How to Synchronize two OEM4 GPS receivers

Here are the configuration setups for using the ADJUST1PPS command with an external clock:

MASTER: ~~~~~~~

LOG COM2 TIMEA ONTIME 10

CLOCKADJUST DISABLE

EXTERNALCLOCK OCXO 10MHZ (or 5MHZ)

 

SLAVE: ~~~~~~

INTERFACEMODE COM2 NOVATEL NOVATEL OFF

CLOCKADJUST DISABLE

ADJUST1PPS MARKWITHTIME

EXTERNALCLOCK OCXO 10MHZ (or 5MHZ)

 

CONNECTIONS: ~~~~~~~~~~~~

- 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 connected to SLAVE MKI (Mark Input). Check to see which pins these are on the receiver. (Board or IO port on the enclosure)

- 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 TIMEA 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 TIMEA 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 ADJUST1PPS command in conjunction with the 1PPS and MKI I/O strobes to keep two OEM4 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 OEM4 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 OEM4 GPS Cards will only be able to work with external clocks of frequency 10 MHz (default), or 5 MHz. For additional information, please see pages 50 to 51 of the OEM4 Volume 1 manual which can be downloaded from the following location. http://www.novatel.com/Documents/Manuals/om-20000046.pdf