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. Back-scatter, commonly called radar, is where the radio signal is broadcast from a transmitter at the same location as the receiver, 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 radiometeor@popastro.com.

Equipment

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: The choice of this may be influenced by the availability of suitable recording and analysis software, not all of which will work on the more recent types of PC, for example. Naturally the machine must have sufficient ports and facilities to cope with the signal input from your receiver too. 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 (see here for details). The computer needs to run special software such as Meteor V8.0 or Colorgramme 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. However, both require either DOS or Windows 98 (not XP or Vista) to run.

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

Transmitter Options

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. Observers hoping to use these must be aware that many TV services are migrating to digital broadcast signals, which cannot be used for meteor-detection, and the existing analogue services will not be around for many more years.
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.
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: Some observers have recently started to use a French space-surveillance radar as their target transmitter. The radar transmission is an unmodulated carrier wave, which is swept across the sky using a phased-array transmitter aerial.

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.

Once your report is ready, send it by e-mail to radiometeor@popastro.com. You may also wish to submit reports monthly to the Radio Meteor Observations Bulletins (RMOBs).

Radio Meteor Observing

Contents

Introduction