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--- communication:piii_experiance [2006/07/16 17:22] – rocketboy
+++ communication:piii_experiance [2006/07/16 22:15] (current) – removed jcoxon
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-====== Pegasus III – The Radio Tracking Experience ======
-Some time ago I did some path loss calculations to see if a 10mW 434MHz licence exempt transmitter was viable for balloon to ground communications. The maths indicated that with narrow-band communication it should be feasible over that distance using a modest sized yagi as a receive antenna. To prove my calculations I conducted some tests using a 10Km line of sight path – the results were very good.  I added a 20dB attenuator – thus simulating a 100Km path – the signal was still of respectable strength.  Finally I tried a further 20dB of attenuation (simulating a 100Km path) – the signals was still audible. So much for the theory – when the opportunity of Pegasus III came up I offered to build and track a transmitter.
-I built the radio tracker using a licence exempt narrow-band 10mW 434MHz transmitter, a PIC microprocessor (to provide a modulating signal and power management) and a Photo-Lithium Battery(which should have been sufficient for almost a weeks operation).
-{{:communication:txnbat.jpg|:communication:txnbat.jpg}}
-The antenna is a simple ¼ wave vertical over a 4 radial ground plane. This was mounted upside down (i.e the radiating element pointing down) under the payload.
-{{:communication:txant.jpg|:communication:txant.jpg}}
-This type of antenna was chosen because of its radiation pattern – maximising the range at low angles - also no power is directed toward space.
-{{:communication:groundp.gif|:communication:groundp.gif}}
-The balloon was released about 7:35am – climbing relatively slowly – heading off about 10 degrees south of west – the maximum signal strength seemed to be inclined at about 30 degrees.
-Over the course of the next hour or so the point of maximum signal strength steadily swung round toward the south and the inclination of the signal seemed to climb slightly.  By 9:10 the signal was being received due south – inclined by about 40 degrees.
-At 9:30 I got in my car to head off south.  The signal was still very good– enough to hear it weakly with just the whip antenna inside the car (with the radio rig lying on the passenger seat).
-I headed south on the M11 – I had intended to pull off at Junction 9 – but found that you cant do this heading south – so I had to carry on.  As I drove on the signal got stronger until it reached a maximum and then got weaker again – I got off the motorway at the next junction (J8 – Bishops Stortford) and parked up at the nearby Motorway services.
-:12 (M11 J8 Motorway Services) – I found a corner of the car park with a large patch of grass – laid the compass on the grass and took a bearing with Yagi – more or less due north and inclined by about 50 degrees . By my reckoning this put the balloon somewhere between Saffron Waldon and myself. The signal was a very good strength (definitely good enough to carry data).
-I headed north on the M11 again – this was a mistake as after only a few miles the signal was completely lost.  I pulled of the M11 at J9 and found a place to park about 10:45 – but despite a sweep of the sky nothing was heard.  I headed south on the B1383 – stopping off here and there to take a sweep – nothing.  After a while the main balloon team gave me a call to say that it was down and had reported its position just south of  Stocking Pellham village.
-As I got close to the village I started to pick up the signal again and at one point It became very strong (this would have certainly be strong enough to carry data) so I parked up and started tracking with the yagi.  As I walked about I started to get some quite confusing signals – I tried attenuating the signal – but with little effect. After about 20 mins of  walking about I got a strong and clear reading and within a minute or so  I found PIII in the corner of a Oilseed-Rape field almost directly under some power lines.  I suspect it was the power lines had been carrying the signal form PIII that had given me the false readings. Fortunately PIII had landed upside down – which meant that the antenna was on the top side – had PII landed right way up the signal would have been a lot weaker.
-Frequency shift – most of the flight the frequency kept relatively stable – shifting a couple of hundred Hz. On landing the frequency jumped about 1KHz higher.