Electronics > Metrology

Synserver S200 GPS lock question

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I looked into the Furuno GT-8031 Motorola binary protocol


I found a drop-in replacement for the GT-8731 which is the GT-8736.  The 8736 uses the same M12 board formfactor and works great in Symmetricom time servers.

Lock is much faster, more satellites are locked, and the GPS week rollover issue won't impact these receivers until the 2030's.    Having said that, it was hard to find any GT-8736 boards in stock.  I did end up finding a UK distributor who had a few and purchased 5 to repair some S300/S350 and S200/S250.   

The GT-8736 installed in a S300/S350 returns the unit to full operation.  When installed in a S200/S250 there is a GPS Unlocked state the never clears even though the GPS receiver itself reports lock.  This could be software issue or an issue with the FPGA.  I didn't look into what controls the GPS lock state on the S200/S250, but the more powerful S300/S350 just works.


Hi, im new to the forum (and metrology), but have been reading a few messages on the S200 since i had one sent over from the US.
Didnt do proper due diligence though...  :)
found out it suffered the week rollover issue recently.

Now i am trying to get it to work.
No luck with the built-in Furuno GT-8031F. Don't know if its an antenna, cabling problem or an issue with the GPS receiver itself.
No lock (0 satellites!), even after a few hours. But i have to say that i dont have a free view of the sky. Must be 50% free; pretty congested here.

Based on experience on the forum i've ordered a few replacement GT-8736's. I kinda hope that they are drop-in replacements, but your message doesn't sound good.
Does it still give correct time, even though it's not locked?
What firmware version do you have on your S200? Maybe that is a factor here too.


The GT-8736 works.  I may have said GPS lock fails on the S200/250, but that is not correct.  GPS works fine and it's the Hardware Clock that never locks.  As far as I can tell, Hardware Clock Lock is when a timing source has updated the time and date of the devices " hardware clock" which is a normal time and date tpe clock.   

The GT-8736 locks onto satellites quickly and enters a position hold mode which has improved 1 PPS output.    What I didn't figure out is how the Hardware Clock Lock is determined or if the Hardware Clock Lock is tied to the control voltage used to steer the TCXO, OCXO, or Rubidium oscillators. 

What I observed is that the S200 upgraded to be a S250 with an Rubidium oscillator had a stable 10Mhz output.  No measurable drift between the S200/S250 with Rubidium and a S350 with factory Rubidium oscillator. 

So with a Rubidium oscillator the S200 is a good 10Mhz source.  But without having an accurate time and date the S200 is not an acceptable NTP source or IRIG source. 

Based upon how the operation of the device is described in the manual, the GPS receiver should have set the S200/S250 hardware clock to GPS time if GPS was set as the primary time source.  The S200/S250 should have set the hardware clock to IRIG time if the IRIG included date and time, and GPS was not valid.  In my testing, neither the GPS receiver of the IRIG receiver would set the Hardware Clock to the correct time which limits the usefulness of the S200 as a stratum 1 NTP timesource.

Long story short, the GT-8736 works fine.  The S200/S250 seems to have a software issue which is preventing the hardware clock from being set to GPS time or IRIG time. 

Though from what I recall, when the S200/S250 acts as an NTP client the hardware clock is updated and this allows the S200 to provide NTP time and IRIG time but with some additional error due to time transfer.

The S200/S250 has the latest software available.  It can found on this forum under a post about upgrading the S200 to an S250

Hello everybody,
on my Symmetricom SyncServer S250 I replaced the Furuno GT-8031 GPS module with an Oncore i-Lotus M48M version IL-GPS0050-B
Status is always "Receiver Down" and I don't know how to fix.
I am aware that the M12M IL-GPS-0030-B module should work on this Symmetricom unit but this module is nowhere to be found for me.
Any suggestions to fix the problem?
Thanks in advance.

Hi all,

I've been trying to get my S200 to work properly.
So far i've updated the GPS board with a Furuno GT-8736. This worked wonders for finding some satellites. The GPS reports status 'locked'.
However, are reported by others aswell, the hardware clock is not synchronized with the GPS time.

Any ideas on how to get that working?


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