mission requirements.  The advantage of the differential method is that an

overdetermined position solution (additional range measurements) is not required

as in RAIM to maintain positioning integrity.  If the number of available satellite

corrections exceeds the number of tracking channels in the user receiver, these

position error estimates can also be used to select the set of satellites that provide

the most accurate position solution.  Normally, the "all in view" position solution is

most accurate, and the correction UDRE values are near the same magnitude.  If

one or more corrections have a relatively large UDRE, a subset of the satellites

may provide a more accurate solution.

The standard maritime DGPS design, being implemented by several countries

throughout the world, includes a differential user receiver located at a nearby

surveyed site to serve as an integrity monitor.  The integrity monitor receives and

applies the corrections and develops a differential position solution in the same

manner as any other user receiver.  This position solution is then compared with

the known antenna location.  If the difference exceeds the allowed differentially 

corrected position error, the transmission of all corrections is terminated.  A

message is also sent to the users warning them to stop using all previously

transmitted corrections.  Although this method protects well against large sudden

errors, rigorous integrity is only provided for a user that determines a position using

the same set of satellite corrections as the integrity monitor (typically the full set). 

Receivers that use only a subset of the satellites and transmitted corrections will

have a higher DOP value for position  determination and may have a significantly

greater position error than the integrity monitor.

A more effective integrity method is to check the integrity of each individual

correction in the range domain, rather than the position domain, providing integrity

for users of any correction subset.  This method also uses multiple receivers in the

reference station (a minimum of two), but each receiver generates an independent

set of corrections.  The receivers also use different antennas, sited a sufficient

distance apart that multipath effects are likely to be independent as well.  The two

sets of corrections are directly compared prior to transmission.  The difference

between each pair of corrections is a direct measurement of most of the

components of the actual instantaneous differential range error seen by the user

receiver.  If a pair of corrections disagree by more than a predetermined amount,

for example, 3 X UDRE (that is, 3 sigma UDRE), correction transmissions are

interrupted for that satellite.  In this way, the user does not receive or use any

corrections until the integrity is checked, and the user can have confidence that the

broadcast UDRE is a true measure of the expected range error.  The reference

station also has the ability to continue broadcasting the corrections that remain

valid.  This technique is particularly useful for applications that must meet short

integrity warning times, must provide a highly confident estimate of UDRE, must

minimize service interruptions, or that have slow correction transmission rates.

Additional enhancements to this technique can include the use of dissimilar

receivers to prevent common mode errors within the reference station, the use of

more than two receivers to "vote out" anomalous measurements, or the calculation

of an "average" correction between the reference station receivers.

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