Satellite Weather Pictures First-Hand, Part 1

Transcription

1 Satellite Weather Pictures First-Hand, Part 1 How to receive NOAA weather satellites using a home-made turnstile antenna Inspired by an article in the German magazine Funkamateur (issue 7/05, p. 674), I wanted to try capturing pictures from weather satellites as seen on TV. The following article describes my experiences; however, I have omitted details that can be found elsewhere. The operation of the PC software has been described in the above article and can be looked up in the software Help files. The terms following in italics can be found via Google. Only the antenna used is described in detail, as I found only little information for home-making one. What is the NOAA? It is an American Authority with various functions (see wikipedia.org) which operates, among other things, several own weather satellites. These are transmitting in the 137 MHz band and can be received legally and cost-free by everyone. Reception uses PC sound card and software The receiver should be capable of receiving 137 MHz and be set to Narrow FM. The audio signal is fed into a PC sound card. The PC runs the software WxToImg which shows not only when and on what frequency a satellite can be heard, it also starts and ends the recording automatically. Live reception of an NOAA Satellite using a Yaesu VX-5 and WXToImg The pictures received can subsequently be edited and improved, e.g. by false colour display. The basic version of WxToImg is freeware. You can purchase several improved versions which promise more options and a better picture quality. February

2 Another very useful software is WXTrack. It shows the flight curves of all official satellites (not only weather sats), also of the International Space Station ISS. This has enabled me to copy the ham traffic coming over the ISS repeater when it flew over my location. Screen shot: WXTrack displays NOAA 15 over the radio horizon of Berlin (+). The window in the back is WxToImg, building the picture currently received by NOAA 15. The NOAA sats are orbiting the Earth with relatively high speed from South to North or vice versa. They are rising over the horizon, flying over your location in about 10 minutes, then descending on the other side of your horizon. They continuously transmit a multi-channel picture (infrared and daylight views with up to four sensors) which builds up line by line on the screen. First Reception Attempts With my large 2 m/70 cm vertical antenna (Diamond X-300) I could hear the sat s audio signal, but the picture was garbled. May be a directional antenna with position tracking would help? WxToImg and WXTrack both display the sat s elevation (the angle over ground when looking at it from your location) and azimuth (position on a 360 degree compass rose). Beforehand, you need to download the Kepler data from the internet. This way you always know not only when the sat will show up and disappear, you can also tell its current position in the sky. February

3 This helps a lot to evaluate the reception (e.g. the signal gets weaker when the sat disappears behind a high-rised building) and for tracking the sat with a directional antenna. Both programs can do this fully automatic if you have the relevant equipment unfortunately, azimuth/elevation rotors and related computer interfaces are very expensive. I therefore tried a manual solution. Manual Satellite Tracking For manual tracking I used a small compass and an enlarged compass scale printed on paper as well as a 90 degrees angle scale. With this simple equipment I manually tracked the satellites with a 2m directional antenna while watching the elevation and azimuth display, enlarged by the Windows Screen Loupe. This resulted in a slightly better signal strength but still delivered poor pictures. You Do Need an Antenna with Circular Polarization! The satellite transmission antennas have circular polarization, so I built myself a crossed dipole antenna, also known as turnstile antenna. I found instructions under I modified the suggested design to be able to use only materials I had in stock. I mounted the dipole ends inside the junction box with small clamps onto a piece of wood. February

4 The antenna pole is a very old PVC tube with 40 mm diameter. Inside the pipe is the transformation line going downwards from the junction box and a PL connector for the regular coax cable leading to the receiver. This way everything was easily made rain safe as well. Measuring the antenna with an analyzer showed an SWR < 1:1.4 in the entire 137 MHz range. The junction box is laying loose on top of the pipe which facilitates stowing and transportation of the antenna (R. H. picture). The reflector is mounted 80 cm underneath the crossed dipole. You may want to drill holes at several heights into the PVC pipe to optimize reception by trying different distances between reflector and dipole. The following page shows a couple of details. February

5 If you drill the holes in the junction box smaller than the diameter of the dipole elements, the soft PVC material will provide for some rain sealing. To protect from condensation water, I drilled another two holes into the bottom, small enough to prevent insects from nesting inside ;-) Detail of the reflector. Both metal bars are held together using two cable ties, tied crosswise. To ensure the reflector is horizontally, you have to carefully balance the free hanging metal bar ;-) Of course, you could fix it to the pipe using more cable ties, but then the antenna will no longer fold for storage February

6 And now some of the NOAA satellite pictures I received with this antenna: If desired, WxToImg puts a map overlay over the picture received. The yellow cross indicates my location. This picture was taken in the evening. You can clearly see the shadows of some cloud towers over Germany, generated by the sun dawning in the West. The cloud swirl over the Mediterranean Sea was surely not a tornado The best picture quality can only be achieved with certain scanner models (listed on the WxToImg web site) or with specialized weather satellite receivers, since the sat signal is up to 30 khz wide. With common ham equipment, like my IC-706 and VX-5, having only 10 to 15 khz bandwidth, you have to live with less contrast in the picture. Switching to WFM does not help. The author of WxToImg promises that the upgraded version (approx. 50 Euros) delivers a better picture with narrow banded receivers. February

7 The bar code on the border is telemetry data. False colour display showing interesting cloud formations. Looks like it was very windy over the Irish Sea, as the clouds appear so stretched. February

8 The horizontal picture disturbances were generated by ground reflections and deletions of the radio waves. Since every sat pass is on a different flight curve, this can always happen. The misalignment between map and actual land borders can be corrected using the WxToImg menu item Move Map Overlay. February

9 As is often the case in February-March, Germany is almost completely covered by clouds whereas in France and Italy, the sun is shining Fluffy clouds over the Irish Sea. February

10 Depending on the flight curve of the sats, the high-rised buildings North West of my location in 100 and 200 m distance disturb reception when the satellite is declining in the North. Future Plans When the weather turns better in Spring, I will try mobile reception from an elevated location. I hope for pictures with less distortion. This is covered in part 2. Conclusion Capturing weather sats is fun, even if you are not a meteorologist! It fascinates me when a satellite, announced exactly to the minute by the computer, shows up on a normally empty frequency, with me being able to decipher the strange audio signals (it sounds like a metallic, high-pitched singing with hectic tic-tocs. These are the telemetry signals showing as barcode on the picture border.) I have learnt a lot about satellites, built an interesting antenna and enlarged my radio knowledge. Beforehand, it did not know what azimuth and elevation mean now I can point with the finger to the position up in the sky where the satellite is just now! February

11 Even if you do not have a ham radio license, you can capture satellites with your radio scanner the software needed is free on the internet! Enjoy! Disclaimer: You are welcome to do and try what I did, but please understand that you will do so at your own risk. Thanks! I appreciate your comments and questions. Please send an to <mycallsign> <at> darc.de (this spelling of my address shall protect me from spam bots). 73, Andy, DL4AND February