How to Synchronize Two OEM3 GPS Receivers

Here are the configuration setups for using the SETTIMESYNC command with an external clock:

MASTER: ~~~~~~~

LOG COM2 TM1A ONTIME 10

CLOCKADJUST DISABLE

EXTERNALCLOCK OCXO

EXTERNALCLOCK FREQUENCY 10 (or 5)

SLAVE: ~~~~~~

ACCEPT COM2 COMMANDS

CLOCKADJUST DISABLE

SETTIMESYNC ENABLE

EXTERNALCLOCK OCXO

EXTERNALCLOCK

FREQUENCY 10 (or 5) CONNECTIONS:

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-Null Modem connects MASTER COM2 and SLAVE COM2

- OCXO signal is sent through a splitter to feed both the MASTER and the SLAVE external clock inputs.

- MASTER 1PPS (pin2) connected to SLAVE MKI (Mark Input, pin4)

- GPS signal is sent through another splitter to feed both the MASTER and the SLAVE RF connector; i.e., both GPS units must share the same antenna (zero baseline). Make sure that you connect everything first before applying power; if power is applied and the GPS receivers have acquired satellites before the OCXO and / or 1PPS -> MKI is setup, then the times reported by the TM1A log will still diverge. We noted that after the clock model was stabilized at state 0, the time difference between MASTER and SLAVE reported by the TM1A log was less than 10 ns. The external clock must be connected to both the master and the slave units; you cannot just connect it to the master unit unfortunately. You will need some sort of external clock in order to use the SETTIMESYNC command in conjunction with the 1PPS and MKI I/O strobes to keep two MiLLennium GPS receivers' time difference less than 30 ns max; 5 ns typ. You cannot get this level of accuracy using the 1PPS and MKI I/O strobes alone. OCXO stands for Oven Controlled Crystal Oscillator. It doesn't really matter what you use for an external clock source as long as it is stable, and has low phase noise. The MiLLennium GPS cards have built in support for OCXO, Rubidium (Rb), and Cesium (Cs) external clocks running at 5 or 10 MHz. You can use a custom clock if you know the "h" noise elements and enter them as parameters to the EXTERNALCLOCK command accordingly. Please keep in mind that the MiLLennium GPS Cards will only be able to work with external clocks of frequency 10 MHz (default), or 5 MHz.