What is a meteor radar

A meteoric radar is an apparatus that exploits a physical property of the atmosphere: when a body (meteor, debris, etc.) enters the upper layers of the earth's atmosphere at very high speed, it impacts the first molecule of the atmosphere it encounters along the I walk.

The consequence of this impact is the disintegration of the corpuscle which generates free ions and electrons. A long, narrow cylinder made of this plasma is then created along the meteor's trajectory. Its persistence is normally short-term, just the period of time needful for the recombination process of the ions with the free electrons to dissolve it.

For the duration of this short period the free electrons, if they are invested by an oscillating electromagnetic field (transmission) are induced to oscillate with the same transmission frequency and polarization. They therefore behave, in turn, like a transmitter, or if you like, like a repeater.

Consequently, from the radioelectric point of view this cylinder is a reflecting object, similar to an airplane, a satellite or any other flying object. This phenomenon is called “meteor scatter” (Figure 1). If there is a radio transmitter that illuminates a certain portion of the sky and a receiver, tuned to the same frequency, the receiver can pick up the signal emitted by the transmitter for the entire period of time in which the free electrons remain in this form, before their recombation with the ions thus dissolving the radiometeora.

Listening techniques

There are two types of meteoric radars: professional and amateur ones.

Professional radars are “bistatic”, ie they are made up of a transmitting station and one or more receiving stations. They are normally managed by research institutions or universities.

Normally these radars transmit modulated impulses on a fixed frequency and this allows the receivers, suitably placed on the ground at several tens or hundreds of kilometers away, to calculate all the meteors phisical characteristic parameters, namely: amplitude, duration, height, direction of origin and trajectory.

Obviously building or buying a transmitter of the necessary power is out of the reach of amateurs. Therfore, amateur observers listen meteors echoes using other people's transmitters.

Normally this listening takes place by tuning a receiver in SSB (Single Side Band Modulation) about one Kilohertz above or below the carrier frequency of the chosen transmitter.

Thanks to this trick, the receiver, in the presence of a meteoric echo (or of other origin), generates a sound equal to the difference in frequency between the received signal and the tuning frequency.

If this difference is one KHz, the radio emits a note (a whistle) on that frequency.

Almost all amateurs stop here, simply listening, or recording the video screens generated by audio signal plotting programs.

Counting and measuring radio meteors

But is it possible to do more?

Even without achieving the result of professional radars that are able to identify the trajectory of the meteor and therefore to attribute it to a swarm, it is possible to count the radio meteors to create a sort of HR (Hourly rate) radio, and moreover, is it possible to measure the echo duration  and amplitude, thus deducing a valid indication on the kinetic energy of the meteor?

The experience we have pursued over the years has therefore been to go beyond simply listening to the audio emitted by a receiver in SSB and try to measure the phisical quantities, record them on a log file and then analyze with computer programs representing them with different types of graphics. This work has led to the creation of RAMBO (Radar Astrofilo Meteorico Bologna) to which a section of this website is dedicated, containing the description and the data daily recorded in years of operation ( Click here for RAMBO. )

The RAMBO experience

As you can see in the appropriate page of this website, RAMBO exploits the signal emitted by the military radar transmitter of Graves (France, Figure 3) which transmits continuously in the VHF at very high power (the frequency is approximately 143 MHz).

Its transmission is directed upwards and therefore, both for this reason and for the opposite shielding from the Alps, it is not admissible from Bologna directly. Our receiver has a 10-element directive Yagi antenna (Figure 4) pointed in azimuth in the direction of the transmitter, and in declination at about 25 degrees, where we calculated to be the reflection point with the atmosphere  upper layers.

Our receiver is a Yaesu FT 857

The RAMBO electronics

For the analysis of the radio receiver audio output we have chosen the Arduino microprocessor with which we first sample the signal using Arduino as an analog to digital converter and then we perform the measurements througt it.

Over the course of months and months of attempts we have created various versions with the aim of both extracting the maximum of available information and eliminating the false positives (many) generated by the surrounding natural and anthropic environment.

With the sixth version of RAMBO we have finally reached up the ultimate project. With it Arduino no longer functions only as an amplitude meter (V meter), but simultaneously also acts as a frequency meter.

A RAMBo feature is given by the use of ArduinoYun, with this device, in addition to the operations listed above, we can perform the daily sending of the day's data as an attachment to an email.

Upstream of ArduinoYun we have created a board in which two differential amplifiers and a potentiometer for adjusting the audio level and seven LEDs to monitor the Arduino operations live have been inserted.

One of the recorded meteoric echoes is shown in Figure, in its representation as a function of time and the amplitude of the audio signal coming from the radio.

Note how this echo (hyperdense radio meteor) faithfully approximates the trend of similar hyperdense meteor recorded by the CNR of Vedrana di Budrio (Bologna, Italy) radar.

Beyond the RAMBo experience

The RAMBo experience was an extremely positive experience: with it we have recorded and measured almost a million meteors every year from 2014 until today.

We have calculated the RZHR of many showers, we have recognized the filaments of some meteor showers, with its data we have participated in two editions of IMC (the congress of the International Meteor Organization) and we have written a couple of publications.

However RAMBo has some limitations.

First of all it is difficult to reproduce: The sound card is our prototype, its assembly and its use require knowledge of electronics not present in all amateurs.

Moreover it is not cheap: the radio we use is extremely expensive and other devices are even more expensive.

Third limit: the measurement takes place on an audio signal generated by the radio: we can assume a strong proportionality with the real radio frequency signal, but we cannot be certain of this.

For this reason we have tried to develop a receiver that exceeds these three limits, and for this reason we have entered the SDR world.

What SDR is

A radio set up as we know can be defined as a set of analog components that each perform a certain function.

For example, if we think about the classic superheterodyne radio we find: an input amplifier, a local oscillator, a mixer, a band filter, an intermediate frequency amplifier, a double half-wave rectifier and an integrator.

Then, got the "low frequency", again an amplifier.

All these functions, although performed by analog devices, are actually mathematical functions: these are multiplications, divisions, subtractions, trigonometric function generators, integrators, etc.….

As is known, to multiply, subtract, divide, generate sinusoids or integrate and derive, today it is much simpler and more efficient to use the computer than an analog circuitry.

The SDR (Software Defined Radio) in fact performs this function: once the antenna signal has been digitized, it is processed by a computer using special algorithms that allow the computer to be used as a radio.

The first SDR studies date back to 1970 in the USA and the first SDR transceiver was made in Germany in 1988.

In the 2000s, the advent of the RTL2832U processor made it possible for thousands of amateurs and enthusiasts to access the world of digital radio.

Today the SDR devices are in all televisions, digital radios and smartphones.

CARMELO (Cheap Amatorial Radio Meteor Echoes LOgger)

In recent years, the Python RtlSDR library has been developed.

With this library it is possible to write Python applications that allow a computer to interface with a Dongle equipped with an R820T2 tuning circuit and with the RTL2832U. The combination of these two integrated devices allows both the tuning and the sampling of the radio signal.

Once we developed this program and tested it on a computer, we chose an inexpensive Raspberry to develop a suitable and inexpensive device.

After a few months of experimentation, we can say that the device is solid (it never crashed) and has never stopped.

Carmelo works as a radio receiver which is always tuned to the selected broadcast station. It continuously analyzes all the signals that "emerge" from the background noise and through the FFT (Fast Fourier Transform) records the waveform of the signal reflected by the radiometeora, then write a log file.    

Obviously the sensitivity of such a simple device does not reach the performance of a professional radio like the Yaesu FT 857 or other more modern and expensive radio equipment. Hence the idea that "unity is strength" and that is that multiple receivers of this type spread throughout the territory can, once networked, provide such coverage as to make up for the lower sensitivity with a greater contribution of observations.

For this reason we propose to use for this network, unlike RAMBo which receives with a directive antenna oriented towards the transmitter, the use of simple omnidirectional dipole antennas, which in addition to being cheaper and easier to assemble, cover the territory in a homogeneous and non-directional way.

This will allow to have congruent observations between the various receiving stations, allowing the comparison between the various observing points.

CARMELO network

The astrofiliabologna.it website was also created to host the observations regarding the radio meteor echoes made with Carmelo.

The program loaded on the Raspberry mini computer, every time a meteoric echo is recorded, sends the corresponding log file to the site via ftp. On this site there are specific pages that will report in graphic form the observations from wherever in the world they come.It is known that the meteor observation, as well as astronomical observation in general, is obviously characterized by the observer's position on the globe as well as by the time and day. These data determine the horizon and therefore what is observable and what is not. It follows that the observation of a given swarm can be optimal in Europe and impossible in the US this year, while another year it can be at the opposit and so on. A global network, with the data considered as a whole, can record meteoric events regardless of the geographical positions of a single observer, but acting as if there were a single "observer earth" that meets and records the meteor showers it encounters along its path within the solar system. Creating such a global network requires a certain number of enthusiasts willing to join the network, and a simple, robust and economical project within everyone's reach, even those who are not radio technicians. In our opinion Carmelo summarizes these three characteristics in himself and can therefore aspire to be the instrument for this type of global observation. 

Below is a description of how to make a CARMELO; if you are not interested in self-construction you can skip to the last paragraph.

1) First of all, you need a known transmitter that is in continuous operation on the VHF (Very Hight Frequency) that is the radio band in which meteoric echoes have always been observed. This transmitter must emit on a known frequency, it must be tens or hundreds of km away from the receiving point and possibly must not be in sight. If we are tuned to Graves, which transmits in circular polarization, the polarization in reception is not very important, but the vertical one is better: both because it implies a simpler assembly, and because, on average, the meteoric reflections are higher in vertical polarization.

2) The receiving antenna must be placed in a point as free from obstacles as possible. Obstacles are essentially manufactured or steep mountains in the immediate vicinity that severely hinder the observation horizon. Thus, the best location can be the roof of the house, but it can also be in a large garden, if there are no buildings close to it. For example: reception on a low floor in a city district involves the loss of a lot of signals.

3) About the antenna installation: the choice we propose for this project is a vertically polarized omnidirectional antenna built to receive the transmitter frequency. This type of antennas is also the simplest and cheapest. They range from "discone" type antennas whose price is around 80 euros, up to balcony antennas that cost less than 30. Do not forget that the antenna is the most important component of this project: do not save neither on the purchase nor on the mounting position

Self-construction should not be underestimated at all.

The self-construction of a "ground plane" type antenna has two advantages: first of all, it is very cheap, since it can be built with a few euros.

Then, it seems absurd but it is not: its efficiency is better than commercial ones.

The explanation of this apparent paradox lies in a simple consideration: commercial antennas are almost never designed to operate on a single frequency but are used to allow radio amateurs to receive (and transmit) on many channels.

An antenna is equivalent to an LC circuit whose electrical characteristics determine the frequency on which it resonates.

A “pure” LC circuit has a narrow and high Gaussian curve of the Q factor (similar to gain) on the central frequency.

A “loaded” LC circuit distributes the gain on a greater basis: the Gaussian is much wider, thus allowing us to tune effectively also on the adjacent frequencies, but also lower and therefore loses gain at the central frequency.

A final consideration: commercial antennas are also designed to transmit and this is the feature that most affects their price due to the necessary construction accuracy; for simple reception no sophisticated antennas are required. To confirm the aforementioned reasons, an homemade ground plane like the "Carmelina_143" proved to be more performing than all the other commercial omnidirectional systems we have tested.

5) The dongle must be based on the RTL2832U. We have tried a few, what we recommend is the NooElec NESDR Smart v4 SDR, it costs 32 euros, is stable and reliable and it is the one with the lowest noise among those we tested.

6) As mentioned, the minicomputer adopted is the famous Raspberry Pi 4 B, 2GB is fine: on it we have to install a:

7) mini SD board it does not matter that it is high capacity: all our software need 2 Gigabyte.

8) The mini computer must be powered with a 5V power supply with USBc socket (i.e. that of second generation smartphones). Warning: Carmelo absorbs about 75 milliamps which at 5 v means a power of less than 400 mW.

9) Last but not least: you need a LAN cable that connects Carmelo with a modem that allows its access to the Internet. This cable can also be particularly long, indeed the advice we give is to place Carmelo as close as possible to the antenna and to reach the modem with the necessary length of network cable. We have not enabled the wireless function: after all Carmelo is still a radio receiver and the fewer transmissions there are in the immediate vicinity the better is.

The total of these purchases amounts to approximately 200 euros.

Hardware

CARMELO hardware consists of a common internal part and an two options for external, depending on the antenna chosen.

The components list of the inner part is as follows

1 - Raspberry Pi 4 model B 2GB
2 - NooElec NESDR Smart v4 SDR
3 - Rg 54 coaxial cable sma female – sma male
4 - Lan cable
5 - Micro SD card (recommended 16 GB)
6 - Plastic support
7 - Project picture
8 - CARMELO board
9 - 4 screws - 2x6 mm

When assemble the components, pay attention to the fact that the contacts are perfect: both that of the antenna cable, both the usb and the LAN.

Fix the components and the cables to the plate: in this way any traction on the cables will not lead to variations in the contacts: always keep in mind that CARMELO is not a normal microprocessor, but its union with the dongle makes it a real radio receiver.

If you have the micro monitor, plug it in the multi connector. Monitoring isn’t essential: with or without CARMELO it works anyway.

When everything is fastened turn on the device via the power supply connected to the micro usb. If you have the micro monitor you will notice that the yellow LED lights up dimly. Then, as soon as the operating system and the program have loaded, all three LEDs will light up. When all the Python libraries are correctly loaded the red LED turns off. When the dongle is revealed the yellow LED is turn off. Then when the receiving station data file has been loaded the last one turns off and CARMELO sends a message to the server saying that it has been turned on, and then starts to work.

Every 200 samplings CARMELO calculates the noise average and turns on the green LED for a while. In that way you can see that CARMELO works correctly. Whenever a sample satisfies the set criteria (power above the threshold and frequency close to that of the transmitter) the yellow LED lights up. If the reception comes from a real meteor echo CARMELO writes the data file and turns on the red LED for a while. Then the sequence resume normally.

The dongle produces heat, don’t worry if you feel is hot: its two ICs work by producing a Fast Fourier Transform every 33 milliseconds!

The external part of CARMELO is divided into two options according to the type of antenna chosen: directive or omnidirectional..

In the case of a directive antenna, the components list is as follows

1 - Yagi antenna
2 - Flange for inclination
3 - Antenna mast
4 - Support for antenna mast
5 - Pl male to sma male adapter

In the case of an omnidirectional antenna, the components list is as follows.

1 - Dipole rod
2 - Male PL connector for dipole rod
3 - Sma female jack to pl female plug connector with 4 holes flange panel
4 - Sma male to sma male adapter
5 - octagonal stainless steel plate
6 - N° 8 aluminium slats
7 - N° 4 screws 3x6 M whit washers and bolts
8 - N° 4 screws 4x18 M whit washers and bolts
9 - N° 12 screws 4x10 M whit washers and bolts
10 - Table leg 250 mm heigh
11 - Antenna mast
12 - Support for antenna mast

Software

The operating system must be loaded on the SD card (Pi OS Lite 32 bit), than the necessary libraries and our installation file must be installed, in the end it will have to be customized by entering the station data (different for each observer). All this is described step by step in a file that will be sent to you upon request by contacting us at carmelometeor at gmail.com (remember to replace at with @).

How to buy a CARMELO finished

If you don't want to build the device yourself all the necessary materials – both individual components and the complete kit - will be available for purchase contacting us at carmelometeor at gmail.com (remember to replace at with @).