The past weekend some friends and I tried attaching a Windsond to a sky lantern. Sky lanterns are made of a light paper bag over a suspended piece of wax that burns to fill the bag with hot air, creating a small hot air balloon. Sky lanterns are very light-weight and don’t carry much fuel to keep them afloat — would it be able to lift a Windsond?
Winter had finally arrived in Sweden and it was about -6 °C with fresh snow covering the landscape. Beautiful as it was, a wind of 5 m/s made the open landscape less pleasant to experience this day. We huddled behind one of the cars to light up the sky lantern. The Windsond was attached with a thin thread.
After a couple of minutes, the sky lantern was filled in spite of the chilling wind and we let it go. It had a hesistant start, then rose by a full 2 m/s as the wind carried it away. It rose to 110 m AGL, then appeared to run out of steam and started to sink again. Unfortunately it never recovered and continued to descend back to ground level by about 1 m/s. The flight was over in 3 minutes but the sonde still travelled 1 km.
Since the flight was over so quickly, we didn’t get a chance to cut-down the sonde and we couldn’t choose the landing location. Knowing the GPS coordinates, with maintained radio contact and the sonde blinking and beeping, it was no problem to find the sonde after walking across a snowy field. The sonde must have been attached to the sky lantern all the way to the ground since the fall was so slow, but we didn’t find the sky lantern and may never learn what happened of it.
The sonde carried the new high-accuracy humidity and temperature sensor which performed perfectly. The sonde was also equipped with an extra memory that logged more data than the radio link has capacity to carry. The firmware was extended to log a number of parameters with timestamps; full GPS readings, different types of temperature readings inside and outside the sonde, pressure, battery voltage and error conditions (although none occurred).
All in all, the results are mixed. Windsond did prove light enough to be carried by a sky lantern. It’s not necessary to bring a helium canister and the price of helium and sky lantern is about the same. On the other hand, sky lanterns are sensitive to winds and high humidity. The open flame presents a risk of starting a fire on the ground. And not least, the altitude of a standard sky lantern is very limited.
A video clip of the launch:
Windsond now has the option for a sensor with especially high guaranteed accuracy and resolution. By placing the sensor 10 cm out on a light-weight boom, the risk of the sonde affecting the temperature and humidity measurements is minimized.
The sensor is Sensirion SHT25, with 1.8% accuracy and 0.04% resolution of relative humidity. Temperature has a typical accuracy of 0.2 degrees C and resolution of 0.01 degrees C.
The high-accuracy sensor is available as an option for all new sondes.
There’s an article about Windsond in the 2013 autumn edition of the Swiss Ballooning magazine Contact. It makes a detailed comparison between Windsond and the method of finding wind directions by manually aiming a measurement device, theodolite, towards a small rising balloon and sending the observed angles to a computer.
The author finds that the theodolite method lacks in reliability since it relies on the rising balloon to maintain a constant rise speed. As Windsond demonstrates, this is not always the case. There’s one disadvantage to Windsond — sudden changes in conditions between two layers of air are diluted by the smoothing of data samples by the Windsond computer software. In response to this, I’ve now developed a more sophisticated algorithm which detects and preserves sudden changes. After testing and optimization, this will be released to all customers.
The magazine is in German. See the Windsond article as pdf. The magazine also mentions how Windsond was used in the European Ballooning Championships 2013 (article as pdf). Or see the full magazine as pdf, from the Swiss Ballooning Association (SBAV).
The Mountain Meteorology Group at the University of Utah is evaluating Windsond for their meteorology research. During one test, the sonde failed to detach from the balloon. They continued to track the balloon to an altitude of 9700 m MSL, at which point the sonde battery was drained. This is close to the altitude of the tropopause, the important point where the temperature reaches a local low point.
The last message before the sonde shut down was transmitted at a distance of 49 km, where the sonde was still rising at 1.5 m/s and travelling at 17 m/s. This is a great result for the basic whip receiver antenna. The reception was becoming increasingly spotty towards the end and the contact wouldn’t be possible without the error-correcting code used in the radio link. The range could easily be improved a lot by using a directional antenna.
To reaching high altitudes on purpose, a bigger balloon would be used to gain a higher vertical speed and to allow the contained helium to expand the balloon further before the balloons bursts. A high-altitude radiosonde commonly use a balloon weighing 100 g, compared to the 8 g balloon commonly used for Windsond. Officially supporting high altitudes would also require proper testing of all components in a climate chamber.
I’m now doing all I can to avoid the mishap from repeating:
- Cut-down is done by heating a thin metal wire until it melts the wire tethering the sonde to the balloon. The duration of the heating will get progressively longer if the detaching fails.
- Before launch, the sonde will detect if the mechanism is broken to alert the user.
- There’s now support for backup cut-down hardware.
- If the sonde still continues to fail to detach it will eventually stop trying. That will save the battery from draining as it did this time. The balloon will eventually pop and with battery to spare it’s still possible to recover the sonde.
EDIT: Windsond has since been used at higher altitude.
The European hot air ballooning championships took place in Wloclawek, Poland on September 6-14. The Swiss teams used Windsond to help them plan their flights. They fared well; all four Swiss teams scored in the top third of the field of 81 competitors (see the results). A part of this favorable result is attributable to Windsond.
Here is an account of how the Swiss teams had good use of Windsond during one of the tasks, as told by participant Léon André.
The flight No. 3 of the 18th FAI Hot Air Balloon Europeans Championship was an evening flight with three tasks. The second task was a “Maximum Distance Double Drop” (MDD).
The distance between entrance point into the scoring area and exit point should be as long as possible. The task before the MDD was a “Hesitation Waltz” (the pilot had to fly as near as possible over one of thee goals). For the following MDD the pilot had to enter as near as possible at the optimal entrance point and to fly without leaving the scoring area in direction of the optimal exit point. Of course it helps a lot, if the pilots know exactly the winds in the different altitudes.
We measured with a Windsond a short time before the pilots completed the “Hesitation Waltz”. So they had the time to plan the best way thru the scoring area with the existing winds.
We sent the wind data to our teams in the following form: [Note by Anders: This view is created by the Swiss teams and not part of Windsond]
The scoring area ended at 2000 ft. But between 1500 ft and 2000 ft there was a wind layer approximately in the direction of the axis of the scoring area.
In the ranking of task 12 our pilots Stefan Zeberli and René Erni has been on place 1 and 2.
— Léon André
The Swedish Hot Air Ballooning Championships is taking place now between April 27 and May 1. The pilots and crew of nine hot air balloons compete for the top position. The hosting organization is Dalslands ballongklubb. The location is the plains and forests around Kroppefjäll, to the west of Sweden’s biggest lake Vänern.
Windsond is used by the competition meteorologist along with other data sources to get a picture of the expected conditions for the next flight. Each flight of the competition needs to be planned ahead to be challenging and fair for all teams, and the weather conditions play an important role in the decision. Here’s a picture gallery in Swedish of how they use Windsond.
Two of the teams also cooperate in using Windsond to get detailed weather information during their flight. Up to four receivers are used to get real-time wind data to pilots and crews at the same time.
Of the seven soundings made so far, all sonds have been recovered and set up for new soundings. Predicting and controlling the landing location is working out well. At one point the competition organizers saw a sond drifting towards lake Vänern and could abort the sounding to save the sond.
A customer deeply involved in hot air ballooning and gas ballooning sent me the story of his first test of Windsond. Naturally the system has already undergone a lot of testing but it’s reassuring to read his account.
Today it looked like a nice evening for a test launch, so I readied my balloon chase car, got my old ACER Travelmate C312XMI charged – and brought my 220v inverter with me.
It is pretty dark out, but nonetheless, I would not need to rely on visual contact with the sond, since the system would tell me exactly where this device is floating.
Ok, system running in the car, antenna mounted, external power supply working, GPRS mobile data service, map loaded, all systems green, one more beep – and release (for flight)!
As I have hoped, very slow wind conditions.
As I have feared with saving precious helium, a very slow ascent (1 m/sec), but that doesn’t hurt.
Ok, we now see clearly in physical reality — and on the Google earth display — the typical fine weather easterly flow, my first Windsond drifting very gently to the west. Daddy was the driver, I was glued to the Laptop Display, my mother in the back row.
Now it was easy to predict where we should go (read the position off GEarth 😉
We went to a dark place, since then we would be able to observe (visually) better.
Then hopped to a next further stopping point, near the estimated landing positions, if we would release soon.
Bad thing was the local Fussball-game with its bright lights, and we were on the wrong side of the stadium, blinded by those beams. Anyway, I could not wait for the set altitude of 1500m AGL and sent the ‘cut’ command at about 1600 MSL or thereabouts. Too bad the wind shifted in those last 20 seconds of flight, although still slow, the track curved towards the stadium (we imagined trying to find the Windsond among 4.500 white foam cups of the visitors of the game …. 😉
So cutting was sensible, or wait for the Windsond to go over the city – I did not want to climb to any roofs in the night. We could even follow the fall quite nicely, the flashing makes sense (for night trainings, anyway)
Then repositioned the car to a closer road, hopped out (with Laptop, GPRS Modem but no external power supply) and got our feet dirty… Estimated landing was about 90 meters short, but no problem, we could even hear the “find me” sound and see the flashing of the Windsond.
Retrieved in no time, it was like finding lost markers, better than Easter anyway.IT IS A GREAT SYSTEM, I TIP MY HAT FOR YOU, THE CREATOR, DESIGNER, THINKER.
It is well thought through, it is reliable, it is usable, it is a dream come true – for me.
The customer suggested some improvements for the software which I’ll incorporate. I’ll also look into improving the 90 m inaccuracy of the landing prediction.
The team and I launched a sond a few days ago by inflating a condom with helium. Condoms are much lighter than regular balloons but above all they improve the mood of the mission participants considerably. 😉 The helium volume was estimated to 17 liters. This gave an average ascent rate of 1.5 m/s. The ascent rate given the neck lift 15.1g should be 1.0 m/s so the shape of the condom probably helped to improve the speed.
The condom burst a bit prematurely at 2400m AGL, a few hundred meters before the sond was instructed to let go. The temperature had dropped from 0*C on the ground to -12.4*C at this altitude. Although it’s doubtful condoms are designed to “operate” at freezing temperatures, the gas expansion at the altitude is the probable cause of the burst. On the other hand, a ground test showed a condom of the same model could contain over 40 liters of air before popping but maybe there are variations between copies. I considered to write a complaint to the manufacturer over this blatant product failure. 😉
The launch was done in the evening, after dark. While the sond was still falling, I activated the beacon strobe by a button in the GUI. We were still 3-4 km away from the falling payload but the brief flashes of light were clearly visible, like a shooting star. The sond landed 65 m from the location originally predicted at 2400m altitude. In the dark, the blinking was very effective to locate the sond. The whole enclosure lit up and illuminated a meter of the ground around it. In the open landscape, the beeping was faintly audible at 200 m distance.
Another Windsond was also launched to compare the results of two data sources. They coincided very closely. The profiles of wind direction, wind speed, temperature and humidity were found to agree within tight margins. The second sond was launched while the first was still in the air which also demonstrated the ability of the system to handle multiple soundings at the same time. For the second sond I picked a landing location on a field close to a minor road. We picked up the sond 50 meters from the estimated position.