Assembling an HRPT System

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Remotelmaging Group Journal

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Assembling an HRPT System CheaplgBy GeraldlhningerSome excellent articles on the subject of home-made HRPT systems have been published in the RIG journal in the last few years. In the year 2000, when I started with HRPT, my target was a very cheap system which would be useful for beginners without much electronic skill. This system should include simple kits and cheap ready-built equipment without the need for test equipment. Additionally, I wanted to have the best performance I could obtain while using only a single PC for both dish tracking and data recording. Not possible, you would think! Then let me tell you how I achieved this at a cost of only 450 euro (other than 1O0O euro for an antenna rotator).

My first operational HRPT system was made using rny 7.2 m Meteosat dish but this proved too big when there was a strong wind. So I bought an 87 x 72 cm offset dish at my local electronic shop for only 25 euro and this now became my starting point. Next I considered the performance and decided that I required noise-free images down to 5'. Using rny Satlink software, I calculated that I required a low-noise-amplifier with 0.6 dB noise figure and an HRPT receiver with 3 MHz bandwidth plus a digital decoder with a 'bit

The offsetdish

integration and dump'function. A11that remained now was to look for cheap HRPI parts to achieve this performance. HRFT System Parts

For the offset dish I used a circular helical feed because of the circularly polarised satellite signal and tried some designs which I found on the Internet. I bought a stove-pipe cap to use as a reflector, 1.5 metres of 6 rnrn2 earthing wire, an N-plug as connection to the low-noise-amplifier and some screws, all for about 15 euro. I wound the earthing wire counterclockwise around a 50 mm diameter former until I had 8 turns. The helix sprang out to about 58 mm when winding was complete and I then carefully stretched it until the turns were spaced 20 mm apart. Finally, I cut off the first half turn and the last half turn to produce a7-twrn helix, drilled a hole 29 rnrn to one side of the centre of the reflector, mounted the N-plug and soldered the helix to the 46

The helical feed

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N-plug. A T-rod in the middle of the reflector gave the helix good stability. The offset dish was then mounted and turned by 90' with the helical feed to the right or left side of the dish. The next step was to find the low-noise-amplifier. Either you use the Sam Elsdon LNA design which is available as a kit, with the ready-built RIG downconverter, or you can look for a much simpler design which is completely ready-built. I have tried both and for the second option found a supplier in Italy called Nuoua Elettronica who offer a combination low-noise-amplifier and downconverter with 0.6 dB noise figure and about 50 dB gain. Power supply of 15 V comes via the co-ax from the receiver. The cost of this ready-built downconverter (TV97O) is 83 euro, but you do have to protect it against rain (e.g. use a waterproof box). For best performance it should be directly connected behind the reflector of the helical feed.

TheTV970

One of the hardest parts is obtaining a suitable HRPT receiver. Two designs are available as kits but of course these need some electronics construction experience as well as test equipment to adjust the receiver. There is, however, a good ready-built receiver that can be bought with your hard earned money, the LX. 1495, again from Nuoua Elettronica, for 2O7 euro. Nuoua Elettronica have their own bimonthly journal in ltaly. After I had read the Italian article about this receiver, with some language troubles, I decided to buy one. Mr. Leonardo Righini, one of the company directors, helped me to order the receiver via credit card, and only one week later I had it in my possession. The first thing I did with it was to remove all PCB's and make a shielding layer inside its plastic box using a copper spray and then connected this shield to earth.

Features of the LX.1495 Receiver . . . . . .a

tuneable from 135 - 155 MHz frequency LED display frequency steps: 1, 10 and 100 kHz 4 programmable frequency memories frequency and channel scan power supply via co-ax to the downconvertersignal meter and AFC BNC input, 2 outputs oscilloscope) suitable for CHRPT (decoder and/or The LX.1495 receiver

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The Decoder I decided to build the Rob Alblas digital decoder, which cost me about 115 euro. This decoder is suitable for HRPT, CHRPT and HRI. Rob uses an EPROM for the decoder and he has also designed a test version with a 'bit integration and dump' function to get less speckles in the image. Some people reported that although this test version produced fewer speckles it gave more bad lines (even at a high signal level) and that their images were not useable. But I wanted to give it a try to see this behaviour for myself. After the first HRPT images with Rob's standard EPROM I changed to his test EPROM and what a miracle - it was running without any bad lines! I improved my system performance by up to 2 dB and feel that its performance almost matches that of an analog decoder. But Rob Alblas, Gotz Romahn (who made some decoder measurements) and I don't know why it is working so well with my Nuoua Elettronica receiver. Then Bob Barnes used the 'bit integrator and dump' EPROM after some modifications of the Sam Elsdon receiver (see RIG 721 and now it also is working without bad lines. It seems that the IF filter modification was the reason for this success and I hope more people can verify this result. Because of the non-inverted receiver output there was no change at the decoder but I did have to build a simple comparator (LM311 and some resistors) with a low-pass-filter to feed the decoder input. The -l[on Plus Utra is an antenna rotator system similar to the G5500 rotator and costs about 950 euro. I have such a rotator and built the FodTrack rotator interface. This interface is really easy to build, cost me only 25 Euro and connects to the LPT2 port of the PC. Wrtrack is used to feed the FodTrack rotator interface with the required azimuth and elevation data. System AdJustments After I had built the decoder, comparator and low-pass-filter into the receiver box and connected all the units together, I searched for the Meteosat-7 signal with an APT receiver, adjusting the helical feed position (focal length) and azil:rutlr/elevation of the dish for the best signal. With the WxSat software I saw a completely noise-free Meteosat-7 image. Next I tracked a NOAA satellite and used its signal to adjust the decoder VCO trimmer for the best signal lock. I did this with a low satellite pass (elevation about 5" to 10"). Fina1ly, I started up my 4OOMHz PC, connected the decoder to LPT1 and the FodTrack interface to LPI2 and ran WxTrack (David Taylor), Wsat (Rob Alblas). I then recorded the first HRPT data with my new system and got a brilliant image with the HRPT reader (David Taylor).

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Now I had noise-free images down to 5" with NOAA 77 and NOAA 16, about 6" \Mith NOAA 12/14 and 15' with NOAA 15. Summary 81 cm x72 cm offsetdish 25 Euro Home-made helicalfeed 15 Euro Ready-builtLNA/downconverter http://www.nuovaelettronica.it 83 Euro Ready-builtHRPTreceiver-http://www.nuovaelettronica.it 207Euro Decoderkit - htto://www.alblas.demon.nl/wsat 115 Euro That totals around 450 Euro and only the helical feed, comparator and decoder were home-made projects. If you want to use a rotator plus tracking interface you need an extra budget of around 1OO0Euro. Have a look at my website http tlm em bers.s urte u.aUgerald_i h n i nge r to see many details about my HRPT system and additional information including the Nuoua Elettronica HRPT equipment.

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