This is a short guide to receiving live tracking data from a balloon sending out GPS fixes. Covering setting up a radio, computer and the software to upload the tracking data to a central server.
Another excellent guide can be found at Project Horus
How does this all work? We are using the receiver, soundcard, and computer to do Software Defined Radio .
This is using the computer to do a lot of the heavy lifting, rather than the traditional way which might be an integrated radio and modem, which is a lot less flexible (and more expensive!).
SSB (single sideband) radio was initially developed as a low power way to send voice. The exact way it is encoded and decoded are irrelevant.
We use it simply to capture a small part of the radio frequency spectrum, and shift it into a range the soundcard can accept.
How this works is that the transmitter broadcasts a '0' at 434.075MHz (for example) - the SSB radio is tuned to 434.074MHz (1000Hz lower). The SSB receiver outputs a tone that is simply the input frequency minus the tuning frequency.
1000Hz and 1200Hz are easily in the audible range (the receiver will only output from around 300-3000Hz) - and are passed to the soundcard.
The tones are then used by fldigi to decode the RTTY signal - which is basically just RS232 - old fashioned serial - at 50 bits a second. The two tones appear in the above 'waterfall' window of fldigi (see below) as two yellow bands (other signals are interference and noise) and are what the decoder lines should be centred over.
There is a 30-sec recording of live data mp3 version and wav version . These files are about 500kB.
Please practice decoding in OFFLINE mode to eliminate any confusion with old data from this clip keep appearing on the map! You can do this by unchecking “online” on the “DL Client” dropdown. Settings to decode this should be 425Hz shift, 50 Baud, 8 bit ASCII , no parity and 2 stop bits. Decoding the data should look like this .
Unless you are very close, you won't hear anything unless you have line of sight. This means that the balloon usually needs to be above your horizon.
(The telemetry audio recording above was made by Daniel Richman during one of Robert Harrison's Icarus launches on Saturday 26th September 2009. The full “un-cut” two minute versions are mp3 and wav . Please note that these are about 1MB and 2MB respectively)
dl-fldigi is an adapted version of the excellent free FLdigi soundcard decoding software. It takes the audio from your radio, decodes the balloon's signal, and then sends the telemetry it's found over the internet to a server running habitat, which plots the payloads position on to the SpaceNear map.
Tracking with r85 of dl-fldigi
Not all of the modes are available using the Auto-config facilities, however there is the RxID facility where by each transmission is preceded by a short burst of data that identifies the mode that will follow. It also has the feature of being tracked by the AFC so it will also allow the receiver to be adjusted to the correct centre frequency. To select this feature, turn on RxID as below:
To setup the configuration for RxID, select the Configure | IDs option:
Turn on all the modes and select the options required, the “Search all Passband” and “Marker” are good defaults to choose:
Finally Save the configuration and close the window, these should then remain as your default settings.