Thursday, 24 August 2017

The TT7F1 Flight

Date: 2017-08-25
Time: 6:00 GMT (8:00 CEST)
Place: Roznov pod Radhostem, CZ
Live Tracking: or

Callsign: TT7F1
Frequency: 434.264MHz
Params: 100 baud, 450Hz shift, 8n2
Preceded by: 5 blips (~6s)
Packet: callsign, count, time, date, latitude, longitude, altitude, sats, Vsolar, Vcap, Tmcu, Ttx
Example: $$TT7F1,58,11:06:11,20170620,49.46890,18.15049,353,5,921,2708,37,29*A926

Callsign: OK7DMT-11
Frequency: Geofenced local APRS frequencies
Packet: Base-91 position and telemetry, Backlogged position and telemetry in Comment section
Example: !/5KyHS)='OJ5W 1F94!5MI,S(LWHC!+=7+;0c!'|!4!-<x+S1%!(!$|

Initial Prediction: 2017-08-24
Power Scheme
If the voltage on the supercapacitor is higher then 2.5V, the tracker transmits both APRS and RTTY once per minute. If the voltage is less, only APRS is transmitted in 2 minute cycles. The tracker manages to operate for about 1-2 hours after local sunset. It then wakes up again after local sunrise. The startup duration is dependent on the current rate and maximum angle the sun rises above horizon.

The construction was described in The Low Power Setup blog post. It is equipped with two 52x38mm polycrystalline solar cells at 0° angle to charge one 15F supercapacitor. As an antenna it uses a 2-meter band vertical dipole.
Mass: 13.4g

The main function running the tracker can be found in ARM_TT7F1.c on GitHub. A detailed description of the program flow is in The Low Power Setup blog post.
The balloon is the first finished envelope described in The Superpressure Balloons blog post. It was made from 50 micron PE/PA/EVOH/PA/PE material and it is 1.33m in diameter. The estimated stretch is 1.94 Gamma. The chart above shows the stretching profile for this specific envelope. A small weld opening had to be repaired after the first stretching. The second attempt took about 50 hours.
Expected Float Altitude: 13800m
Expected Ascent Rate: 1m/s
Expected Superpressure: 2098Pa (at 17°C supertemperature)
Gas: Hydrogen
Free Lift: 10g

In contrast to last year, I managed to get hold of a reasonably small hydrogen cylinder (10l at 20MPa advertised as 1.8m^3 of gas) which was much easier to transport and handle. I aimed for 10g of free lift, however the improvised filling gear and my heavy hand led to something about 11.2g in the end. I heat sealed the filling tube with several parallel welds to make sure and cut the extra bit. Since this was a supercap tracker version, I had to wait for a while for the skies to clear, so the sun could quickly charge it. The launch itself went fine as there was little wind this early in the morning. I lost line of sight with the balloon relatively quickly as there was a mountain in its path and the horizontal winds were quite strong. That meant the end of the RTTY reception, while the APRS packets continued to pour in from a number of stations since as early as 700m AMSL (300m above ground level). Nothing in the telemetry suggested any problems. All the more baffling it was when the tracker was not heard from since a little over an hour into the flight and about 4000m in altitude. With no new packets arriving for the remainder of the day, I looked anxiously towards the next morning which would provide a forced MCU reset. Fortunately, when I checked the tracker in the morning, the balloon was already transmitting from above the Black Sea just off the coast of Ukraine at about 11.2km in altitude. This floating level in contrast to the expected altitude suggests the method I used while making the balloons quite overestimates the achieved stretch. At the time of writing the balloon has been afloat for three days with the last after sunset transmission putting it above eastern Ukraine. There also seems to be an issue with the date in some of the backlogs.

APRSfi_TT7F1.txt - collected raw packets.
Python parser of raw packet format.
The positional and telemetry data extracted from received APRS packets and decoded backlogs:

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