Linux Unwired [Electronic resources] نسخه متنی

10.2 A GPS Glossary

Here are some GPS terms
that you will encounter when you use GPS and GPS software:

8/12 channels receiver

An 8-channel receiver uses 8 channels to
access 8 different satellites at any one time. A
12-channel receiver can access 12
satellites at once.

Bearing

The direction you are aiming for.

CEP, RMS, and 2D RMS

Circular Error Probable (CEP),
Root Mean Square (RMS), and
2D RMS are all
measures of the accuracy of a GPS receiver. CEP represents the radius
of a circle containing 50% of the GPS readings. RMS represents the
radius of a circle containing 68% of the GPS readings. 2D RMS
represents the radius of a circle containing 98% of the GPS readings.
If three GPS receivers each claims to have 2m CEP, 2m RMS, and 2m 2D
RMS respectively, then the third one is the most accurate, because it
has readings accurate to within a 2-meter radius 98 percent of the
time.

DGPS

Differential GPS is an enhancement to the
satellite-based GPS that makes use of receivers on fixed reference
points on the ground and improves accuracy to within 3-5 meters.
These receivers transmit error-correcting information to DGPS
receivers to enhance the information supplied by the satellites.

Fix

A location returned by the GPS receiver after
processing the readings of at least three satellites.

Heading

The actual direction you are traveling towards. It is not the same as
bearing. Bearing is your desired direction, but you may not be
heading
towards the desired direction due to factors such as obstacles (e.g.,
water, fences, and mountains). Therefore, you have to momentarily
head in another direction in a bid to get to your destination.

Latitude, longitude, and altitude

The coordinates of a specific location on earth. These three pieces
of information together define a point in the three-dimensional
space.

National Marine Electronics Association (NMEA)

The NMEA-0183 standard has been universally adopted by GPS
manufacturers and virtually every GPS product for exchanging
navigational information between devices. NMEA-0183 defines a
"sentence" format (using printable
ASCII text) describing navigational information.

Route

A collection of waypoints representing the path that you
would like to take.

Selective Availability (SA)

The degradation of GPS data for nonmilitary use. See the sidebar
"GPS Accuracy" earlier in this
chapter for more information on SA.

Time to First Fix (TTFF)

The least amount of time required to obtain a fix by the minimum
number of satellites required for triangulation. Normally, it takes a
few minutes before you can receive a fix.

WAAS

WAAS is an enhancement similar to DGPS that uses fixed reference
stations on the ground to enhance accuracy to under 3 meters.

Waypoint

A location that you store in your GPS system (as coordinates).
Examples of waypoints are a hiking location, camping
ground, church, or any place of interest to a GPS user. You normally
add a waypoint to your GPS before you start your traveling. You can
also add one during your travel when you locate a place of
interest.