Page 418 - Automotive Engineering Powertrain Chassis System and Vehicle Body

Global positioning technology C HAPTER 14.1

vehicle, may be able to access DGPS correction data on relative cost of refining the positioning information
the Internet, or have it delivered on a subscription basis for each mobile unit at the central location is minimal. Of
from a private differential correction service provider. course, with the discontinuation of S/A, DGPS re-
With the discontinuation of S/A, using the DGPS finement may no longer be necessary for many of these
positioning technique will still provide enhanced posi- applications.
tioning accuracy, since other timing errors are inherent in
the SV broadcasts that DGPS may help correct. How- 14.1.3.2.4 Server-assisted GPS positioning
ever, these much smaller improvements in accuracy may Server-assisted GPS is a positioning technique that can
no longer offset the additional cost of receiving and be used to achieve highly accurate positioning in
processing the DGPS correction information for many obstructed environments. This technique requires a spe-
applications. cial infrastructure that includes a location server, a ref-
erence receiver in the mobile unit, and a two-way
14.1.3.2.3 Inverse differential GPS positioning communication link between the two, and is best suited
Inverse differential GPS (IDGPS) is a variant of DGPS for applications where location information needs to be
in which a central location collects the standard GPS available on demand, or only on an infrequent basis, and
positioning information from one or more mobile units, the processing power available in the mobile unit for
and then refines that positioning data locally using DGPS calculating position is minimal.
techniques. With IDGPS, a central computing centre In a server-assisted GPS system, the location server
applies DGPS correction factors to the positions trans- transmits satellite information to the mobile unit, pro-
mitted from each receiver, tracking to a high degree of viding the reference receiver with a list of satellites that
accuracy the location of each mobile unit, even though are currently in view. The mobile unit uses this satellite
each mobile unit only has access to positioning data from view information to collect a snapshot of transmitted
a standard GPS receiver (see Fig. 14.1-5). data from the relevant satellites, and from this calculates
This technique can be more cost-effective in some the pseudorange information. This effectively eliminates
ways than standard DGPS, since there is no requirement the time and processing power required for satellite
that each mobile unit be DGPS-enabled, and only the discovery and acquisition. Also, because the reference
central site must have access to the DGPS correction receiver is provided with the satellite view, the sensitivity
data. However, there is an additional cost for each mobile of the mobile unit can be greatly improved, enabling
device, since each unit must have a means of communi- operation inside buildings or in other places where an
cating position data back to the central computer for obstructed view will reduce the capabilities of an
refinement. For applications such as delivery fleet man- autonomous GPS receiver.
agement or mass transit, IDGPS may be an ideal tech- Once the reference receiver has calculated the pseu-
nique for maintaining highly accurate position data for doranges for the list of satellites provided by the location
each vehicle at a central dispatch facility, since the server, the mobile unit transmits this information back to
communication channel is already available, and the the location server, where the final PVT solution is

Fig. 14.1-5 IDGPS positioning.

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