Glossary

Pulse-per-second. Used in hardware timing. A pulse is generated in conjunction with a time stamp. This defines the instant when the time stamp is applicable.

A file that contains orbit information on all the satellites, clock corrections, and atmospheric delay parameters. The almanac is transmitted by a GNSS satellite to a GNSS receiver, where it facilitates rapid acquisition of GNSS signals when you start collecting data, or when you have lost track of satellites and are trying to regain GNSS signals.

The orbit information is a subset of the ephemeris/ephemerides data.

GNSS signal spoofing is an increasing concern. The latest firmware implements various mitigation techniques to detect and eliminate spoofing. The detection techniques are a combination of signal processing and navigation filter updates. If the receiver detects it is tracking a spoofed signal, it attempts to find the correct signal from the satellite and relock the channel. If it is unable to identify the correct signal, it sets the RAIM flag for the satellite in the streamed data observable data and eliminates it from the position solution. Disclaimer: While new GNSS anti-spoofing feature is a vast improvement, no anti-spoofing feature can guarantee full protection of spoofing or location hacking.

Also called reference station. In construction, a base station is a receiver placed at a known point on a jobsite that tracks the same satellites as an RTK rover, and provides a real-time differential correction message stream through radio to the rover, to obtain centimeter level positions on a continuous real-time basis. A base station can also be a part of a virtual reference station network, or a location at which GNSS observations are collected over a period of time, for subsequent postprocessing to obtain the most accurate position for the location.

The BeiDou Navigation Satellite System (also known as BDS) is a Chinese satellite navigation system.

The first BeiDou system (known as BeiDou-1), consists of four satellites and has limited coverage and applications. It has been offering navigation services mainly for customers in China and from neighboring regions since 2000.

The second generation of the system (known as BeiDou-2) consists of satellites in a combination of geostationary, inclined geosynchronous, and medium earth orbit configurations. It became operational with coverage of China in December 2011. However, the complete Interface Control Document (which specifies the satellite messages) was not released until December 2012. BeiDou-2 is a regional navigation service which offers services to customers in the Asia-Pacific region.

The third generation of the system (known as BeiDou-3) consists of 3 geostationary satellites, 3 geosynchronous, and 24 Medium Earth orbit (MEO) satellites. BeiDou-3 is fully operational as of June 2020.

An internet server that manages authentication and password control for a network of VRS servers, and relays VRS corrections from the VRS server that you select.

A radio wave having at least one characteristic (such as frequency, amplitude, or phase) that can be varied from a known reference value by modulation.

A wireless adapter that connects a laptop computer to a cellular phone system for data transfer. Cellular modems, which contain their own antennas, plug into a PC Card slot or into the USB port of the computer and are available for a variety of wireless data services such as GPRS.

When enabled, the receiver clock is steered to GPS system time rather than periodically introducing 1 ms steps and constraining the clock to ± 0.5 ms. Disabled by default; this setting does not affect performance.

Compact Measurement Record. A real-time message format developed by Trimble for broadcasting corrections to other Trimble receivers. CMR is a more efficient alternative to RTCM.

A real-time message format developed by Trimble for transmitting more satellite corrections resulting from more satellite signals, more constellations, and more satellites. Its compactness means more repeaters can be used on a site.

A unique identifier for the CMR message. It can be any value between 0 through 31.

Shows whether or not CMR corrections are being used from a specific base station.

Code differential solution. Typically a single-frequency solution.

An external height constraint for the antenna position. The receiver will produce a height value within the constraints provided by the external application.

A statistical measure of the variance of two random variables that are observed or measured in the same mean time period. This measure is equal to the product of the deviations of corresponding values of the two variables from their respective means.

Also called geodetic datum. A mathematical model designed to best fit the geoid, defined by the relationship between an ellipsoid and, a point on the topographic surface, established as the origin of the datum. World geodetic datums are typically defined by the size and shape of an ellipsoid and the relationship between the center of the ellipsoid and the center of the earth.

Because the earth is not a perfect ellipsoid, any single datum will provide a better model in some locations than in others. Therefore, various datums have been established to suit particular regions.

For example, maps in Europe are often based on the European datum of 1950 (ED-50). Maps in the United States are often based on the North American datum of 1927 (NAD-27) or 1983 (NAD-83).

All GPS coordinates are based on the WGS-84 datum surface.

Withdrawal of all electrical energy to the end-point voltage before the cell or battery is recharged.

See real-time differential GPS.

Differential correction is the process of correcting GNSS data collected on a rover with data collected simultaneously at a base station. Because the base station is on a known location, any errors in data collected at the base station can be measured, and the necessary corrections applied to the rover data.

Differential correction can be done in real-time, or after the data is collected by postprocessing.

See real-time differential GPS.

Dilution of Precision. A measure of the quality of GNSS positions, based on the geometry of the satellites used to compute the positions. When satellites are widely spaced relative to each other, the DOP value is lower, and position precision is greater. When satellites are close together in the sky, the DOP is higher and GNSS positions may contain a greater level of error.

PDOP (Position DOP) indicates the three-dimensional geometry of the satellites. Other DOP values include HDOP(Horizontal DOP) and VDOP (Vertical DOP), which indicate the precision of horizontal measurements (latitude and longitude) and vertical measurements respectively. PDOP is related to HDOP and VDOP as follows: PDOP² = HDOP² + VDOP².

A type of receiver that uses both L1 and L2 signals from GPS satellites. A dual-frequency receiver can compute more precise position fixes over longer distances and under more adverse conditions because it compensates for ionospheric delays.

European Geostationary Navigation Overlay Service. A Satellite-Based Augmentation System (SBAS) that provides a free-to-air differential correction service for GNSS. EGNOS is the European equivalent of WAAS, which is available in the United States.

The vertical distance from a geoid such as EGM96 to the antenna phase center. The geoid is sometimes referred to as Mean Sea Level.

The angle below which the receiver will not track satellites. Normally set to 10 degrees to avoid interference problems caused by buildings and trees, atmospheric issues, and multipath errors.

An ellipsoid is the three-dimensional shape that is used as the basis for mathematically modeling the earth’s surface. The ellipsoid is defined by the lengths of the minor and major axes. The earth’s minor axis is the polar axis and the major axis is the equatorial axis.

Height above ellipsoid.

A list of predicted (accurate) positions or locations of satellites as a function of time. A set of numerical parameters that can be used to determine a satellite’s position. Available as broadcast ephemeris or as postprocessed precise ephemeris.

The measurement interval of a GNSS receiver. The epoch varies according to the measurement type: for real-time measurement it is set at one second; for postprocessed measurement it can be set to a rate of between one second and one minute. For example, if data is measured every 15 seconds, loading data using 30-second epochs means loading every alternate measurement.

A feature is a physical object or event that has a location in the real world, which you want to collect position and/or descriptive information (attributes) about. Features can be classified as surface or non-surface features, and again as points, lines/break lines, or boundaries/areas.

The program inside the receiver that controls receiver operations and hardware.

Galileo is a GNSS system built by the European Union and the European Space Agency. It is complimentary to GPS and GLONASS.

The geoid is the equipotential surface that would coincide with the mean ocean surface of the Earth. For a small site this can be approximated as an inclined plane above the Ellipsoid.

Height above geoid.

Global Orbiting Navigation Satellite System. GLONASS is a Russian satellite navigation system comparable to the American GPS system. The operational system consists of 24 operational satellites in 3 orbit planes.

Global Navigation Satellite System.

Global Positioning System. GPS is a space-based satellite navigation system, owned by the United States government, consisting of multiple satellites in six orbit planes.

General Serial Output Format. A Trimble proprietary message format.

Horizontal Dilution of Precision. HDOP is a DOP value that indicates the precision of horizontal measurements. Other DOP values include VDOP (vertical DOP) and PDOP (Position DOP).

Using a maximum HDOP is ideal for situations where vertical precision is not particularly important, and your position yield would be decreased by the vertical component of the PDOP (for example, if you are collecting data under canopy).

The vertical distance above the ellipsoid. The classic ellipsoid used in GPS is WGS-84.

Internet Base Station Service. This Trimble service makes the setup of an internet-capable receiver as simple as possible. The base station can be connected to the internet (cable or wirelessly). To access the distribution server, the user enters a password into the receiver. To use the server, the user must have a Trimble Connected Community site license.

The primary L-band carrier used by GPS and GLONASS satellites to transmit satellite data.

The secondary L-band carrier used by GPS and GLONASS satellites to transmit satellite data.

A modernized code that allows significantly better ability to track the L2 frequency.

The third L-band carrier used by GPS satellites to transmit satellite data. L5 will provide a higher power level than the other carriers. As a result, acquiring and tracking weak signals will be easier.

Every single Ntrip Source needs a unique mountpoint on an Ntrip Caster. Before transmitting GNSS data to the Ntrip Caster, the Ntrip Server sends an assignment of the mountpoint.

MTSAT Satellite-Based Augmentation System. A Satellite-Based Augmentation System (SBAS) that provides a free-to-air differential correction service for GNSS. MSAS is the Japanese equivalent of WAAS, which is available in the United States.

Mobile-Satellite Service. A radiocommunication service between mobile earth stations and orbiting space stations. Most commercial mobile-satellite services are provided by systems operating in the L-band spectrum between 1.5 and 2.5 GHz, with the upper portion often referred to as the S-band. Trimble RTX and OmniStar corrections are transmitted via MSS as are Fugro MarineStar corrections.

Interference, similar to ghosts on an analog television screen that occurs when GNSS signals arrive at an antenna having traversed different paths. The signal traversing the longer path yields a larger pseudorange estimate and increases the error. Multiple paths can arise from reflections off the ground or off structures near the antenna.

The Indian Regional Navigation Satellite System (IRNSS) with an operational name of NavIC (Navigation with Indian Constellation, also known as sailor or navigator in Sanskrit, Hindi, and many other Indian languages) is an autonomous regional satellite navigation system that provides accurate real-time positioning and timing services to India and surrounding areas.

National Marine Electronics Association. NMEA 0183 defines the standard for interfacing marine electronic navigational devices. This standard defines a number of 'strings' referred to as NMEA strings that contain navigational details such as positions. Most Trimble GNSS receivers can output positions as NMEA strings.

Networked Transport of RTCM via Internet Protocol (Ntrip) is an application-level protocol that supports streaming Global Navigation Satellite System (GNSS) data over the internet. Ntrip is a generic, stateless protocol based on the Hypertext Transfer Protocol (HTTP). The HTTP objects are extended to GNSS data streams.

The Ntrip Caster is basically an HTTP server supporting a subset of HTTP request/response messages and adjusted to low-bandwidth streaming data. The Ntrip Caster accepts request messages on a single port from either the Ntrip Server or the Ntrip Client. Depending on these messages, the Ntrip Caster decides whether there is streaming data to receive or to send.

Trimble Ntrip Caster integrates the Ntrip Server and the Ntrip Caster. This port is used only to accept requests from Ntrip Clients.

An Ntrip Client will be accepted by and receive data from an Ntrip Caster, if the Ntrip Client sends the correct request message (TCP/UDP connection to the specified Ntrip Caster IP and listening port).

The Ntrip Server is used to transfer GNSS data of an Ntrip Source to the Ntrip Caster. An Ntrip Server in its simplest setup is a computer program running on a PC that sends correction data of an Ntrip Source (for example, as received through the serial communication port from a GNSS receiver) to the Ntrip Caster.

The Ntrip Server - Ntrip Caster communication extends HTTP by additional message formats and status codes.

The Ntrip sources provide continuous GNSS data (for example, RTCM-104 corrections) as streaming data. A single source represents GNSS data referring to a specific location. Source description parameters are compiled in the source-table.

The OmniSTAR HP/XP service allows the use of new generation dual-frequency receivers with the OmniSTAR service. The HP/XP service does not rely on local reference stations for its signal, but utilizes a global satellite monitoring network. Additionally, while most current dual-frequency GNSS systems are accurate to within a meter or so, OmniSTAR with XP is accurate in 3D to better than 30 cm.

The Orthometric elevation is the height above the geoid (often termed the height above the 'Mean Sea Level').

Position Dilution of Precision. PDOP is a DOP value that indicates the precision of three-dimensional measurements. Other DOP values include VDOP (vertical DOP) and HDOP (Horizontal Dilution of Precision).

Using a maximum PDOP value is ideal for situations where both vertical and horizontal precision are important.

Postprocessing is the processing of satellite data after it is collected, in order to eliminate error. This involves using computer software to compare data from the rover with data collected at the base station.

Quasi-Zenith Satellite System. A Japanese regional GNSS, eventually consisting of three geosynchronous satellites over Japan.

Also known as real-time differential correction or DGPS. Real-time differential GPS is the process of correcting GPS data as you collect it. Corrections are calculated at a base station and then sent to the receiver through a radio link. As the rover receives the position it applies the corrections to give you a very accurate position in the field.

Most real-time differential correction methods apply corrections to code phase positions.

While DGPS is a generic term, its common interpretation is that it entails the use of single-frequency code phase data sent from a GNSS base station to a rover GNSS receiver to provide submeter position . The rover receiver can be at a long range (greater than 100 kms (62 miles)) from the base station.

A rover is any mobile GNSS receiver that is used to collect or update data in the field, typically at an unknown location.

Roving mode applies to the use of a rover receiver to collect data, stakeout, or control machinery in real time using RTK techniques.

Radio Technical Commission for Maritime Services. A commission established to define a differential data link for the real-time differential correction of roving GNSS receivers. There are three versions of RTCM correction messages. All Trimble GNSS receivers use Version 2 protocol for single-frequency DGPS type corrections. Carrier phase corrections are available on Version 2, or on the newer Version 3 RTCM protocol, which is available on certain Trimble dual-frequency receivers. The Version 3 RTCM protocol is more compact but is not as widely supported as Version 2.

Real-time kinematic. A real-time differential GPS method that uses carrier phase measurements for greater accuracy.

Satellite-Based Augmentation System. SBAS is based on differential GPS, but applies to wide area (WAAS, EGNOS, MSAS, and GAGAN) networks of reference stations. Corrections and additional information are broadcast using geostationary satellites.

SNR. The signal strength of a satellite is a measure of the information content of the signal, relative to the signal’s noise. The typical SNR of a satellite at 30° elevation is between 47 and 50 dB-Hz.

The satellite skyplot confirms reception of a differentially corrected GNSS signal and displays the number of satellites tracked by the GNSS receiver, as well as their relative positions.

See signal-to-noise ratio.

The Ntrip Caster maintains a source-table containing information on available Ntrip Sources, networks of Ntrip Sources, and Ntrip Casters, to be sent to an Ntrip Client on request. Source-table records are dedicated to one of the following:

• data STReams (record type STR)

• CASters (record type CAS)

• NETworks of data streams (record type NET)

All Ntrip Clients must be able to decode record type STR. Decoding types CAS and NET is an optional feature. All data fields in the source-table records are separated using the semicolon character.

A type of receiver that uses three carrier phase measurements (L1, L2, and L5).

Universal Time Coordinated. A time standard based on local solar mean time at the Greenwich meridian.

Trimble xFill^® is a service that extends RTK positioning for several minutes when the RTK correction stream is temporarily unavailable. The Trimble xFill service improves field productivity by reducing downtime waiting to re-establish RTK corrections in black spots. It can even expand productivity by allowing short excursions into valleys and other locations where continuous correction messages were not previously possible. Proprietary Trimble xFill corrections are broadcast by satellite and are generally available globally where the GNSS constellations are also visible. It applies to any positioning task being performed with a single-base, Trimble Internet Base Station Service (IBSS), or VRS RTK correction source.

Virtual Reference Station. A VRS system consists of GNSS hardware, software, and communication links. It uses data from a network of base stations to provide corrections to each rover that are more accurate than corrections from a single base station.

To start using VRS corrections, the rover sends its position to the VRS server. The VRS server uses the base station data to model systematic errors (such as ionospheric noise) at the rover position. It then sends RTCM correction messages back to the rover.

Wide Area Augmentation System. WAAS was established by the Federal Aviation Administration (FAA) for flight and approach navigation for civil aviation. WAAS improves the accuracy and availability of the basic GNSS signals over its coverage area, which includes the continental United States and outlying parts of Canada and Mexico.

The WAAS system provides correction data for visible satellites. Corrections are computed from ground station observations and then uploaded to two geostationary satellites. This data is then broadcast on the L1 frequency, and is tracked using a channel on the GNSS receiver, exactly like a GNSS satellite.

Use WAAS when other correction sources are unavailable, to obtain greater accuracy than autonomous positions. For more information on WAAS, refer to the FAA website at http://gps.faa.gov.

The EGNOS service is the European equivalent and MSAS is the Japanese equivalent of WAAS.