is often associated with a position.

Position Dilution of Precision (PDOP)

See Dilution of Precision. Measure of the geometrical strength of the GPS satellite configuration for 3-D positioning.

Post-Processed GPS

In post-processed (Differential or Relative ) GPS the base and user (or roving or mobile) receivers have no data communication link between them. Instead, each receiver records the satellite observations that will allow differential correction (in the case of pseudo-range-based positioning), or the processing of double-differenced observables (in the case of carrier phase-based positioning) at a later time. Data processing software is used to combine and process the data collected from these receivers.

Precise Positioning Service (PPS)

The most accurate Absolute Positioning possible with GPS navigation receivers, based on the dual-frequency encrypted P-Code. Available to the military users of GPS. Typical accuracy is of the order of 10-20m.

Pseudolite

A ground-based differential GPS receiver which transmits a signal like that of an actual GPS satellite, and can be used for ranging. Originally intended as an augmentation for Local Area Augmentation Systems to aid aircraft landings. However, pseudolites may also be used where signal obstructions are such that insufficient GPS satellites can be tracked. In fact, pseudolites are feasible in circumstances where no satellite signals are observable, e.g. for indoor applications.

Pseudo-Random Noise (PRN)

A binary signal with random noise-like properties. It is generated by mathematical algorithm or "code", and consists of repeated pattern of 1's and 0's. This binary code can be modulated on the GPS carrier waves using Binary Shift-Key (BSK) modulation. The C/A-Code and the P-Code are examples of PRN codes. Each satellite transmits a unique C/A-Code and P-Code sequence (on the same L1 and L2 frequencies), and hence a satellite may be identified according to its "PRN number", e.g. PRN2 or PRN14 are particular GPS satellites.

Pseudo-Range

A distance measurement based on the correlation of a satellite's transmitted code (may be the C/A-Code or the encrypted P-Code) and the local receiver's reference code (for that PRN satellite number), that has not been corrected for errors in synchronisation between the transmitter's clock and the receiver's clock. Hence a pseudo-range measurement is a time-error biased distance measurement. The precision of the measurement is a function of the resolution of the code, hence C/A-Code pseudo-range measurements may have a "noise" at the few metre level for standard GPS receivers (and at the sub-metre precision level in the case of so-called "narrow correlator" GPS receivers).

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