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30/10/2020

TimeTools T1000/2 GPS Receiver

I've recently got around to attempting to revive the Time Source project at the London Hackspace that was originally started by russss - a GPS synchronised Lucent/Symmetricom KS-24361 REF-0 reference clock with a 10MHz output (GPSDO). 

Russ was originally using a TimeTools T1000/2 GPS Receiver, a device with very little information available online. I contacted TimeTools for a copy of the manual but got told rather bluntly that it was over 10 years old, no longer supported, and probably didn't work anyway because of the GPS Rollover.

Its a fairly simple device so I figured it wouldn't take too much to figure out, and it turns out it does still work after all. 

TimeTools GPS Reciever
TimeTools GPS Reciever


Hardware

The GPS receiver is a simple aluminium box with a DC input / DB9 connector on one end, and what I believe is a TNC connector and 2 LEDs on the other.

It requires 12V DC to operate, and it seemed to run fine on a 300mA power supply I had available. The DB9 is a proper RS232 serial interface, with proper RS232 voltages!

On the other end the TNC connector is for connecting to a GPS antenna. As far as I understand it supports active and passive variants. 

The two LEDs show the status. One is just an indication the unit is powered up, and the other flashes with the PPS signal. I forgot which way around they are but it should be pretty obvious when you power the unit on.

Inside the unit is a Navman Jupiter 21 Module. 

The PPS signal is output on the CD pin of the DB9 connector which is the top right pin as you look into it. It's normally at 0V, pulsing to -20V each second.


Oscilloscope Showing PPS


The internal GPS board


The internal carrier board. Power and Level Conversion

Does it work?

To test the unit I powered it on and stuck the antenna on the balcony and forgot about it for a while whilst it got a lock. There's no battery in the device so I assume that means its a cold start each time which can take some time. After a while the PPS LED began to flash suggesting it had a lock.

I connected up with a RS232 to USB converter and used gpsmon to check the NMEA output which confirmed that satellites were visible and the time was correct, so it looks like the device hasn't been affected by GPS Rollover yet.

Below shows the redacted output of gpsmon. I have no idea why its blurry, apologies.


One observation is the unit appears to output a PPS signal before its got a proper lock on satellites (although not immediately when its powered on) . I don't know how critical this is but its worth keeping in mind.

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