SETI League Technical Manual -- Software
FFTDSP Update
by Mike Cook, AF9Y
October 1996 Update:
The SETI League members might be interested in the recent upgrade to the FFTDSP program. The upgrade includes the following:
- Data Logging for long term unattended monitoring (such as SETI)
- A replay program for the data file is included.
- On screen Object tracking by RA and DEC entry (such as Mars Observer)
- Setup allows offset hours from local to GMT
- Correction of moon tracking error for non US stations
Available Download Sites for FFTDSP are:
http://www.webcom.com/af9y
ftp.webcom.com/pub/af9y/www
February 1996 Update:
This is to inform SETI League members that a new
beta version of the FFTDSP program is available for downloading
from my webpage. Here are a few of the new features in FFTDSP41:
- Record and Playback WAV files for post spectral analysis
- Automatic Color Gradient for optimum visibility
- Integration mode for signal detection below the noise
- Mouse point and click for selective recording
- On screen Moon position and TOD tracking
- True S/N (in 100 Hz Ref) bar graph
- Selective area and mode for S/N display
- Display Smoothing Filters
- Zoom In/Out for wider frequency coverage
- Improved Graphics Interface and Setup Screens
- Find Call feature from partial information
I've noticed that some of the SETI League members are confused
about the term "below the noise level". Weak signal detection
of a carrier is limited by the effective channel bandwidth
and frequency stability. Here are the key considerations:
- FFT Processing per time element
Example: 4096 Pt FFT at 8192 Sample Rate = 2 Hz Channel (0.5 Second update rate)
- Signal stability, including doppler, must be less than the channel bandwidth for best detection. For the above example, the signal should not drift more than 4 Hz per sec.
- Higher levels of FFT processing will reduce channel bandwidth but will demand greater signal stability.
- Integration increases signal detection will average down in all channels which do not contain a carrier. This has the effect of bringing the signal out of the noise.
- During integration, the signal must remain in the channel for best detection. If the 4096 Pt FFT is averaged over 100 time periods (50 seconds), the detection is improved by 10dB. The signal would have to stay in the 2 Hz channel for the entire 50 seconds. The drift rate should be no more than 0.04 Hz per sec.
It is apparent that equipment stability and doppler consideration
are key factors in any SETI attempt. I believe that compensation
could be made for doppler (if significant) during the integration
process. This possibility and any other recommendations for program
improvement is welcomed.
Mike Cook, AF9Y
Webpage: http://www.webcom.com/af9y
Home Email: mwcook @ cris.com
Work Email: mwcook @ itt.com
