Ground winds

Some data formats include a field for the ground wind speed and direction. At first glance, Windsond should be able to report this… but it can’t, since the wind is sensed by the balloon floating with the air movements (i.e. winds) as it rises. Before the balloon starts rising it’s anchored (typically by someone holding it…) and can’t measure winds.

But wait a moment, what exactly is meant by winds on the ground? Due to the physics of fluid mechanics, wind movement very near the ground will be much lower than expected, even approaching zero extremely close to the surface. The ground drag is felt up to 200-400m height, depending on the type of terrain. As it turns out, the standard is to measure ground wind at 10 meters height. Here’s another discussion, by Belfort Instrument. It takes Windsond several seconds to reach 10m height so the GPS has time to sense the change in speed and direction.

Still, the wind close to the ground is more turbulent than at altitude, so a single reading from Windsond cannot capture average conditions like a ground-based weather station can.

By the way, the GPS senses the movement by doppler effect. This is more accurate than calculating heading and distance between successive position reports — GPS position reports may experience sudden jumps and even at the best of times they have a degree of uncertainty that would severely limit the time resolution possible.

At the moment Windsond measures wind once a second and makes an average over the last three seconds when sending data to the ground station. A software extension could transmit all three values to the ground station. A future experiment is to measure five times a second and save this to on-board storage for downloading after the sounding finished.

Detachable Battery

Looking down into the sonde, with the new battery mounted

From now on, Windsond sondes use a mini connector for the battery, and the battery itself is fastened by velcro. To charge the battery, remove it from the sonde and connect it to the USB charger. Previously, the battery was soldered in the sonde and the USB charger connector needed to extend down into the sonde encasing.

This change removes the concern whether a particular sonde is charged and even enables the same sonde to be launched multiple times in succession by changing to a fresh battery. The battery can be removed for repairs, or replaced in case it’s damaged from deep discharge. For different trade-off between weight and running time, other battery sizes can be employed.

Another issue was the strict shipping regulations for LiPo batteries that affect world-wide shipping since Jan 1, 2013 to handle the risk of spontaneous battery combustion that have occurred aboard aircrafts. The rules are complex and cause headaches for many businesses. Even though the Windsond battery is miniscule and hardly constitutes a risk for anything, limitations may that apply — depending on whether you turn to the Swedish Postal Service, DHL, UPS or Fedex. Separating the battery gives the option to dropship the batteries directly from China which bypasses the problem altogether since China has less strict shipping regulations than Europe.

The battery assemby is custom-made for Windsond and I’m happy about how this change makes the system more modular and easier to handle.

Even charge three batteries at the same time from a USB port

Mailing Tubes

Mailing tubes for Windsond

From this point, Windsond sondes will come individually packaged in stiff cardboard mailing tubes. An effect of the low payload weight is certain fragility in the sondes. The new packaging will simplify both shipping and handling.

Windsond and sky lantern

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:

 

High-accuracy sensor

High-accuracy sensor on boom

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.

Windsond in Swiss Ballooning Magazine

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).

 

Windsond altitude record

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.

Sounding wind speed

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.

European hot air ballooning championships 2013

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).

Part of the task sheet for flight No. 3, with the description of 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.

Drawing of the scoring area (overlap of the two circles) on the competition map

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.

The way of the sounding balloon during the measurement

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 measured wind data

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

Swedish Ballooning Championship

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.

Windsond evaluated

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.

The launch

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)

The recovery

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.

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