By Flavio Falcinelli
A very interesting activity is the radio observation of astronomical phenomena that influence the Earth's ionosphere.
In this case, you should not talk about radio astronomy, if by that term we indicate the observation of cosmic sources, outside the Earth's atmosphere. However, it is customary, at an amateur level, including in the field of radio astronomy also the analysis of astronomical phenomena that produce measurable effects in the radio spectrum, such as ionospheric perturbations induced by radio-meteoric events or the activity of the Sun.
It is interesting to illustrate the Meteor Scatter technique that uses a bi-static radar configuration to record the radio reflections produced by the ionized contrails that are formed (at about 100 km altitude) when very fast objects from outer space get consumed in entering the atmosphere Earth.
The image of the bistatic radar geometry was taken from the document: "Sistema a uso didattico per la ricezione di echi radar meteorici" - G. Pupil, C. Bortolotti, M. Roma - IRA 483/14. The spectrograms shown below are records of meteor echoes made by our experimental VHF station.
The receiver is tuned (typically in the VHF band, from about 30 MHz to 200 MHz) on the frequency of a powerful radio transmitter, far enough not to be normally received in direct wave because of the Earth's curvature. The transmitter constantly "enlightens" a large portion of the sky. Only when the meteoric track reflects or diffuses obliquely (forward scattering) the incident radio waves generated by the transmitter, they can reach the receiver and produce an echo radar.
The phenomenon (with a typical duration from fractions of a second to a few seconds) is studied by analyzing the evolution in time of the spectrum associated with radio reflection of ionized meteor trail (spectrogram), using free software available on the web. Measuring the received signal strength and its Doppler frequency shift, you can obtain important information on the source movement.
The system is conceptually simple and economical, to the reach of all: it is sufficient to have a good VHF receiver operating on the same frequency of the transmitter, an antenna (typically a yagi) and a PC equipped with appropriate software.
You can choose among several transmitters as sky "illuminators": commercial FM broadcast stations operating in the range 88-108 MHz (it is not always easy to do, given the in-band overcrowding), analog TV broadcasters (unfortunately there are few left, all in Eastern Europe), or dedicated transmitters as that of the French GRAVES radar, operating at 143.050 MHz and used to control the space debris in orbit around the Earth.
For our testing we opted for this choice.
It is very important that the transmitter guarantees a continuous service and it is at a distance, from the receiving station, comprised between 500 and 2000 km. It is desirable that the transmitted signal power is stable, at non-modulated continuous wave (CW) and that the transmitter antenna beam "illuminates" always the same area of the sky, without spatial variations.
Much documentation is available on the web that illustrates the principles and techniques of this interesting research, especially accessible for enthusiast. The following images illustrate our experiments. Further documentation can be found in the in-depth section.
Our experimental Meteor Scatter station operating at 143.050 MHz, optimized for receiving meteoric echoes produced by the French GRAVES radar. The receiver and antenna are built "ad hoc." The station operates continuously recording, at regular time intervals, the received spectrograms. Every day the data are downloaded from the station PC and analyzed. We are performing counts of events, spectral analysis and statistical analysis.
Technical characteristics of the receiving station:
ANTENNA: dipole with balun;
RECEIVER: at triple frequency conversion, with DDS as first local oscillator;
Output audio baseband [0-48 kHz], quadrature signals I & Q;
ACQUISITION: external 24-bit audio card, 96 kHz sample rate;
SOFTWARE: Spectrum Lab (free by DL4YHF).
Example of spectrograms documenting considerable meteoric events captured by our experimental station.
The graph shows the updated results of the counting of “radio meteoric” events recorded from our station.
The research highlights the main meteor showers: during the expected days of peak, the number of recorded events is much higher than the background value due to sporadic flow.
Here are some technical considerations on the research, which is still in progress.
As you can see "wandering" through the web, there are many excellent and varied experiments in this area, most led by radio amateurs. Hereby I will highlight the peculiar aspects of this project, with reference to the experience gained during the long and patient work of analysis of the results.
A lot of attention has been paid to the construction of a receiver "ad hoc" (very stable, without any automation in the frequency and gain control), centered on the 143.050 MHz frequency of the French radar transmitter Graves (as the crow flies, about 725 km from my station). The antenna is a simple specially built dipole. To attenuate the low antenna gain, a well-filtered pre-amplifier was included.
The large receiving lobe of the dipole is a great advantage: if suitably oriented with respect to the transmitter and positioned at a proper height from the ground, it allows the reception of meteoric echoes without too many limitations from the orientation point of view, safely when more violent weather events occur. When such antennas are coupled with a sensitive receiver, optimized in its pass band (only the one needed...), combined with a spectrogram acquisition software suitably configured as Spectrum Lab (excellent job by Wolfgang Büscher - DL4YHF), the results are great, at least for the goals of our project.
Even accepting a reduced antenna sensitivity, we could see how a sufficient distance from the transmitter (Graves, in our case), such as to prevent direct reception of its carrier (along with all the spurious radio echoes, including those caused by air traffic) and a perfect antenna-receiver pair are crucial to discriminate, without errors, the radio reflections caused by the meteors.
By choice, we avoided any automatic system for the counting of events: after many tests, we configured Spectrum Lab for a complex acquisition (I & Q) of the baseband channels of the receiver (acquired from an audio card external to the PC, connected via USB) with rejection of the frequency image, enhancing the resolution in the spectrogram frequency and setting a periodic and automatic data logging (a spectrogram every 3 minutes).
The use of TeamViewer software enables the remote control of the acquisition PC, with the ability to download every day the spectrograms recorded.
The following data analysis is visual: you need to patiently check the individual records to detect spectral "signatures" of radio-meteoric events. This approach, certainly more laborious than the automatic counting, is very accurate and reliable (it minimizes, above all, the counting of "false" events), even if it requires an initial period of "running-in" to gain experience.
Much more could be said.... We will talk about it in the next articles.
The tests go on.
The radio-meteors recorded every hour by our station were counted, in a day. You can see how the ultimate meteor stream happens to the early hours of the morning, while the minimum to the afternoon and evening hours: although the daily number of radio-meteoric events is characterized by large diurnal variability, the average ratio of the maximum and the minimum peak is of the order of 5. The number of events observed during the day will be greatest during the early morning hours, when the observer "impact" head-on with the meteor stream due to the motion of the Earth movement in space.
In this graph we were counted the most significant meteoric echoes (generically called "radio-fireballs") recorded in a month. The classification criterion is empirical, based on the visual analysis of spectrograms recorded every day byour fixed station: all events were counted, while only the pictures that show the spectrograms recording the more intense and longer lasting radio reflections were saved.
So interesting events deserve, undoubtedly, far more accurate and complete analysis of those shown on this page, which are to be regarded as a stimulus and a starting point to experience in this fascinating activity.
We want to highlight that it is actually easy and economical to install a permanent station dedicated to the study of radio-meteoric events through the Meteor Scatter technique in the VHF band, activities that can be coordinated with other observers, both in the visual, both in the radio, in order to verify possible correlations of events, especially during major meteor showers of the year.
In conclusion, we report some animations which show, in sequence, the most important monthly radio-meteoric events, for intensity, shape and duration. These records, which we will update regularly, are very important because they represent the "spectral signatures" of the observed phenomena and are the basis for their classification and interpretation.
In addition to the fixed station, it was built a different mobile receiving station, always tuned to the frequency 143.050 MHz, used occasionally for demonstration purposes during the recurring periods of major meteor showers.
The mobile station includes a yagi antenna tuned on the 143.050 MHz frequency and positioned on the roof of a car via an adjustable support. The receiver is the SDR type that receives the supply voltage directly from the USB port of a laptop used for the acquisition.
The system is very simple, practical and economical, suitable for demonstrations "on the field" when the major meteor showers of the year recur: it is interesting and educationally useful to show the evolution of the meteor phenomenon in the radio band in parallel to the traditional visual observations .
Recordings of remarkable meteor echoes acquired from our mobile station during the Perseids swarm 2016.
We report, in sequence, the most important radio-meteoric events registered by our mobile station in the month of August 2016 (Perseids).
The metallic structure of the International Space Station (ISS) is able to reflect the radio signal transmitted from the Graves radar.
From the following pictures you can see the reflection of the radio signal that shows the transit of the International Space Station (ISS) over the Marche skies looking on the Adriatic sea (Senigallia-Ancona). It is clearly visible the Doppler shift in frequency of the reflected signal due to the movement of the object. Since the antenna of the receiver system (mobile station with Yagi 3 elements) is vertically oriented and situated not far from the ground and in a buildings screened area, the system is only sensitive to metal objects (or ionized trail of meteorites) passing near the local zenith.