- Choice of Target Transmitter
- Receiver System Options
- Transmitter Options
- What to Record and Report
- Further Reading
|Figure 1: Colorgramme plot showing hourly radio counts detected by Bill Ward (Kilwinning) via forward-scatter from the GRAVES radar during December 2014.|
Radio meteor observing is technically challenging, but allows continuous meteor observations to be made regardless of the weather or daylight. To perform it, you will need a radio receiver. Unfortunately, most household radios, especially DAB ones, are not suitable for this. It will need to be fitted with an outdoor aerial appropriate for the type of observing you hope to accomplish, and the receiver connected to a computer to record your results. Ideally, the receiver and computer should be run continuously. If this hasn’t put you off trying, you are already part-way to becoming a radio meteor observer!
As explained in the Meteor Section’s booklet “Observing Meteors“, a meteoroid entering the Earth’s upper atmosphere excites the air molecules, producing a streak of light – the meteor – and leaving a trail of ionisation behind it. This ionised trail may persist for less than a second up to several minutes in the case of a rare, very bright, larger event. Reflections of radio waves from the ionisation trail make detection of meteors possible using a radio receiver.
There are two main radio observing methods, back-scatter and forward-scatter.
|Figure 2: A forward-scatter meteor echo, showing an ‘epsilon’ shape, detected by Alexei Pace (Malta) on 2013 Aug 14 (vertical axis shows signal frequency (1450-1600 Hz) ; horizontal axis covers 5 minutes).|
Back-scatter, commonly called radar is where the receiver is at the same location as the broadcasting transmitter and so the radio waves are ‘scattered’ (that is, reflected) back to where they started, after bouncing off a meteor’s ionisation trail.
In forward-scatter, the transmitter and receiver are separated often by hundreds of kilometres or more, so the broadcast signal is reflected forward to the receiver from a meteor’s ionisation trail, which must lie somewhere between the two places.
Amateur radio meteor observers use primarily the forward-scatter method, as being both more practical and less expensive. This webpage provides some suggestions for how you can make, record and report such forward-scatter radio meteor observations. While technical jargon has been kept to a minimum, or explained, radio work commonly uses terms which may be unfamiliar to amateur astronomers. For example, frequent use is made of acronyms from the world of amateur-band (HAM) radio enthusiasts, such as “Tx” for “transmitter” and “Rx” for “receiver”. This makes communication of ideas swifter and easier, but does require some familiarity. If you come across a term you do not understand, or simply need more advice about choosing, setting up or operating your radio equipment, please e-mail email@example.com.
Choosing the right equipment for what you hope to achieve in radio meteor observing is essential, and needs careful thought before-hand. Cost may be one key issue. Space for setting it up, leaving it set up and running all the time another, but the points considered here relate chiefly to the physical requirements for meteor observing.
Receiver: The ideal radio receiver for meteor work should have the following characteristics:
- Mains powered
- Synthesized tuning
- Options for Continuous Wave (CW) and Single Sideband (SSB) demodulation of the radio signals
- VHF coverage in the frequency range 30 MHz to 150 MHz
- An external aerial input socket
- An audio line output for connecting to a computer
- An adjustable audio filter for the signal, allowing it to be taken down to 3 KHz or less of bandwidth
Aerial: The aerial should be of an appropriate type and size for the frequency you are hoping to receive. It should be located to match the direction and orientation of the area in the sky from which meteor reflections should be received. These aspects will all be determined by the transmitter you intend to try to receive signals from.
Computer: Good news is that you don’t need an expensive high spec PC running the latest version of Windows. A PC from a decade ago will most likely suffice. The choice of computer will be influenced by the availability of suitable recording and analysis software. This associated software will quite probably have been designed to work from DOS or with Windows versions from the early 21st century and may well be incompatible with Windows 7 and 8. Naturally the machine must have sufficient ports and facilities to cope with the signal input from your receiver. Some of the software aspects are considered below under “Receiver System Options”.
Choice of Target Transmitter
Selection of a suitable target transmitter is fundamental to successfully operating a forward-scatter meteor detection system. For the system to work reliably, the distance between the transmitting and receiving stations needs to be right. Too close and you’ll receive the transmission continuously by non-meteoric means. Too far and you’ll not detect any signal reflections from meteors. As the meteor region lies approximately 100 km above sea level, using basic maths we can calculate the distance to this effective ‘radio horizon’, the maximum distance from the transmitter or receiver to the meteor, which is approximately 1150 km. Twice this distance, 2300 km, is the upper limit for the transmitter-receiver separation, sometimes called the baseline. If the baseline is greater than this, there will be no overlap between the two radio horizons, and thus no meteors will be detected.
Vertical section through the transmitter-receiver axis
The actual usable baseline depends on the power of the target transmitter. For a high power broadcast transmitter, a suitable range would be 600 to 2000 km. For a modest power amateur, or dedicated meteor, beacon, a suitable range would be 100 to 600 km.
Other transmitter requirements are that it should operate continuously, 24 hours a day, 365 days a year, and transmit in all directions equally. Such omnidirectional transmission allows the signal to illuminate the meteor region evenly. Coastal broadcast stations will typically concentrate their broadcast signals towards the populated coastal regions and may not be ideal.
The aerial choice for longer baseline observers is relatively straightforward. Reflections will originate from an area of the meteor layer where the two circles of approximately 1150 km radius centred on the observer’s location and the transmitter’s location overlap. The aerial should have a beam-width which matches this area as projected onto the observer’s sky.
Overlap area for a typical long baseline system
As the baseline decreases, the potential reflection area increases and extends across more of the observer’s sky. In this case, the receive aerial can be less directional and aimed at an higher elevation.
Overlap area for a shorter baseline system
As the baseline decreases, the potential meteor reflection area increases, extending across more of the observer’s sky. In this case, the receiver’s aerial can be less specifically directed, and may be aimed at a higher elevation.
Receiver System Options
There are three basic arrangements of automated meteor forward-scatter detection systems.
- System 1: The audio output from the receiver is connected to the audio input port of the computer’s sound card. The computer needs to run spectrum-analysis software such as SpectrumLab. The receiver should be operated in its upper side-band mode, with the receiver frequency slightly offset from the target transmitter’s carrier frequency. The software may be set-up to automatically detect and count either the increased audio level, or the frequency-change, that results from a meteor trail reflection. System 1 is relatively easy to implement. The PC must be equipped with a sound card. Unix users may consider using Baudline in the same arrangement.
Receiving System Option 1
- System 2: The audio output from the receiver is connected to the RS232 port of the computer via a special interface box . The computer needs to run special (free!) software such as Meteor V8.0 or Colorgramme(available via http://radio.meteor.free.fr/fr/en/ ) which will count the meteor signals detected. The receiver should be operated in upper side-band mode with the receiver frequency slightly offset from the target transmitter’s carrier frequency. The interface box allows the PC to detect frequency-changes in the audio signals from the receiver due to meteors. Meteor and Colorgramme will run on a basic PC, but the PC must be equipped with a serial data port. Note, however, that Meteor runs under DOS and Colorgramme requires DOS, Windows 98 or XP (not Windows 7/8 or Vista) to run. Instructions also exist for the running of Colorgramme under Linux (see Colorgramme under Linux ). In addition to counting the number of meteor signals received, observers may also want to investigate the frequency spectrum of the signals. This can be achieved via software such as HROfft (works well with Colorgramme). HROfft is free but you need to contact the Japanese author directly to get a copy (see HROfft availability ).
Receiving System Option 2
- System 3: This involves using a proprietary interface between the receiver and the computer, plus a software application running on the computer to allow the computer to monitor the received signal levels. This can be achieved by modifying the receiver to allow access to its automatic gain control (AGC) signal and equipping the computer with an analogue-to-digital interface to read the AGC signal level. There are numerous options available to implement a system of this type, but it will require reasonably advanced electronic and software skills. One slightly simpler option is to use one of the software-controlled radios and write a software application to interrogate the receiver, using serial-port data communication, to obtain the receiver’s signal strength information. This removes the need to modify the receiver.
Receiving System Option 3
These give some notes on the transmitter types currently available for use in radio meteor work from Britain.
- VHF Analogue Television Broadcast Stations: This uses the vision-carrier signal from a distant television transmitter. Unfortunately, most analogue TV services have in recent years been migrated to digital broadcast signals, which cannot be used for meteor-detection.
- VHF FM Radio Band II: Although ideally suited to System 3 meteor-detection, this band is not suited to either Systems 1 or 2. The VHF FM band is generally congested too.
- Amateur Radio Beacons: There are a number of 6-metre (50 MHz) and 2-metre (140 MHz) amateur beacons that are suitable for meteor work. The use of wavelengths to indicate frequency ranges is a common amateur radio convention, incidentally. One such example is the beacon operated by BRAMS in Belgium (see http://brams.aeronomie.be/ ), which can be monitored by observers in southern and eastern England.
- VVS Beacon: This is a beacon dedicated to meteor work, which operates on 49 990 kHz.It is a pure carrier-wave signal with circular polarization, and ‘upward’ beaming at 50 Watts, located at Astrolab Iris (50° 49′ N, 2° 55′ E) in Ypres, Belgium.
- Space Radar: Many amateurs now use GRAVES – a French space-surveillance radar near Dijon in central France – as their target transmitter. The radar transmission is an unmodulated carrier wave, which is swept across the sky using a phased-array transmitter aerial. It’s frequency is 143.050 MHz. The receiver should be set to CW. With GRAVES being a military set-up, detailed specifications for the radar system are not readily available. Hence meteor observers have had to deduce how its beams sweep across the sky. The belief had been that arc across which the beams swept was southwards from Dijon. However, there is considerable evidence, based on observations by Richard Fleet and others, to suggest that some of the transmissions “leak back” northwards and thus it should be possible to detect meteors over northern France and possibly also the English Channel using GRAVES as the “beacon”. This opens up the possibility of correlating radio detections with meteors imaged from the UK. Obviously, the further that you are located from the transmitter, the less favourable the geometry for picking up the echoes. However, as an indication as to the possible use of GRAVES from the UK, Bill Ward (located near Glasgow) successfully makes use of this transmitter.
What to Record and Report
Radio meteor observers generally record the number of meteor reflections detected in a fixed time interval, usually either an hour, or ten minutes if activity is high. The reflection counts should be recorded with the time in UT at which the recording interval started. To be scientifically useful, radio meteor counts should cover at least several days to either side of the expected shower peak activity being observed, and the receiving system parameters should not be changed while data is being collected.
Other information which can also be useful to record is the duration of meteor echoes found during the same time interval as the counts. The time of any particularly strong, or long reflections can be important too, as these may be produced by fireball-class meteors. It may be possible to correlate these with other types of observation of the same fireball later. A sequence of audio-spectrum images can be very helpful in this, as can a recording of the audio signal from the receiver as a WAV file. Both of these require a substantial amount of disk space to be available on the computer, however.
The observer should also report any unusual radio propagation conditions, or interference, that may have been noted.
In addition to the counts and the date and time information, each report should include the following information:
- The observer’s name and observing location, preferably giving both the latitude and longitude, and the name of the nearest town or city, and county if in Britain.
- The type of receiver used.
- The frequency observed at.
- The aerial type, the azimuth bearing of its horizontal direction in degrees east of north, and its elevation in degrees above the horizontal.
- The type of any pre-amplifier used.
- The observing system used – e.g. SpectrumLab, Meteor v8.0, etc.
- Any other remarks, such as interference or equipment problems.
Notes of caution: Radio meteor detection does not differentiate between shower meteors and sporadics and therefore your counts will represent the total meteor activity detected. The number of echoes detected will depend on many factors, including the sensitivity of your equipment, your distance from the transmitter, the direction of the shower radiant relative to the transmitter and your receiver, and, of course, the activity level of the meteor shower. The more sensitive radio systems are also capable of picking up echoes related to particles smaller than those that produce naked eye meteors and therefore the relative strengths of meteor showers seen by radio observers can differ from those seen by visual observers. Radio results can, for example, show the southern Delta Aquarids (rich in faint meteors) out-performing the Perseids (rich in bright meteors) during the first week of August.
- “Meteor Science and Engineering”, by D W R McKinley, McGraw-Hill Book Company Inc, 1961. Despite its age, this is still considered a classic reference by many radio meteor observers.
- “Proceedings of the Radio Meteor School 2005”, edited by C Verbeeck & J-M Wislez, International Meteor Organization, 2006. This details many of the technical aspects and physics of radio meteor observation and analysis. Although it concentrates on the use of back-scatter, which is easier to analyse the results from, forward-scatter is covered too, while the physics involved are the same for both.
- “Amateur Radio Astronomy”, by John Fielding, ZS5JF, Radio Society of Great Britain.
- “Meteor Burst Communications”, by Jacob Z. Schanker, Artech House.
- “Meteor Astronomy” by Sir Bernard Lovell, Clarendon Press, 1954. Now old, but a good technical read as to how radio meteor astronomy got underway.
- RMOB – Set-up to collect and publish monthly radio meteor data in 1993, this continues in the same format, and provides a means of contacting other enthusiasts to swap data and ideas for observing and analyses.
- Radio Meteor Observatories On-line – Contains “live” radio meteor data published on the Internet.
- Colorgramme/HROfft software example – A guide to the use of the Colorgramme & HROfft software.
- CMOR intensity plot Canadian Meteor Orbit Radar (CMOR) daily intensity. Be aware that although the timestamp regularly updates and the constellations move across the sky, the intensity are only updated once a day.
By David Entwistle (with updates by Tony Markham, Aug 2014)