Using the Motorola GT Receiver

These GPS units were available from Tanner Electronics in Dallas at the end of 2002. They were used in an OEM application and came as a PCB only (no antenna). They were supplied with a photocopy of how to get them going, also available as using_gt.pdf. Experience has shown that this document with the following notes together may be enough to get one going.                           

Interface

The first item of business is to build a converter to change the TTL level signals supplied by the unit to the RS232 levels the computer expects. I decided to use the MAXIM MAX232 rather than the MAX233 suggested in the document as they were cheaper and more readily available. The only advantage of the MAX233 is that it needs no external capacitors; the disadvantage is that it is in a bigger package.

Power

The unit is powered by 5V so a 7805 regulator was used. Due to the units power needs, the regulator needs mounting in the box to dissipate heat.

Housing

As with the Tanners $25 (Trimble SV-6 GPS) the unit will be  housed in a 6x3x2" diecast box which is just big enough. These are available in a number of varieties with very similar outside dimensions; make sure the one chosen does not have internal PCB mounting ribs or mounting the antenna jack through the box sidewall will be a problem

Testing

Initially the unit was connected via the regulator to a 12V  supply and just the MAX232 IC interface circuit.  Upon initial switch on nothing will appear from the rs232 port of any significance as the unit is in Motorola binary mode. Using the visualgps program choose settings> <GPS settings> click the gps initialisation box and select <Motorola VP/GT> and hit ok. The GPS unit will be put into NMEA mode and a terminal set for 4800-8-n-1 will start to see NMEA strings after they are turned on. 

1 PPS output

Next attention were turned to the 1PPS outputs. Unlike the SV-6 this unit produces a 100ms TTL level pulse. This may be fed directly to the MAX 232 to be converted to RS232 levels and thence to the computer

Final design

As the unit had now been successfully interfaced  the final complete circuit was drawn up as shown in Fig 1.

LED1 is also driven by the 100mS wide 1PPS signal to indicate activity. V1 is a 3.5V NiCd pack which is used to batter backup the receiver, which retains the Satellite Ephimerisis data making the unit find the satellites quicker on power up. It also retains the current settings (which/how often NMEA strings are sent etc) so is more vital than one for the SV-6

It was found that the same PCB employed for the SV-6 could be used by cutting off the part with the unneeded pulse stretcher  The track layout is shown in Fig 2, the component overlay in Fig 3, both showing where the board is cut.

The complete unit with interface takes 0.17A at 12V. Note the original MCX input connector is unused, an SMA socket is mounted on the box wired in parallel with the MCX connector. I did try removing the MCX connector from the top of the board with various handtools but that caused much damage!

Conclusion

The GT+ has improved performance over the SV-6 module in that provision is made to more easily customise the setup of the module, however it needs battery backup to retain settings.

It is good for navigational purposes and for driving such programmes as NMEATIME. However the unit cannot be put into Fixed position mode, a requirement for accurate 1PPS output for driving 1PPS locked frequency references. It can however be used as the basis for a GPS clock as described in gt clock.htm