(This is AA6E's original write-up from 2010.)
We have written cross-correlation and frequency-searching software and tested it by autocorrelating data recorded at a single station. We get a clear peak provided that frequency can be corrected to within about 1 Hz. The peak, as expected, defines the relative signal delay (zero, in this case) to about (receiver bandwidth)**-1, i.e. about 300 µs. We want to see how well this result carries over to real crosscorrelation, where the two data streams have independent noise, receiver passbands, frequency offsets, etc.
First Experiment: Recording W1AW Bulletin Transmissions
This experiment attempts to answer the first question above: "What are the limitations imposed by the ionosphere, imprecise receiver tuning, etc.?" We are soliciting the help of a small group of radio amateurs who can receive and record the 80 M W1AW SSB Bulletin.
Concept: Use W1AW SSB Bulletins as a standard voice transmission that can be recorded at multiple receiving locations.
What do you need to participate?
- W1AW receive capability on specified bands and specified dates.
- Want "good" propagation conditions - preferably S8, S9, or better.
- Ideally, a location outside the very local zone - 50 miles or more from W1AW?
- Accurate tuning ability (within ±10 Hz, preferably)
- Ability to make computer WAV file recordings at 48,000 samples/second, 16 bits per sample (Note changed sample rate.)
- Availability to record during a small number of recording sessions to be announced in advance.
I am looking for help from a number of stations to make as many simultaneous recordings as possible of W1AW SSB Bulletin transmissions on the 80M band. The recordings will be sent to AA6E by email, where we will cross-correlate the results. We are specifically looking for how well we can define the peak correlation of signals between stations and how well we can compensate for small frequency mistuning.
These preliminary recordings will validate our software methods and allow us to see how well we can correlate "real life" HF signals, although without a "time tagging" capability, we can not (yet) get "time of arrival" information.
In the future, we are looking at using GPS time sync pulses to tag our audio recordings so that we can get precise time of arrival information that could let us use the recordings to find the location of an unknown transmitting station, such as a "jammer".
All stations providing data will receive full credit in any publications. Thank you for considering working with our project!
Setting up your Receiver and Computer
At present, we are planning an 80M recording (3.990 MHz LSB). Anyone who can reliably receive W1AW's 9:45 pm (Eastern) transmission on 80M is welcome to participate. (We are looking for stations in the "single hop" skip zone, probably within 600 miles of Newington, CT.) The date will be selected after we have found enough volunteers.
Your receiver should be set up for normal SSB reception, around 2.4 kHz bandwidth, slow AGC (or AGC off), with no noise reduction or other "fancy" filtering.
Accurate tuning is very important. If you can accurately set your rig in advance for 3.990000 MHz, that is the best. (That may mean checking your rig's VFO against WWV, for example.) If you need to tune in the signal in "real time", it is important to do this the night before, and record your setting for the live run. We want an accurate and unchanging VFO setting right from the start of the Bulletin transmission. (Voice Bulletins are usually pretty short!) If frequency setting is a problem for you, please discuss it with me before time.
We want to use the computer to record a 48,000 Hz (samples per second), 16 bit, mono signal. It should be written to disk as an uncompressed ".WAV" file.
First, connect your soundcard mic or line input to your receiver's audio output. You can probably use the headphone output, but it might be easier to monitor your reception if you can find an audio output that does not mute your radio's loudspeaker. (In a pinch, you could place a computer mic next to your receiver's speaker, but we don't encourage that!)
In the following, I assume your computer runs Windows XP. (Macintosh and Linux computers most likely have similar sound recording options.)
- Start the Sound Recorder program. (Start-->All Programs-->Accessories-->Entertainment-->Sound Recorder --or-- you can run the application "sndrec32".)
- When the Sound Recorder window comes up, select the menu File-->Properties.
- Click the "Convert Now..." button. A dialog box called "Sound Selection" comes up.
- Under "Name", select "Radio Quality". Under "Format", use the default "PCM". Under "Attributes", select "48.000 kHz, 16 Bit, Mono".
- Then press "OK" and "OK" again.
You should now be ready to record. Try recording and playing back SSB transmissions until you are comfortable with the software. You may need to adjust the audio settings to get the levels right. (Edit-->Audio Properties brings up the Audio Devices screen.) Make sure you can get good audio volume (visible on the Sound Recorder scope display) without any clipping or distortion.
After you have a good recording, you need to save it to disk. Use "File"-->"Save as..." and give it an appropriate name, like W1AW_test.wav. I recommend you verify your recording procedure, by sending me a copy of a test recording, well in advance of the scheduled run.
- Note on sound recording software: If you use Sound Recorder in the simple way, you may notice that a recording is limited to 60 seconds. For this project, we would like to record the full W1AW Bulletin -- or the first several minutes, at least. There is a way to record for longer than 60 sec. Basically, you create a dummy 60 second WAV file and then "include" it in Sound Recorder. Each time you include this file, the recordable length increases by 60 sec! (Don't ask me why. :-) More details are available at this Microsoft page.
- If you would like to use a more full-featured sound recorder (and editor), I would encourage you to look at Audacity, which is a free-to-download program that runs on many flavors of Windows, Mac OS X, and Linux. Alternately, you can use most any recording software, as long as it can generate the required WAV file.
Live Recording Runs
- Check the date and time of our scheduled recording run. (We will provide it in email.)
- Well in advance of 9:45 pm, set up your receiver and computer. Set your VFO the to receive 3.990000 MHz LSB as accurately as possible. (I.e., include any known frequency offsets.)
- Start your recording as close to 9:45:00 pm Eastern as possible. Stop at the end of the Bulletin or after 5 minutes, whichever comes first.
- Make a little log of start time and stop time - noting general signal quality, receiver setup, and any other information of interest.
- Save your recording and play it back to be sure it sounds good.
- Mail log to email@example.com with the WAV file as an email attachment.
Thank you for participating in our Time of Arrival project!