- I -
Isobath
A line on a map connecting two points of equal water depth.
Isogloss
A line on a map between linguistically varied areas.
Isogone
A line on a map used to show characteristics of the earth's magnetic field.
Isogonic lines
A line on a map where the magnetic deviation is the same.
Isogram
A line on a map connecting points of equal value.
Isograph
A line on a map indicating areas that are linguistically similar.
Isogriv
A line drawn on a map such that all points have equal grid variation.
Isohel
A line on a map connecting points receiving equal sunshine.
Isohyet
A line on a map connecting points having equal rainfall.
Isotach
A line on a map connecting points of equal wind velocity.
Isotherm
A line on a map connecting points of equal mean temperature.
- J -
JPALS
Joint Precision Approach and Landing System - A differential GPS that will provide a rapidly deployable mobile, adverse weather, adverse terrain, day-night, and survivable precision approach and landing capability that supports joint service, civil, and multi-national interoperability.
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).
- K -
Karst
An area of irregular limestone in which erosion has produced fissures, sinkholes, underground streams and caverns.
Kinematic GPS
Observations while a receiver is in motion.
Kinematic Positioning
Kinematic Positioning refers to applications in which the position of a non-stationary object (vehicle, ship, aircraft) is determined.
Knot
A unit of speed of one nautical mile per hour, equal to 1.15 statute mile per hour (1.85 kilometers per hour).
- L -
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.
Lagoon
A shallow body of water, especially one separated from a sea by sandbars or coral reefs.
LBS
Location-Based Services - A service which uses knowledge provided about where a user is located in order to deliver information relevant to the user's position.
Leg
The portion of a route between waypoints.
Lidar
Also called laser radar - A method of detecting distant objects and determining their position, velocity, or other characteristics by analysis of pulsed laser light reflected from their surfaces.
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.)
- M -
Magnetic Declination
The difference, in degrees, between magnetic north and true north.
Magnetic North
A point on the earth about 500 miles from the north pole toward Hudson Bay in Canada; this is where a compass points. In the U.S., magnetic north can vary up to approximately 22 degrees from true north.
Magnetic Variation
Same as magnetic declination.
Maidenhead Grid Squares
A system designed to facilitate the designation of geographical positions for use within the amateur radio community.
Map Projection
The method used to convert the spherical shape of the earth onto the plane of a map.
Map Projection Deformation
The error or distortion that results when the spherical shape of the earth is projected onto the plane of a map.
Mask Angle
See Cutoff Angle
Mean Sea Level
The average height of the sea surface, based upon hourly observation of the tide height on the open coast or in adjacent waters that have free access to the sea. In the United States, it is defined as the average height of the sea surface for all stages of the tide over a nineteen year period. Mean sea level, commonly abbreviated as MSL and referred to simply as 'sea level,' serves as the reference surface for all altitudes in upper atmospheric studies.
Mercator Projection
A cylindrical map projection devised by Gerardus Mercator in 1569. Its parallels and meridians are straight lines, and the unavoidable east-west stretching away from the equator is accompanied by a corresponding north-south stretching, so that at each location the east-west scale is the same as the north-south-scale.
Meridian
An imaginary line circling the earth, passing through the north and south poles; a line of longitude.
MGRS
Military Grid Reference System. A metric grid system that is a subset of the UTM system. It uses letter pairs to represent 10km squares.
Microstrip Antenna
A GPS antenna constructed of one or more elements that are photoetched on one side of double-coated, printed-circuit board.
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).
Minute
A unit of measurement; one degree contains 60 minutes.
MOB
Man Overboard - A GPS receiver function that allows marking a specific location so that you can return to that location at a later time.
Moraine
Stones, boulders, and other debris deposited by a glacier.
Morrow
A rounded hill.
Moving Map
A program that displays your position in real time on the top of either a Vector or Raster map using GPS input.
MSAS
MTSAT Satellite-based Augmentation System - Japanese version of WAAS - covers parts of Asia and the Pacific
MTSAT
Multi-functional Transport Satellite - A satellite with two main functions: a meteorological mission conducted by Japan Meteorological Agency; an aeronautical mission conducted by Japan Civil Aviation Bureau of the Ministry of Land, Infrastructure and Transport of Japan (MLIT).
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.
- N -
NAD 27
North American Datum 1927
NAD 83
North American Datum 1983
Nadir
A point on the celestial sphere directly below the observer, diametrically opposite the zenith.
Nala
A ravine.
National Differential Global Positioning System
A network of ground- and beacon-based differential stations that increases GPS accuracy.
National Marine Electronics Association
A US standards committee that defines the data message structure, contents, and protocols that allow GPS receivers to communicate with other electronic equipment.
Nautical Mile
a unit of length used in sea and air navigation, based on the length of one minute of arc of a great circle - equal to appr. 1852 meters (6076 feet).
NAVD 88
Vertical reference system (elevation) for U.S., Canada, and Mexico; orthometric heights are derived from a particular point on the St. Lawrence Seaway.
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.
Neatline
Line separating the body of a map from the map margin. On a standard quadrangle map, the neatlines are the meridians and parallels delimiting the quadrangle.
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).
NGS
National Geodetic Survey - An agency of the United States NOAA that defines and manages a national coordinate system. This network, the National Spatial Reference System (NSRS), provides the foundation for transportation and communication; mapping and charting; and a multitude of scientific and engineering applications.
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".
NSDI
National Spatial Data Infrastructure - The set of base geographic data necessary for effective operation of the United States federal government and its suppliers, made accessible as a distributed database.
NSRS
National Spatial Reference System - A system administered by the NGS that provides the foundation for transportation and communication; mapping and charting; and a multitude of scientific and engineering applications.
- O -
Oceanic Survey
Survey or examination of condition in the ocean or any part of it, with reference to animal or plant life, chemical elements present, temperature gradients, etc.
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.
Orthographic Projection
The two-dimensional graphic representation of an object formed by the perpendicular intersections of lines drawn from points on the object to a plane of projection.
Orthometric Height
The distance of a point above the geoid.
Orthophotograph
Photograph having the properties of an orthographic projection. It is derived from a conventional perspective photograph by simple or differential rectification so that image displacements caused by camera tilt and terrain relief are removed.
Orthophotomap
Orthophotographic map with contours and cartographic features related to standard reference systems.
Orthophotoquad
Monocolor orthophotgraphic map presented in a standard quadrangle format and related to standard reference systems. It has no contours and little or cartographic treatment.
Orthophotoscope
Photomechanical device used in conjunction with a double-projection stereoplotter for producing an orthophotograph.
OSGB
Ordnance Survey of Great Britain - A system similar to the U.S. UTM for Great Britain.
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).
Overedge
Any portion of a map lying outside the nominal map border (neatline).
- P -
Packet
A unit of data that is routed between an origin and a destination on the Internet or any other packet-switched network.
Parallel of Latitude
An imaginary line circling the surface of the earth that is parallel to the equator; a line of latitude.
Laramo
A treeless alpine plateau of the Andes and tropical South America.
Patch Antenna
An antenna based on a plated section of metal on a circuit board and its associated electronics used for receiving GPS signals.
PCMCIA
Personal Computer Memory Card International Association - A standard for a credit card-size memory or input/output device that fits into a notebook, laptop or personal computer.
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).
- Q -
Quad
See quadrangle.
Quadrangle
A four-sided area of the surface of the earth used as a unit in mapping.
Quadrifilar Helix Antenna
A type of GPS receiver antenna in which four spiraling elements form the receiving surface of the antenna.
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