NSL-81

NSL-81, flown the morning of May 30, 2020, tested a tiny new APRS tracker.

Tim W. and Paul L. had been working on building several 'next generation' trackers over the fall/winter. This new APRS tracker was dubbed a "Femto Pecan" by its creator Sven DL7AD. Sven is known in the hobby for his work on the small Pecan tracking units (Pico Pecan etc).

This tiny board was run off of 3 AAA batteries and had enameled 30awg wire for the APRS, and GPS dipoles. It was programmed to transmit APRS position, temperature, and other telemetry every minute.

The flight train consisted of the veteran rig from NSL-80, launched under a Kaymont 600g H₂ cell. This included a 90cm parachute, a small box with an AP510 tracker, then a larger box with a Pi-in-the-sky LoRa tracker. As the day was predicted to be 100% overcast, only a Mobius Mini camera was added to the box. Below all of this hung the Femto Pecan.

Tim and Paul each had a teenage son with them to help out. New to the team, Christopher R. was interested in learning about High Altitude Ballooning from an education perspective, so he offered up his yard in Apex as the launch site. He and two of his sons joined in on the flight as well. The gear was all integrated in Christopher's yard and balloon fill proceeded without issue. The PITS tracker struggled to maintain lock on satellites, but all of the other trackers behaved well.

Flight projections showed a simple meandering flight up to Bunn, NC about 2.5 hours later. Contrary to forecast -- patches of blue sky showed at launch. Maybe the camera will capture something after all.

Minutes into the flight, things started to go wrong:

• The reliable AP510 tracker went silent only 2.5 minutes into the flight.

• The new Femto tracker wasn't showing up on HABHUB or APRS.FI -- even though it could be received by the chase team.

• The PITS tracker stopped sending photo images, but at least it was now sending GPS data with only 5 satellites in lock.

The chase team proceeded to the landing area in their three chase cars. At least the PITS tracker was sending tracking data!

...And then, 30 minutes into the flight, the PITS lost GPS lock and starting sending the last know position over and over...

Thankfully, just over an hour into the flight, the PITS regained GPS lock and started sending new position data!

...But 15 minutes later it lost lock again. The only thing to do now was get to the projected landing area and see what could be spotted. At least they might use radio direction finding to locate the payload.

Meanwhile, the payload continued upwards towards burst. The Mobius Mini camera recorded a beautiful day below.

Christopher and Tim's chase cars reached Bunn, NC and headed to the last predicted landing spot. The APRS frequency was quiet!

Paul's chase car lagged behind, having stopped to drop off the H₂ tank and fill gear. On board, son Matthew double-checked the PITS receive gear and assembled the big Yagi antenna -- they would need it. This entire time, the LoRa receivers displayed the same stale coordinates over and over. The only data changing were the inside and outside temperature readings of the payload. This at least gave hints that burst had occurred. Having a LoRa telemetry receiver, they hung back 8km southeast of Bunn. LoRa signals registered stronger to the northwest up towards the others.

Still APRS remained quiet. Slowly the teams converged at Bunn. Then suddenly the LoRa signal died. Typically signal strength will drop-off dramatically when the payload lands -- Perhaps this could be used to narrow the search!

A plan hatched, the three chase cars split up again. Each headed out to patrol a different sector around Bunn. HAM radios, set to the LoRa frequency, were held out the car windows in hopes of hearing the weak LoRa signal. Moments later, the "southern patrol" picked up the LoRa signal, just 2km southeast of Bunn. All teams converged at a farm stand beside the road.

Then out of nowhere came a squawk on the radio at the APRS frequency!

APRS decoders soon picked-out the GPS coordinates of the still functioning Femto tracker across the street! Of course, this pointed to a stand of tall trees on the edge of the wide-open fields. Tim negotiated land access and the team set up for retrieval in the woods.

The payload was soon located far up a pine tree. Upon gathering of supplies, Tim shot an arrow up over the payload and hauled up a thicker line. After much pulling and prodding, the entire payload came down.

What caused the strange issues with the trackers?

• The AP510 appeared fine and still had a full battery. No idea what shut it off. Perhaps a piece of packing foam snagged the power switch? Better put this tracker in the "suspect bad" pile.

• The PITS tracker had only 5 satellites in view at best. This tracker has a sorted history, so it is probably time to retire it and to move to a different GPS receiver module.

• Quite some time after the flight, many of the Femto tracker's packets suddenly appeared on APRS.FI. We suspect that there was some sort of APRS iGate delay (network outage?) around the area. Inspection of the packets reveal several reboot events during ascent. Review of the flight video shows reboots happening around the time when the small tracker was being violently tossed about at the far end of the payload train. Perhaps this was causing g-forces on the spring loaded battery holder resulting in power hits??

This violence could have also tangled the dipole antennas leading to a drop in signal strength.

Flight visualization using recovered data from the three trackers. AP510 (bright green), PITS (dark blue), Femto (cyan)

Interpolating timestamps, ascent rates, and video footage puts the burst altitude around 29km (95kft).