2.3  AUTONOMOUS INTEGRITY MONITORING TECHNIQUES

GPS receivers may track additional satellites for integrity monitoring purposes.  This

function is independent of receiver architecture. Integrity monitoring receivers derive

multiple position solutions by excluding one satellite at a time.  Inconsistencies in the

results are used to identify and exclude a faulty satellite.  In general, at least five

satellites must be tracked to detect an integrity failure, and at least six satellites must

be tracked to exclude an erroneous satellite. Other measurements, such as altitude or

time, may be substituted for satellites in the integrity algorithms, much in the same

manner as these measurements are substituted in the PVT solution. In doing so, the

integrity of the aiding sources is checked as well.  The integrity monitoring algorithms

are commonly referred to as Fault Detection and Exclusion (FDE) algorithms or as

Receiver Autonomous Integrity Monitoring (RAIM or AIM) algorithms. These algorithms

are typically executed on each new set of measurements, thus protecting the integrity

of each PVT data set output by the receiver.  For additional discussion of integrity, refer

to Chapter 12.

2.4  TIME TRANSFER RECEIVERS

One of the more common uses of GPS is for precise time dissemination applications.

Several manufacturers offer this type of equipment commercially.  These precise time

GPS receivers need only one GPS satellite for precise time dissemination if the

receiver is stationary on a precisely known location and the only "unknown" is its own

clock offset from GPS time and therefore from UTC.  To obtain the necessary precise

position, the receiver either receives it as an operator input or uses four satellites to

determine its own position. These receivers typically include an internal oscillator or an

optional external frequency source (rubidium or cesium). Whenever the receiver is

tracking a satellite, it generates 1, 5,  or 10 MHz reference frequencies that are

synchronized to UTC time.  If no satellites are visible, the reference frequencies are

derived from the internal or external frequency source. The receivers can provide either

stand alone (uncoordinated) or coordinated time transfer operations.  In SPS receivers,

use of SA will reduce the time and position accuracy available.  The manufacturers of

time transfer receivers claim time accuracies in the 20 to 50 nanoseconds range, but

this accuracy requires algorithms that average pseudorange measurements over time

(10   60 minutes).  A stand alone PPS time receiver normally provides time accuracy in

the 100 nanoseconds range.  The advantage of having an external frequency source

interface designed into the receiver is that the long term error in the frequency source

can be adjusted when the receiver has satellites in view.  A stationary PPS GPS

receiver with a precise time and time interval (PTTI) interface should be able to provide

UTC to an accuracy of 50 to 60 nanoseconds.

2.5  DIFFERENTIAL GPS (DGPS) RECEIVERS

DGPS receivers are used in applications where enhanced accuracy of the PVT solution

is required or desired.  DGPS is based on the principle that receivers in the same

vicinity will see similar errors on a particular satellite ranging signal.  In general, the

DGPS technique

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