Tria GPS Success in the REF-0

I made some good progress yesterday on the Tria GPS firmware. I have been finding and fixing little bugs over the past few days, and finally got to the point where I could test my Oncore message formatting routine. This is an entirely new chunk of code that creates Oncore message strings in real-time using data sent from the U-blox GPS. That is a very different approach from my previous REF-0 standalone techniques, which used canned strings to make a REF-0 lock up. I am now sending real data in all of the messages, including SV info, position, date/time, and configuration settings.

I dropped the Tria GPS into a REF-0 and launched SatStat to view the status screen. To my slight surprise, nearly everything was populated correctly. I had to make some minor adjustments, but the new Oncore message formatting routine is a success. Here's a screen capture of SatStat for the REF-0 with the Tria GPS:

Status Screen for a REF-0 with the Tria GPS

The SV info is little wonky. Partly that is because I have to adapt satellite info for a 16 channel receiver to 8- and 12-channel messages. I plan to do some more sorting to ensure that the top 12 SVs that the U-blox has acquired appear in the status screen of the GPSDO.

With my message formatting routine working, I am closer to the end goal of full Oncore compatibility. I now need to write some code to allow the GPSDO to send commands to the Tria GPS and configure it. Once that is done, I will be able to make a REF-1 lock up using the new GPS. That will be exciting.

Thanks for reading!

- Dan W.

PCB Progress on the Tria GPS

Lots of progress today on the Tria GPS board. I got the PCBs back and assembled the first one. So far it tests out good.

SMT is so clean. My least favorite part of assembly is

soldering on the through-hole bits.

Now begins the long process of completing, testing, and verifying the firmware...

- Dan W.

Measuring Time Takes Time

Here's a little bit of data on the Denuo GPS in a REF-0 during the warm-up period.

Classic "lots of zeros" picture of the REF-0 10MHz output.
Does this shot have much meaning? Not really.

I've been collecting phase data on the REF-0 and Denuo GPS as the pair warms up. My equipment is quite limited, and it's difficult for me to collect useful frequency data in the E-13 range. So I thought I'd just measure the phase difference between the REF-0 PPS and a stock REF-1 with the Oncore GPS. Perhaps measuring the two relative to each other will provide some insight on how a REF-0 performs with a non-timing GPS.

No data was collected for the first two days. I did monitor it casually on the counter, and it was exactly as you'd expect. Lots of big swings and settling.

Below are traces showing T + 48 to 72 hours, and T + 72 to 96 hours.

Notice the "pop" of noise during day three of the warmup.

Make sure to notice the scale when comparing this plot to the first one.

It's difficult to know what's causing what in these plots. The REF-1 has a 15 year old GPS receiver. The REF-0 has a non-timing GPS with no position hold, but great sensitivity. Better references and lots more time will be required to know more.

Though, something interesting happens when you overlay the two plots.

Plots from two consecutive 24 hour periods shown together.

The consistency between them in many spots is easy to see, even though the traces are separated by exactly 24 hours from each other. Especially during the middle of the plot, which would have been the overnight hours, the two follow each other closely. I'm not sure if there's actually anything useful to glean from that.

That's all for now. There's not much data yet, but any comments are welcome. We'll see how the unit does after a week of being on, then a month, then.... 

Thanks for reading!

- Dan W.

Denuo GPS Retrofit Board

Here is another quick post about some progress I've made toward developing a retrofit board for the Lucent KS-24361 REF-0.

Denuo GPS Retrofit Board for the KS-24361 REF-0

The Denuo GPS is a drop-in retrofit board for the REF-0. It uses the u-blox NEO-6M GPS module. There is a microcontroller on the board that translates the UBX serial messages from the u-blox to the Oncore format for the REF-0. The microcontroller also replicates control functionality of the REF-1, in that it automatically sends the appropriate messages with the correct timing for the GPSDO to operate. This enables standalone operation. While the NEO-6M is not a timing GPS, the board is still able to send sawtooth correction and other timing-related information.

The board is a significant step up from my previous prototyping efforts. It installs just like an Oncore GPS would in a REF-1. Power and signaling are handled through the 10-pin header used by a real Oncore. There are a small number of modifications that need to be made to the REF-0 to enable this, such as mounting standoffs and removing two SMD jumpers. The normal antenna mounting location is not present on a REF-0, so I opted for a u.FL connector and a pigtail to the outside. I think the finished install looks pretty nice. What do you think?

The Denuo GPS board mounted inside of a REF-0.

The board also has some neat features that are enabled by the modern GPS receiver. Sensitivity is hugely improved compared to the Oncore. Time to first fix is several seconds instead of several minutes. Also, the board breaks out the USB connector of the u-blox GPS. I can now monitor GPS status and tweak settings using the free u-center application.

So you might be thinking, "Where's the beef?" Or rather, where's the timing GPS module? The REF-0 is a GPSDO after all, and the original Oncore GPS was of course made for timing. Don't worry, that version of the board is in the works too.

A sneak peek at the Tria GPS Timing Module.

So that's it! I have a REF-0 with the Denuo GPS humming away in a quiet corner. Over the next few weeks, I'll be collecting data on the performance of the GPSDO with this retrofit board.

More to come.

Thanks for reading!

- Dan W.