For some applications (such as aircraft precision approach), it is essential to be

able to solve for the integer ambiguity at an unknown location or while in motion (or

both).  In this case, solving  for the integer ambiguity usually consists of eliminating

incorrect solutions until the  correct solution is found.  A good initial estimate of

position (such as from ranging code differential) helps to keep the initial number of

candidate solutions small.  Redundant measurements over time and/or from extra

satellite signals are used to isolate the correct solution.  These "search" techniques

can take as little as a few seconds or up to several minutes to perform and can

require significant computer processing power.  This version of the carrier phase

DGPS technique is typically called kinematic differential GPS.

If carrier track or phase lock on a satellite is interrupted and the integer count is

lost, then the initialization process must be repeated for that satellite (known as

cycle clip).  Output data flow may also be interrupted if the receiver is not collecting

redundant measurements from extra satellites to maintain the position solution.  If a

precise position solution is maintained, reinitialization for the "lost" satellite can be

almost immediate.  Developing a robust and rapid method of initialization and

reinitialization is the primary challenge facing designers of real time systems that

have a safety critical application such as aircraft precision approach.

10.3.3  DGPS Data Link Implementations

DGPS can also be implemented in several different ways depending on the type of

data link used.  The simplest data link is no data link at all.  For non real time

applications, the measurements can be stored in the receiver or on suitable media

and processed at a later time.  In most cases to achieve surveying accuracies, the

data must be post processed using precise ephemeris data that is only available

after the survey data has been collected. Similarly, for some test applications the

cost and effort to maintain a real time data link may be unnecessary.  Nevertheless,

low precision real time outputs can be useful to confirm that a survey or test is

progressing properly even if the accuracy of the results will be enhanced later.

Differential corrections or measurements can be uplinked in real time from the

reference station to the users.  This is the most common technique where a large

number of users must be served in real time.  If the user receivers are passive as in

GPS itself, an unlimited number of users can be served.  For military purposes and

proprietary commercial services, the uplink can be encrypted to restrict the use of

the DGPS signals to a select group of users.

An uplink can be a separate transmitter/receiver system or the DGPS signals can

be superimposed on a GPS like D band ranging signal.  The uplink acts as a

pseudo satellite or "pseudolite" and delivers the ranging signal and DGPS data via

the RF section of the user receiver, much in the same way the GPS navigation

message is transmitted.  The advantages are that the additional ranging signal(s)

can increase the availability of the position solution, or decrease carrier phase

initialization time, and a separate data link system is not required. However, the

reference station and user receivers become more complex and the pseudolite can

become a GPS jammer if it overpowers the GPS satellite signals.

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