The TT7B3 Flight

Date: 2019-10-13
Time: 08:00 GMT (10:00 CEST)
Place: Roznov pod Radhostem, CZ
Tracking: http://aprs.fi and http://wsprnet.org

Transmissions
Type	APRS		WSPR
Callsign	OK7DMT-3		OK7DMT
Frequency	144.39-145.57	MHz	14.097,045	MHz
Period	1	min	10	min
Transmit Power	2.3	mW	3.0	mW

The APRS packets are a combination of Base91 encoded position and custom telemetry data in the comment section of the packet. An online decoder and example packets can be found at this link. The tracker transmits on local APRS frequencies. In case of GPS failure, telemetry without position is transmitted. Packets contain backlogged data from the past 7 days.

The tracker also transmits a standard WSPR message every 4th minute in every 10 minute period and an extended message with a 6-digit locator every 8th minute in every 10 minute period. The power level field in these messages is used to encode the balloon's altitude with a 52m resolution.

Flight Parameters
Lifting Gas	hydrogen
Envelope Volume	1.497	m3
Envelope Mass	274.0	g
Payload Mass	8.2	g
Free Lift	13.0	g
Neck Lift	21.2	g
Initial Ascent Rate	0.95	m/s
Expected Float Altitude	15190	m

The balloon on this flight was a Mylar balloon shaped envelope 2.04m in diameter (flat) from a 40μm multilayer (PE/PA/EVOH/PA/PE) film. The making of the envelope is in detail described in this blog post. A calculator was used to obtain the flight parameters based on previously measured envelope volume, mass and payload mass. Latest atmospheric sounding data (Poprad, 20191013 0:00Z) were used to calculate the expected float altitude.

Initial ascent trajectory was modeled with CUSF Landing Predictor, while the long term float predictions with HYSPLIT Trajectory Model. The HYSPLIT prediction contains three altitude levels. One 500m below and another 500m above the expected float altitude.

The design of the tracker is in detail described in this and this blog posts. This time, the primary battery was replaced by a pair of solar cells (52x38mm) and a small SPV1040 circuit with a 1F supercapacitor. The output of the converter was set to 2.0V while the tracker's boost converter and hence its main operating voltage to 2.1V. Since there no longer was a need for saving power, the GPS module runs continuously for higher accuracy, and APRS packets are transmitted once per minute at the 41st second. Both APRS and WSPR transmissions are carried out by the same radio module, and the dipole antenna in terms of length (2x 4.9m of 0.118mm magnet wire) is a compromise that should in theory transmit about 56% of the delivered power at 14MHz and 9% at 145MHz. The new and a few updated libraries, just as the Eagle files for the SPV1040 board, can be found on github.

The fact that the launch site is relatively shaded this time a year combined with some issues verifying the tracker was transmitting led to the actual launch happening an hour and a half later than planned. By that time the surface wind picked up somewhat, but with a bit of patience the balloon was launched at the right time and cleared the nearby forest.

Day 1: The balloon ascended at an average rate of 0.98m/s (maximum 2.0m/s) and headed to Poland in accordance with the prediction. Early into the flight it was clear that the APRS transmissions were quite weak and received by only the closest stations. The furthest reported reception of a packet was at a distance of 43km. At 12:03 (GMT), while still ascending, the last packet was received at an altitude of 9898m with nothing in the data suggesting any issues. No high altitude clouds that could overshadow the solar cells can be seen on the satellite images either.

Information from the APRS data suggests that the tracker was regularly transmitting WSPR messages. Nevertheless, only a few were actually received. On the other hand, all the reported packets were received at substantial distances of 1252 to 1441km with SNRs between -9 to -19. Since this tracker was a last minute and rather improvised setup that hadn't undergone extensive testing, I am not certain about the source of the problems. At this moment, there is still a possibility it will be received again at some point in the upcoming days.

All the received APRS packets can be found in this text file: TT7B3_packets_aprs.txt
All the received WSPR messages can be found in this text file: TT7B3_packets_wspr.txt
And all the decoded live and backlogged data can be found in this csv file: TT7B3_data.csv