User`s guide

Chapter 3 - Receiver Descriptions

Note that a receiver operating in differential mode will discard corrections once they have aged

more than 90 seconds. This is normally not a problem as corrections are typically applied every

5-20 seconds, but if you have a poor RF link between the Base Station and the rover, this

condition may occur. The receiver will automatically switch between differential and autonomous

modes as corrections are received or time out.

M12+ as a Differential Base Station

In order to generate the most accurate corrections, the M12+ being used as a Base Station must

be put in Position-Hold mode with the antenna at an accurately surveyed position. The positioning

accuracy of all of the rover receivers is limited by the accuracy of this position. To set up a Base

Station, the following steps should be followed:

• Disable the ionospheric and tropospheric corrections by invoking mode 0 in the @@Aq

Ionospheric Corrections Select Command.

• Set the satellite mask angle to 10 degrees using the @@Ag command.

• Enter the Position-Hold-Position using the @@As command. The coordinates can either

be determined by a professional site-survey, or you may use the Base Station receiver

along with a software program such as WinOncore12 to develop a reasonably accurate

position by allowing the receiver to run for 12 - 24 hours and utilize the Mean position

calculated by the software.

• Place the receiver in Position-Hold mode by invoking mode 1 of the @@Gd Position

Control Message.

• Enable the output of differential corrections from the base station using the @@Bh

command. Allowable update rates are once per second to once per 255 seconds. As a

practical matter, corrections are usually sent out every 3-20 seconds. Any longer than a

20 second update rate may tend to cause larger errors in reported position. Once the

@@Bh command is invoked, the base station will start issuing @@Ce correction

messages at the requested rate. Two @@Ce messages will be issued back-to-back if

more than 6 satellite corrections are available as the @@Ce message format only

handles a maximum of 6.

• If the rover is receiving the corrections, it will respond to the @@Ce messages with an

@@Ck acknowledgement message.

M12+ as a Precision Timing Receiver

As received, the M12+ Timing Receiver default operating parameters are already set up for

optimal operation. There is no need to set the Mask Angle to 10 degrees as this is the default

condition for this receiver.

• Enter the Position-Hold-Position using the @@As command. The coordinates can either

be determined by a professional site survey or you can use the Auto-Survey function of

the M12+ timing receiver. Invoking this function (mode 3 of the @@Gd command) will

automatically average 10,000 position fixes and then force the receiver into Position-

Hold.

• Set the timing parameters using the @@Gf, @@Ge, and @@Hn messages

@@Gf – This message is used to set the T-RAIM alarm limit. The receiver defaults time

is 1000ns, but the user may select any value between 300 and 1,000,000ns using this

command. Typical values are between 500 and 1000ns.

Motorola GPS Products - M12+ User's Guide Revision 6.X 09FEB05