First time I had this idea was when I found out that ISS sends SSTV images from space, many years ago… back them the ISS was transmitting SSTV images in ROBOT36 mode.
I wanted to try something similar, to send images from high altitude. My rockets hardly get more than few hundred meters so the only other option was to transmit images from a weather balloon. Launching a weather balloon is easy, just buy the balloon and helium + some paperwork for permission. The hard part was to get the images send out in a way radio amateurs to be able to decode them.
Back then 2012, 2013 were no open source SSTV projects so I started from scratch. First successful SSTV transmitter using ROBOT36 mode was published in IEEE.
Short story long: we use a USB webcam to acquire images, Raspberry Pi 2 to process the information and an external USB sound card to generate the audio signal… the on board PWM audio of Raspberry created “snow” on images and I was too busy with other things to make any fancy filter to improve the situation.
The transmitter was based on a rfPIC and a 1w RFPD PA to get the signal in ether on 70cm HAM band
I choose to transmit images in the same manner as ISS hoping that more radio amateurs will have the setup ready to acquire this kind of images.
Anyway, time passed and ISS switched SSTV transmission from ROBOT36 to PD120, now images have a higher resolution: 640×493. Also the transmission frequency is in the 2m HAM band.
in 2019 I decided to redesign the entire system and to create a Raspberry Pi shield: “2m Radio Shield” this included:
- 2m PLL with VCTXO controlled through SPI
- Audio codec controller though I2S and I2C
- 7W hybrid amplifier
- barometer and 2 temperature sensors
Fig1: “2m RF Shield” prototype in tests
I had to develop two application for Raspberry: APRS and SSTV. These two apps were written in C in Eclipse CDT.
Since the PLL is controlled from Raspberry, I had the option in the script to shift the frequency. First an APRS signal was transmitted on 144.8Mhz. After this the SSTV will be transmitted on 144.75Mhz, the allowed frequency by IARU Region 1 for SSTV transmission
APRS is getting data from GPS, barometer and temperature sensors and creates a .wav file that will be played by a python script. the audio signal will be generated by the audio codec. audio signal will be fed to a VCTXO to modulate the frequency and to achieve FM modulation on 144.8Mhz – European APRS frequency
Fig2: APRS.fi report of the balloon position and payload conditions in comments
The SSTV is reading an JPG file and the GPS data to create another .wav file that will be played by the audio coded included on the “RF shield” . The JPG file is transferred by the Python script from an Nikon camera though USB into Raspberry by using functions from GPhoto2lib. The transmitted image will have the altitude overplayed in the upper left corner
Fig3: example of received SSTV from the balloon
Unfortunately some local RF interference made the images unclear. The receiver was more than 50km away. Lab image looked like this:
Fig4: SSTV image received in lab conditions
The flight took place on 23 June 2019. Unfortunately the battery stopped providing power after a short time so max altitude was not reported by APRS.
Fig 5: balloon, payload and the Moon
But we got some nice pictures
Andrei Hapenciuc 