Dropsonde Micro

One of the key measurements I need is windy speed and direction. The most effective method for measuring wind in a sonde situation (where you are moving with the wind) is to measure your displacement. If you drift a certain distance in a certain time, over a certain altitude, the movement of air can be inferred. 

I picked up two low-cost GPS modules from Amazon, a BN-220 (~$17) and a GT-U7 (~$15). They were chosen somewhat arbitrarily, since they should all perform similarly. I tested the two units on my bench. This is indoors, so results are probably worse than they would be outdoors. VisualGPSView was used to display the position and GPS performance information. The GT-U7 has an integrated USB port, while the BN-220 requires an FTDI RS-232 to USB adapter to function with a computer. This doesn't really factor into integrated work, but is an interesting note.

Model	BN-220	GT-U7
Networks	GPS, GLONASS, Galileo, BeiDou, QZSS, SBAS	GPS, GLONASS, QZSS
Fix time	15-20s	15-20s
Form	Small, integrated board	Larger board, u.fl connector for antenna
Satellites after one hour	12 used, 21 in view	8 used, 12 in view
Scatterplot

Interestingly, the GT-U7 has a tighter distribution than the BN-220 even with less satellites and a poorer DOP figure. Further tests with the systems outdoors are needed to provide more information, as well as tracking quality under motion.

The idea for this sonde is a minimal cost system that transmits data to ground from at least 5000ft. The device should be recoverable and reusable. 

The desired measurements are as follows:

• Temperature
• Pressure
• Humidity
• Wind Strength
• Wind Direction

P/T/H can be accomplished with a single chip such as the BME280.

Wind measurements can be derived with some accuracy using a GPS and integrating the movement of the system as it falls.

The descent rate of the system will provide the time for GPS movement, but with some caveats; the slower the sonde descends, the more it will drift. A balance needs to be struck between ease of retrieval and  measurement quality.