Almanac

A set of parameters transmitted by each GNSS satellite that enables a receiver to predict the approximate location of the satellite. The data includes orbit information on all the satellites, clock correction and atmospheric delay parameters. This data is used to facilitate rapid satellite vehicle acquisition. The orbit information is a subset of the ephemeris data, with reduced accuracy.

Ambiguity

The unknown integer number of carrier phase cycles in an unbroken set of GNSS measurements. In GNSS processing mathematical calculations are made to compute this number, thus 'resolving the ambiguity'.

Baseline

The three-dimensional vector distance between a pair of stations for which simultaneous GNSS data has been collected and processed with static differential techniques. This is the most accurate GNSS result.

Coarse/Acquisition (C/A) Code

Two pseudo random noise (PRN) codes are transmitted by each GPS satellite, C/A and P (Precision). C/A is the simpler, non-military code, which is modulated onto the GPS L1 signal. The code is a sequence of 1024 pseudo random binary bi-phase modulations of the GPS carrier at a chipping rate of 1.023MHz, thus having a code repetition period of one millisecond. This code was selected to provide good acquisition properties.

Carrier

An unmodulated radio wave having characteristics of frequency, amplitude, phase.

Continuously Operating Reference Station (CORS)

A network of RTK base stations that broadcast corrections, usually over an Internet connection. Accuracy is increased in a CORS network because more than one station helps ensure correct positioning and guards against a false initialisation of a single base station.

Cycle slip

The loss of lock of the satellite signal by the receiver. When lock is resumed the fractional part of the measured phase would still be the same as if tracking had been maintained. The integer number of cycles exhibits a discontinuity or 'cycle slip'.

Differential positioning

Determination of relative coordinates of two or more receivers that are simultaneously tracking the same satellites. Dynamic differential positioning is a real-time calibration technique achieved by sending corrections to the roving user from one or more monitor stations. Static differential GNSS involves determining baseline vectors between pairs of receivers.

Differential processing

GNSS measurements can be differenced between receivers, satellites and epochs. Although many combinations are possible, the present convention for differential processing of GNSS phase measurements is to take differences between receivers (single difference), then between satellites (double difference), then between measurement epochs (triple difference). A single difference measurement between receivers is the instantaneous difference in phase of the signal from the same satellite, measured by two receivers simultaneously. A double difference measurement is obtained by differencing the single difference for one satellite with respect to the corresponding single difference for a chosen reference satellite. A triple difference measurement is the difference between a double difference at one epoch of time and the same double difference at the previous epoch of time.

Dilution of precision (DOP)

A computed unitless scalar value which describes the geometric contribution to the uncertainty of a GNSS position solution. For any GNSS fix a DOP value is computed. It is usually either geometric DOP (GDOP), position DOP (PDOP) or horizontal DOP (HDOP). In addition, other values exist such as vertical, time and relative DOP. See definition of PDOP for further details.

Double difference method

A method to determine that set of ambiguity values which minimises the variance of the solution for a receiver pair baseline vector.

Dynamic positioning

Determination of a time series of sets of coordinates for a moving receiver, each set of coordinates being determined from a single data sample, and usually computed in real time.

Earth-centred earth-fixed (ECEF)

Cartesian coordinate system where the X direction is through the intersection of the prime meridian (Greenwich) with the equator. The axes rotate with the Earth. Z is the direction of the spin axis.

Elevation

Height above a defined level datum, e.g. mean sea level or the geoid.

Elevation mask

The lowest elevation in degrees above the horizon at which a GNSS receiver is set to track a satellite. It is usually set to 10 degrees or 15 degrees to avoid atmospheric effects and signal interference. A lower mask angle would increase ionospheric distortion and also tropospheric effects.

Ellipsoid

In geodesy, unless otherwise specified, a mathematical figure formed by revolving an ellipse about its minor axis. Used interchangeably with spheroid.

Ephemeris

The set of data that describes the position of a celestial object as a function of time. The GNSS ephemeris is used in the processing of GNSS observations. Either the broadcast ephemeris from the satellite navigation message or a precise ephemeris calculated from GNSS tracking stations can be used, depending on application.

Epoch

A point in time that is the reference for a set of coordinates. The measurement interval or data frequency, as in recording observations every 15 seconds. In this example loading data using 30-second epochs means loading every other measurement.

European Geostationary Navigation Overlay Service (EGNOS)

A European operated, satellite based real-time differential GNSS system. EGNOS transmits a signal containing information on the reliability and accuracy of the positioning signals sent out by GPS and GLONASS.

Float solution

A baseline solution that does not fix the integer ambiguity values to whole numbers. The values are left as non-integer real numbers giving the baseline a higher RMS. than a fixed baseline. In general float solutions are not acceptable as final baseline measurements.

Galileo

The European Commission's GNSS system.

Geodetic datum

A mathematical model designed to best fit part or all of the geoid. Conventional datums depended upon an ellipsoid and an initial station on the topographic surface established as the origin of the datum. Such datums were defined by the dimensions of the spheroid, by the geodetic latitude, longitude and the height of geoid above the ellipsoid at the origin, by the two components of the deflection of the vertical at the origin, and by the geodetic azimuth of a line from the origin to some other point. Geocentric datums are designed to give the best possible fit worldwide rather than to depend upon values determined at an initial station. Their origin is the geo-centre of the earth.

Geoid

The particular equipotential surface that most closely approximates to mean sea level in the open oceans and which may be imagined to extend through the continents. This surface is everywhere perpendicular to the force of gravity.

Geometric dilution of precision (GDOP)

The relationship between errors in user position and time and in satellite range. GDOP = PDOP + TDOP. See PDOP.

Global Navigation Satellite System (GNSS)

The generic term for satellite navigation systems, including GPS, GLONASS, Galileo and Compass.

Height - ellipsoidal

The distance above or below the ellipsoid measured along the normal to the ellipsoid at that point. Not the same as elevation above sea level. GNSS receivers output position-fix height as the height above the ITRS ellipsoid.

Initialisation

The moment when a rover GNSS receiver in a high precision real-time dynamic system (RTK) solves the integer ambiguity and gains a real-time high precision fixed baseline solution.

Ionospheric delay

The ionosphere is a non-homogeneous (both in space and time) and dispersive medium. A wave propagating through the ionosphere experiences variable delay. Phase delay depends on electron content and affects carrier signals. Group delay depends on dispersion in the ionosphere as well, and affects signal modulation. The phase and group delay are of the same magnitude but opposite sign.

Kinematic surveying

A dynamic method of GNSS surveying using carrier phase observations in which one receiver is moving and one receiver is stationary. It is a highly productive survey method, useful for ground control or camera positioning, but is sensitive to high DOP values, multipath interference and loss of signal lock. Operational constraints include starting from or determining a known baseline, and tracking a minimum of four satellites. One receiver is statically located at a control point, while others are moved between points to be measured.

Multipath errors

Signals can arrive at a GNSS receiver either by direct line of sight or can be reflected off nearby objects (hills, buildings, etc.), in which case the differences in path length will cause interference at the antenna and corrupt the pseudorange measurements and subsequent positional reliability. (An interference similar to ghosting on a television screen).

Narrow lane

A baseline solution that is a linear combination of the L1 and L2 frequencies. It is often an intermediate solution used for statistical testing in the process of obtaining a final L1 or iono free fixed solution.

Network RTK

The networking of GNSS base stations to enable real-time corrections to be generated and transmitted to users anywhere in the area covered by the base station network. Users receive GNSS corrections from a central source, not directly from individual base stations. The systems work by using the GPS observations at known network stations to model the unknown bias sources across the network area. From these, positional corrections can be generated and delivered to the rover as either a set of multiple reference stations or as the corrections that would be generated from a 'virtual' base station adjacent to the user. Actual raw base station GPS data or virtual GPS observation data is also transmitted to the rover.

Point positioning

A position produced from one receiver in a stand-alone mode.

Position dilution of precision (PDOP)

PDOP is a unitless scalar value expressing the relationship between the error in user position and the error in satellite position. Geometrically, for four satellites PDOP is proportional to the inverse of the volume of the pyramid formed by unit vectors from the receiver to the four satellites observed. Values considered good for position are small, say 3. Values greater than 7 are considered very poor. Thus, small PDOP is associated with widely separated satellites.

PDOP is related to horizontal and vertical DOP by PDOP2 = HDOP2 + VDOP2. Small PDOP is important in dynamic surveys, which are sensitive to larger PDOP values, but much less so in static techniques.

Pseudo random noise (PRN)

PRN is a sequence of binary digits that appear to be randomly distributed. This is used in the GNSS C/A and P codes, with each GNSS satellite transmitting a unique PRN. GNSS receivers use this PRN to identify which satellites they are tracking. The important property of PRN codes is that they have a low auto correlation value for all delays or lags except when they are exactly coincident. Each NAVSTAR satellite has its own unique C/A and P pseudo random noise codes.

Pseudorange

The apparent distance from a satellite to the phase centre of a GNSS receiver antenna. This is computed from the C/A or P code which gives a signal propagation time. This time can then be multiplied by the speed of light to give an apparent distance, which is not the true distance.

Pseudorange differs from the actual range by the amount that the satellite and user clocks are offset, by propagation delays, and other errors. The apparent propagation time is determined from the time shift required to align (correlate) a replica of the GNSS code generated in the receiver with the received GNSS code. The time shift is the difference between the time of signal reception (measured in the receiver time frame) and the time of emission (measured in the satellite time frame).

Radio Technical Commission for Maritime Services - State Space Representation (RTCM SSR)

A high accuracy data output from International GNSS Service (IGS) that allows users to access information on satellite orbit errors, satellite clock errors, satellite signal biases, ionospheric propagation delays and advances, and tropospheric delays.

Receiver Independent Exchange format (RINEX)

A set of standard definitions and formats to promote the free exchange of GNSS data and facilitate the use of data from any GNSS receiver with any software package. The format includes definitions for three fundamental GNSS observables: time, phase, and range.

Reference frame

The realisation of any particular coordinate reference system by the measurement of points using survey instruments. There can be several realisations of any system as survey techniques and methods change.

Reference system

A mathematical definition of the particular coordinate system, including the origin, scale position and orientation of the reference ellipsoid.

Relative positioning

The process of determining the relative difference in position between two points with greater precision than that to which the position of a single point can be determined. Here, a receiver (antenna) is placed over each point and measurements are made by observing the same satellites at the same time. This technique allows cancellation (during computations) of all errors which are common to both observation sets, such as satellite clock errors, satellite ephemeris errors and the majority of propagation delays, etc.

Root mean square (error) (RMS, RMSE)

In general, when accuracies or tolerances have been specified, they refer to vector errors and are defined statistically as root mean square errors (RMSE), or as maximum tolerances. The RMSE is equivalent to a 67% tolerance, and a 90% tolerance is 1.65 times the RMSE when a representative sample of points is tested. Thus an RMSE of ± 0.01m indicates that in a representative sample of 100 points, it is expected that not less than 67 will be correct to better than ± 0.01m, and not less than 90 points will be correct to better than ± 0.016m. Any errors exceeding three times the RMSE, in this case ± 0.03m, can be regarded as mistakes.

Sigma (one sigma)

The 68th percentile or one standard deviation measure in a statistical population.

Static positioning

Positioning applications in which the positions of static or near static points are determined.

Tropospheric correction

The correction applied to the measurement to account for tropospheric delay. This value is obtained from a model such as that of Hopfield.

WGS 84 World Geodetic System (1984)

The geocentric datum used by GNSS since January 1987. It has its own reference ellipsoid. WGS 84 is fully defined in publications by the US. National Imagery and Mapping Agency (NIMA).

Wide lane

A linear combination of L1 and L2 observations (L1-L2) used to partially remove ionospheric errors. This combination yields a solution in about one-third the time of a complete ionosphere-free solution.