Fullscreen HTML5Med Res (??MB)HTML4

By Bruce Mitchell Everyone knows that a large asteroid or comet probably killed off the dinosaurs. But did you know that the Earth gets hit by countless small meteors every day? This article tells you how to observe and count them using readily available “junk” and a little ingenuity. Y ou’ve probably noticed by now that the Earth wasn’t destroyed by the Leonid meteor shower last November. But all this could change. Out there in space there are enough big lumps of rock (aka asteroids) to keep at least a few researchers on the lookout for the sort of encounter that would make nuclear war seem a pleasant alternative. These scientists sift through all kinds of astronomical observations, trying to predict and identify asteroids that could make an unwanted entry into the Earth’s comfort zone. In this article we’ll first look at how meteors, asteroids and comets are related and then look at ways of automatically counting meteors using a passive radar technique. It’s not 6  Silicon Chip a cut-and-dried list of instructions on how to make a fully-featured meteor detector. It provides some background information, describes one (of many) approaches to the task and mentions a few practicalities on the way. It assumes a moderate degree of competence in electronics and construction and an honours degree in the fine art of scrounging. Be prepared for disappointments, frustration, lots of reading and hopefully, a sense of accomplishment. You’ll certainly learn more about astronomy, electronics and computing on the way. Did you feel that? We’re seldom aware of it, but as the Earth orbits the Sun it keeps hitting things. There are some boulders, quite a few lumps the size of pebbles and lots of dust that nobody’s got around to cleaning up yet. The pebbles and dust are nothing to worry about unless you earn your living in a space shuttle but as anyone who’s worked in a mine knows, a stray boulder can really ruin your day. Way out in space they’re hard to see even with a large telescope, because on the astronomical scale of things they’re not all that big, maybe no more than a kilometre across. Asteroids and comets leave wispy trails of debris in their wake, so one sign of their passing is higher-than-usual numbers of meteors entering the atmosphere at certain times. These trails persist for a long time. For example, the Leonid meteor shower results from debris associated with comet 55P/Tempel-Tuttle. This insignificant little comet orbits the Sun every 33 years and each November the Earth passes through a small cloud of its debris. Unexpected increases in meteor numbers can indicate the Earth is passing through the trail of debris left by an unknown object. NASA coordinates a project that collects meteor counts from volunteer observers around the world. Each month these observers email an hour-by-hour summary of their observations to NASA’s Ames Research Center for inclusion in the various models used to study meteor and asteroid distribution in and around the orbit of the Earth. Analysis of this sort of data can help identify the orbits of previously unknown asteroids and comets. Just what to do if someone does find an asteroid heading straight towards your place is probably more of a political than scientific decision, given the size of the issues and budgets involved in trying to avoid it. Counting meteors is easy! Try it tonight: lie down in your back yard and make a mark on your notepad every time a “shooting star” appears. (Making a wish is optional.) You’ll soon find, however, that long-term activities of this kind have a few drawbacks. Relationships suffer (“Where were you last night?”); careers can be affected (yawning while the boss tells a joke is risky); it’s cold out there in winter and even the most enthusiastic observer can get discouraged after three weeks of non-stop rain. Oh, and it’s desperately hard to spot them during the day, but that doesn’t matter because you’ll be in bed catching up on lost sleep. Video and photographic methods also have severe limitations (daylight and clouds being the most obvious), so can electronics offer an alternative? Well, at this stage you can’t walk into your local hobby shop and buy a $99 meteor counter because the demand just isn’t there. But anyone with an interest in electronics and computing definitely can make one for that sort of money if they’re prepared to tinker and fiddle, scrounge and improvise. Fortunately, for those of us who prefer bed to backyard during the dark hours, a meteor leaves a telltale trail that can be detected using radio waves. How does this happen and how can we detect it? Frying high Meteors that get into the Earth’s path appear to be moving pretty fast compared to the speeds at which humans operate. The Earth wobbles along around the Sun at something like 100,000km/h, so anything it happens to bump into is going to suffer in a way that can’t be ignored. An innocent grain of space dust suddenly finds itself rubbing against an increasingly dense collection of molecules around 100km above the ground. This friction quickly gets converted to heat so intense that electrons get stripped off some of the molecules in the vicinity, ionising a small patch of sky. The ionised matter may disperse in only a fraction of a second if the dust particle is small but larger ones generate more energy and it may take a few seconds before things are back to normal up there. The same process produces light, which is why we see the familiar streak when a meteor hits. Bigger ones (and we’re talking about ball-sized stones here) can even reach the ground before they vaporise completely, emitting lots of light and even sonic bangs on the way. Every few hundred years a really big one impacts spectacularly, becoming a useful source of hyperbole for bad TV docos (and even worse movies) about the Impending End of Civil- isation As We Know It. And every few million years... well, just don’t mention the dinosaurs! How can we detect that brief signature high in the atmosphere? At light wavelengths we could use our eyes or a camera, techniques which are fine as long as it’s dark and not cloudy and as long as the observer is looking at the right part of the sky and hasn’t fallen asleep, frozen to death or been divorced. Infrared detectors might also work but are subject to the same limitations as visible light detectors in cloud or sunlight. Further down the electromagnetic spectrum, things look more promising. Radio wavelengths aren’t swamped by solar interference during the daytime and can penetrate cloud. It’s Fig. 1: meteor trail reflecting FM signal to a receiver beyond the horizon. FEBRUARY 2001  7 even possible to use radar to pick up the ionised trails. A few observers still do but the ionised trails don’t reflect microwaves or UHF signals all that well. It turns out that some of the most strongly reflected frequencies are in the low VHF band, between 40 and 150MHz; the lower the frequency, the longer and stronger are the reflections. For decades, radio amateurs have bounced short messages off meteor trails but not everyone has the financial or technical resources to use specialised transmitters and receivers to detect meteor trails. Free radar, anyone? What we need is a reliable and powerful source of VHF signals and a simple receiving setup that can look after itself. No problem! All around the world there are thousands of 50100kW VHF transmitters pumping out FM radio signals 24 hours a day at between 88MHz and 108MHz. Despite the best efforts of antenna designers, not all of the signals transmitted by these stations travel near the ground. Some get radiated straight up and unless something gets in the way, they continue off into space to become interstellar electronic pollution (see Fig.1). At SILICON CHIP we’ve long maintained that old computer cases are too good for the tip . . . here’s proof – two receivers, using the XT’s power supply! Occasionally an aircraft reflects FM signals back to the ground, causing that familiar ghosting on TV screens and flutter in FM receivers’ sound (socalled multi-path reception). Aircraft are seldom more than 12km above the ground, so those signals aren’t reflected very far. The line-ofsight range of an FM station is a couple of hundred kilometres at best and an aircraft reflection may double this. But a meteor trail is anywhere from 60 to 120km above the ground, so they can reflect signals up to 2000km. There’s little chance of direct reception by a VHF receiver of a transmitter more than 500km away or even reflections from aircraft. Any signals received would have to come either from a meteor trail reflection or from sporadic ionisation of the ionosphere’s “E” layer. And it’s easy to tell the difference: sporadic E reception lasts for minutes or even hours, whereas meteor reflections seldom last longer than a couple of seconds. What about interference? TV video signals are (amplitude modulated) AM, so they are subject to interference from electrical noise. This can be bad in the lower VHF bands, especially if there’s a busy road or industrial complex nearby. FM receivers don’t have this problem because just about all AM interference is removed by their limiter stage. In practical terms, perhaps the most difficult source of interference to eradicate is cross-modulation in the receiver’s front end from nearby stations. In cities, this can be a serious challenge to overcome. Some observers use preamps with bandpass filters, while others opt for more subtle approaches. Mine was to set up the observation site at the bottom of a valley about 80km from the nearest powerful transmitter. Although not necessarily a cheap or convenient solution, it sure works. Choosing a transmitter A three-element Yagi cut for 89MHz, mounted seven metres above the ground. The preamplifier is protected from rain and sun by a piece of PVC drainpipe. The boom has been left a bit longer than necessary to make room for experiments with spacing, or maybe another element. 8  Silicon Chip There are hundreds of FM stations in Australia and New Zealand, many of which are very powerful. You may be lucky enough to have access to a comprehensive list of frequencies such as those published for scanner enthusiasts. Another approach is to look up the list of transmitters on the ABA’s website (www.aba.gov. au/what/bro-plan/broadcasting_stations/ind-ex.htm). There are lists sorted according to both frequency Fitting the receiver(s) into the computer case leaves lots of room for future expansion. and exact location. Your browser will need the Acrobat Reader plugin to read this info. When you’ve done that, take out your Jacaranda Atlas and try to find a transmitter somewhere between 750 and 1500km from your location, ensuring it uses a frequency well clear of local stations. This is not a trivial task but in places like the USA and Europe it’s almost impossible to find clear channels so consider yourself lucky. You then need to step through every channel on your digital FM receiver (all 200 of them) and make a note of those that seem to be free of interference. With luck, there will be at least one distant transmitter available on a clear channel. In my case, I eventually opted for ABC FM on 88.3MHz, transmitting from near Cootamundra with a power of 50kW ERP (Effective Radiated Power). Its transmitter at Mt Ulandra is about 980km from my observing location just north of Brisbane. Using The interface board, data and coax connectors inside the XT case. (NOT a pretty board.) a nationally networked station is a big advantage. In the early stages of setting up, its signal can be compared with the same content coming from a local transmitter. Otherwise it’s pretty hard to identify a station when the bursts last less than a second and are spaced minutes apart! By the way, there’s nothing wrong with observing two or three stations on the same frequency, as long as they’re all a long way from the receiver. In fact, this a very desirable setup because it increases the amount of data available for collection. The dish? Relax – you don’t need one. A suitable antenna is a three to 5-element Yagi cut to a frequency within a megahertz or so of the station(s) you’ve chosen as your “radar transmitter”. It only needs to be a few metres above the ground and pointing accuracy isn’t all that important either. Some observers elevate the front of the boom ten or twenty degrees if they’re observing transmissions from less than 600km or so but it didn’t make any difference in my case. There’s no shortage of software to help design a Yagi. A DOS-based package called Quickyagi (http:// www.raibeam.com/wa7rai.html) is well worth looking at. Or even easier: SILICON CHIP March 1998 issue had a design for a 5-element build-it-yourself Yagi antenna for the FM band. A larger antenna will bring in more signal but that may not be a blessing if your receiver’s front end gets swamped by other transmitters. More useful is a preamp at the antenna to boost the signal-to-noise ratio. For cheapness, reliability, ease of construction and performance it’s hard to beat the venerable VK5 2-metre preamp. Contact the VK5 branch of the WIA (see references for further details). The coils will need an extra turn or two so that they resonate in the FM band. (Don’t buy the relays for it Fig.2: a starting point for your data interface. Depending on your logging software’s requirements, you may need to reverse the op amp’s inputs. FEBRUARY 2001  9 Fig. 3: a typical day’s plot. More meteors are detected near dawn than dusk. – you’re not going to be transmitting!) See the references at the end of this article for an alternative design. Our photograph shows a homemade three-element Yagi cut for 89MHz, mounted seven metres above ground level. The preamplifier is protected from rain and sun by a piece of PVC pipe. The boom has been left a bit longer than necessary to make room for experiments with spacing, or maybe another element. Yagi construction needs only basic metalworking skills and the only design challenge I had was keeping water out of the preamp box since the rainfall at my location can be over two metres per year and usually arrives by the bucketful. Eventually, I used a small diecast aluminium box mounted inside a 30cm length of 90mm PVC drainpipe, capped by an overhanging “roof” of UV-resistant plastic sheet. The bottom has been left open to help with cooling and to drain any leaks. The entire assembly hangs from the antenna boom. All connections are coated in self-amalgamating tape and neutral cure silicone sealant. Use decent quality coax for the run to your receiver and don’t waste money on cheap connectors. Avoid the clunky old PL239 and SO239 types: BNC or F styles work well and are much neater. The expense of type N connectors is not warranted at these frequencies. Choosing a receiver If you have a high performance digital communications receiver you can spare for 24 hours a day, 365 days a year, then by all means tune it to the 10  Silicon Chip transmitter of your choice and leave it. The rest of us need something a little less capital-intensive, which is where a talent for sniffing out recycled treasure is vital. Most car radios have excellent sensitivity and if you’re using a preamp at the antenna, their signal-to-noise ratio isn’t a big issue. Garage sales, school fairs and car wreckers offer a selection of junked car radios. But choose a digital one: nothing else will do. (If you need convincing, try tuning an analog receiver to a station that’s irregularly audible for 200 milliseconds once every five minutes or so.) Make sure the FM section is working and don’t pay too much for it. Old hifi FM receivers may be OK but they must be digital and sensitive. I was lucky enough to buy two matching Pioneer units for $10. Experience has shown that nearby lightning strokes can make a real mess of the meteor data, so a future enhancement will use the spare (and desensitised) receiver to monitor an unused frequency in the AM band and log local thunderstorm activity. If you have a frequency counter or digital VHF communications receiver, it’s worth checking the frequency accuracy of the car radio before doing anything else. The local oscillator frequency should be 10.7MHz away from what ever frequency the radio shows on its display, usually higher. (For example, if the display shows 100.0MHz, the local oscillator should be running at 110.700000MHz.) If the frequency is more than a couple of kHz off, try a ceramic capacitor in the 2.2 - 47pF range across the crystal, to pull it back into line. I fitted a receiver into an old desktop XT computer case rescued from our suburb’s annual roadside junk collection. It has a good power supply, the case provides plenty of ventilation and there’s heaps of room to mount everything. It also looks slightly less ugly than a nest of cables and boxes side by side and makes the whole setup easy to transport. The antenna lead attaches to a BNC socket in one of the card slots in the back panel and the two-wire data cable to the computer leaves from the slot containing the interface card. All 5V and 12V power wires connect to a chunky great terminal block mounted down the middle of the computer case. That way it’s easy to fiddle with various sections without resorting to a soldering iron and it’s all very accessible. Power for the masthead preamp also comes from the XT supply via some RF filtering and is fed up the coax in the usual way. Connect the computer’s internal speaker to one of the receiver’s audio channels so you can monitor the channel when necessary. Most of the time you’ll want the audio turned right down. Set the mode switch to “mono” and if there’s a “Local/DX” control, set it to “DX”. If the receiver defaults to a particular frequency on power-up, make sure you configure this to be the one you’re observing so that it will be able to keep observing after power failures. Getting a signal out of the receiver You’ll need some kind of data logger to record the time when the system picks up a signal. That requires a digital output, a feature I’ve yet to see on any car radio, so it’s time to open the case and start poking around with a multimeter or scope. You’ll be looking for a mute signal or failing that, an AGC line. This may be easy to find if you have a circuit diagram but it’s unlikely you’ll be that lucky. Take some time to look at the circuit board layout. First, try to identify the RF section (the antenna lead is a giveaway), then the frequency synthesiser (probably near a crystal) and audio sections. The signal you want will probably not be in these sections, so now you know where not to start. It’s more likely to be near a large IC containing the IF and demodulation components. Just measure the voltage on each pin methodically. Tune in a local signal and try to find a pin where the voltage level changes when you switch to an unused frequency. It’s easy to be fooled by voltages that change gradually as you shift frequency: these control the local oscillator. My Pioneer receiver didn’t appear to have a mute line but the AGC wasn’t hard to find. It varied from 1.4V with no signal down to 50mV on a very strong station. Data logging Although there are alternatives, the obvious way to go here is to use a computer. Any old computer will do, as long as you can keep its RF interference out of the receiver. Even the slowest XT or early Mac would be fine. The early Macs featured excellent RFI shielding, which makes them quite attractive for this application. (If you intend using a Mac or Linux system, bear in mind that supplies of readyto-go shareware for observing meteors seem to be pretty sparse, if they exist at all, so you’ll be writing your own.) All your computer needs is a reliable clock, a few megabytes of disk space or even just a floppy and an operating system that doesn’t fall flat on its face twice a day. Avoid operating systems like Win 95/98, which are much too unstable for this kind of application. Use DOS as early as version 3.3 or if you must use Windows, opt for Windows for Workgroups. Systems 6 or 7 should be fine on older Macs. Another advantage of an older OS is that, in the event of a power failure, you can configure your machine to reboot itself in seconds. Display quality is irrelevant, because 99% of the time the monitor won’t even be switched on. Low power consumption is important because this device is going to be on all the time. An old laptop running from a float-charged battery would be ideal. Data interface There’s no need to rush out and buy an A/D conversion board. All we’re dealing with here is an on/off signal that needs to be sampled 100 Fig. 4: Leonid shower 15-17th November 2000 (freq = 88.3MHz). Area observed: NE NSW. The red vertical lines show the number of meteors detected per 10 minute period. The green vertical lines show midnight local time. FEBRUARY 2001  11 times per second at the most. This is not leading-edge stuff, so you can interface to the computer through just about any port. I chose the games port as it’s electrically pretty basic, has multiple data lines and happened to be available on my machine, but most observers’ setups use a COM port. If you’re using a Mac, its serial port would be the obvious choice. To tell the computer there’s a signal present we need some kind of threshold detector. Its trigger point has to be set to an arbitrary level, ideally just above the receiver’s background noise. Too low a threshold leaves you with megabytes of false data, while a higher threshold ignores data from weak reflections. As with any piece of real-world equipment, judging where to set it is an art based on experience. A suitable comparator circuit can be based on the design on page 75 of the December 2000 issue of SILICON CHIP. Whether you use an inverting or non-inverting comparator depends on the logic level required by your data logging software. An alternative comparator circuit suggestion is shown in Fig.2. The three diodes in series act as a 2.1V zener, preventing minor offset voltage variations in the op amp from affecting the optocoupler. The comparator circuit was built on a piece of scrap Veroboard and uses power from the XT supply. It’s well worth including an optocoupler on the data output to isolate the receiver from your computer. Keeping computer noise out of the receiver I could say I was lucky to find a cheap DEC 486 that was screened by a high-quality metal case but actually it took several weeks of searching classifieds and making phone calls to find one of that quality at a sensible price. Was it worth it? Unquestionably! Any computer with a unscreened plastic case will need lots of work to keep its RF emissions inside the box. As it was, even the superbly engineered DEC required a ferrite toroid on the data lead (scrap figure-8 speaker cable) as it left the case, along with a choke and filter capacitors at the receiver end. It also helped a lot to put the receiver on the opposite side of the room, as far away from the computer as possible and to keep the data lead well away from other leads. Yes, it would have been a lot smarter to use shielded data cable but I’m a slow learner. Software Data logging software is not all that difficult to write, though a medium level of programming ability is helpful. It needs to record when each event occurs and its duration. This means sampling the digital output of the receiver at regular intervals. Most observers sample every 10 to 40ms which is within the capabilities of even the slowest machines. It’s important to save to disk at regular intervals so that a minimum of data is lost when (not “if”) there’s a power failure or system crash. Save your data in a format that doesn’t leave you with hundreds of megs of data to wade through each month. It may be fun the first time but most people soon tire of unnecessary drudgery. Aim to keep your monthly files under 500Kb; that way they can be saved on a floppy with room to spare and then on newer systems you could set your BIOS to turn off the hard disk once the program is running. My software uses 16 bytes to record the time (expressed as the number of seconds after midnight on 1 January 2000) and the length of the burst in milliseconds. This is probably more detail than is needed but it only comes to a couple of hundred kilobytes a month and leaves open the opportunity to analyse the data in great detail should this be necessary. A conversion program summarises this data in a simple comma-separated variable (CSV) text file. Part of a typical summary looks like this: 355530, 355540, 355550, 355560, 355570, 355580, 355590, 355600, 355610, 355620, 0.012, 0.002, 0.003, 0.001, 0.005, 0, 0, 0.005, 0.001, 0.006, 11, 4, 5, 3, 5, 0, 1, 5, 2, 2, 0.2 0.025 0.075 0.025 0.2 0 0.025 0.175 0.025 0.35 The first column shows the start of the observation period in minutes since midnight UT on 1 January 2000. The second column shows the total duration of reflections during that period in seconds. The third column shows the number of hits detected during the period. The fourth column shows the duration of the longest reflection during the period, in seconds. Other observers directly record their data in this format, which is all that NASA’s survey requires. Re- Useful sources of inspiration and information Global MS-Net (details of observers’ setups): http://www-space.arc.nasa.gov/~leonid/Global-MS-Net/GlobalMSNet.html Monthly summaries: rec.radio. amateur.space newsgroup SILICON CHIP March 1998 issue: Building a 5-Element Yagi Antenna for FM Radio ARRL Handbook (any recent edition) for meteor scatter background info and tips on building Yagi antennas International Meteor Organisation (IMO): http://www.imo.net/radio/ Ilkka Yrjola’s meteor site (includes a preamp design and interfacing info): http://www.sci.fi/~oh5iy/ Society of Amateur Radio Astronomers (SARA): http://www.bambi.net/sara.html Meteor scatter communication and background: http://www.borg.com/~warrend/metburdu.html ABC FM station frequencies (sorted lists): www.geocities.com/meteorcount/abcfm.htm Wireless Iinstitute of Australia, VK5 Branch (2-metre preamp kit): WIA Equipment Supplies Committee, PO Box 789,   Salisbury SA 5108. http://www.sant.wia.org.au/esc.htm 12  Silicon Chip member that Universal Time is always used, so you must set the logging computer’s clock accordingly. Data analysis More talent is needed for writing data analysis software. Mere mortals can write something to produce simple monthly summaries such as the one in Table 1, while those with time and talent can create applications that show fancy graphs and hourly distributions. When you have a few days of observations on file, look for a variation in counts showing a peak at dawn and a trough around sunset. Fig.4 shows a typical day’s plot. As the Earth rotates on its axis, it encounters more cosmic debris around sunrise. By sunset you’re looking into the Earth’s wake, so there will be much less material to bump into. This daily cycle is a good way of confirming you really are observing meteors and not a neighbour’s arc welder in action. If all the fun of writing your own data analysis software seems like something you could do without, just use a spreadsheet to analyse your data. Versions of Lotus 123 are available as freeware these days and have powerful graphing and date manipulation functions. Another excellent freeware package is StarOffice, which is available for Linux as well as Windows. On my setup, Lotus is quicker to load and remarkably stable, so that’s what I use. All spreadsheet programs readily import CSV files. The graph in Fig. 4 was created from a CSV file using Lotus 123. Reliable data logging software for DOS is available from the website of Finnish meteor guru Ilkka Yrjola (www.sci.fi/~oh5iy/), along with masses of far more useful information than I could possibly provide here. Another good package with useful self-adjustment features is Meteor (radio.meteor.free.fr/us/accueil. html), though it helps if you can read documentation in French. Its companion analysis package Colorgramme (pierre.terrier.free.fr/ meteor/us/art.htm) is also available and this pair may well meet all your needs. R_Meteor (sapp.telepac.pt/coaa/r_ meteor.htm) is designed to be used with WinRadio cards or sound cards connected to a communications Table 1: Part of a typical observer's monthly summary 2000 Sep UT 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 —————————————————————————————————— 00 23 18 14 12 38 48 21 30 30 4 12 4 8 6 22 01 76 61 6 127 13 17 16 14 7 36 9 11 14 22 40 02 13 17 128 24 118 3 22 24 9 7 20 15 17 8 36 03 13 6 25 2 80 14 33 41 22 7 31 54 67 9 25 04 13 15 20 31 21 20 10 10 17 3 47 20 36 9 25 05 5 31 33 33 9 14 15 5 3 8 3 4 18 6 11 06 7 18 31 13 15 11 28 13 10 5 9 21 10 26 5 07 13 10 29 11 7 53 17 7 10 4 13 33 55 4 9 08 17 24 25 13 12 62 28 4 7 7 10 35 7 6 10 09 20 11 19 66 30 20 28 43 24 15 34 12 13 14 14 10 23 15 21 20 14 37 37 8 27 22 19 32 12 51 9 11 52 56 77 42 37 52 41 98 8 27 20 25 16 23 44 12 65 60 17 47 58 26 851? 24 27 19 5 12 20 31 23 13 27 46 26 18 22 36 839? 10 8 16 258 13 13 6 16 14 42 40 87 18 21 31 67 19 57 30 15 13 9 5 7 15 63 19 77 15 182 55 35 38 18 17 60 20 17 24 97 16 24 102 64 62 75 64 48 31 44 45 12 57 28 180 37 17 48 80 62 96 57 58 56 29 56 21 34 102 93 24 36 18 170 75 147 33 40 47 43 45 76 47 20 22 53 165 31 19 106 56 *E* 49 32 61 30 47 54 41 145 74 33 59 99 20 78 118 62 49 45 35 28 45 111 70 36 54 133 118 106 21 36 90 100 65 73 54 431? 41 78 60 42 128 23 57 119 22 105 50 84 80 61 36 29 16 38 41 87 107 29 34 10 23 41 16 19 91 28 33 14 19 20 9 22 22 13 17 17 ——————————————————————————————————— *E*denotes probable sporadic E ? denotes possible sporadic E (As published in the rec.radio.amateur.space newsgroup.) receiver tuned to a shortwave AM station. It displays the Doppler shifts of ionisation trails and other moving objects that cause reflections. (You can also use it to detect aircraft thousands of kilometres away but that’s another subject altogether.) If you can’t find a suitable VHF transmitter to monitor, shortwave techniques could be a good alternative. World Distance, by Eric J. van Drop, is a handy little shareware utility for Windows that calculates the distance between any two points on the Earth’s surface. Visit www.zdnet. com/ downloads/ and search for “distance”. Is it worth the effort? It certainly has been for me. I get huge satisfaction from making something unique from old bits and pieces. At times I had to brush up on theory I should never have forgotten and that can’t be a bad thing. From a geek’s viewpoint it sure is satisfying to hear the hum of a good computer in the background as it logs fiery events happening hundreds of kilometres away in the outer reaches of the atmosphere. Data pours in each day and at the end of the month there’s the challenge of matching up the summaries and graphs with various meteor showers, turfing out the bits affected by sporadic E and thunderstorms and then comparing it with those of other observers around the world. For software addicts, there’s that added attraction of knowing your analysis package will always have room for one more feature. But do remember that observing meteors is not a short-term proposition. Long runs of data spanning over several years, rather than weeks or months, are vital. While it’s exciting to hear those first bursts from the edge of space, you must be seriously committed to a sustained effort if your data is to be of any real use. By all means give it a try. Scour the Web and read widely before you begin and take it one step at a time. There is precious little specific help available but that makes it all the more satisfying when your system finally comes together. SC FEBRUARY 2001  13