Appendix D - Glossary

accuracy absolute accuracy— the accuracy of displayed lat/lon coordinates of your present position as compared to true lat/ lon determined from a chart or other reference source.

repeatable accuracy— the ability to return to the same location using a receiver’s previously-obtained coordinates.

acquisition The procedure—and the time it takes—during which a GPS receiver tries to track satellites, but before it actually computes a position fix.

almanac Data describing the precise orbits of GPS satellites, obtained automatically from satellite signals and stored in a GPS receiver. The first time the receiver is turned on, it requires about 15 minutes to collect almanac data. Thereafter, it usually can use the almanac data previously collected.

Avoidance point A waypoint that you enter into the database and define by its radius and either its lat/lon, or loran TDs, or distance and bearing. It indicates an area into which you specifically want to avoid navigating.

beacon

A radio transmitter, operated by the U.S. Coast Guard, which supplies differential corrections to GPS receivers.

bearing

The magnetic course from your present position to a waypoint.

cartography

The art or technique of making maps or charts.

chain

A group of loran transmitters comprising a master station and at least two secondaries.

COG

C ourse O ver G round. The actual path of your vessel relative to the surface of the earth; that is, the direction your vessel is actually moving.

coordinates

A position defined by either latitude and longitude; TDs; or distance and bearing. The positions of lat/lon and TDs are referred to as absolute, and distance and bearing are relative

(that is, distance and bearing are in relation to a certain reference point).

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course-up

An orientation on the chart screen in which the viewing angle matches your true course during the last few seconds, as determined by the GPS receiver.

coverage

The extent to which a geographical area is encompassed by usable GPS or loran signals.

cross-track error

The difference between your desired track and your actual track; that is, how far off you are from your intended course line.

cursor

The display panel indicator that shows which character or character field is ready to be changed, acknowledged, or entered by the user. It’s activated and positioned by the user when required. The 958’s cursor appears as an arrow or a flashing block, depending on whether you’re choosing or entering data. Also known as the red circle on the CHART screen, which you manually activate and position.

DGPS

D ifferential G lobal P ositioning S ystem. A technique developed by the U.S. Coast Guard for correcting basic GPS position to increase its accuracy from 20-30 meters (65-100 feet) to 1-3 meters (3-9 feet).

ephemeris

Data used to calculate exact satellite positions, continually updated and derived from information relayed to the 958 in each satellite’s data stream.

EBL

E lectronic B earing L ines. Markers that you can place on top of the radar image to aid in navigation and in keeping track of your surroundings.

ETA

E stimated T ime of A rrival at the next waypoint, as calculated by the 958 according to your present speed of advance (SOA) and distance to the waypoint.

ETE

E stimated T ime E nroute to the next waypoint, as calculated by the 958 according to your present speed of advance (SOA) and distance to the waypoint.

GPS

G lobal P ositioning S ystem. A navigation system using signals from earth-orbiting satellites. The system is capable of providing high accuracy under all weather conditions.

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great circle

Shortest possible path between two points on the surface of a sphere.

GRI

G roup R epetition I nterval. The characteristic identifying period of signals emitted by each loran transmitting chain. The identifier 9960 (Northeast US) indicates that the gap between the start of any two adjacent master signals is 99,600 microseconds long.

HDOP

H orizontal D ilution of P recision.

heading

The compass direction toward which the longitudinal axis of a vessel points.

knot

One nautical mile per hour.

latitude

Imaginary lines on the earth's surface running East/West and expressed in degrees (0-90) north or south of the Equator.

Used with the North/South lines of longitude to determine position.

leg

The line, or path, between any two adjacent waypoints in a route.

leg-up

An orientation of the CHART screen in which the viewing angle matches your desired track when navigating, on a leg-by-leg basis.

longitude

Imaginary lines on the Earth's surface running North/South and expressed in degrees (0-180) east or west of the Prime

Meridian (a line running from the North to South Pole, passing through Greenwich, England).

Loran-C

LO ng RA nge N avigation. A system of land-based radio transmitters emitting precisely timed pulses that are received and translated into position “fixes” by a loran receiver. The 958 can convert and display its GPS coordinates into Loran-C position measurements, or TDs.

loran chain

A group of loran transmitters comprising a master station and at least two secondary stations.

loran receiver

An electronic position finding device that receives pulses from loran stations.

magnetic variation

The difference between magnetic North and true North. Since the difference varies according to geographic location, the

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Northstar 958 automatically calculates magnetic variation and uses it to display bearings as magnetic north. The variation at any location changes slowly over a period of years, and the unit compensates, according to the current calendar year.

master

The primary loran transmitter in a “chain” of at least three stations which initiates a series of coded pulses and is followed by the secondaries.

mile (statute)

A unit of measurement (distance) equal to 5,280 feet (1,609.3 meters).

nautical mile (nm)

An international unit equal to 6,076.115 feet (1,852 meters) used officially in the U.S. since 1959. A nautical mile is about

800 feet more than a statute mile.

nav aid

Aids to navigation often used on coastal and inland waters.

Buoys, lighthouses, and daymarkers are nav aids.

NMEA

N ational M arine E lectronics A ssociation. The industry group that establishes the standards for exchanging data between electronic navigation equipment. The standard that’s currently in wide use is NMEA 0183, which specifies the format for sentences containing specific types of data.

Phantom Loran

Northstar’s exclusive loran-simulation feature that mathematically converts GPS lat/lon coordinates to loran

TDs—in real time, as you travel—enabling you to display your current position as TDs.

PPNM

P ulses P er N autical M ile.

radar

Ra dio D etecting A nd R anging.

rhumb line

A constant, true direction to your destination, which appears as a straight line on a Mercator grid. Compare with great circle .

route

A group of two or more pre-defined waypoints that form a connected path to and/or from specific locations. In its simplest form, a route can be a straight line to a single waypoint.

satellite

An object orbiting the earth. GPS satellites are controlled by ground stations, and transmit signals that can be used to calculate position and speed. WAAS satellites.

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secondaries

The “slave” loran transmitters in a chain of at least three stations.

SNR

S ignal-toN oise R atio. Guide number for determining the relative quality of GPS, DGPS, or loran signals as compared to background radio “noise.”

SOG

S peed O ver G round. The actual speed of your vessel relative to the sea bottom.

TD

Time Difference. The measured amount of time—in millionths of a second—between loran signals received from the master and the secondary stations.

target

Boats or ships, navigational markers, land masses, or any other object detected by radar.

track

A desired line of travel, or path already travelled.

VRM

V ariable R ange M arker. A circle on the radar, surrounding one of the end points of the electronic bearing line, used to measure distances from that end point to any other point of interest on the screen.

WAAS

W ide A rea A ugmentation S ystem.

waypoint

A particular location (defined for navigation purposes by its lat/lon, distance and bearing, or TDs), used as an intermediate or final destination.

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