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The WSPR-TX Pico is a solar powered WSPR transmitter that is custom-made for Balloon flights.
It has the the following characteristics:

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1. Is a low power autonomous solar powered shortwave WSPR transmitter for the 20m and 30m amateur bands that continuously transmits its position and altitude during daylight. 

2. Has an onboard GPS module and antenna for calculations of position and altitude.

3. Runs open-source software on an Arduino compatible micro controller.

4. The transmitter is the well known Si5351 and Phase-Locked-Loop oscillator that uses a Lab-calibrated Temperature Compensated Crystal reference Oscillator (TCXO) for exact transmissions.

5. Weight 10.5gram

6. Transmits its altitude using the WSPR Powerfield 

7. Configuration is done with a Windows program. No programming skills required.

8. Pre-built, tested and calibrated. Solar cells not soldered to PCB to make it sturdier during shipping, see assembly doc for instructions.  

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Data:

• Tranmission Frequency:
  10 and 14MHz with on-board low pass filter.

• RF Power output:
  Slightly more than 10mWatt.

• Size:
  45x50x25mm.

• Power usage:
  0.3W (5V 20mA at idle, 60mA at transmit) Powered by solar cells or by USB to Serial converter

• Weight:
  10.5g with solar cells

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The WSPR Pico is a custom designed WSPR transmitter for balloon flights and uses the power encoding capability of he WSPR  to send its current altitude to complement the position report that is transmitted as a Maidenhed grid.  

The altitude is converted to dBm by internally dividing the altitude in meter by 300.

The result is rounded to the nearest value in dBm in the table.

To take an example - lets say the transmitter is on 11500m of altitude it will report that its power is 37dBm or 5W. (11500 divided by 300 is 38.33dBm - nearest value is 37dBm)