Calibration

2014 Nov. 21

We calibrated the USRP + Furuno radar combination to allow estimates of incoming radar power from the sample values returned by the USRP plugin.

The setup:

• USRP-1 with resistor ladder front-end board and custom FPGA build as used by the radR USRP plugin
• Furuno FR-1964 radar, set to short-pulse mode but left on standby.
• Wavetek 907A-S-792 signal generator; tuned to 9.49 GHz (fine-tuned to maximize USRP output values); pulsed output; 6.25 microsecond pulses @ 500 Hz
• 12' of LMR400 Ultraflex coax cable
• N-connector to waveguide adapter.  UG-446A/U  Mfg Code: 01882  VSWR: 1.25
• a short R script ran the test_usrp_bbprx program (a diagnostic tool distributed with the radR USRP plugin) to capture digitized samples
• the trigger sync signal from the Wavetek was connected to the USRP's trigger input
• the video signal from the Furuno was connected to the USRP's video input
• the USRP sampled at 64 MSPS for 1024 samples per pulse.
• USRP video-channel gain was set to 0 dB
• 201 samples from the middle, stable region of each pulse were captured
• 20 pulses approximately 1 second apart were sampled at each power level
• at each power level, mean and SE of the USRP digitized value was calculated.  (Mean was mean across 20 * 201 samples; SE was mean SD across 20 pulses, divided by sqrt(20)

Results:

The loss for the cable and N-waveguide adapter is estimated to be 1.53 dB (1.48 dB for the cable, as calculated by some online RF cable attenuation calculators, and 0.054 dB loss in the adapter).

Here is the plot of signal value imputed for a range of sample values returned by the USRP:

This is the power arriving in the flexible wave guide attached to the feed horn behind the dish antenna. 
Note that dBm is itself a logarithmic scale, with 0 -> 1 milliwatt, -10 -> 0.1 milliwatt, -20 ->0.01 milliwatt, etc.
To get the incoming power from the target, you need to also calculate the antenna gain factor.  For the 55 cm
dish, this is around 25 or 30 dB (I have an estimate of the value somewhere - bug me.)  So if the usrp.value
maps to an incoming power of -60 dB, and the antenna has a gain of 25 dB, then incoming power at the antenna,
before focussing (i.e. before the antenna gain comes into play) is -85 dB.