[ From the Motorola Oncore Manual]

USING THE GT ONCORE IN DGPS MODE

The GT Oncore can be used in differential GPS (DGPS) mode for applications
that require 1 - 5 meter accuracy. The GT Oncore uses corrections in the
Motorola binary format or RTCM SC-104 Type 1 and Type 9 messages. A VP
Oncore GPS receiver must be used as a base station to generate corrections
in the Motorola binary format; a GT Oncore cannot generate corrections. This
section describes how to set up a differential system using a VP Oncore as a
base and GT Oncore receivers as mobile units. The VP Oncore should have
firmware version 8.4 or newer in order to support all of the commands
listed. The GT Oncore should have firmware version 1.6 or newer to support
the Motorola binary differential mode. VP Oncore Base Station Setup In order
to generate the best possible corrections, the VP Oncore must be put in
position-hold mode with the known location of the antenna. The accuracy of
the mobile receivers is limited by the accuracy of the known location. The
following steps should be used:


1. Disable the ionospheric correction algorithm using the @@Aq command.
Operating in differential mode will cancel out the ionospheric delay, whereas
the ionospheric correction algorithm is not as effective.

2. Set the mask angle to 10 degrees using the @@Ag command. This will
prevent the base station from tracking those satellites that are near the
horizon, which tend to have the most error due to atmospheric delays and
multipath.

3. Set the Satellite Selection Mode to Highest-In-The-Sky using the @@Ah
command. This will cause the receiver to select the satellites that have the
highest elevation angles, which are the satellites that will have the least
amount of multipath. The GT Oncore always operates in the highest-in-sky
mode. With the base and mobile receivers in the same mode, they will be
most likely to select and track the same satellites.

4. Use the @@Ar command to set the receiver to N-In-View satellite
selection, to
disable the position filter, and to disable AFC tracking. This will force the
receiver to use all possible satellites, disable the smoothing, and track with
the phase-lock-loop only for the lowest receiver noise, respectively.

5. Set the correction threshold to 32 seconds using the @@AD command. This
will require the receiver to track a satellite for 32 seconds before
reporting a
correction for that satellite. The delay will allow the carrier-aided-tracking
filter to tighten up which reduces the receiver tracking noise to a minimum
before sending out a correction.

6. Enter the position-hold position using the @@As command. The more
accurate the position used is, the more accurate the corrected mobile fixes
will be.

7. Enable the position-hold mode using the @@At command. This will force the
receiver to solve for the satellite range errors rather than the receiver
position.

8. Start the differential correction output using the @@Bh command. If more
than six satellites are being tracked, then the response message @@Ce will be
sent twice in order to output all of the corrections.

GT Oncore Mobile Setup

The GT Oncore must also be configured for optimum operation in differential
mode, which is done as follows:

1. Initialize the receiver with date, time, position, and almanac if required.
Entering these values may speed up the time to first fix (TTFF).

2. Disable the ionospheric correction algorithm using the @@Aq command.
Operating in differential mode will cancel out the ionospheric delay, whereas
the ionospheric correction algorithm is not as effective.

3. The GT Oncore is configured to accept the differential corrections using
the
@@Ce format. The corrections from the base station should be entered
directly into the input port of the GT Oncore. The GT Oncore will
automatically enter DGPS mode when enough corrections are available. If
there are more than six satellite corrections, two of the messages will be
required.

If the RAM is not backed up using a battery when the main power is down, the
setup of the GT Oncore will not be retained. If this is the case, the
ionospheric correction algorithm will be set to the default setting of on.
For this reason, the ionospheric corrections should be disabled every time
the unit is powered on if used in a differential system. The GT Oncore
receiver uses a position filter to smooth out the effects of constellation
changes, multipath, receiver noise, and selective availability (SA). In
differential mode, the position filter is disabled. Whenever the receiver
switches from non-differential mode to differential mode, the position will
ësnapí to the new position. When the receiver switches from differential
mode to non-differential mode, the filter will make a smooth transition.

There are a few conditions that will cause the GT Oncore to compute a
non-differential
fix:

1. There are no differential corrections available.

2. The last differential corrections received are more than 90 seconds old.

3. There are three or fewer satellites being tracked that have corrections
available.

One way of improving the differential performance in dense urban canyon
areas is to send the satellite ephemeris information to the mobile units
over the transmission link. This does not improve the accuracy of the DGPS
solution; it only helps the mobile get started if it is subject to satellite
blockage during startup. The base station can be used to output the
satellite ephemerides on a regular basis. These ephemerides can be input to
the GT Oncore using the @@Bf command. In this manner, the mobile will always
have satellite ephemerides available. The GT Oncore will continue to
download ephemerides from the satellites. The ephemerides in use will not
change until the receiver decodes ephemerides with a new IODE number. The GT
Oncore will store up to eight satellite ephemerides in memory. It will only
store the ephemerides that correspond to the satellites that are currently
assigned to one of the eight tracking channels. When the GT Oncore is first
powered on, the satellite assignments change frequently as the as the
startup search algorithm is employed. For this reason, it may be best to
hold off on inputting the satellite ephemerides until the receiver is in 2D
or 3D fix mode and has a valid almanac. If the system is designed to send
out the satellite ephemerides on a regular basis, then this is not an issue.