Glossary

+ A-F

Acquisition
Algorithm
Almanac
Ambiguity
Autonomous Position
Azimuth
Base Receiver
Carrier Phase
CDMA
Channel
Choke Ring Antenna
C/A-code
Cold Start
Control Point
DGPS
DOP
Elevation Mask Angle
EGNOS
Ephemeris
Fixed Solution
Float Solution

+ G-O

Galileo
Geodesy
Geodetic
GLONASS
GNSS
GPRS
GPS
GPS Time
GPS Week
GSM
Hot Start
Initialization
L1 Frequency Band
L2 Frequency Band
L2 (C) Frequency Band
L5 Frequency Band
Line-of-Sight
Modulation
Multipath
Network RTK
Noise
NTRIP
Obstructions
On-the-Fly

+ P-Z

Post-Processed Kinematic Surveying
Post-Processing
P-Code
Pseudorange
PRN Number
Range
Rapid Static
RTCM
RTK
Receiver
Reference Station
RINEX
Rover Receiver
SBAS
Selective Availability
Single Point Positioning
Static Positioning
Tolerance Range
Triangulation
UHF Radio Modem
UTC Time
Warm Start
WAAS
WGS 84
Y-Code

Acquisition: The ability to find and lock on satellite signals for ranging.
Algorithm: A special method used to solve a mathematical problem for a variety of surveying functions.
Almanac: Message transmitted by each satellite that gives the approximate location of all GNSS satellites. The almanac is provided so that the signals for individual satellites can be acquired rapidly. This tool helps predict satellite positions for planning purposes.
Ambiguity: The unknown integer number of cycles of the reconstructed carrier phase. The carrier phase ambiguity is inherent in a continuous set of carrier-phase measurements from a single satellite pass at a single receiver. Also known as integer ambiguity and integer bias.
Autonomous Position: Derived from a single receiver without using any differential correction. Autonomous position is the least accurate method of satellite positioning.
Azimuth: The angle in a clockwise direction between the north and the location of the object of interest (e.g. GNSS satellite).
Base Receiver: A receiver that acts as the stationary reference. It has a known position and can be used to transmit messages for the rover receiver to use to calculate its position.
Carrier Phase: The phase of either L1 or L2 carrier of a GNSS signal, measured by a receiver while locked onto the signal. Also known as integrated Doppler.
CDMA: Code Division Multiple Access. A method whereby many radios use the same frequency, but each one has a unique code. GNSS uses CDMA techniques with codes for their unique cross-correlation properties.
Channel: One channel is allocated to track one satellite at one or two GPS carrier frequencies. Thus, a typical 12-channel GPS receiver tracks up to 12 GPS satellites.
Choke Ring Antenna: Particular form of omnidirectional antenna for use at high frequencies. It consists of a number of conductive concentric cylinders around a central antenna. It is used in GNSS applications to reject multipath signals.
C/A-code: Course/Acquisition Code. Modulated onto the GNSS L1 signal. Also known as Clear/Access Code.
Cold Start: The ability of a GNSS receiver to start providing position updates without the assistance of any almanac information stored in its memory. The GNSS receiver has to attempt to lock a satellite signal from all of the satellites. This re-acquisition of a satellite lock takes the longest because there is no known information.
Control Point: A point on the ground whose horizontal and vertical location is known and used as the basis for detailed surveys.
DGPS: Differential GPS is a technique to improve GNSS accuracy. It primarily uses pseudorange errors at a known location to improve the measurements made by other GNSS receivers within the same general geographic area.
DOP: Dilution of Precision. A description of purely geometrical contribution to the uncertainty in position fix. The geometry of the visible satellites is an important factor in achieving high-quality results. The geometry changes with time due to the relative motion of the satellites.
Elevation Mask Angle: An adjustable feature in GNSS receivers that determines if a satellite is going to be included in the position solution. A satellite must be at least a specified number of degrees above the horizon before the receiver uses the signals from the satellite. Satellites at low elevation angles, usually five degrees or less, have lower signal strengths and are prone to loss of lock and multipath.
EGNOS: European Geostationary Navigation Overlay Service. A satellite-based augmentation system (SBAS) that provides a free-to-air differential correction service for GNSS users in Europe. EGNOS is the European equivalent of WAAS, which is available in the United States.
Ephemeris: A set of satellite orbit parameters used by GNSS receivers to calculate GNSS satellite positions and velocities. The ephemeris is used in the determination of the position solution and is updated periodically. Available in real-time as “broadcast ephemeris” or as post-processed “precise ephemeris.”
Fixed Solution: Processing of GNSS vectors produces many solutions for the vector at different stages of the processing. One of the parameters being solved during the processing is the integer ambiguities. A fixed solution is a vector solution where the integer ambiguities have been correctly determined and used to constraint the solution. The fixed solution for a vector is most often the best solution. If for some reason the ambiguities could not be correctly solved, the final solution for the vector will be a float solution.
Float Solution: Processing of GNSS vectors produces many solutions for the vector at different stages of the processing. One of the parameters being solved for during the processing is the integer ambiguities. A float solution is a vector solution where the integer values for the ambiguities could not be determined; therefore they are not fixed to a specific integer and the solution uses real values for the integer ambiguities.
Galileo: A global positioning system planned by European Union countries. It will join the American GPS and Russian GLONASS systems as a third satellite-based positioning system.
Geodesy: The brand of applied mathematics that deals with the measurement and the representation of the Earth.
Geodetic: Pertaining to geodesy. Latitude and longitude readings are geodetic coordinates.
GLONASS: GLObal NAvigation Satellite System / GLObalnaya NAvigatsionnaya Sputnikovaya Sistema. A satellite-based radio navigation system launched in 1982 and run by the Russian Ministry of Defense. Similar to the Global Positioning System (GPS) in the U.S., GLONASS permits 3D positioning anywhere on earth; however, typically it is used as an augmentation alongside the GPS system. The GLONASS constellation in 2009 includes 20 satellites, each traveling in a circular orbit at 19,140 kilometers above the Earth in three circular planes. The satellites are positioned so that four are observable nearly 100 percent of the time from any point on Earth.
GNSS: Global Navigation Satellite Systems. The collective of current and proposed satellite navigation systems, including GPS, GLONASS and Galileo.
GPRS: General Packet Radio Service. Technology that allows mobile phones to be used for sending and receiving data over an Internet Protocol (IP)-based network.
GPS: Global Positioning System. A satellite-based radio navigation system owned and operated by the United States Department of Defense. The GPS constellation in 2009 includes 30 satellites, each traveling in a circular orbit at 20,200 kilometers above the Earth. GPS signals are freely accessible for civil applications, allowing land, sea and airborne users to determine their exact location, velocity and time 24 hours a day in all weather conditions anywhere in the world. The satellites are positioned so that four are observable nearly 100 percent of the time from any point on Earth.
GPS Time: The time system upon which GPS is based. GPS time is an atomic time system and is related to International Atomic Time (IAT) in the following manner: IAT = GPS + 19.000 sec. Note: IAT and Universal Time Coordinated (UTC) are closely related. The difference is that UTC has leap seconds put in to adjust for changes in the Earth’s rotation.
GPS Week: GPS time started at Saturday/Sunday midnight on January 6, 1980. The GPS week is the number of whole weeks since GPS time zero.
GSM: Global System for Mobile Communications. A digital cellular phone technology based on time TDMA digital transmission technology. First introduced in 1991, the GSM standard has been deployed at three different frequency bands: 90 MHz, 1800 MHz and 1900 MHz. GSM uses narrowband TDMA, which allows eight simultaneous calls on the same radio frequency.
Hot Start: The GNSS receiver remembers its last calculated position and which satellites were in view, the almanac used, and the UTC Time. It then performs a reset and attempts to acquire satellites and calculate a new position based upon the previous information. This is the quickest re-acquisition of a GPS lock.
Initialization: The first time a GNSS receiver orients itself to its current location and collects almanac data. After initialization has occurred, the receiver remembers it location and acquires a position more quickly because it knows which satellites to seek.
L1 Frequency Band: The 1575.42 MHz GNSS carrier frequency which contains the C/A-code, as well as encrypted P-code and navigation messages used by commercial GNSS receivers.
L2 Frequency Band: A secondary GNSS carrier at 1227.60 MHz that presently contains only the encrypted P-code. A C/A-code will be available on the L2 frequency in the future.
L2 (C) Frequency Band: Contains two distinct Pseudorange Noise Number code sequences, the Civilian Moderate (CM) and Civilian Long (CL) length codes, to provide range information. Transmitted on the same frequency as the L2 frequency band.
L5 Frequency Band: The 1176.45 MHz GNSS carrier frequency which contains two Pseudorange Noise Number ranging codes: the in-phase (I5-code) and quardra-phase (Q5-code) codes.
Line-of-Sight: An unobstructed view from the point being measured from the instrument doing the measuring.
Modulation: A method of encoding a message signal on top of a carrier, which can be decoded at a later time.
Multipath: The reception of a satellite signal both along a direct path and along one or more selected paths. Reflecting surfaces near the GNSS antenna causes this type of reflected signal. The resulting signal results in an incorrect measurement and thus errors in position estimates.
Network RTK: A reference network comprised of permanent or mobile reference stations, communication and processing infrastructure to provide GNSS users with centimeter-level positioning accuracies for their rover receiver positions.
Noise: An interference that tends to mask the desired signal at the receiver output and which can be caused by space and atmospheric phenomena, can be human made or can be caused by receiver circuitry.
NTRIP: Networked Transport of Radio Technical Commission RTCM via Internet Protocol. An emerging standard for delivery of real-time kinematic (RTK) corrections over the Internet.
Obstructions: Objects such as trees, mountains and houses limiting the satellite visibility at a user site. The fewer obstructions that are present, the better the satellite geometry (DOP) and better the user’s positioning accuracy.
On-the-Fly: The term used to identify a technique that resolves differential carrier phase integer ambiguities without requiring a GNSS receiver to be stationary at any time.
Post-Processed Kinematic Surveying: Kinematic surveys require occupations as short as a few seconds but with a moderate reduction in accuracy. During kinematic surveys, a base station records control data over a known point while one or more rovers takes brief GNSS readings at each unknown position. GNSS data is collected while rovers are stationary or in motion. The data from base and rover receivers are later processed. Each point where the rover position is fixed yields an accurate measurement. Because many points may be quickly surveyed, kinematic surveying allows for high productivity and is useful for surveys, such as topographic or as-built surveys, that require a large number of points over a local region. Also known as stop-and-go surveying.
Post-Processing: The processing of GNSS data after the data was actually collected in the field. Post-processing is usually accomplished on a computer in an office environment where appropriate processing technique is employed to achieve optimum position solutions.
P-Code: Precise Code. The protected or precise code used on both L1 and L2 GPS frequencies. Also known as protected code.
Pseudorange: The calculated range from the GNSS receiver to the satellite. It is determined by taking the difference between the measured satellites transmit time and the receiver time of measurement and multiplying it by the speed of light.
PRN Number: Pseudorange Noise Number. A pseudo-random number used to identify each satellite. Typically, a particular satellite will keep its PRN (and hence its code assignment) indefinitely, or least for a long period of time. Also known as satellite number.
Range: A fixed distance between two points, such as between a starting and an ending waypoint or a satellite and a GNSS receiver.
Rapid Static: A form of static GNSS positioning which requires minutes of observations instead of hours due to special ambiguity resolution techniques which use extra information such as P-code measurements or redundant satellites.
RTCM: Radio Technical Commission for Maritime Services is an organization that developed and defined the SC-104 message format for differential positioning.
RTK: Real-Time Kinematic. The relative positioning technique whereby carrier phase measurements or corrections are transmitted in real-time from a Reference or Base Station to the user's roving receiver. Centimeter accuracy is achieved without the need to record and post-process double-differenced carrier phase observables. RTK often uses the on-the-fly integer ambiguity resolution approach.
Receiver: Calculates its position by precisely timing the signals sent by the GPS satellites orbiting the Earth. Each satellite continually transmits messages containing the time the message was sent, orbital information (the broadcast ephemeris), the satellite’s health and rough orbits of all GNSS satellites - the almanac. The receiver measures the transit time of each message and computes the distance to each satellite. Geometric trilateration uses these distances together with the locations of the satellites to determine the receiver's location. Many GNSS units also show derived position information in addition to direction and speed.
Reference Station: In differential positioning, a reference station is the end of the baseline that is assumed known and its position fixed. It is the GNSS receiver, which is acting as the stationary reference. It may be used to transmit correction messages to the rover receiver, which uses the information to calculate its position. Sometimes referred to as a base station.
RINEX: Receiver INdependent EXchange format. 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: observation time, carrier phase and pseudorange. There are three basic types of RINEX file formats: the observation data, the navigation message and the meteorological data.
Rover Receiver: The GNSS receiver that moves from site to site during a real-time kinematic (RTK) survey. The receiver does not know its position and needs to receive measurements from a base station to calculate differential GNSS positions at each point.
SBAS: Satellite-Based Augmentation System. System that supports wide area or regional augmentation through the use of additional satellite broadcast messages. Such systems can consist of networks of ground stations, geostationary satellites designed to broadcast corrected time and distance measurements to end users. Various implementations of SBAS include WAAS, EGNOS and other commercial services.
Selective Availability: The method used by the United States Department of Defense to control access to the full accuracy achievable by civilian GPS equipment, generally by introducing timing and ephemeris errors. A Department of Defense program controls the accuracy of pseudorange measurements, whereby the user receives a false pseudorange which is in error by a controlled amount. Differential GPS techniques can reduce these effects for local applications. Selective Availability was disabled on current satellites in May 2000.
Single Point Positioning: The determination of the coordinates of a receiver with respect to the Earth's reference frame by processing the signals from four or more satellites.
Static Positioning: Static positioning or static surveys are often performed as control surveys or boundary surveys. During static surveys, two or more receivers are placed at the ends of the baselines being measured and each receiver collects data for a session (15-60 minutes depending on baseline length). This process can be repeated for a number of lines, yielding a set of connected baselines that form a survey network. Static surveys require longer periods of occupation to provide centimeter-level results.
Tolerance Range: Tolerance range is the difference between the upper specification limit and the lower specification limit.
Triangulation: 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. Triangulation can use distance and/or angles.
UHF Radio Modem: Transmits data over a wireless connection to another radio modem over a point-to-point or multipoint link. They are independent of mobile and satellite network operators and have no cost associated with transferring data. Operation range varies depending on the transmission power, antenna gain, mast height and environment. Users in rural areas with a 1 W radio modem with a line-of-sight radio link may range more than 20 km, while users in dense metropolitan areas with the same radio modem may see a corresponding range that varies from several kilometers to more than 10 km.
UTC Time: Universal Time Coordinated Time is an international time standard based on the International Atomic Time (IAT). Because of Earth’s slowing rotation, UTC sometimes adjusted by an integer second. The accumulation of these adjustments compared to GPS time, which runs continuously, has resulted in a 15 second offset between GPS time and UTC in 2009. 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.
Warm Start: The ability of a GNSS receiver to begin navigating using almanac information stored in its memory for previous use. The GPS receiver remembers its last calculated position and almanac used and knows the UTC Time but does not know which satellites were in view. It then performs a reset and attempts to obtain the satellite signals and calculate a new position. The receiver has a general idea of which satellites to look for because it knows its last position, and the almanac helps identify which satellites are visible in the sky. This takes longer than a Hot Start but not as long as a Cold Start.
WAAS: Wide Area Augmentation System is a Satellite-Based Augmentation System (SBAS) developed by the Federal Aviation Administration in concert with other agencies and provides Wide Area Differential GPS corrections - additional ranging signals from geostationary satellites and integrity data on the GPS and GEO satellites.
WGS 84: Word Geodetic System 1984 is a geocentric coordinate system designed to represent the shape of the Earth. It is often used as a reference on a worldwide basis, while other coordinate systems are used locally to provide a better representation to the Earth’s surface in a local region.
Y-Code: The encrypted P-code.