KIWI - GPS Timing Hardware

KIWI - Precision GPS Timing - the hardware pics.

KIWI Geoff holding his two GPS units. Either GPS unit combined with the KIWI software allows precision timing or time signal generation. On your left is the Motorola (GT+), on the right is the Trimble (SV6).

Motorola GT+ on the left, Trimble SV6 on right, the Motorola has a 6 metre cable on the patch antenna, the Trimble has about a 6cm cable to the patch antenna.

A view of the "Micro Pulse" active patch antenna, mounted on the top of the case, with double sided sticky foam. There are two cables into the case. A medium grade speaker cable is used to carry the unregultaed 9 Volts DC from the mains plugpack. The grey cable is a four conductor (individually shielded) that carries, 1PPS, battery backup, serial Tx, and serial Rx and the the ground to the PC. The two cables are both 10 metres long to allow placing the GPS in a good reception position.

A closer view of the antenna, the actual pickup is the slightly darker colour on the pink ceramic substrate, you can see where the wire is soldered to the metal. Although simple looking, the size of the metal "square" is quite critical, it is slightly longer in one dimension than the other. This causes one length to be capacitive, the other to be inductive, resulting (at the GPS frequency of 1575.42 MHz ) in an antenna with right hand circular polarization. Underneath the pink ceramic is the LNA (low noise amplifier). As shown, the antenna is not water or weather proof - it needs a plastic radome for finishing off. However I just wrap the whole box in a plastic bag when putting the case outside. Later I want to do various tests on GPS reception which is why I have left it "naked" as you can see.

It looks like the case is 200mm long!

The antenna connects to the SV6 module via a SMB connector.

This end is wrapped in Black insulation tape, why? because I had no white insulation tape!

Looking inside the SV6 case. The Trimble SV6 module is on the left, the PC interface circuit on the right.

A side view of the SV6 module, from left to right, +5V, Battery Backup, GND, 1PPS, Serial Rx, Serial Tx.

In the foreground is the LM7805 5 Volt regulator, mounted to the chassis to achieve a large heatsink. The circuit draws about 300 mA

A closeup of the SV6 to PC interface. The 74LS123 is used to invert and stretch the 1 microsecend width 1PPS from the SV6. The time constant components (100K and 4.7 uF) give about a 200 mS pulse. The MAX233 converts the SV6 CMOS/TTL to RS232 so that direct connection to a PC/Laptop can be achieved. The 233 is more expensive than the standard 232, but you dont need the 4 capacitors (they are inside the chip). On the Motorola interface (see below) I used a 232.

The Motorola GT+. The diecast aluminium box has from left to right unregulated DC, a high intensity LED to show the 1PPS, and the 1PPS digital signal on a 3mm socket.

Inside the Motorola case. The GT+ on the left, the PC Interface on the right. The "veroboard" contains the MAX232 CMOS/TTL to RS232 converter, and a single transistor inverter to supply the active low 1PPS required by the KIWI software. A DB9 on the right side of the case, takes the serial data to the PC.

Thanks to:
Barry Rutter for the photos using his very nice Canon 4 Megapixel digital camera.
Dave Gault for supply of the aluminium case for the SV6.
Dave (Blue Mountains, Australia) has a very colourful webpage detailing his project using the SV6 and KIWI software.
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