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[Discuss-gnuradio] UHD loses GPS lock - N210 and GPSDO

From: Kathy Hertzog
Subject: [Discuss-gnuradio] UHD loses GPS lock - N210 and GPSDO
Date: Tue, 10 May 2016 10:38:04 -0700


(I've posted this question on the USRP mailing list but have had no responses. I'm hoping someone here has some advice - thanks.)

I have a very simple GRC project with a UHD:USRP Source Block, a QT GUI Frequency Sink Block and two function probes into the UHD's function "get_mboard_sensor". When it works, two QT GUI Labels are filled with the output from calls to get the "gps_time" and "gps_lock".

I'm connected to an N210 with GPSDO. 

GNURadio, UHD 3.09.02, Ubuntu 14.04.4 trusty

Even with very slow (once every 4 seconds) access to the "gps_time" and "gps_lock" sensor values this example eventually loses lock and stops reporting time. The rest of the flowgraph keeps running. It almost always starts out locked. If I don't use a function probe but have an OOT block that creates an instance of a uhd::usrp::multi_usrp::sptr to access the call to "get_mboard_sensor" I see the same behavior.

At first there are UHD Warnings: "get_time: ValueError: get_nmea(): no GPRMC message found". Then it throws a "RuntimeError: ValueError: Timeout after no valid message found" from the "get_mboard_sensor('gps_time')" which halts writing to the QT GUI Labels. The spectrum keeps updating.

If I add a try/except for the RuntimeError in the Python code for the get_time function probe I eventually still get continuous UHD Warnings about "no GPRMC message found" but, of course, the exception is caught. Just adding a pass doesn't fix it though. Right now this is an unrecoverable error - the system never gets lock back and the time is never updated again.

Is there anything I can do in the "except" that will allow the GPS to lock again? Or better, why is this happening!!

I do have a good GPS antenna and signal with a re-radiator by the puck as well as other equipment on the same antenna that is not having a problem staying locked.

UHD Setup:
Device Address:      addr0=
Sync:                       unknown PPS
Mb0 Clock Source:  O/B GPSDO
Mb0 Time Source:   O/B GPSDO
Mb0 Subdev Spec:  A:A A:B
Num Channels:        2

Thanks for any suggestions.

Kathy Hertzog

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