11.2.1.3  Narrow Channel Maritime Operations

There are two types of narrow maritime channels    natural confined waters and

channels swept through mine fields.  Large ships, both military and civilian, will operate

in both types of narrow channels in wartime and will need all the position accuracy they

can get.  Use of DGPS will reduce the requirement for the width of the channel to be

swept.

11.2.1.4  Reference Station for Testing/Calibration of Navigation Equipment

A Reference Station can provide the "ground truth" necessary for testing GPS and

other navigation equipment.  It can also be used to calibrate other terrestrial navigation

system transmitters such as Loran C and Omega.

11.2.1.5  Surveying for Mapping and Positioning

The use of DGPS to collect data for post  processing (not real time DGPS) is a common

operating method for mapping and geograph ical surveying purposes.  The usual

method is to use one mobile GPS receiver, one stationary reference GPS receiver, plus

the necessary data recording and data processing equipment.  The mobile receiver is

moved around to those points that will be surveyed.  GPS data (pseudoranges and

deltaranges) are collected at both receivers and the actual DGPS process is done

when the data from the two receivers is brought together at a later time.  This technique

eliminates the data link; despite this, very  accurate DGPS data can be obtained within

minutes of data collection.

11.2.1.6  Blind Take Off

GPS is inherently most accurate in the horizontal plane.  If the horizontal accuracy is

enhanced by DGPS techniques, an aircraft could use the improved accura cy to

navigate down a runway for take off in zero/zero conditions.

11.2.2  DGPS Data Link

The transfer of data from the reference receiver to the mobile receiver can be done

using any communication system capable of transfer ring digital data.  It can be via

telephone lines, radio or satellite communications.  Military users of a data link may

wish to consider encrypting the link to provide protection from imitation of the signal. 

Once the data is received, it can be loaded into the receiver by using existing

interfaces such as the Instrumentation Port (IP), a MIL  STD 1553 data bus, or a special

interface dedicated to DGPS data.

The use of pseudolites transmitting GPS NAV msg "look alike" data on L1 or L2

excludes the need for extra radio equipment on the user vehicle.  The range of

operation will also be very limited due to the high frequency of the pseudolite signals. 

The problem with pseudolites is that because of their high signal power relative to the

received satellite signals, they can "jam" the user receiver if the receiver comes too

close to the pseudolite. The user will then only receive the Reference Station

(pseudolite) signals and not the satellite signals.

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