FHALP-3 was launched on 26/04/08 from EARs in Cambridgeshire, shortly after launch the flight computer died but we are able to track visually and also by the radio beacon for approximately 50 minutes until all contact was lost. However on landing the flight computer turned back on and transmitted its co-ordinates and were able to retrieve it from 52.332,0.888
Picture downloaded from the camera can be found on flickr
Instead of using a GM862-GPS module which has a SIRF III chipset and therefore a 24km altitude limit the plan is to use a GM862 module with a separate GPS module. There are a variety of GPS modules available which run at TTL (the same voltage levels as the GM862 therefore removing the need for additional voltage shifting) however they are either SIRF III or ublox and so require a PCB to be made. Instead the plan is to use an older GPS module which doesn't have this altitude restriction but there are very few off the shelf TTL modules.
Luckly a ublox RCB-LJ gps module and board came up on ebay, its ripped out of a dev system but has ttl inputs and is happy to run off the LiPo. One thing that needs to be done is that you need to provide a voltage source for the active antenna or it won't run, this again came from the LiPo power source.
Have finished the radio beacon, it uses the radio module from FHALP-2 (Circuit Design CDP-TX-04S-R 434.075mHz) and is attached to a CMOS 555 timer which has been set up to work at approximately 1Hz. It runs off 2 AAA lithium batteries (separate power supply so that if the main flight computer fails it'll continue to work) and the circuit is mounted on the back of the battery holder.
After returning my original GM862-QUAD-Py module I have finally managed to set up the replacement module with my development system. The GM862-QUAD-PY modules are not just GM862-GPS modules without the GPS modules but mine have different hardware and therefore different firmware. You can check to see whether you have the old or new versions by checking the firmware using the AT command 'AT+CGMR', the old version is 0.6xx while the newer version is 0.7xx.
While the modules are claimed to be pin backward compatible there are still some differences:
The radio module i'm using is a Radiometrix NTX2 which is being used to transmit Morse. The radiometrix module can be used for long range RTTY tranmission with the correctly programmed circuit so is future proof. The radio module can be turned on and off using the 'En' pin allowing it to be continously attached to a power source. The module transmits at a frequency of 434.650MHz.
The module has 7 pins:
The module has been placed at the back of the board and directly wired in, with the wires travelling underneath the breakout board. Its been tested and works.
The cutdown is the same charger circuit as was used in FHALP-2 but this time is being mounted internally. It has 3 wire, Vcc, GND and a GPIO trigger input. For more information see the ukhas wiki.
Right so the launch…
Actually the payload itself was suprisingly easy to set up, on arriving I attached the freshly charged LiPo by its PP3 connector which powered everything up including the GPS but didn't switch on the gm862 module. This allowed the GPS to sort itself out, to check everything worked I turned on the module, it transmitted some radio and sent a text. I then just turned it off and set up the flight train while the others inflated the balloon. After inflating one balloon to find it had a hole we moved sites to get out of the wind, inflated the balloons attached the payload and rather chaotically let go.
The payload was transmitting coords for about the first 3 minutes of the flight and then it died but the radio beacon continued though this stopped after about 30 minutes. I got one text at 347m altitude which is normal for a flight. We were able to track it visually for a good 50minutes until it disappeared into clouds.
After returning for brunch we were just going to find an internet connection to run the forecast modules when sms started to arrive reporting the correct position so after a bit of google maps we jumped in the car drove over to the landing site and didn't have too much difficulty finding the payload though it did require a bit of rally driving down a bumpy track. It was as a whole a pretty successful flight though certainly not perfect.
Once I got back I powered the module up and extracted the logs - there was nothing for the flight, it had died shortly after take off and restarted on landing, also some data that we collected at launch wasn't there suggesting that the logging to flash overwrote it. The pictures again were blurry and cutout probably at apogee.
* GM862 - While the concept is good in practice the Telit modules drive me crazy,
After a bit of research (Deconstruction of a Pretec CF GPS and that the drivers required are just for the CF→serial adapter so that it appears as an additional serial port (an old hack I did for Peg II on the gumstix)) I found out that Compact Flash GPS units are just TTL serial GPS modules with a CF → Serial adapter in between.
I'll add pictures soon (once I get my xD card back.)
Further investigation has suggested that SIRF II GPS units don't work above the dreaded 18km altitude limit which means that using this gps is as good as using the built in SIRF III module on the GM862-GPS. One GPS unit that does work above 18km is the Lassen IQ module which can be bought from Sparkfun and Diamond Point - will investigate further…