Author: ibisek

One of the first questions ever asked was What range is the tracker compared to FLARM. At fist it was a pretty insidious one as nobody actually knew.

As we learned already in the kindergarten, there are numerous parameters entering the radio-signal propagation equation and having almost zero tools it is not that easy to come up with reasonable answers.  It is a matter of a good antenna its counterbalance and location of the device in the fuselage so neither the GPS nor the OGN antennas were obstructed.

The first attempts to discover range of our box were udertaken during days not feasible for gliding. We’ve placed a tracker on-board of an UL-Fox (that time with it’s engine still working somehow fine) and flew away from the airfield (LKKA OGN receiver) until PK (that’s a friend’s nickname) sitting on the control tower told us over radio we are not on the tracking website anymore. Consequently, we kept flying across various tracks back and there towards the airfield till we finally appeared  on the web again. Hitting the boundary of the signal we’ve somehow established range of less the tracker and more the receiver antennas on the tower. Tough life of the experimenters 🙂

A few months later we’ve encountered the KTRAX Range Analyzer where one can establish the actual range way better and perhaps also more objectively.

So what is the range if the first experimental tracker built?

Hell great! 🙂 The signal strength is way high above the average (on the chart above – the green line is average signal strength in the OGN network, the blue one is us!). Of course, it depends also on the receiver location, antenna, LNA or other details. Nonetheless, the KTRAX record is based on a long-term statistics so this effect is hopefully negligible.

What about FLARMs? It is difficult to asses. I’ve taken 30 randomly chosen IDs and looked on their properties while encountered a lot of differences.

I’ve chosen the DD932F as a typical example but we could easily use also the DD94DD or DD9A1C. In 40km distance only on at the edge of 10dBs.

At this spot I boldly dare to say the FLARM range is a half of our OGN tracker’s range. 🙂 That either means a job well done or I just misinterpreted what the KTRAX stats actually show. Please correct me in the comments if I am wrong.

Those who have ever seen the guts of a FLARM box would probably not be so surprised. Antenna in form of an ugly rubber-wire connected to the PCB, which could (does it?) serve as a dirty counter balance (judging by the size of copper fill on the back side of the board). As my education in the field of radio black magic is very limited and the guys and gals from Switzerland have their work very well reqarded I can only assume the know their craft.

Very important aspect is also the unit’s antenna location. During the competition I’ve seen a FLARM antennas fastened to an iron-beam with a metallic tape or another one hidden under a carbon coaming. And with one contestant even a combination of those above!

Very weak examples can be DDEB14 or DDDDC9 where the signal strength is most likely limited by antenna location and obviously also the pilot him/herself. And there are even a worse examples to be discovered.

If there is enough time I will try to analyse signal strengths from the data acquired during this summer season (I’ve got some 20GBs from 150km around LKKA) and statistically compare the range of FLARMs and various OGN trackers also made by all the folks around. If there were someone who wanted to do such analysis (mainly due to impartiality reasons – don’t trust to stats which you’ve not tampered personally) the data is available upon request ( .. of course to trustworthy individuals only 😉 )

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With the culminating preparations of regionals in Křižanov someone suggested to build some more trackers for the competition.

Hence I’ve started to gather all requisites – meaning the parts for the PCB. And yet another trouble was to be encountered. Arrow advertises an “overnight shipping” which was to save my day, however, I did not expect the “overnight” means the package will travel to various locations around the globe every night until it finally arrived to my doorsteps after 20 days of adventurous journey.

It was a season of vacations meaning Laďa was gone and so his 3D printer. That meant to search for a backup one resulting into use leftovers of various materials of variable qualities and properties. After having printed numerous experimental enclosures the material choice for the competition trackers seem to become a spool of white ABS with lids of multifarious colours.

The source of screws to fix the lid originating from a salvaged microscope camera was already depleted so also the fastening had to be changed. Although a click-mechanism would be better, the lid is now glued in the box with acetone as the printer cannot print  such small details.

I’ve also experimented with translucent PETG (polyethylene terephthalate glycol-modified). Unfortunately, as it cannot be neither smoothened with acetone nor polished without losing its crisp translucency so the final decision went to snow-white ABS for the boxes and red, green and blue (yes, RGB 🙂 ) ABS for the lids.

The original mini-usb connector needed to be replaced for a fixed cable as some trackers were losing contact while the connector slid out. On the other hand the power from the USB battery was left as it kept the options open when needed to replace batteries in case discharge of failure.

A situation that appeared clumsy at first turned into and advantage. The two thingies – the tracker and the battery – loosely connected with a cable let us to put the battery into a side pocket or anywhere else while the tiny tracker can be placed on top of the stuff piled up in the centroplan to have a clear view for the GPS and OGN antennas while making all the organizers and the contestants happy.

Eight new OGN trackers found its way among the contestants while having served well reliably till the last competition day, last landing and last glass of beer.

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April turned to May and the bombastic success was coming – first PCB (of ten) was populated, brought to life and even tested in the air by flying once around the airfield in Fox. The last step in development was closing – what enclosure to use for the tracker? This turned out it should had been the first step of the design..

The mounting holes were originally intended to fix the antenna counterbalance from the bottom side of the PCB. Dimensions of the already existing board do not fit any existing box or the available are  either too hideous (large and just ugly) or incredibly expensive (and still too big; e.g. Fischer elektronik AKG 55 24 xx – I have bought one but it’s not really a fit).

Purely by coincidence a friend of mine -“Glidertools” Laďa – went by saying – “Man, what’s the trouble? I can print you one!”. And thus we started fitting the tracker board into an experimental box made on a 3D printer.

At the beginning it was kinda disaster. We were fiddling with available materials (PLA sucks), ABS, ASA and later we also experimented with PETG. The main goal at that time was to find a way how to separate the antenna counterbalance from the PCB so nothing went off in magic blue smoke. At the same time the box had to be closed for which a printed lid and three screws from salvaged microscope camera served very well. The gray ASA (acrylonitrile styrene acrylate) quickly proved as a well suited material resistant to high temperatures and therefore the first two trackers now fly in this suit.

Another struggle was the OGN antenna. The GPS module with integrated antenna is soldered on the PCB in horizontal position and shall be situated facing the sky, or space respectively. Therefore the box must be oriented the same way (image facing skywards). The radio antenna, on the contrary, is supposed to be vertically polarised – samewise as the receiver’s antennas. That results into requirement of an antenna that could be bent or even better with a joint.

Searching for such one took me almost a year. Again, by a pure coincidence I have found a splendid one when visiting the Ampér 2017 fair, where the exhibitors did not want to understand why I demand an immediate sample and do not want to wait for order from their e-shop to arrive..

Perfect, ain’t it? 🙂

April debugging smoothly transcended to highway to hell. Over the last fortnight trying to find a bug in SPI communication and that bastards is hiding so well that even colleagues MacGyver cooperating with Chuck Norris would be troubled.

All can be blamed to hasty transition from F042 to F103 and to that related change of architecture resulting into libraries refactoring. I’ve already deployed all I’ve got and even borrowed some heavy guns.. and still no idea, nothing changed.

Should start some other business.. beekeeping perhaps?

The first prototype was created in April 2016. Initially based on Atmel’s ATMEGA8 which had to be replaced once the program did not fit its 8kB of FLASH by ATMEGA328 (on which, by the way, the famous Arduino is based). It was time of struggle with timers, SPI and I2C registers that maliciously differ in teeny tiny details.

Two trackers in form of wiring hell and radio modules on long stretching legs can be seen in the front. A quarter-wave antenna on the top, an almost half-wave counterbalance became one of the legs. One of the trackers still based on M8, the other already running on M328. A bit chaotic setup, but the end justifies the means, right?

And because everything worked as intended it was the right moment to put it all together and up in the skies!

Powering the box from USB cable was the best initial idea (which actually proved to work very well). Later on all other components were completely replaced (besides the radio module). Starting with GPS module with antenna borrowed from local bakery owner, pressure sensor from an experimental variometer, perfboard made of paper, all inside a soap box.  It was a great prototype which served perfectly and reliably.

Now, under the cumulus clouds, gray shit or in clear blue skies can be seen with the “PK” livery 🙂