[USRP-users] underrun problem in GPS record and replay system with USRP/WBX

Fabrizio Turin fabrizio.turin at rina.org
Mon Nov 27 10:19:46 EST 2017



Hi all
for a research project I'm trying to record and replay few seconds of GPS signal. The recording part has been done correctly, a 40 seconds GPS acquisition has been tested against a GPS-SDR receiver obtaining a correct position fix. Now I'd like to transmit the recorded signal through RF cable and
see if a commercial GPS receiver is able to get the position fix as well. The problem is that, as soon as I start the simple flow graph in GNU radio I get the "U" underrun warning (side note: PC became un-responding too) and it seems I'm not transmitting at all.
Here's the complete description of the problem:

Front-end: USRP N210 with WBX

Recorded file:

	data type: complex float32
	centre freq: GPS L1
	sampling rate: 10 MHz
	acquisition length: 40 seconds
	recorded file size: 3.2 GB

Host PC:

	OS: Windows 10 Pro (64bit)
	CPU: Intel Core i7-7820HQ
	RAM: 32 GB
	HD: Toshiba THNSNK512GCS8 (545 MB/s write speed)
	network adapter: gigabit capable

USRP Sink configuration:

	sampling rate: 10 MHz
	input type: complex float32
	wire format: automatic
	centre freq: GPS L1
	Ch0:Antenna TX/RX
	(everything else as default)

>From the numbers I wouldn't say that the bottleneck is the HD (in any case I've already tried loading the recorded file from a RAMdisk with the same problems...) or the network speed.
Any idea on what I'm doing wrong and how to get rid of the underrun warning?
Thanks in advance

Fabrizio

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