17 December 2011

Using a GPS tracker for flight monitoring - Part 2

December 2011 Today, the weather cleared sufficiently for me to fly my long-awaited test sortie with the GPS tracker. Although the cloud cover was scattered at 4,000 ft, I expected cruising at 3,500 ft would be high enough to get good GSM network coverage.

After take-off, I headed out west towards Mayfield before turning southwards for the next waypoint at The Oaks. Once overhead the township, I headed out to intercept the southern end of Lake Burragorang before turning north to follow its course towards Katoomba.

Nearing the Three Sisters, and having been satisfied the tourists were behaving themselves, I turned DZY back on a dog-leg track towards Warragamba Dam.

On the way back to Bankstown between Warragamba Dam and Prospect Reservoir, I did my usual practice forced landing over a paddock near St Mary's airstrip before heading onto Bankstown.

After an uneventful landing, I called my pilot friend to check if he had received SMS messages giving updates on my position. When he replied that he hadn't, I wondered if it was due to the lack of GSM coverage. If it was, then my experiment had failed.

Upon returning home, I checked the GpsGate tracking software and found that the experiment was actually successful in that my entire flight had been tracked properly via the GPRS data link.

However, I still needed to understand why the parallel SMS updates hadn't been sent. While reviewing the MT90 tracker's settings, I discovered that I simply missed out one digit while entering my friend's phone number in the tracker - duh! So the problem wasn't due to equipment failure but brain failure on my part.

Overall, the experiment to use a GPS tracker for flight monitoring was successful. At altitude, GSM network coverage is enhanced due to greatly reduced terrain masking effects. It is therefore best to trigger a MAYDAY alarm with such devices before descending into zones of poor coverage.

For my experiment today, I set the distance interval between reports to 500m and the time interval between reports to 20 seconds. What these settings mean is that a position report is relayed back to the tracking centre (in this case, my home computer) every 20 seconds or every 500m; whichever occurs first.

Doing so means that the tracker's last known position is broadcasted as close as possible to the time at which GSM coverage is lost in areas of poor close to ground coverage (as the aircraft descends). Of course, this is a non-issue if a forced landing occurs in areas of good ground coverage.

My next test sortie will be flying the same route but using the Optus network; as today's test used the Vodafone network.

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