previous techniques which takes advantage of the strengths of both and appears to

greatly improve the probability of continuing successful navigation.

BIM can take several forms, but is closely related to differential GPS/GNSS.  In

general, any system that provides differential GPS or GNSS corrections also

provides BIM if it provides an assessment or guarantee of residual range errors

after the differential corrections are applied to the receiver range measurements. 

The receiver can then estimate the residual position error using the observed

satellite geometry and compare it against the current integrity alarm threshold.  The

BIM system can also make the integrity decision and issue use/don t use messages

for individual satellites, but there can be a significant increase in the false alarm

rate for many users since the BIM system must make a conservative assumption

regarding the user s satellite geometry and number of satellites in the user s

position solution.  Use/don t use messages can still be valuable to indicate

satellites that are not monitored or that are exhibiting extremely erroneous or erratic

behavior.

BIM functions can be incorporated in either a local area or wide area differential

system. The U.S., European nations, Japan, and Australia are currently planning or

developing terrestrial networks of differential GPS/GNSS receivers with

differential/integrity broadcasts via geo stationary communication satellites.  It is

planned that the satellites will broadcast the messages superimposed on a ranging

signal that emulates actual GPS ranging signals.  These additional ranging signals

will significantly enhance the availability of the SPS position solution in the

coverage areas of the satellites.  It is also likely that differential/integrity data may

be broadcast by terrestrial stations in northern regions where geostationary satellite

signals are intermittent or subject to obscuration.  The U.S. and Canada are

planning to have a cooperative wide area system operating by 1998, and the

European Community and Japan by the year 2000. 

AIM and BIM can be supportive of each other, and may be used in combination to

meet the most stringent integrity requirements.  In addition, the U.S. DoD is

implementing upgrades to the Control Segment monitoring and failure response

time, to minimize the problem at the signal source.  One such solution (nicknamed

"satzap") involves commanding the satellite to change its PRN number to one that

is unused, in the event a specific URE threshold is exceeded.  The threshold would

be chosen to protect non precision approaches, which is the most restrictive

integrity requirement for non differential GPS.  The unused PRN number would be

permanently set unhealthy in the satellite almanacs so a properly operating

receiver would never try to acquire it.  Immediately after an upload, the failure

response time would be extremely short since the ground antenna would already be

in contact with the satellite.  The technique would also be effective against slowly

increasing errors (e.g., clock drift failures) since the satellite could be contacted

and "zapped" before the error exceeded the URE threshold.  Swiftly increasing

errors and large step errors will normally cause a user receiver to lose lock. 

Fortunately, these types of errors can be easily detected by AIM, or a Kalman filter,

in cases where a user receiver might acquire or reacquire the satellite before the

control segment can correct or neutralize the problem.

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