Summary of RFI Data Samples at Green Bank

Rick Fisher, NRAO, Green Bank, WV

Contents

Introduction

This web page is a brief summary of data files that contain samples of signals seen at the NRAO observing site at Green Bank, WV, USA. Many of these signals are transmissions for valuable commercial or government purposes such as communications and radio navigation, but to the radio telescopes at Green Bank they are seen as radio frequency interference (RFI), sometimes covering up weak radiation from celestial radio sources. The main intent of these data samples is to aid engineering research into ways of separating man-made from cosmic signals.

RADAR at 1292 MHz

In the afternoon of February 6, 2001 a 10 MHz passband centered on 1293 MHz from the GBT's 1.15-1.7 GHz receivers was sampled at baseband with the Signatec PDA500 data acquisition board. This passband contains an air surveillance RADAR signal with a nominal carrier frequency of 1292.01 MHz. The RADAR pulse sequence is randomly selected with an average repetition rate of 341 Hz and a pulse width of 2 microseconds. The RADAR antenna sweep period is 12 seconds per rotation, and the beamwidth is 1 degree. The RADAR antenna is 104 km south-southeast of Green Bank at an azimuth of 163.87 degrees. The GBT was pointed near the zenith at the time.

Four data files were recorded. All contain 8-bit A/D data samples of baseband voltages from the GBT receiver. Zero voltage has a value of approximately 115. The receiver white noise spans about 5 counts peak-to-peak. All four files contain 102,415,360 bytes.

Our data acquisition software was pretty primitive so the data sequences were initiated manually. Hence the start times with respect to the RADAR sweep passing over Green Bank are only approximate. The available computer memory limited each data sequence to about 5 seconds, so a full 12-second run was not possible to record in a single file. The files were recorded about 5 to 15 minutes apart. The files are

sweep_minus1s.dat - data begun about 1 second before RADAR beam passed over Green Bank. 20 megasamples per second.

sweep_plus1s.dat - data begun about 3 seconds after RADAR beam passed over Green Bank. 20 megasamples per second.

sweep_plus6s.dat - data begun about 6 seconds after RADAR beam passed over Green Bank. 20 megasamples per second.

noise_cal.dat - calibration white noise source followed by a 12 MHz low-pass filter. 31.25 megasamples per second. These data show any internal spurious signals in the data acquisition board. One at 180 kHz plus two or three harmonics are known to exist.

Please let me know if you'd like a copy of any of the files and I will either give you the directory paths, arrange to make them ftp accessible, or make a CD-ROM.

A preliminary analysis of the 'sweep_minus1s.dat' data set can be found here.


Last modified February 10, 2001

rfisher@nrao.edu

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