c. Friendly forces  position/course/speed

d. Geographical obstacles in possible weapon trajectories

e. Other information vital to the operator (safety zones, waypoints, weapon

engagement zones, etc.).

13.4  OTHER ISSUES CONCERNING DIGITAL MAPS AND GPS

The use of digital maps and GPS raises at least three questions/prob lems to be

aware of:

a. Electrical interfaces between GPS receiver and digital map systems

b. Digital map accuracy

c. Map datums

13.4.1  Electrical Interface Between the Digital Map Display and the GPS

 Receiver

Most of the available GPS receivers provide an external interface capability that

can be used to integrate the receiver with a digital map system.  These interfaces

generally conform to the NMEA 0183 format.  Differential receivers will also provide

 an RTCM SC 104 message.  These inter faces may be usable to transfer GPS

position, speed over ground, heading, etc. to the digital map.  If you wish to

integrate GPS with a digital map system, make sure that one or more of the GPS

receiver interfaces are compatible with an interface on the digital map system in

terms of:

a. Electrical characteristics

b. Functional characteristics

c. Data format

d. Data update rates

13.4.2  Digital Maps Accuracy

Digital maps that are produced by the raster scan process from ordinary paper

maps cannot be more accurate than the source documents from which they are

prepared. Some of the paper maps used for navigation today are based on

surveying data that is 80 to 100 years old. The quality of these maps leaves much

to be desired when used for today s navigational needs.  Additionally, inaccuracies

in the optical reader system and operator  induced inaccuracies can produce errors

which when combined are less accurate than the original paper map. It is therefore,

very important to display the "age of the mapping data" on the digital map display to

alert the navigator to the mapping inaccuracy.

13.4.3  Map Datums

The GPS receiver employs ECEF coordinates which are based on the WGS 84 map

datum. Most receivers and digital mapping systems have the capability to convert

from the WGS 84 datum to any one of a number of user selected datums.  When a

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