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3DM-GX4
™
-45
™
Inertial Navigation System
User Manual
OEM System Integration
54
7.3
Alternate GPS Equipment
Other (external) GPS receivers, antennas and/or cables can be used with the 3DM-GX4-45
™
.
When using third party antennas the antenna cable length, antenna gain, and antenna power
must be considered. For antennas with internal LNA (Low Noise Amplifiers) the power
requirements should not have a minimum voltage below 3 volts and the current draw should not
be over 20mA. The longer the cable the lower the signal strength, and including the antenna
cable offset in GPS outputs is advised (
see GPS Antenna Offset on page 41
). The loss of signal
strength can only be empirically determined by trying out a particular installation and monitoring
the number of satellites links and quality of data reception from those satellites. This can be
done in the MIP Monitor software (
see Global Positioning System (GPS) Outputs on page 25
).
When using a different GPS antenna, always use the non-magnetic MMCX-to-SMA adapter
supplied with the 3DM-GX4-45
™
unless the magnetometer is not used in the end application.
7.3.1
GPS External Receiver
To use an external GPS receiver, the internal one must be disabled and a serial link
established between the receiver and the sensor through the host computer. A program is
then written using the LORD MicroStrain
®
MIP Data Communications Protocol to port the
data from the GPS input to the host to the sensor serial port and to translate the
GPS receiver data into the message structure the sensor processor can interpret. The data
cannot be sent any faster than 5Hz.
To set the GPS receiver to external, open the Device Settings menu in the MIP Monitor
software by right-clicking on the sensor name in the main window (
Figure 37 - GPS Source
Select
). Select the Estimation Filter tab and then EF Options. Set the GPS Update Source
to External GPS .
Test external GPS messaging using the Advanced > GPS External Input interface. This
command will send a fixed external GPS message to the device and mimic a pulse per
second clock input (
Figure 38 - External GPS Data
).