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Navis导航术语(二)

发表 2017/01/21

masks See satellite masks

maximum PDOP A measure of the maximum Position Dilution of Precision (PDOP) thatis acceptable in order for the GPS processor todetermine a locationsolution (see PDOP)

NAVSTAR The name given to the GPS satellites,built by Rockwell International,which is an acronym formed from NAVigationSystem with Time AndRanging.

NMEA National Marine Electronics Association.An association that defines

marine electronic interface standards for thepurpose of serving thepublic interest

NMEA 0183 message NMEA 0183 is a standard for interfacing marine electronics

navigational devices. The standard specifiesthe message format used tocommunicate with marine devices/components.

packet An “envelope” for data, which contains addresses and error checking

information as well as the data itself.

parity A scheme for detecting certain errors indata transmission. Paritydefines the condition (i.e., even or odd偶数或奇数) ofthe number of items in a set

(e.g., bits in a byte).

PDOP Position Dilution of Precision. PDOP is aunitless figure of merit that

describes how an uncertainty in pseudo-range affects position solutions.

PDOP constellationswitchA value, based on PDOP, that defines when theGPS receiver/processorshould switch between 2-D and 3-D GPS modes.The PDOP

constellation switch is only active when theGPS mode of operation isset to Auto.

PRN Pseudo-random noise. Each GPS satellitegenerates its own distinctivePRN code, which is modulated onto each carrier.The PRN code servesas identification of the satellite, as a timingsignal, and as a subcarrierfor the navigation data.

伪随机杂讯,其序列可鉴别卫星。

protocol A formal set of rules that describe amethod of communication. The

protocol governs the format and control ofinputs and outputs.

pseudo-range A measure of the range from theGPS antenna to a GPS satellite.Pseudo-range is obtained by multiplying thespeed of light by theapparent transit time of the signal from theGPS satellite. Pseudo-rangediffers from actual range because the satelliteand user clocks are offset弥补from GPS time and because of propagation传播 delaysand other errors

RAM Random-Access Memoryrandom-access memory Memory in which information can be referred to in an arbitrary任意orrandom order. The contents of RAM are lost whenthe System Unit isturned off

range A term used to refer to the distanceradio signals can travel before they

must be received or repeated due to loss ofsignal strength, the curvature曲率of the earth and the noise introduced becauseof moisture in the airsurrounding the earth's surface.

range rate The rate of change of range betweenthe satellite and receiver. The

range to a satellite changes due to satelliteand observer motions. Rangerate is determined by measuring the Dopplershift of the satellite beacon信标carrier.

read-only memory Memory whose contents can beread, but not changed. Information isplaced into ROM only once. The contents of ROMare not erased when

the system unit's power is turned off.

real time clock An electronic clock, usually batterypowered, that keeps current time.

Used by a GPS receiver during a warm or hotstart to determine whereto search for GPS satellite signals.

Relative positioning The process of determiningthe vector distance between two points andthe coordinates of one spot relative toanother. This technique yields

GPS positions with greater precision than asingle point positioningmode can.

rise/set time Refers to the period during whicha satellite is visible; i.e., has an

elevation angle that is above the elevation mask. A satellite is said torise” when its elevation angle exceeds themask and “set” when theelevation drops below the mask.

ROM Read-Only Memory.

RS-232 A communication standard for digitaldata. Specifies a number of signal

and control lines. RS-232 is often associatedwith a 9 pin connectorcalled a DB-9

RTCM Radio Technical Commission for MaritimeServices. Commission thatrecommends standards for differential GPSservices. “RTCMRecommended Standards For Differential GPSService,” prepared by

RTCM Special Committee No. 104 (RTCM SC-104),defines acommunication protocol for sending GPS differential corrections from adifferential reference station to remote GPSreceivers.

satellite masks As satellites approach thehorizon, their signals can become weak anddistorted, preventing the receiver fromgathering accurate data. Satellite

masks enable you to establish criteria forusing satellite data in aposition solution. There are three types ofsatellite masks: Elevation,SNR, and PDOP.

SA Selective Availability. This is the name ofthe policy and theimplementation scheme by which unauthorizedusers of GPS will havetheir accuracy limited to 100 meters 2D RMShorizontal and 156 meters2D RMS vertical.

SEP Spherical Error Probability.球面误差概率The radius ofa sphere such that 50% of theposition estimates will fall within the surfaceof the sphere.

Serial communication A system of sending bitsof data on a single channel one after the other,rather than simultaneously.

serial port A port in which each bit of informationis brought in/out on a single

channel. Serial ports are designed for devicesthat receive data one bit ata time.

signal to noise level GPS/GLONASS signals withSNRs that do not meet the mask criteriaare considered unusable.

signal to noise ratio A measure of the relativepower levels of a communication signal andnoise on a data line. SNR is expressed indecibels (dB).

SNR Signal to Noise Ratio.

spread spectrum The received GPS/GLONASS signalis a wide bandwidth, low-power

signal (-160dBW). This property results frommodulating the L-bandsignal with a PRN code in order to spread thesignal energy over abandwidth which is much greater than the signalinformationbandwidth. This is done to provide the abilityto receive all satellitesunambiguously and to provide some resistance tonoise and multipath.

SPS Standard Positioning Service. Refers to theGPS as available to theauthorized user.

start bit In asynchronous transmission, thestart bit is appended to the beginning

of a character so that the bit sync andcharacter sync can occur at thereceiver equipment.

stop bit In asynchronous transmission, the stopbit is appended to the end of

each character. It sets the receiving hardwareto a condition where it

looks for the start bit of a new character.

SV Space Vehicle (GPS satellite).

Synchronous communicationA method of sending digital data in which thebits come at fixed, ratherthan random, times and are synchronized to aclock.

URA Satellite user range accuracy. The URA issent by the satellite and iscomputed by the GPS operators. It is astatistical indicatory of thecontribution of the apparent clock andephemeris prediction accuraciesto the ranging accuracies obtainable with aspecific satellite based onhistorical data.

UTC Universal Time Coordinated. Uniform atomictime system/standard that

is maintained by the US Naval Observatory. UTCdefines the local solarmean time at the Greenwich Meridian.

UTC offset The difference between local timeand UTC (Example: UTC - EST = 5hours).