Hi long duration ballooners,
This may be of interest to some of you. I launched a buoy last week 430 miles SW of San Diego. It sends two WSPR signals on 20 meters every 10 minutes just like the balloons do.
The payload board was made by Kazu, AG6NS. Kazu and I worked together to optimize the firmware for buoy use. The considerations were: buoys are always at zero altitude, move around 1 km/h or less, operate in a different temperature range than balloons, have large GPS antennas and are powered from a 12 Volt system with batteries with solar charging.
We reassigned the two WSPR power values from altitude to add a 7th and 8th character to the grid square position for a 10x finer position resolution (balloons report to the Subsquare, the buoy to the Extended square).
Voltage from solar cells are encoded on the 4th character of the second WSPR transmission (range A-Z) and for balloons cover a range of 3.2 to 5.7 in 0.1 steps. A 12V to 5V converter is used to power the payload board and connects where the solar cells for a balloon payload would. If we transmitted the DC voltage supplying the payload board it would always report 5 Volts until just before the 12 Volt batteries were dead. For this buoy we changed the voltage divider resistors and the voltage sensing A/D monitors the 12 Volt system. We rescaled the voltage range from 9 to 16 volts in 0.3 Volt steps. We typically see the voltage drop to 13.2 Volts overnight and quickly rise to 14.1 the next morning.
For balloons, the 2nd character of the second WSPR transmission (36 values from 0-9, A-Z) combines GPS status and temperature with a range of -30C to +25C in 5C steps. For the buoy we don’t send the GPS status because it will always be good given the large antenna. We use all 39 values to cover +4C to +39C in 1C steps.
For the WSPR to APRS conversion, the default WB8ELK decoding requires both the callsign and telemetry messages to be received to upload to APRS.fi for that cycle. Often during times of poor propagation only one of the two is received. Since little changes with the buoy over 10 minutes, we have modified the decoding software to replace a missing message with the one received 10 minutes ago.
The buoy can be tracked at https://aprs.fi/#!call=a%2FKQ6RS-12&timerange=604800&tail=604800
73,
Randy KQ6RS
SF-HAB 