Instruments for High-Altitude Balloons

For many of us, an important motivation of launching balloons high in the atmosphere is to put instruments up there. Of course, the fact that instruments are heavier than air means they resist attempts to get them in the atmosphere and are prone to falling out of it. So, airborne sensors need to be light enough to be lifted by an affordable balloon and cheap enough to make risk of crash acceptable. I spent a big chunk of the last couple years designing and building a custom instrument with these characteristics in mind; this post describes how I did it.

My particular field is infrasound, sound waves whose pitch is too low for humans to hear. Infrasound waves normally come from very large sources like lightning, volcanoes, nuclear bomb tests, windmills, and even large animals like tigers or elephants. Unlike audible sound, infrasound waves can propagate long distances without dissipating as heat, which makes them useful for monitoring nuclear and volcanic blasts. Putting microphones high in the air could help improve geographic coverage in our otherwise land-based monitoring networks.

Solar balloons offer an affordable and flexible means of putting cheap, lightweight sensors in the sky. However, commercial infrasound instrumentation falls short in those aspects. To be sure, we do have a few high-quality data loggers and sensors to choose from, but none is lightweight or cheap enough to fly on solar balloons. The closest is the Omnirecs DATACUBE-3 (below, right). Including batteries and sensor, it weighs more than 1 kg and can cost a few thousand USD (depending on quantity and options chosen).

My normal research is on volcano acoustics. I’m not normally worrying about flying microphones on balloons, but I faced similar instrument limitations in volcano fieldwork. For example, my most recent project involved a hike with 2000 m of elevation gain to install 17 instruments near the vent of Tungurahua volcano. On a hike like that, the last thing I need is to be weighed down with heavy instruments, or even heavy batteries for the instruments. Plus, the instruments near the vent are definitely in harm’s way, and I don’t want to accidentally blow up an expensive sensor.

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The upper slopes of Tungurahua: loose, rocky, steep, and prone to ballistic fall during eruptions.

The answer was that I needed to design my own instrument. This process had two parts: firmware (programming the microcontroller) and hardware (finding the right components and connecting them together). Fortunately for me, I began this project around 2012-2013 when a revival in hobbyist electronics was in full swing, and had access to components that are powerful but still novice-friendly.

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The various companies and online forums I used to design the instrument. Most of these cater to hobbyists and beginners.

The Arduino system makes firmware way easier than it used to be. Arduino is a combination of a microcontroller (very, very basic computer), user-friendly circuit board, and programming interface that makes it easy for hobbyists to program the microcontroller and connect peripherals to it. It’s perfect for a beginner. In my case, the firmware’s job was to read data from several inputs, do a little processing, and write output data to a micro SD card. Inputs included an analog-digital converter listening to the infrasound microphone, a GPS (for precise timing and location), a temperature sensor, and a battery voltage tracker.

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The Arduino IDE speeds up the process of writing and testing code.

Without the Arduino system to simplify this process, it wouldn’t really be accessible to someone like me. I’m a geophysicist, not an electrical engineer! But it still wasn’t easy, and it took me a couple years of off-and-on work before I really got it working well.

The hardware part of this project began with a bunch of user-friendly breakout boards wired to the Arduino. A breakout board is a little circuit board that includes some principal component (say, a micro-SD socket, or a GPS) and all the various minor components needed to make it work. The user simply then has to make a few connections (i.e., power, ground, and communication wires) between the microcontroller and breakout board to make it work. Breakout boards are important because a bunch of little things–for example, bypass capacitors and level shifters–have to be added for the principal components to work properly. These little things are often obvious to electrical engineers but can be overwhelming to beginners. Platforms like Arduino and Raspberry Pi might be the foundation of hobbyist electronics these days, but breakout boards make it possible to do cool things with those platforms.

One bit of advice to the DIY instrument designer: if you base your design on the Arduino, don’t use an actual Arduino in the final product. Use a bare-bones variant instead. Real Arduinos have a lot of nice features that unfortunately burn a lot of power, and you can reduce power draw by up to a factor of 30 by using a bare-bones knockoff. I used the Diavolino from EvilMadScientist at first, but am now designing my own board with a built-in efficient power supply instead.

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Top left: GPS breakout board. Top right: analog-digital converter breakout board. Bottom: Arduino Uno, the most common Arduino version.

After getting the breakout boards to play nicely with the Arduino, it was time to design a printed circuit board (PCB) to house them. This might have been the most daunting part of this process. Two circuit design programs are common among hobbyists: KiCAD (free and open-source), and the light version of Eagle (free, not open-source, restricted to non-commerical use, but popular). Like a lot of scientific software, these programs take a while to learn.

In each program, the first step is to draw a schematic showing conceptually how components connect to each other. So, for example, if your circuit included a resistor and capacitor, your schematic would include the resistor symbol connected to a capacitor symbol, but no information about the size, shape, or position of the components.

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Schematic editor in KiCAD

After drawing the schematic, the user decides how to lay out the circuit in physical space–specifically, where the components and electrical connections are located on the top and bottoms surfaces of the circuit board. The first step is choosing a physical footprint for each component. Then, a PCB editor window helps the user place component footprints on physical locations on the board and draw wires connecting them. The files created in this process can be sent to a manufacturer (I use Advanced Circuits, based in Aurora, Colorado, USA).

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PCB Editor in KiCAD

The final product is called the Gem infrasound logger (named for the “Gem State” Idaho, where it was developed). Parts for it cost about $250 and it takes a few hours to assemble. This is about an order of magnitude cheaper than the cheapest commercial alternative. On the other hand, I put an uncountable number of hours (hundreds, at least) into designing and revising the Gem. On the other other hand, I learned some valuable skills in the process, and my next electronics project (a power-efficient Arduino board) was much easier as a result.

And of course, it’s turned out to be useful in applications beyond volcanoes. Danny Bowman (of Sandia National Labs, and this blog) has launched them on balloons. Tim Ronan (grad student at UNC-Chapel Hill) used them in his research studying river rapids. (Turns out that being lightweight is important when microphones have to be installed by kayak through class-IV whitewater!) Danny and Tim both helped me a lot by testing Gem versions that were still in development, for which I am grateful.

Was it worth it? As a grad student, my time is cheap and half my job is to learn new skills, so spending a lot of it learning how to do this was a good investment. If I was a professional scientist whose labor was more expensive, it might not be such a good decision: better to pay an engineer to do it, even if the product costs more to develop.

In any case, the final product justifies the effort, regardless of who did it. I now have a cheap logger that I can put high on volcanoes and on balloons. It’s been used in several projects around the world now, and it will remain a key part of my infrasound research.

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A solar balloon carrying a Gem lifting off in Albuquerque, NM, USA. Photo by Danny Bowman.

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A subset of the many sites where the Gem has been tested.

How to build a high altitude solar balloon

This balloon can deliver a 2 lb payload to 72,000 ft (22 km) and fly for as long as the sun shines.  It can be hand launched by two people without the aid of electricity and lift gas.  The envelope is built from cheap, easy to find materials.  Total construction time is 4-6 hours for a team of two.

It’s best to find a large area, such as a gym, for building the envelope.  Take care to not damage the plastic – I recommend taking your shoes off so that you can step on the balloon material without ripping it.  When it comes time to darken the envelope, I highly recommend *not* doing it somewhere where lots of charcoal dust will cause a problem.  We recently did it in a parking deck, which meant that spills were no big deal and also kept the wind from blowing the balloon around.

Materials:

-400 x 12 foot sheet of 0.33 mil plastic sheeting (sold as “light duty paint dropcloth” at hardware stores)

-Several rolls of heavy duty clear packing tape, such as these on Amazon.

-Scissors

-Permanent marker

-Tape measure

-Heavy duty string or cord (I use parachute cord)

-Air float charcoal, at least 1 lb

Building the envelope:

This is where you cut the plastic into the required shape, tape it together into a balloon, and check it for holes.  Click here to see a time lapse of this process.

Step 1: Cut the plastic sheeting into five 30 ft sections.  This is around the 10 second mark in the video above.

Step 2: Unfold them until you have five 30 x 12 rectangular sheets of plastic.  This is around the 15 second mark in the video above.

Step 3.  Fold each sheet once across the longer section and once across the shorter section.  Now you have five 15 x 6 ft sections.  This is between the 15 and 25 second mark in the video.

Step 4.  Lay the five sheets on top of each other, all facing the same way.  Find the corner that forms the center of the original sheet (this is where the fold seams all meet each other).  That corner is in the video around the 28 second mark, next to the guy in the blue shirt.  We spend

Step 5.  Consider the corner described in the previous point as the origin, the long part of the sheets as the X axis, and the short part of the sheets as the Y axis, draw the following points using a permanent marker (units in inches):

X Y

0 72

18 71

36 68

54 64

72 58

90 51

108 42

126 33

144 22

162 11

180 0

These points describe a half gore pattern, which is how we turn two dimensional objects (plastic sheets) into a 3 dimensional object (a spherical balloon).  Here, it happens to be a sine curve.  This is from the 40 to the 50 second mark in the time lapse.

Step 6:  Carefully insure that all sheets in the stack are lined up with each other (from about 28 to 40 seconds in the video).  Then, using a pair of scissors, carefully cut along the curved line defined by the points drawn on the top sheet (50-52 second mark).

Step 7: Unfold the sheets; you should have 5 diamond shaped ones.  These are gores, and they form the envelope of the balloon.  The other, roughly triangular pieces of plastic are trash (52-54 second mark).

Step 8:  Tape one edge of the first gore to one edge of the second gore using packing tape.  The seam should be centered in the tape, with no gaps between successive pieces of tape.  We have one person hold the two sheets together and the other tape them together, see photo below:

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Taping the gores together.  If you have more people, you can have multiple teams going at once!  Photo by Mary Lide Parker, UNC Research Communications.

Step 9.  Add the next three gores successively, to make an ever larger sheet of plastic.  Finally, tape the two ends of the sheet together: you’ve now made a ball a little more than 19 feet across (the envelope of the balloon!)  This process takes up from the 1 minute to about the 2 minute mark in the video.

Step 10:  Find one of the two “poles” of the balloon (where the taped seams all meet).  Cut the pole off to make a hole about 5 feet across.  This will become the bottom of the balloon, and allow you to fill it with air.  We do this at 2:14 in the video.

Step 11:  Carefully tow the balloon back and forth, holding the hole open.  It will begin to fill with air.  This is from 2:15 to 2:17.

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Filling the balloon with air prior to checking for holes.  Photo credit: Mary Lide Parker, UNC Research Communications.

Step 12:  At this point, it should be pretty clear whether or not you built the balloon correctly.  If everything looks good, send a brave soul inside to check the envelope for holes (gaps in seam tape are the most common culprits).  Someone on the outside can fix the holes as they are found.  Be careful, of course, since the air supply in there is finite.  This is from 2:18 to 2:36 in the video.

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Checking each seam for holes.  Photo credit: Mary Lide Parker, UNC Research Communications.

Step 13:  Deflate and pack the balloon.  Start from the pole opposite the hole, and slowly push air towards the open end of the balloon.  Don’t go too fast or you’ll pop sections of the balloon.  It’s pretty simple to then stuff the balloon into a big garbage bag for storage.  This is from 2:37 to the end of the video.

Rigging the balloon:

The open hole on one side of the balloon is very weak and susceptible to tearing.  Also, it does not provide any means of attaching a payload.  Thus, we need to reinforce it and provide a way to attach our equipment.

A simple way to do this is to run some tape around the bottom, poke some holes in the tape, attach some string, and tie your payload on.  Our first versions had this system, but it was not ideal; in fact it is probably why we had an “unscheduled rapid disassembly” at 72,000 ft last May.

A much better way is to tie a length of strong cord (parachute cord, for example) into a loop slightly larger than the opening of the balloon.  Pull the opening through the loop, fold it around the loop, and tape the edge of the opening to the outer envelope of the balloon.  This provides a very strong lining system for the bottom.  A payload can be attached by tying guy lines onto the cord loop.  I believe the best place for these guy lines is right at each seam, since the seam tape provides a means of distributing the load along a relatively strong portion of the envelope.  The photo below shows one edge of the balloon with the parachute cord folded in, as well as one payload attachment string.

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Darkening the envelope:

This is the most fun part (besides launching).  Find a place that is protected from the wind but will allow you to make a big mess.  As mentioned earlier in the post, an indoor parking deck is ideal.

Unpack the balloon and lay it out on the ground. Throw a generous quantity of air float charcoal into the open end, and shake it all the way through the balloon.  The charcoal is so fine it will coat the interior of the balloon, changing it from white to dirty gray.

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Midway through darkening our solar balloon.

Then, wait for good weather conditions:

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Two solar balloons and their payloads in storage.

Launching:

The launch procedure is simple: tow the balloon back and forth until it fills with air, attach the payload, let the whole thing heat up for a bit, and off it goes.   Here’s a video of us doing it.  Simple, right?  No.

Actually, launching solar balloons is hard.  It’s a lot harder than helium balloons, since ground conditions are much more restrictive.  With this in mind:

An ideal day for solar ballooning has clear skies and calm ground winds.  This is actually pretty rare, and you may have to wait several weeks for an opening.  If you start to get impatient, keep in mind that even winds barely strong enough to move leaves can make handling a 20 foot tall balloon very dicey.  Early mornings (just after dawn) are best.

An ideal site for a launch is a large open field, where slowly rising balloons will not get caught in trees, power lines, etc.  An alternative is a parking lot between tall buildings, since wind tends to go around them.  This is risky, though, since the balloon can still hit and potentially snag on them.

Finally, if you are planning on recovering your payload, realize that the balloon will fly until the sun sets.  This means that even a 10 mph wind can carry the balloon 120 miles, assuming 12 hours at float.  Many times, the winds in the upper troposphere/lower stratosphere are much stronger.  Careful consideration of the wind profile from 0 to 100,000 ft above sea level is thus imperative before attempting a full day flight with payload recovery.

Happy ballooning!

Predicting the Flight Path of a Solar Balloon

Balloons that fly on solar power – what could be better?  Unfortunately, the physics of these balloons is complex and reliable data on how they fly is hard to come by. On this blog, we’ve discussed our attempt to come up with a numerical model of solar balloon flight as well as the data set from our solar balloon that made it to 22 km (72,000 ft) last May.  In this post, we present an empirically-derived solar balloon flight model based on data we collected on the high altitude flight mentioned above.

First off: a disclaimer.  This model is generated from data collected by one flight of a specific design of solar balloon.  While we hope that it captures some general features of a solar balloon flight, we can’t be sure.  Furthermore, different balloon designs and atmospheric conditions will cause inevitable (and at this point, unpredictable) deviations from the model we describe.  Take it with a big grain of salt.

Ascent Rate

We smoothed the GPS altitude data from our high altitude launch and performed a 1st order Tikhonov regularization to derive an ascent rate versus altitude model for our solar balloon.  The plot is below, if you want the numbers click here.

Modeled ascent rate versus actual ascent rate for our high altitude solar balloon.

Modelled ascent rate versus actual ascent rate for our high altitude solar balloon.

The solar balloon initially ascends at about 1.5 meters per second, reaches a maximum of about 2.5 meters per second at around 15 km elevation (just below the tropopause), and rapidly decreases as the balloon approaches neutral buoyancy at 22 km elevation.  This is fortuitous, since the strongest winds in this elevation profile typically exist in the region where the balloon is ascending the fastest.  In contrast, the lower stratosphere (18-22 km) is usually calmer.  The exciting possibility in this trajectory data is that we may be able to park future balloons in this low wind “sweet spot”, and thus recover payloads relatively close to the launch site.

The modelled ascent rate lacks the vertical velocity oscillations in the real data; this is by design.  We suspect those oscillations are either due to gravity waves in the atmosphere or uneven heating of the balloon envelope.  In any case, we would not expect to see the exact same ones on future flights.   Furthermore, removing the oscillations has little effect on the accuracy of the ascent rate model, as the plot below shows.

Modeled elevation vs actual elevation for the May 29, 2015 high altitude solar balloon flight.

Modeled elevation vs actual elevation for the May 29, 2015 high altitude solar balloon flight.

However, a comparison between the actual ground track of the balloon versus one using horizontal winds calculated from archived weather forecast data shows a considerable deviation:

Modelled versus actual ground path of the high altitude solar balloon.

Modelled versus actual ground path of the high altitude solar balloon.

This may be because of the poor spatial resolution of the archived Global Forecast System model we used (1 x 1 degree) as well as the lack of a precise match between the prediction time and the flight time.  Although the two trajectories are rotated, their general form is similar.  While the forecast skill is not ideal, it still gives a reasonable approximation of what we could have expected on launch day.  To this end, we are utilizing the simple vertical velocity model derived above in combination with predicted wind speeds to generate potential flight paths.  Here’s an example from September 28, 2015:

Example of a modelled solar balloon flight path. The green diamond is the launch site, the red line is the ascent, the dashed line is 8 hours of drifting at neutral buoyancy, and the blue circle is the predicted balloon location around sunset.

Example of a modelled solar balloon flight path. The green diamond is the launch site, the red line is the ascent, the dashed line is 8 hours of drifting at neutral buoyancy, and the blue circle is the predicted balloon location around sunset.

It may not be perfect, but it sure beats what we used to use.

Stratospheric Solar Balloon Flight

It took three years and several false starts, but we finally got a solar balloon with a video camera on board to take off successfully.  Not only that, but the balloon reached an elevation of over 22,000 meters (72,000 feet), well into the stratosphere!  Then, it suffered what Elon Musk calls a “rapid unscheduled disassembly,” sending the payload into a 22 kilometre free fall that ended in the muddy banks of a cow pond.  Despite this, the photos, video, and GPS track were all recovered.  Here’s the flight video:

The balloon envelope consisted of a 19 foot diameter sphere of 0.31 mil clear plastic paint drop cloth.  Since the drop cloth comes in 12 x 400 foot sheets, we constructed the envelope from 5 gores that were 12 feet wide at the equator.  We attached the gores together using clear shipping tape and darkened the interior of the balloon with black paint pigment.  Each gore seam had a string attached to the bottom with black duct tape.  The opening at the bottom of the balloon is about 6 feet across, allowing two people to inflate it by hand (check out the full inflation and launch video).

The solar balloon in flight just after launch.  Image credit: Mary Lide Parker

The solar balloon in flight just after launch. Image credit: Mary Lide Parker

The payload consisted of a SPOT satellite tracker for recovery, an Arduino Uno with Adafruit High Altitude GPS Shield for trajectory determination, and a Raspberry Pi with camera module for video and stills.  The system was powered using a lithium battery pack meant for recharging cell phones (10 ampere hours).  All this was contained in a Tupperware box with a small hole to admit the camera lens.  We attached it to the balloon using four strings taped onto the box with white duct tape.  The four strings led to a fishing swivel to keep the payload from spinning too much.  We clipped it to the strings on the envelope using a black carabiner. Total payload weight was 800 grams (1.75 lbs).

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Payload with the lid off, showing flight instrumentation (left) and payload just before launch (right).

We kept an eye on the wind profiles for about 5 months, since most of the time winds in the troposphere blow out to sea.  Finally, we waterproofed the payload and decided to risk a flight even if some winds were going east.  On May 29, the winds in the troposphere were pretty low (max 10 m/s or so) and with varying azimuth.  The stratosphere had a steady breeze going west, so we figured if we made it that high, we’d head back over land.  However, we had a much faster ascent rate and reached a much higher altitude than we anticipated, so we ended up not flying very far from the launch site.  You can download the trajectory data in text format here or Google Earth KML here.

trajectory

Ground flight path (left), launch site is the origin. Altitude versus time (centre), local time was GMT – 4. Ascent rate versus time (right), local time was GMT – 4. I obtained the ascent rate by calculating a 1 minute moving average and dividing elevation by time.

The photos and video were very good quality considering that we were using the Raspberry Pi camera module (not the world’s most advanced camera).  The troposphere was pretty misty, and it seems like we even passed through a haze layer on the way up.  However, it could be that the lens fogged up temporarily.  Once we entered the stratosphere, the pictures are much better:

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The view from 22 kilometres in the sky.

You can watch a slide show of all the photos here, and all the video clips stitched together here.

Just as we approached neutral buoyancy, the payload unexpectedly separated from the envelope and fell back down to Earth.  We kept GPS tracking until about 18 kilometres elevation, and as far as I can tell the box was falling at about 320 km/hr (200 mph).  The Arduino kept track of time even after losing GPS fix, continuing to record until the moment of impact.  Thus, we know that the payload fell the remaining 18 km in about 12 minutes.  The impact speed was probably less than 100 km/hr (60 mph).  The Tupperware payload box was cracked, and everything except the SPOT tracker stopped working.  Had we landed 10 centimetres or so west, we would have splashed down in a pond.  Luckily, we hit the mud on the pond’s edge (and missed the cows that were in the area).

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The payload box at the impact site in Snow Camp, North Carolina.

We consider this flight mostly successful.  Our main objectives were to launch, recover, and extract data from an instrument package lifted with a solar balloon.  Furthermore, we wanted to inflate the solar balloon by hand.  Both of these objectives were reached. Our secondary goals were to fly until sunset, not land in the ocean, and make it to the stratosphere.  The flight was only about 2.5 hours, so we did not fly all day as we hoped.  However, we made it well into the stratosphere and were never in danger of ending up in the ocean.

We had a slightly tense launch when the payload got snagged on the eaves of a nearby building, but the balloon built up enough lift to detach itself in about 20 seconds.  The SPOT tracker did not record any positions during flight, so we did not know where the balloon was and were not even sure if the tracker was working.  In fact, it did not record positions until about an hour and a half after impact. The unexpected flight termination was upsetting as we did not anticipate having the payload detach from the envelope.  We assumed that either the envelope would rupture due to sun-induced heating at altitude or that it would deflate at sunset.  In either case, the payload would have had a large plastic streamer to slow it down to safe velocities.  Instead, it appears that the black duct tape we used to attach the payload strings to the envelope got too hot in the intense sunlight at 22 km.  This caused the payload to come loose from the balloon.  The lesson we learned from this is to never use dark coloured tape if there’s a chance the flight system will make it to extreme elevations.

A flight aboard the NASA High Altitude Student Platform (HASP)

The NASA High Altitude Student Payload (HASP) project provides a spot on a high altitude balloon payload for undergraduate and graduate students. When I heard about this last year, I gathered a team together, we applied and were accepted into the program. Our project: launch infrasound microphones into the stratosphere. Infrasound (sound at frequencies below audio range) is usually measured at the Earth’s surface, but we know it propagates hundreds of kilometers upward into the atmosphere. Our goal is to measure these sound waves as they cross the stratosphere.

The HASP project was definitely a commitment. As the team leader, I was required to write a monthly status report letting the HASP project leaders know what I was up to. I had to build my payload box under strict power draw, weight, and size limits. I also had to learn electronics from the ground up. Thankfully, another member of our team had lots of experience in electronics, so it wasn’t so bad.

Our Omnirecs DataCube logger installed in the payload box.

Our Omnirecs DataCube logger installed in the payload box.

A few weeks ago, I traveled to Palestine, Texas to bring my payload to the Columbia Scientific Balloon Facility (CSBF). There, our payload was subjected to extreme temperatures (ranging from -50 to 50 Celsius) and pressures (sea level to stratospheric).  We passed the test, recording the 8 Hz signal from the vacuum pump clearly even when the pressure was around 5% of sea level.  This was an important milestone: not only did it clear us for flight, but it also showed that our differential pressure microphones (constructed by Dr. Jeff Johnson at Boise State University) would operate in a near vacuum, something they were not designed to do.

Our payload about to face the thermal/vacuum test.

Our payload (the white box with the UNC logo) about to face the thermal/vacuum test.

From Texas, I traveled to New Mexico to launch our payload into the stratosphere.  I ended up staying in my home town, about 2.5 hours from the launch site at (a CSBF facility in Ft. Sumner, NM).  Needless to say, I spent a lot of time on the road!  I had to drive there and back three times: once to put my microphones on the flight ladder (see below), another time to make sure everything worked during the “hang test” (a dry run for launch), and finally for the big day itself – the flight.

CSBF staff mounting the infrasound microphones on the flight ladder.

CSBF staff mounting the infrasound microphones on the flight ladder.

Like any balloon flight, this one depended on the weather.  This time, the news was not good.  One group was ahead of HASP, and they had dibs on each launch window.  They tried twice, and were not able to fly both times.  I had to fly home on Saturday, so I showed CSBF and HASP personnel how to set up my payload, and I resigned myself to not seeing the balloon fly.

But as luck would have it, the previous team decided to wait, and a launch window opened Saturday morning, the day I was scheduled to fly home.  Since the flight was early in the morning and my plane ticket was for early afternoon, I decided I was going to go see the launch.  I drove out, arrived in Ft. Sumner at about 10 PM, slept in the back of the car for a few hours, then got up at 2:45 AM Saturday morning to start getting ready.

It's about 3:45 AM, and CSBF staff secure the microphones after I powered them up about 15 minutes before.  The wheel on "Big Bill" (the launch vehicle), is taller than I am.

It’s about 3:45 AM, and CSBF staff secure the microphones after I power them up. The wheel on “Big Bill” (the payload vehicle), is taller than I am.

The launch was touch and go the whole time – we had to wait for the winds to all blow in the same direction for the first 1000 ft in order to start the inflation process.  As luck would have it, they did straighten out, and the call was given to roll out and start inflating the balloon.

The balloon reaches full inflation about a half hour after sunrise.

The balloon reaches full inflation about a half hour after sunrise.

The launch was spectacular.  The balloon was released and drifted into the air.  Big Bill started driving in the direction that the balloon was going, and just when it was overhead, the payload was released.  The entire structure (800 ft high!) was now in free flight.  It seemed to climb slowly, but that was an illusion – when the balloon was 12,000 ft above the ground it still seemed close enough to touch.

The radio crackles and says "it's your balloon."  With that, the 3 million cubic ft envelope is released and begins climbing into the air.

The radio crackles and says “it’s your balloon.” With that, the 3 million cubic ft envelope is released and begins climbing into the air.

Here we go!

Here we go!

 

The balloon flew for about 8 hours, and was terminated over northeast Arizona.  Once the recovery team picks it up and ships us our data logger, we can find out what we heard up there.

A big thanks to the Louisiana State team for running HASP, and all the great people at CSBF who made it all happen!

The Search for our Missing Balloon: Closing in on the Landing Zone

In late May, Jared Sabater of Soleil Multimedia and I launched a high altitude balloon from south Chapel Hill, North Carolina. The balloon was carrying three cameras to capture spectacular high resolution video images of North Carolina from 20 miles in the air. We also expect to see the black sky of near space and a slightly curved horizon. However, the satellite tracker package fell off immediately after launch, and the balloon — cameras in tow — disappeared into the sky.

We set off to the expected landing zone — Harnett County, North Carolina. Needless to say, the search made looking for a needle in a haystack sound easy, and we returned home with nothing. I posted a desperate plea on my blog (read it here) and crossed my fingers, hoping someone would find the payload and get in contact with me. A reporter from the Daily Record in Dunn, North Carolina, came across my blog post and wrote a story about it.  A day later, I got a call from a woman living in Coats, North Carolina.

“I saw your balloon,” she said.

I didn’t believe her at first.  The balloon was supposed to pop in the stratosphere, not come down intact.  She said she saw it at sunset, a full twelve hours after launch — I figured it should have been way out in the Atlantic by then.  But as she described what she saw, I realized that there were only two possibilities:
1.  She saw our balloon.

2.  Someone else launched a high altitude balloon that just happened to come down in Coats.

The probability of #2 is vanishingly small — smaller even than #1, so I was forced to conclude that, in fact, our balloon was somewhere by Coats.  Jared, my friend Xiao, and I drive out to Coats to talk with the witness.  She was wonderfully nice and amazingly observant.  We stood by the window from which she saw the balloon, and she started telling me what she saw.  First of all, her pastor had also seen it crossing a road to the north, so we had a bearing.  Second, she said it had something sticking off to the side.  Third, it had remained in the same place for about an hour.

I didn’t know what to make of the second and third statements.  Balloons don’t fly with things sticking sideways, that makes them unbalanced and they tend to rotate so that whatever it is sticks straight down.  Also, even when the wind is imperceptible at the Earth’s surface, it’s strong enough above the surface to move the balloon out of her viewing area in less than an hour.

That’s when I realized what she’d seen:  Our balloon had already landed by the time she glanced out her window!!

It was simple in retrospect.  By the time the witness looked out her window and saw the balloon, the payload was on the ground or snagged in a tree.  The balloon envelope was still buoyant, hanging above the landing site like a giant flag — pulled slightly sideways and hooked over by the wind (hence the thing sticking out) and more or less stationary (why she was able to see it for an hour in the same place).

Here’s what we know right now:

The balloon envelope is cream to reddish colored and probably up to 20′ across.  It’s likely shredded into bits due to UV light and wind action.  The payload is 40′ from the envelope in a red lunch box with cameras attached.  The envelope is likely draped over a tree, and the payload is probably in the upper branches of another tree.

The area visible to the witness starts at 35.389756 N latitude, 78.638441 W longitude, and extends due east.  We can define sight boundaries by using buildings and trees that obscure her view to the east and the west (see figures below).

We searched several fields in the sight line and didn’t find anything, so the payload’s likely in forested areas.

Another witness saw the balloon crossing Red Hill Church Road at very low elevation, heading towards the first witness’ house, so that road defines a hard eastern boundary to the search area.

The first witness says the balloon was west of Black River – this provides another search area boundary, but with less confidence.

Elevation profiles (see below) and distances to the Black River make it even more unlikely that the witness could have seen the balloon on the ground if it were in the Black River valley.

Here are a series of maps of the search area:

view_areaThe red triangle defines the region visible from the first witness’ house, with the east boundary defined from the road which the balloon crossed per the second witness’ description.  Green polygons enclosed regions we were able to search on our first trip out to Coats.  The pale blue arc is 1 mile from the first witness’ house.  It’s hard to imagine being able to make out as much detail as she reported if the balloon is beyond this circle.

Elevation profiles along the north and south sight lines are available here:

north

south

These profiles suggest that the Black River area would be difficult to see from the first witness’ location.

landing_zone

The four yellow polygons show the most likely landing zones based on the analyses described above.  Areas A and B are by far the most probable locations for the payload, based on the witness’ statements and distance from where she saw it.  Any search should carefully investigate these regions.  Area C is also possible, but rather less probable because it is significantly below (and thus probably invisible from) the witness’ house.  Also, it’s quite far away, which would make the details she described hard to see.  Finally Area D cannot be eliminated, but it is the least likely due to elevation and distance.

Our next step is to go out and search.  If you’d like to join, or if you have some other information for us, let us know!

Cellphone reception in the air measured with helium and solar balloons

The maximum height of cellphone reception varies quite a bit depending on where you are – we’ve seen 30,000 ft in New Mexico and 4,000 ft in North Carolina.  In this post, I’ll give a few examples of cellphone reception from altitude from our own experience.  I’ll also explain what factors probably led to the surprising range of maximum altitudes we observed.

Cell phones have been used to track high altitude balloons for a while now: at least since these guys from MIT launched a camera into near space for $150.  Here at Bovine Aerospace, we used cell phones for three launches: a near-space flight in New Mexico, a solar balloon near Norwood, Massachusetts, and a solar balloon near Chapel Hill, North Carolina.  We used the instamapper app, which went offline in 2012 but apparently is back (just realized this about 2 minutes ago, it’s like hearing back from an old friend).

The Jake 2 high altitude balloon launch carried a SPOT tracker and a Boost Mobile phone.  The phone continued to transmit latitude, longitude and altitude until the balloon reached 20,000 ft, after which the altitude ceiling was reached (some GPS chips stop sending altitude above a certain height).  It continued to report latitude and longitude to approximately 9,000 meters (30,000 ft), when the connection was lost.  The cell phone did not transmit data on the way down (good thing we had the SPOT!).  This is not too surprising because the balloon came down over a remote desert area and ended up in the mountains.

Unfortunately we do not have the cell phone data, because we never copied it down and the online repository expired.  Rookie mistake!

The Jake 3 solar balloon carried my coworker’s old Android phone to about 3,000 m (9,000 ft):

Elevation versus time plot for the Jake 3 solar balloon launch in Norwood, Massachusetts.

Elevation versus time plot for the Jake 3 solar balloon launch in Norwood, Massachusetts.

We had cellphone tracking from the surface (35 meters above sea level) to 350 m (1161 ft) above sea level.  We lost tracking for about twenty minutes, then regained it at 1867 m (6123 ft) above sea level.  Tracking was lost for good at 2755 m (9038 ft) above sea level, after about 43 minutes in the air.  The balloon was visible for 15 or 30 minutes after we lost tracking.  We’re not sure how high the balloon went, but we do know it came down in the Atlantic…7 of the 9 messages in bottles in the payload were discovered up and down the Massachusetts coastline.

The gap in coverage is particularly interesting since the balloon was never more than a mile from the launch site during tracking (despite being well over a mile in the air when contact was finally lost).  So, clearly there is not a simple relationship between cell reception and altitude.  I have a pretty good idea what is causing this – see the discussion at the end of the post.  You can download the position data for Jake 3 here.

We repeated this experiment with a different solar balloon design (and a different Android, obviously) in Chapel Hill, North Carolina a few months later.  Once we lost reception, we never expected to see the balloon again.  However, something went wrong during the flight and the balloon came back down a few miles away.  The resulting altitude plot looks like this:

Elevation versus time for the Jake 4 solar balloon flight in Chapel Hill, North Carolina.

Elevation versus time for the Jake 4 solar balloon flight in Chapel Hill, North Carolina.

Cellphone tracking was present from the surface (100 m, 328 ft) above sea level to 800 m (2624 ft) above sea level.  The balloon was too high for cellphone reception for about 21 minutes.  It reappeared, descending rapidly, at an elevation of 1164 m (3818 ft) above sea level.  We recovered the balloon, cell phone, and the messages in bottles we’d hoped to distribute at sea in a forest about 6 miles away.  Over a thousand foot difference in maximum height of cellphone reception over 6 miles!  Evidently, the maximum cellphone reception height is variable spatially as well.   You can download the position data for Jake 4 here.

While we still have no idea what made it come down suddenly, it is certainly possible to make a rough estimate of the maximum height of the balloon using linear regression.  Turns out it made it almost exactly a mile above sea level before something went wrong!

Maximum elevation reached during the Jake 4 flight estimated using linear regression.

Maximum elevation reached during the Jake 4 flight estimated using linear regression.

Evidently cellphone reception with altitude varies with location (highest in New Mexico, lowest in North Carolina), varies with horizontal distance in the same region (i. e. North Carolina), and can periodically disappear and reappear with altitude (Massachusetts).  All three of these variations make sense.  In New Mexico, cellphone networks must cover large areas of mountainous land with very low population density.  My guess is that cellphone transmitters in New Mexico send a very powerful signal in order to cover the largest land area with the fewest possible towers.  So it makes sense that we would still have cellphone reception above 25,000 ft in New Mexico.

The gap in cellphone transmission in Massachusetts was likely not due to the phone malfunctioning but actually a consequence of how radio antennas operate.  It turns out that directional antennas (such as those used to transmit cell phone signals along the ground) produce side lobes in addition to the intended radiation direction.  So this balloon remained in the main signal zone until about 2600 ft, where it entered a null region.  It then crossed into a side lobe at about 6100 ft, finally losing the signal again just over 9,000 ft above sea level.

Illustration of main and side lobes. Figure from the Wikipedia article on side lobes in radio antenna design.

As an aside, this same radiation pattern shows up in some unexpected places – such as the sound pattern produced by speakers.

The North Carolina launch showed something a bit more mundane – cell phone reception varies with location!