HABAXE - High Altitude Balloon Tracker for PICAXE.


Senior Member
Cut down - balloon release

Cut Down

Just tested the mod to allow a remote cut down (balloon release) of HABAXE.

Remotely operated cut downs don’t seem popular in the UK, which is odd considering the obvious safety benefit of being able to terminate a flight if the balloon drifts where it should not.
One reason seems to be a belief that the transmission of a cut down signal is not allowed, even for a radio amateur. Ofcom seem ambivalent on the issue, and for it to be illegal the receiver on the airborne balloon would need to be deemed part of the ‘amateur radio station’. This seems completely untenable as no license is required to own or use an amateur radio receiver in the first place, so how can a receiver also be considered part of an ‘amateur radio station’ ?

However, whilst the use of an amateur radio license would be needed to if you wanted to use a high power amplifier to send a cut down signal say 1000km (needs about 5W ERP), the RFM22 can be used on its own over shorter distances.

Transmissions of 10mW are licence exempt, and this would have a range of about 15km, ground station to balloon. RC control in the 458Mhz band at 100mW is allowed in the UK, and this would have a range of about 40km.

The RFM22 can easily switch between transmitting on one frequency, license exempt at 434mhz say, but then listen on an amateur radio frequency or the RC control frequency of 458Mhz. Similarly the ground station can easily switch between downlink and uplink frequencies.

Implementing a cut down is actually very simple with a small MOSFET and a Lipo.

You can get ready made nichrome wires, a short length of nichrome with two bits of copper wire welded on, these are ideal for melting a bit of nylon cord.
I measured the current it took to melt a bit of dyneema, its stronger than nylon and has a lower melting point. The cord cut within 10 seconds at a current of 800mA.
I next measured the short circuit current of a small 70mah Lipo weighing 2.2g, it was 6A with the protection circuit removed, that ought to be plenty.
A DMP1045 SOT23 MOSFET will handle 5.5A, has a rated forward resistance of 31mR, and a gate source threshold voltage of 0.55V.

So all we need to do is arrange for the ground station telemetry receiver (see picture) to transmit a command packet at the appropriate time. In the case of HABAXE it goes into listen mode just after the FSK RTTY and telemetry packets are transmitted, at this time just press the button on the ground station receiver, and the cut down packet is sent.

When the cut down command is received, the MOSFET is turned on, the nichrome wire goes red hot and the dyneema cord breaks within about half a second. See the arrangement in the picture, the two black wires connect to solder pins, and you cn just see the nichrome wire wrapped around the blue suspension cord.


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Senior Member
My balloon is lost.

OK, so how to find it ?

You may be lucky and get a good GPS fix as the tracker comes down, either from the balloon bursting or via a remotely transmitted cut down command. However if not you will be relying on picking the transmissions from the downed balloon, maybe with a directional antenna.

What you can do with the RFM22 is make it transmit a series of FM tones that reduce in power and rise in frequency. These can be heard on a simple UHF handheld, as you get closer you hear more of the higher frequency tones. It’s a crude form of direction finding.

HABAXE has been setup to transmit these ‘find me’ tones when it receives the cut down command, and the tones should be helpful in finding a downed balloon.


Senior Member
Test program for the Dorji DRF1278F module is attached.

The Dorji module uses the Semtech SX1278 RF chip, so the same program should work on the Hope RFM98, which also uses the same Semtech chip.

The program transmits at FSK RTTY at 100baud, 7bits, 2stop bits and no parity a message first in FSK RTTY and then AFSK RTTY, followed by a slow Morse message.



Senior Member
If all goes well at the Launch, HABAXE1 should be in the air around 09:00 this Sunday morning.

Its a Pico floater on a 36" Foil balloon.

434.400Mhz, RTTY at 100baud, 7N2, 625hz shift.

Single 350mahr Lipo, should last around 18 hours.

Also outputing the data payload using RFM22M data telemetry at 1kbps.

I will be staying at the launch site to see how far the I can track it using the data telemetry.

Predicted path from Cardiff is over the Severn bridge, Swindon, Oxford, Cambridge then Norwich.

Updates when I return from launch site.


Senior Member
HABAXE - First Flight

HABAXE1 was launched from Caerphilly common just to the North of Cardiff at 09:15 on Sunday 5th October 2014.

This prototype GPS tracker weighed 19.5g with a 340mahr Lithium Polymer battery. The battery was predicted to last 15-18 hours. The balloon was filed with enough helium to give 2g of free lift which should allow the balloon to rise and stabilise at 6000 to 7000km. Thanks to the guys on the UKHAS group for the advice in getting this set up, advice was spot on.

After launch the balloon rose to about 6300M in 2 hours and continued to float along at this altitude.

After 7 hours of flight the balloon appears to have developed a leak since it lost altitude steadily over 3 hours, eventually going out of contact at an altitude of 675M when it was approx. 35km East of Skegness and over the North sea, so presumed lost at sea. It had travelled around 340km.

The FSK RTTY from the RFM22B was troublesome, as expected really, with the centre frequency drifting (after transmit was turned on) first one way then the other, as the temperature of the device oscillated around 0C, the point approximately where the temperature co-efficient switches between positive and negative. However the FLDIGI software does a good job of coping with the shifts and drifts, although it does take a fair bit of manual tuning of the receiver.

By using the special HAB version of FLDIGI, incoming payloads from listening stations all over the UK are decoded, checksum verified and then posted across over the Internet into the Spacenear tracker system. This shows the balloons altitude and other information from the balloons transmitted data payload.

Reception of the RFM22B data packets proved to be very reliable, even at 1kbps and 10mW. With my home station set-up, mast head LNA and 70cm co-linear antenna, I was still receiving data packets when the balloon was 174km away. The data packet option has the distinct advantage that it can be used unattended. Just turn on the receiver and it will log data as it comes in, no other intervention required.

At the launch site car park I was able to use the data packets to track the flight quite easily with portable and battery powered gear, no internet connection required. I had a RFM22B set-up as a receiver. The received packets were translated in the receiver back into a NMEA sentences and sent out to a serial port. Thus the receiver was acting as if it was the remote GPS on the balloons tracker. Memory map displayed the track of the balloons location, direction and altitude on a small Netbook PC. YAAC will do a similar job.
After the packets had failed, the FSK RTTY kept working for another hour or two.

The 36” Qualtex foil balloon cost about £4 and it needed approximately £5 of helium to fill it for the required 2g of free lift.

What next ?

The ‘self tracking’ mode using the RFM22B data packets works well; is easier to use than a FSK RTTY based set-up, does not need an internet connection for tracking, allows for an uplink\command functionality, can operate at much higher data rates and a completely portable set-up is possible. I was getting 178km range when the antenna was a 6dB gain co-linear. With a tripod mounted Arrow satellite yagi, I would expect the range to be 250km. It will not replace the FSK RTTY and Spacenear tracking of course, but you can use both at the same time.

For even longer range, The SX1278 LoRa devices look very promising indeed and whilst the greater sensitivity would allow for more than 250km range, LoRa devices may also make it possible to achieve similar range with a simpler and cheaper receiver set-up.

When FSK RTTY is started up on the RFM22B, the TX power heats the device and it starts to drift in frequency, as much as 200-300Hz. This creates difficulties for decoding and in general the receiver cannot be left unattended for long before a retune is needed. The SX1278 devices do not exhibit such a severe frequency shift, around 15Hz only in a 10 second RTTY send. So if only to make the RTTY more stable, testing of a SX1278 in a balloon would be a good idea.

Foil balloons are cheap, and could be a useful way of testing long distance communications, you may even get them back !
Hi Srnet

I am at a loss to get the Ublox unit into the low power mode. I have connected the U centre software and discovered that my Ulbox has only two modes - continuous and max power. I have copied the data string that Ucentre provides into the code (after formatting it naturally!) and as yet do not see a power reduction.

Could you kindly post the code snippet that shows how to get the Ublox into the low power mode? at the web site https://ukhas.org.uk/guides:ublox_psm, several options are given to reduce the power. I tried the examples and still cannot get it into low power mode.




Senior Member
I used the setup strings from the UKHAS website, worked for me.

There is a link to the code I used and a post about power saving somewhere in this thread.

Be aware that the GPS wont go in to powersave\cyclic mode unless you have good GPS signals.

And there are also quite a few different 'Ublox units' the later ones such as the Max8 are rather better, power consumption wise, than the older versions.
Hi Srnet

My apologies for being vague. I refer to the end of the listing shown in HABAXE V2 Test GPS Collector Cyclic Mode.bas.

'configure GPS for cyclic mode
serout TXterm, baud, ("Set Cylic Mode",CR,LF)
hserout 0, ($B5, $62, $06, $11, $02, $00, $08, $01, $22, $92)
pause configuredelay

'configure GPS for continuousmode mode
serout TXterm, baud, ("Set Continuous Mode",CR,LF)
hserout 0, ($B5, $62, $06, $11, $02, $00, $08, $00, $21, $91)
pause configuredelay

The value of configuredelay is set at 100 at the top of the file. There is a statement "sleep 28 ; halt for 20 seconds" which suggests that the value of configuredelay gives approximately 70 seconds delay for the UBlox to reset.

I will try this.

Thanks for providing the code. It is much appreciated!




Senior Member
The sleep commands have nothing to with configure delay at all, so I dont know why you assumed they were related.

'pause configuredelay' adds a short delay, 50mS if I recall, that allows the GPS time to complete the configuration command just sent. You could wait for the 'OK' response orf course, but the pause is just easier.