Synserver S200 GPS lock question

[ki0kn]

Hello all,
I've been searching/reading this forum for days now (as a guest) and decided to join and ask about this since I am running into a dead end.

I have a Syncserver S200 that's been running great for a long time.  I bumped some cabling in the rack the other day and it went into alarm with a GPS antenna open error.  I found that the BNC center pin had splayed a bit and wasn't making contact.  I rectified this and it rebooted and it found the GPS.

After approximately 20 hours, it changes from Survey to Position hold, has a good position.  GPS lock is "Unlocked'.   I am trying to figure out if I have the Sept 2022 problem with my GPS but am not sure how you determine this. 
Here's the info screen:

Local Time
Sunday January 08 07:49:42 2023 MST
Hostname
CizekNTP1
 
NTP Stratum
3
NTP Reference ID
108.61.73.243
 
Current Sync Source
NTP
Status
Locked
 
GPS Receiver Status
Position Hold
Number of Satellites
4
 
Alarm Status
No Current Major or Minor Alarms
 
Model
S200
Release Version
1.30
Up Time
1 day(s) 12 hours 12 minutes

I do have the 1.36 image and the HDD raw imaging tool (and a delkin card).  I could try the 1.36 upgrade if that has any merit, but I suspect that maybe my GPS has the rollover problem maybe?

Just trying to determine why I can't get lock.

Thanks for any assistance, I've been learning (turning from education to pulling hair out!!) for some days on this, but the only piece I can't find data on is how to determine if your GPS has the issue.  If it does, I assume the easiest fix is to replace the GPS with a new board that doesn't have the issue?

James

[miro123]

Only four satellites.
Do you have the RSSI levels?

[ki0kn]

3   -159   Current
4   -159   Current
7   -156   Current
9   -150   Current
16   -162   Current


It varies because of the shadowing of the antenna... but I see typically 5-6 usually and up to 8 at a time.  How many do you need for a "lock"?  It's been locked for months and has had as many as 8 and as little as 3 visible that entire time.   I watched it go from 2d solution to 3d solution within an hour of reboot and it's been in the state it's in now every time since.

Thanks!
James

[ki0kn]

You got me thinking about the number of satellites.  I added an extension cable to move the antenna out in the clear open.

4   -155   Current
7   -152   Current
8   -151   Current
9   -149   Current
14   -159   Current
20   -162   Current
27   -153   Current
30   -155   Current


8 satellites in view now, full position fix, it's in "position hold" and still will not "Lock"

Wondering if there are other ideas?

Thanks.

James

[The Bootloader]

Hi,

GPSDOs may need hours, if not days, to lock.
I would recommend you wait it out for one more day or two, and come back to report
Very important to not touch ANYTHING during that time, and ensure the receiver has a clear view of the sky during this time.

Additionally, how long is your extension? An extension which is too long can cause problems. Some GPSDOs have an option to compensate for this.

(I have a GPSDO myself)

Also, side note, the "Sept 2022" problem you mention can happen at any date. GPS week number rolls over every 1024 weeks, and GPS receivers all have a reference date in their firmware. Usually, this is the firmware build date, but can be older too. The GPS receivers work for 1024 weeks after this reference date without further information. But after that, they get a wrong date, and this can lead to issues like you mention. Some GPS receivers have a command to send them a new reference date.

[ki0kn]

Thanks Bootloader for the response.  I do understand that it can take some time to lock.   Prior to this last antenna problem, it's never taken more than a few hours to achieve lock after an outage (power or disconnected antenna, etc) so the fact that it won't lock even after days now is concerning. This is an anomaly compared to the last years of it's performance.

The original question I had was to try and understand if I had a GPS board installed in this device that is affected by this:
https://my.microsemi.com/AWelcome/SyncServer%20S2xx%20S3xx%20September%2018%202022%20internal%20week%20number%20rollover%20FSB%20v2.pdf

People on this forum seem to know how to check and that was what I was after.  I know there are some GPS modules that can be used to replace the one in the Syncserver that resolve this.  Given your comments, I am guessing they simply have a much newer date in their firmware. 

I use GPSDOs for other things, and I understand how they work and what it takes to lock, but I know much less about the inner workings of this particular NTP server and was hoping to find some answers from those that have torn these Symmetricom devices down, repaired and upgraded them.  Since this product had long ago been end-of-life and replaced by newer versions, the forum folks are my best shot as getting this thing on it's feet again I believe.

I'll give it some more time and see if it locks.  It is still currently unlocked and has 9 sats visible.   You asked about my extension, I added about 10 ft of LMR240.  The RSSI did not go down noticeably on any of the sats it's tracking.  My RSSI figures are listed in the 2 previous posts.  They range from about -140 to -160.

Thanks for the help.
James

[ki0kn]

Digging into the manual deeper I find that:

Frequency: 1575.42 MHz (L1 signal)
Code: Coarse Acquisition (C/A) code
Tracking: Up to 12 satellites. All 32 PRN's.
Position Accuracy:
Typically <10m w. four satellites. Available from web interface.
1PPS Accuracy:
50 nS RMS, 150 nS Peak to UTC-USNO
Time standard: UTC
Signal strength: 1 sat ≥ -166 dBW to acquire, and ≥ -171 dBW to track.

so I have plenty of signal

and:

Verify the GPS antenna installation:
1. Press the STATUS button on the front panel.
2. Press the up arrow button to display the GPS STATUS screen.
3. When the number of Satellites is equal or greater than "4", Status should be Locked.
4. With GPS locked, the SYNC LED on the front panel should be green within approximately
15 minutes.


so, it looks like the "lock" is not an actual GPSDO lock, simply a satellite lock.   All other things are "locked" right now, the NTP source, the internal clock, just not the GPS.

So I am back to suspecting the GPS board and the rollover.

Would very much still like to verify if I have a receiver that has suffered the Sept 17, 2022 rollover.  Any hints on how to tell if you have an affected receiver?

Thanks again.
James

[ki0kn]

OK,  I tore down the Symmetricom,

It's got a Furuno GT-8031F GPS which does have a rollover of Sept 28 2022.
It's on a Pentair SA101 carrier card.

I will continue to scour the internet, but if there any folks on the forum that have successfully found a replacement GPS that works in this unit, it would be fantastic to hear about it. 

Thanks
James

[mendip_discovery]

Is there not an updated firmware?

I have wondered if the firmware can be modified to include a new rollover date.

[ki0kn]

I wondered that too.  It is end of life, but I wondered if there were any updates before it went that way.  I have a call into the "Furuno GPS guy" and am waiting to hear back.  If I do, and if there is any news, I will post it here.

[JoeFloyd]

I looked into the Furuno GT-8031 Motorola binary protocol

https://www.eevblog.com/forum/metrology/symmetricom-syncserver-s350-furuno-gt-8031-timing-gps-gps-week-rollover/msg4546583/#msg4546583

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.

[Kets_One]

@JoeFloyd

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.

[JoeFloyd]

Hi,

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

[antoine74]

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.

[Kets_One]

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?

[JoeFloyd]

The S200 I swapped out the GT-8731 for a GT-8736, added a rubidium oscillator and 1PPS, 10MHz, and IRIG BNC connectors was fai+ ling hardware clock.

I played around with this device for a while and tried a bunch of stuff.  I provided valid IRIG date and time which showed as locked.  I setup a the device to sync with a known good NTP server.  I had the GPS receiver in position hold mode with more then 5 satellite in lock... 

Nothing I did seemed to fix the hardware clock lock.  BUT, after leaving the thing running overnight it finally did enter hardware clock lock via IRIG.  I disconnected IRIG and it remained in lock via GPS.  I'm guessing that it just takes quite a lot of time for the lock algorithm to decide that the hardware clock has valid time.

The 10MHz output from the S200 is being compared to an S300 with factory rubidium and the two oscillators are very close in frequency, but the relative phase of the two oscillators moves around.  The change in phase isn't fast (~10 deg per min) and is random which is consistent with the error model for GPS precision time transfer and the resulting 1PPS.

Long story short, the only thing that appears to have been wrong with the S200 updated to S250 with rubidium oscillator is operator error (not waiting long enough).