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By JIM ROWE Artist’s impression of a NOAA weather satellite (Courtesy Lockheed-Martin Missiles & Space). What you need to receive weather satellite images Interested in receiving the images broadcast by orbiting weather satellites? It’s now easier than ever, as long as you have a reasonably up-to-date PC fitted with a sound card. In this article, we explain how weather satellites work and tell you what you’ll need to receive their images. And elsewhere in this issue, we describe a weather satellite receiver that you can build yourself. Y ES, IT’S TRUE that you can see weather satellite images (or computer enhanced graphics derived from them) on the TV evening news and you can also download images of “special weather events” like cyclones from sites on the Internet. But there’s nothing quite like the satisfaction of receiving them yourself directly from the satellites, as many radio amateurs and other enthusiasts have been doing for decades. And as it happens, this is now a lot easier to do than it has ever been before. Only a few years ago, you not only needed a suitable receiver and antenna to receive the weather satellite signals but a special decoder box as well, 8  Silicon Chip before the signals could be displayed on a PC (using a specially written program). But now, providing your PC is reasonably up to date and has a decent sound card, the decoder box is no longer needed. Instead, you simply feed the audio signals from the receiver into your sound card and record them on your hard disc. That done, they can be decoded and displayed in one operation, using software that’s freely available on the Internet. So if you’d like to try your hand at receiving weather satellite signals, it’s now all fairly straightforward and can be done at low cost (provided you already have a PC). In this article, we’ll give you a quick introduction to weather satellites, describe how they work and describe the kind of receiver, antenna and masthead amplifier you’ll need to receive their signals. We’ll also discuss the kind of PC you’ll need and tell you about some of the software that’s available to both track the weather satellites (so that you can be prepared when one comes within range) and then decode their signals after you’ve received them. About weather satellites Weather satellites have been orbiting the Earth for over 43 years now, providing valuable information on the world’s weather and other environmental www.siliconchip.com.au This false-colour picture from NOAA17 shows extensive cloud cover over the southeastern corner of Australia. The vertical band at far left shows the sync pulses, while the adjacent vertical black band carries the minute markers (this picture was received over a period of about five minutes). The vertical band at far right represents undecoded telemetry data, which conveys the status of various systems on-board the satellite. events on a 24-hour basis. The first of these satellites was Tiros 1, launched by NASA for the US National Oceanic and Atmospheric Administration (NOAA) in April 1960. Since then, there has been not only a continuous series of NOAA satellites but also many broadly similar satellites launched by the former USSR, Japan, India and the People’s Republic of China. So you mightn’t have been aware of them but at any time in the last few decades there have been quite a number of weather satellites orbiting above us and sending down a constant stream of images and other meteorological data. There are still quite a few satellites in orbit, although some of them (like the Russian Meteors) seem to have reached the end of their operating life and are no longer sending down any pictures. But there are still at least two fully operating NOAA satellites, for example, providing weather images at least twice and sometimes three or four times a day virtually anywhere in the world. By the way, there are two rather different types of weather satellite. www.siliconchip.com.au One type are in equatorial orbits (ie, around the Equator) at an altitude of about 35,800km, so they rotate in synchronism with the Earth itself and are therefore described as “geostationary”. Each of these satellites constantly views a fixed “disc” of the Earth, with its centre point on the equator directly below it. Signals from a geostationary satellite can be received continuously from anywhere inside its field of view. However, receiving their signals isn’t easy because they only transmit in the UHF S-band (typically at 1.691GHz) and the signals are quite weak because they’re coming from so far away. You need a fairly large dish antenna with a low noise down-converter (LNC) just for a start. The other kind of weather satellites are in close to polar orbits (ie, passing over the poles) and orbit at a much lower altitude – ie, around 850km. In other words, they’re “Low-Earth Orbiting” or “LEO” satellites and each circles the Earth many times a day and passes over (or at least near) Fig.1: an APT transmission line starts with a sync pulse burst. This is followed by an 11.3ms section allocated to “space data” and minute markers, then a 218.5ms section with 909 pixels of image data from the Channel A sensor, and then 10.8ms of telemetry data. This 250ms-long data format is then repeated for the Channel B sensor. December 2003  9 This is another false colour picture from NOAA17, this time received over a period of about seven minutes and showing a large part of eastern Australia extending from the Gulf of Carpentaria down to Tasmania. The sudden change in the picture towards the bottom is a result of turning up the RF gain control on the receiver at this point during signal reception. any particular point a couple of times a day. The NOAA satellites are of this type and typically orbit the Earth about 14.1 times a day, or about once every 102 minutes. For example, the NOAA17 satellite currently passes over New Zealand and Australia a number of times during each morning, while the NOAA12 satellite passes over a number of times in the late afternoon or early evening. Since these satellites “precess”, or slowly move around the Earth as they orbit, their “passes” don’t follow the same path every time. However, there is usually at least one pass (and sometimes two or three passes) by each satellite that can be received each day, to provide interesting weather pictures. Another big bonus with the LEO satellites is that they not only transmit weather images in the UHF band (usually on 1.698GHz or 1.707GHz) but also in the VHF band on frequencies such as 137.50MHz and 137.62MHz. And although you need a steerable dish and LNC to track the satellites and receive their UHF signals, the VHF signals are much easier to receive. For VHF, all you need is a fixed antenna with a roughly hemispherical reception char10  Silicon Chip acteristic, plus a masthead amplifier and a suitable VHF receiver. So the polar orbiting LEO weather satellites are of much greater interest to amateur weather satellite enthusiasts, because their VHF signals are a lot easier to receive. And NOAA’s satellites 12, 15 and 17 are of particular interest at present, because they’re the ones that are currently in operation. The NOAA satellites The latest generation of NOAA satellites are fairly large “birds”, powered from a large solar cell array which is attached to one end (see artist’s drawing). They are equipped with quite a range of scanning and sensing subsystems, including microwave and IR sounders, an alpha particle sensor and the main source of meteorological images: the Advanced Very High Res­ olution Radiometer/3, or “AVHRR/3” for short. The data from these sensors is transmitted back to Earth (along with housekeeping telemetry data) via a number of communications links. In fact, each NOAA satellite has no less than 14 antennas, nine transmitters and various receivers (for receiving command data). The AVHRR/3 is mounted at the opposite end of the satellite from the solar array. It is a continuous imager, which uses a rotating mirror scanning system to scan the path beneath the orbiting satellite in “lines” which are perpendicular to the path and stretching from the horizon on one side to the other. The scanning mirror rotates at 120RPM, giving 120 lines per minute – chosen because as the satellite moves in its orbit, this provides the vertical deflection, so each scanning line butts against the last for contiguous scanning. The radiometer’s sensors have quite a small field of view (1.3 x 1.3 milliradians, or about .075° x .075°) and the sensor outputs are sampled on the spacecraft at a rate of 39.936kHz, so there are essentially 2048 samples per sensor per scanned line. There are a total of six sensors in the AVHRR/3 radiometer, three scanning at visible wavelengths near the infrared and three at thermal IR wavelengths. The outputs from any five of these sensors can be transmitted back to Earth at any time on the UHF (1.7GHz) channel. However, the satellite’s APT (automatic picture transmission) signals provided on VHF (137.5MHz or 137.62MHz consist of down-sampled versions of the signals from two of the AVHRR/3 sensors, selected by commands uplinked from NOAA’s control centres. During the part of each satellite’s orbit that is in daylight, each APT line contains data from one visible light sensor and one IR sensor. By contrast, at night the visible light data is replaced by data from a second IR sensor to provide more useful information. The down-sampled APT data derived from the two selected AVHRR/3 sensors is converted back to analog form and then used to amplitude modulate a 2400Hz audio subcarrier, together with synchronisation and timing pulses and other telemetry data. The 2400Hz subcarrier is then frequency modulated onto the VHF carrier signal, for transmission down to Earth via a 5W FM transmitter and helical antenna. APT signal format Fig.1 shows the basic format of the signals conveyed in one APT transmission line (lasting 500ms). The line starts with a sync pulse burst of seven www.siliconchip.com.au cycles of a 1040Hz square wave. This is then followed by an 11.3ms section allocated to “space data” and minute markers, then a 218.5ms section with 909 pixels of image data from the channel A sensor, and finally 10.8ms of telemetry data. The second half then starts with a second sync pulse burst of seven pulses at 832Hz, followed by a second space data and minute marker section of 11.3ms. Then comes another 218.5ms section with 909 pixels of image data from the channel B sensor and finally another 10.8ms of telemetry data. It’s this format that gives the signal a characteristic “tick-tock” sound when you listen to the received 2400Hz audio via a speaker or earphones. Receiving antenna The VHF APT signals from NOAA satellites are strong enough not to require a high-gain tracking antenna. Instead, a low-gain fixed antenna can be used, although it does need to have a hemispherical or “flattened hemispherical” reception characteristic so that it picks up the signals with much the same sensitivity as the satellite passes over. Note that because the signals are transmitted from the satellite via a helical antenna, they are also righthand circularly polarised. This means that the antenna must also be able to pick up signals with this type of polarisation. There are three main types of receiving antenna which meet these requirements: (1) the crossed-dipole or “turnstile” antenna (either alone or combined with a reflector to become a turnstile/reflector); (2) the Lindenblad antenna; and (3) the quadrifilar helix antenna or “QFHA”. Of these, the QFHA probably gives the best performance but is not easy to build because it’s essentially a truncated double helix. The Lindenblad gives reasonable performance but is still fairly difficult to make because it consists of four dipoles in a square array, with each dipole tilted at 30°. It also doesn’t perform well unless it’s mounted very high off the ground and well away from metal roofing. In fact, the author built and tested a Lindenblad antenna for the receiver described elsewhere in this issue but after a lot of frustration, I finally www.siliconchip.com.au A crossed-dipole or “turnstile” antenna coupled to a masthead amplifier are all that are required to “pull in” the signals from the NOAA satellites. Articles describing how to build these items will be published in SILICON CHIP in the next few months. scrapped it and built a turnstile/reflector instead. This was quite easy to make and also gives surprisingly good reception at my location. Now although the VHF NOAA signals are strong enough to be received using this type of fixed antenna, they’re still pretty weak. After all they’re coming from a 5W transmitter which is still more than 800km away even when the satellite is passing directly overhead. The transmitting antenna is also propagating this power in a solid angle of 63°, so by the time it does reach the ground below, the effective path loss is quite high. From a practical point of view, this means that most VHF receivers simply aren’t sensitive enough and don’t have a good enough noise figure to give good reception of the weather satellite signals by themselves. In short, you also need a low-noise masthead preamp, to boost the signals as close to the antenna as possible – and certainly before they have to pass down through any significant length of coaxial cable to the receiver (which introduces losses). So as well as describing an easyto-build turnstile/reflector antenna in coming months, we’ll also be de- scribing a suitable masthead preamp. Stay tuned! The receiver Since the NOAA signals are in the 137MHz VHF band and use FM, you’d expect that almost any VHF communications receiver or scanner would be suitable for receiving them. However, while it’s true that you can receive them reasonably well with some receivers, the results are often disappointing. That’s mainly because the 2400Hz satellite subcarrier signal is modulated with an FM deviation of ±17kHz, so it has a bandwidth of about ±25kHz. This bandwidth is quite a bit wider than that used for narrow-band VHF FM communications but at the same time, it’s much narrower than that used by broadcast FM stations. So a VHF scanner or communications receiver can’t be set to its narrow bandwidth, because this is too narrow to receive the signals without severe distortion. Instead it must be set to WFM (wideband FM), even though this gives a relatively low audio output level and often a fairly poor signal-to-noise ratio. The ideal type of FM receiver to use December 2003  11 Tracking And Decoding Software As you’ve probably guessed already, it’s the 2400Hz subcarrier “audio” signal from the receiver that contains the APT information as amplitude modulation. As a result, it’s this signal which is fed into your PC via the sound card, to be initially stored on the hard disk and then decoded and displayed using the appropriate software. PC requirements WinOrbit 3.6 is a “predictive” freeware satellite tracking program that can be downloaded from www.amsat.org/amsat/ftp/software This readout, taken over a 2-hour period, shows the path and current location of NOAA17, with the large circle indicating the satellite’s current field of view. The readout also indicates the dark and sunlit areas of the Earth, as indicated by the purple/red plot and the Sun symbol (ie, all areas in the middle of the “U” were in darkness when this plot was made). The program can predict the time of the next useful pass of the nominated satellite for a given location and shows lots of other data as well. You don’t need a particularly hot PC to record and decode the APT signals. Almost any reasonably up-to-date machine will do, as long as it’s running Windows 98SE or better, has a sound card and also has a reasonably fast and capacious hard disk so you can record mono audio signals sampled at 11.025kHz (16 bits). Most Pentium II, III and IV machines should be quite suitable, as should many of the machines using Celeron and Athlon processors. Of course, your PC also needs to have a modem and an Internet connection, so you can get on the Internet to download the software you’ll need for both satellite tracking and weather image decoding. You’ll also need the Internet connection to download the orbit update information for the satellites you want to track. Tracking software SatSignal V4.04 is a freeware APT decoder that works quite well. You can download it from www.satellitescience.com or from www.satsignal.net for the APT signals is one with a bandwidth of about ±30kHz, or not much more. There are specially designed weather satellite receivers with this bandwidth available commercially but they’re fairly expensive. Because of this, we’ve developed a small 12  Silicon Chip 2-channel VHF FM receiver which has a bandwidth of about ±35kHz and is therefore quite suitable for receiving the APT signals. This receiver is described in this issue in a separate article, so that you can build your own at a reasonable cost. Because the polar-orbiting satellites move in very well defined orbits, the position of each one can be calculated at any time based on the so-called Keplerian elements (orbit definition parameters) for that satellite. This is done by tracking software, which can also predict when that satellite will pass within your antenna’s field of view, once it knows your longitude and latitude. This calculation is done completely “off line”; you don’t need your weather satellite receiver to be working. There are quite a few freeware and shareware satellite tracking programs available on the Internet. We tested and can recommend WinOrbit 3.6, written by American radio amateur Carl Gregory, K8CG. Once you provide it with the orbital information on the satellites you want to track, it can not only plot their positions at any time on a world map but also predict the next useful pass of any nominated satellite together with the local time, the satellite’s range and elevation and so on. WinOrbit 3.6 is freeware, and you www.siliconchip.com.au Useful Websites If you’d like to get some more information on weather satellites, or to download some satellite tracking or decoding software, here are some useful websites and documents: www.amsat.org http://celestrak.com/NORAD/elements/ www.david-taylor.myby.co.uk/software/ www.drig.com www.geocities.com/SiliconValley/2504/wx.htm www.noaa.gov www.ncdc.noaa.gov http://www.ospo.noaa.gov/ www.riglib.demon.co.uk/index.htm www.satellitescience.com www.satsignal.net http://sattrackhouston.com www.telecable.es/personales/ealbcu/ kepsen.htm http://www.time-step.com/products_ apt.htm can download it as a single zipped file (WINORB36.ZIP – 478KB) from various sites, including www.amsat. org/amsat/ftp/software However, we plan to make a copy available on the SILICON CHIP website, so look for it there first. Two other popular satellite tracking programs are L. Hamilton’s “Footprint V2.08” which can be downloaded from www.riglib.demon.co.uk/ footprint.htm and “WXTrack V3.4.0” which is written by David Taylor of Edinburgh, Scotland and can be downloaded from his website at www.satsignal.net Which ever program you decide to use, you’ll need to provide it with the tracking data for the satellites you want it to track (ie, their Keplerian elements). This tracking data can be downloaded as a text file from various Internet sites. For example, you can get the data for the NOAA satellites from http://celestrak.com/NORAD/ elements – it comes as a text file called “noaa.txt”. This is then simply renamed with a “2li” extension instead of “txt”, after which it can be used by the tracking program. Using the tracking program, you’ll be able to find out when the satellite www.siliconchip.com.au you’re interested in will next be in range. You’ll then be able to receive its signal at the expected time and record it on your PC’s hard disk using an audio recording program. You can use CoolEdit (which can be downloaded from the Internet), for example, or Creative Recorder which comes with most Sound Blaster audio cards. By the way, most weather satellite decoding programs seem to want the signals recorded as WAV files, in mono (left channel), with 16-bit resolution and a sampling rate of 11.025kHz. So that’s the recording format to use and it’s much more economical when it comes to disk space than recording in 44.1kHz stereo. APT decoding software Once you have the signals recorded on your hard disk, you can fire up the decoding program and process them to produce the actual images. So if you don’t have a decoding program as yet, the next step is to download one of the freeware or shareware decoders available on the Internet. There are quite a few weather satellite decoding programs available for free downloading; eg, from sites such as www.satellitescience.com One of the most popular programs is WXSAT 2.59e, written by Christian Bock. It’s free for schools and private/amateur use and has good documentation. It’s also fairly easy to use, although sometimes it seems to have trouble decoding signals where the subcarrier has been Doppler shifted in frequency. After testing several programs, we eventually settled on SatSignal V4.04, written by David Taylor. You can download this program from www.satellitescience.com or directly from David Taylor’s own website at www.satsignal.net All of the weather satellite images shown here were decoded using SatSignal V4.04, incidentally. By now, you should have a good understanding of how weather satellites work and how you can receive images from them using a suitable receiver, a PC and freeware software from the Internet. If we’ve whetted your appetite, the next step is to take a look at the 2-Channel VHF Weather Satellite Receiver described elsewhere in this issue of SILICON CHIP. It’s easy to build and will have you receiving your own weather satellite pictures in next to no time. SC Silicon Chip Binders REAL VALUE AT $12.95 PLUS P &P These binders will protect your copies of S ILICON CHIP. They feature heavy-board covers & are made from a dis­ tinctive 2-tone green vinyl. They hold up to 14 issues & will look great on your bookshelf. H 80mm internal width H SILICON CHIP logo printed in gold-coloured lettering on spine & cover H Buy five and get them postage free! Price: $A12.95 plus $A5.50 p&p. Available only in Australia. Silicon Chip Publications PO Box 139 Collaroy Beach 2097 SC Or fax (02) 9979 6503; or ring (02) 9979 5644 & quote your credit card number. Use this handy form Enclosed is my cheque/money order for $________ or please debit my  Bankcard  Visa    Mastercard Card No: _________________________________ Card Expiry Date ____/____ Signature ________________________ Name ____________________________ Address__________________________ __________________ P/code_______ December 2003  13