Download A brief introduction to the METEORSCATTER-THEORY

A brief introduction to the
METEORSCATTER-THEORY
Meteor Scatter (MS) is known as one of the most exciting ways to make DX contacts in VHF bands.
MS is based upon Scattering and Reflection from ionized trails produced by Meteoroids entering the
Earth’s atmosphere. This phenomenon allows, under precise conditions, long distance VHF contacts
every time during the year.
MS has been used since the 1950’s for low throughput data communications in professional and military
radio links, but soon it became of interest by Radio Amateurs to “Make More Miles on VHF”...
A brief and simplified description of MS physics will follow.
What is a Meteor?
Space around earth is not completely empty, but always crossed by dust and debris, with dimesions
ranging from microscopic particles up to big stones; these particles are generally called Meteoroids.
Some of these meteoroids are able to enter the atmosphere due to gravitational effects, when their orbit
approaches or crosses that of the earth.
When a meteoroid enters (at high speed) atmosphere layers with sufficient density, it dissipates its
energy and substance in a brief blaze, due to friction. The visual result is the so called “shooting star” or
Meteor, a spectacular event that we all have experienced, watching the sky in clear nights.
When the meteor is particularly brilliant it is usually called a Fireball, and if the object appears to
explode is called a Bolide. Very rarely, a residual fragment of a large meteoroid may survive the impact
with atmosphere and drop to the ground as a Meteorite.
This is a rare event, anyway. The most part of meteoroids are very small particles – the smaller the
dimensions, the larger the amount.
Smaller meteoroids are called Micrometeorites. These can be so small that, while entering atmosphere,
they don’t loose too much mass and do not even reach the point of incandescence.
So the smaller particles cannot be detected by means of optical or radio methods.
Bigger particles can instead be observed visually, or by means of photographic, telescopic or radio
techniques. Over entire earth, it is estimated that from 100 to 200 million visible meteors burn themselves
out daily in our atmosphere.
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Sometimes earth crosses some areas in space, where the density of particles is much higher than average.
These zones are streams of particles that encounter earth’s orbit periodically, and the result is a significant
increase of meteors impacting the atmosphere. This phenomenon is known as Meteor Shower.
A Sporadic or Nonshower Meteor is one which is not a member of a recognized shower. Sporadic
meteors are entering the atmosphere continuously during the year, creating a sort of meteor rate
background, having daily and seasonal variation.
The total Meteor Flux is determined by the contribution of Shower and Nonsower meteors: if a shower is
active, its flux is superimposed to that of sporadic meteors.
So the Meteor Rates (the number of meteors detectable by an observer in a specified time interval) is the
sum of sporadic meteor rates and (eventually) the shower meteor rates (if there is a shower active in that
time). In case of multiple showers, total rates will be the sum of all shower meteor rates, plus the sporadic
meteor rates.
The Radiant of a meteor is the point where the meteor path intersects the celestial sphere, so it is the spot
in the sky from which the meteor appears to have come. A similar definition is also valid for showers:
shower meteors , departing from a single spot , are seen crossing the sky in all directions. Their paths,
extrapolated backward, all appear to originate in a common point, known as Shower Radiant. This is an
effect of perspective, since all shower meteors are actually moving in parallel paths.
Sporadic Meteors
Basically, there are six known sources of sporadic meteor activity. They are known as Northern Apex,
Southern Apex, Helion, Antihelion, Northern Toroidal and Southern Toroidal sources. They
correspond to relatively wide specific sky areas, where the sporadic meteor radiants are observed.
Anyway, it must be noted that an appreciable number of sporadic meteors are due to contribution of
minor or unrecognized meteor showers.
The six sources are determined by meteor orbits. The plane containing meteor’s orbit is tilted at an
Inclination i to the plane of earth’s orbit, called Ecliptic.
If i<90° the motion of the meteor is said to be direct, that is, in the same direction of planets. If i>90° the
motion is termed retrograde.
The two Toroidal sources are related to meteors coming with orbits that are highly inclined to the
Ecliptic. Helion source is located close to the Sun’s position in the sky, while Antihelion is located in the
opposite portion of the sky from the Sun, so its activity can be found at night, produced by low
inclination, direct motion meteors.
The Apex sources are due to meteors orbiting in a retrograde motion, encountering the earth in a “headon” direction, while our planet is moving through space. The Apex of the earth’s way is the point of the
celestial sphere toward which the earth is moving at any moment.
Apex sources are characterized by a double radiant located 15 degrees north and 15 degrees south of the
Ecliptic, 90 degrees west of the sun.
Since we can observe meteors (whether visually or by radio) only if their radiant is above observer’s
horizon, Apex sources’ sporadic meteors are detectable only after radiants rise (after local midnight) and
their best rate is in dawn / early morning (local time of the observer), when the radiants reach their
highest elevation in the sky.
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Since Apex sporadic meteors play a significant role in determining total sporadic meteor flux, we can
understand why sporadic rate is higher in morning hours. Imagine the earth as a ball moving in a dusty air
environment. Most impacts with dust particles will obviously occur in the frontal surface of the ball (that
oriented in the direction of motion), instead that in the opposite surface.
Due to rotation of earth around its axis, an observer on earth will find himself on the surface directed
toward motion, only once in a day: in early morning hours (in his local time). So in that time he will
detect a daily maximum of sporadic meteors rates, for the reasons mentioned above. It must be also kept
into account that, for meteors orbiting in a retrograde motion as for the Apex sporadics, the relative
velocity during impact is higher in the “head” portion of earth’s atmosphere. So even small meteoroids
can “burn” easily and so can be detected by means of visual/radio techniques in these conditions, thus
increasing measured meteor flux.
In first approximation, the two major factors affecting sporadic meteors flux are the position of Apex in
the observer’s sky and the actual distribution of meteoroids around the earth.
Assuming a homogeneous distribution of meteoroids around earth, since the Apex corresponds to the
portion of the earth's atmosphere which has the highest mathematical probability of interesecting (or
capturing) meteoroids in a variety of orbits, the higher the Apex elevation is in the observer’s sky, the
higher the sporadic meteors rate is.
The Apex reaches its highest point in early morning (local time of the observer), so this is an alternative
way to explain the diurnal variation in sporadic rates, with the highest sporadic rates occurring in early
morning, and lowest rates occurring in early evening.
Apex position affects also seasonal variation of sporadic meteors. For a northern emisphere observer, at
latitudes greater than 23.5 deg N, an annual peak in sporadic activity will be observed around the
Autumnal Equinox (about September 21). At this time, the Apex point will be above the observer's
horizon for the longest number of hours each day. A corresponding minimum in
sporadic rates will occur around the Vernal Equinox (March 21) each year.
In the southern emisphere, observers below 23.5 deg South would have an annual peak corresponding to
the Vernal Equinox, and a minimum corresponding to the Autumnal Equinox, the opposite of the
northern hemisphere.
So, in first approximation, Sporadic flux vs. Time can be represented by superimposing two quasisinusoidal curves: a short period sinusoid representing daily variation, superimposed to a long period
sinusoid representing seasonal variation.
Anyway, the real meteoroids distribution is not homogeneous, and both shower and non-shower meteors
appear to have a noticeably higher density in the later half of the year, and a lower density in the first half
of the year. Keeping into account minor / unrecognized showers meteors contribution, the annual peak
will be placed even earlier in the summer for northern observers: yearly maximum of sporadic activity (in
the northern emisphere) is around July-August-September, the yearly minimum in February-March.
Please note that sporadic rates indicated above are expected rates! Using a statistic approach, they may
be considered the average level encountered after a couple of observations in the same period each year.
Actually, in a single observation, significant deviations from expected rate may be encountered time to
time. High probability does not mean certainty! That applies also to Meteor Showers...Anyway, if
observations are long enough, one can verify that the mean sporadic rates are really converging to what
predicted using this mathematical model.
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Meteor Showers
During its motion into space, earth sometimes encounters dense streams of dust and debris, able to
increase significantly the number of meteors entering the atmosphere, originating a so-called Meteor
Shower.
There are hundreds of known meteor showers; most of them are minor (or even dispersed) showers,
characterized by a meteor rate barely distinguishable from that of sporadic background.
On the other hand, some spectacular showers are able to increase meteor rates dramatically (by a factor
100 or more) in some periods.
The debris associated to meteor showers, have a cometary origin. They are shed by comets (or even minor
planets) while orbiting relatively close to the sun, and then spread out along the entire orbit of the comet,
thus forming a Meteor Stream.
Meteor streams orbits follow somehow that of comets, but due to gravitational effects they can evolve in
complicate ways. For example, a close approach to a planet makes some meteoroids accelerate and some
others to decelerate. The result is the formation of gaps or high density clusters inside the dust trail.
Gravitational effects by big planets (Jupiter first) can also affect the distribution of meteoroids in a cross
section of the stream.
All that means that a dust trail is typically not homogeneous, neither in transverse nor longitudinal
section: that contributes to make some showers meteor rates vary significantly with time.
Resonance of stream’s orbit with that of Jupiter tends to keep meteoroids in a particular relative position
in respect to Jupiter itself. As a result, a stream component called Filament is originated.
Last but not least, radiation pressure of the sun tends to blow away smaller particles, leaving only bigger
particles in the dust trail. That explains why some showers are rich in bright meteors, while others are
rich in faint meteors. Typically, long-established streams have an excess of bigger meteoroids, thus
originating brighter meteors.
Most ancient meteor streams tend to broaden, until they become dispersed streams; in addition, some dust
trails are not able to intersect earth’s orbit anymore, since gravitational effects have deviated their orbit.
On the other hand, many younger streams are travelling in elliptical orbits, intersecting that of earth
regularly, in the same period every year (annual meteor streams /showers). Other streams encounter the
earth with a periodicity greater than 1 year (Periodic Showers); the most famous among these is Leonids
shower, having a periodicity of 33 years.
A shower is usually named after the constellation in which the radiant appears. If more than one radiant
happens to occur within a given constellation, the name is prefixed in each case by the Greek letter
designation of the nearest prominent star.
As we have seen, a shower may appear in different ways. The cross-section of the stream may have one
or more high-density cores, thus leading to one or more maximums of meteor rates when earth’s orbit
intersects them. Even mass distribution of meteoroids may vary both in cross and longitudinal sections of
the dust trail. Some showers have a very broad maxima, while others have a very sharp peak of activity.
Some are rich in bright meteors, some others only exhibit faint meteors, and so on.
To characterize a meteor shower, several parameters have been introduced. Some of them are timedependant. Here are the most significant:
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Date / Time of the maximum: usually it is given in terms of Solar Longitude (λsol, equinox 2000), a
precise measure of the earth's position on its orbit, which is not calendar-dependant. Actual calendar date
may be derived from λsol, using proper conversion tables.
Duration of the shower: The Duration (in days) is a measure of the sharpness of the peak and is roughly
the interval between the times when the shower activity is one quarter of the maximum meteor rates. It
should be noted that this duration interval may be asimmetrically disposed around peak time, since the
activity of many showers displays a slow rise and a rapid fall.
Radiant Position: Celestial coordinates (RA, DEC) of shower radiant, at the time of maximum. The
radiant drifts slightly across the sky each day (typically at the rate of about 1 deg in RA per day) due to
earth’s own orbital motion around the sun.
Geocentric Entry Velocity, V∞: Apparent meteoric velocity entering the atmosphere, given in km/s.
Velocities range from about 11 km/s up to 72 km/s.
Population Index, r: Takes into account shower's meteor magnitude distribution, and is somewhat related
to mass distribution of meteoroids. r = 2.0 - 2.5 is brighter than average, while r above 3.0 means fainter
than average.
Zenithal Hourly Rate, ZHR: Given in terms of meteors per hour, it’s the calculated maximum number of
meteors an ideal observer would see in perfectly clear skies if the shower radiant were overhead at Zenith.
A visual observer estimates ZHR applying a correction factor to the number of meteor detected, keeping
into account the actual elevation of radiant above horizon.
A radio observer is also able to calculate ZHR, once he has measured Radio Hourly Rate (RHR), using a
scaling formula, which keeps into account the Antenna Beamwidth and the Limiting Magnitude of the
receiving system.
ZHR is a measure of shower’s meteor rates, that is, the number of meteor one can observe in a specified
time interval. Minor showers are typically characterized by ZHR < 10, while bigger showers exhibit ZHR
around 100 or even much more in case of meteor storms.
In relatively rare cases, a shower’s meteor rate reaches significantly higher values in comparison to its
normal level of annual activity. This phenomenon is known as Outburst, and is capable to increase meteor
rates up to storm level in best cases.
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Data for 5 major (annual) showers of the year:
Quadrantids (Bootids)
Parent objects: Minor planet 2003 EH1, comets C/1490 Y1 and C/1385 U1
λsol,max = 283°16 (beginning of January)
Duration of peak = 0.5 days
Radiant (RA,DEC) = 230°, +49°
V∞ = 41 Km/s r = 2.1, ZHRtyp = 120 hr-1
η Aquarids
Parent comet: 1P/ Halley
λsol,max = 45°5 (beginning of May)
Duration of peak = 10 days
Radiant (RA,DEC) = 338°, -1°
V∞ = 66 Km/s r = 2.4, ZHRtyp = 70 hr-1
Arietids (Daytime shower)
Parent comet: 96P/ Machholz
λsol,max = 76°7 (early June)
Duration of peak = 20 days
Radiant (RA,DEC) = 44°, +24°
V∞ = 38 Km/s, ZHRtyp = 60 hr-1
Perseids
Parent comet: 109P/ Swift-Tuttle
λsol,max = 140°0 (mid August)
Duration of peak = 5 days
Radiant (RA,DEC) = 46°, +58°
V∞ = 59 Km/s, r= 2.6 ZHRtyp = 100 hr-1
Geminids
Parent object: Minor Planet 3200 Phaeton
λsol,max = 262°2 (mid December)
Duration of peak = 6 days
Radiant (RA,DEC) = 112°, +33°
V∞ = 35 Km/s, r= 2.6 ZHRtyp = 120 hr-1
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Radio Echo Theory
When a meteor enters the atmosphere, the kinetic energy of the meteoroid is converted to heat, light and
ionization by the collisions with air particles. Atoms of the meteoroid are vaporized from the surface of
the parent body, during a process named Ablation.
During the ablation of the meteoroid, meteoric and atmospheric atoms are ionized, creating a trail of ions
and free electrons. These charged particles are responsible of reflection / scattering of radio waves
incident to the trail.
The electric field vector of the incoming wave makes the charged particles vibrate, and oscillating
charged particles emit electromagnetic waves. The absorbed energy from the incident radio wave is thus
re-emitted in all directions. Theoretically, both ions and electrons in the trail contribute to the scattered
signal. However, the ions are too heavy to produce a significant contribution, especially at higher
frequencies, so only free electrons will be taken into consideration.
This mechanism allows (using proper techniques) long distance radio contacts even in cases when the
radio link is normally not possible, due to curvature of earth and lack of other ways of propagation. Radio
Amateurs have succesfully used MS since many years, for DX contacts in bands ranging from 28 MHz up
to 432 MHz.
The same mechanism allows radio observers to detect a meteor entering the atmosphere: tuning a radio
receiver to a frequency where a known distant transmitter is normally not received, the arrival of a meteor
is marked by a brief interval of reception of the signal coming from the distant transmitter. Scientists
carry on studies on meteors using radars, analyzing their own-produced backscatter echoes.
Echo duration is tipically brief, and geometry/frequency-dependant: it ranges from fractions of a second,
up to minutes in best cases. Signal is scattered until free electron density in the ionized trail remains high
enough for the given operating frequency, to support wave scattering / reflection.
Free electron density decreases with time, due to electrons’ diffusion, recombination with ions and
attachment to neutral atoms.
The reflection mechanism depends on the density of free electrons in the trail; two extreme cases
(Underdense and Overdense trails) will be analyzed, keeping in mind that real meteor trails can be
typically viewed as intermediate cases between these two.
Underdense Trails: are produced by smaller meteoroids. In this approximation, the free electron density
is so low that the radio waves can penetrate the trail without attenuation. Each free electron scatters the
incoming wave individually, and the total signal received down the trail is the sum, taking into account
the phase, of the signals coming from all individual electrons.
A pure scattering occurs in this case. The resulting echo is also named Ping.
The greatest contribution to scattering, takes place in the volume where the electron density is higher (the
trail core and the meteor head). The resulting received power, besides other factors, is dependent of the
position and orientation of the trail, and of the wavelength (received power increases strongly as
frequency decereases, and also echo duration is higher).
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At any time, the free electrons show a circular Gaussian density distribution around the meteoroid path.
Immediately after the trail is formed, the electrons and ions start diffusing in the surrounding atmosphere,
thus decreasing the volumetric charge density. As the radius of the trail grows with time due to diffusion,
phase coherence between scattered waves is lost, thus decreasing (exponentially with time) the total
received power. Meteoroids that are relatively small and fast, tend to ablate at greater heigths, where
atmospheric density is low enough to allow the trail radius grow fastly; for this reason they will provide
shorter and weaker echoes.
The typical profile of an echo from an underdense meteor (Received Power vs Time), is characterized by
a fast rise of the signal and an exponential fall. The rise time carries information about the velocity of the
meteoroid, while the exponential fall is the result of the diffusion of underdense trail in the atmosphere.
The time constant and decay time is related to atmospheric density, and consequently to the altitude of the
reflection point. The higher the ablation height is, the faster the decay time is and the shorter the echo.
Overdense trails: are produced by bigger meteoroids. When the electron density is high, the central part
of the trail behaves like a plasma. Radio waves cannot penetrate the core of the trail anymore, and are
scattered off the trail. The meteor can be approximated by a metallic reflecting cylinder. The incident
radio waves can reach the cylinder without attenuation, but are reflected on its surface. Total reflection
takes place on the cylinder in the trail in which the electron density is higher than a specific Critical
Density (function of wavelength).
In this case, a specular reflection occurs. The resulting echo is also known as Burst, and the associated
received power is, once again, strongly dependant by position and orientation of the trail, and by
frequency (lower frequencies yeld stronger signals).
Typical overdense echo shape may be explained by interference. A few kilometers of the trail actually
contribute to the reflected signal (it is thus more appropriate to talk of a reflecting section instead of a
reflecting point). Waves are reflected from several areas of the trail, but constructive interference only
occurs in the direction predicted by geometrical optics. Main reflected signal (that is, the reflected signal
with shortest optical path) will interfere with signal reflected by more distant areas, thus having a
difference in amplitude and phase. In this way zones of constructive and destructive interference with
main signal (named Fresnel zones) can be defined. During a meteoroid’s flight, alternately constructive
and destructive parts of the trail are formed, leading to a characteristic “ripple” on the echo shape. If full
geometry of the reflection is known, the ripple’s periodicity can be used to derive meteor velocity.
Another way to measure meteor velocity (also in this case, once full geometry is known) is to analyze
Doppler frequncy shift, often found on “head echo” MS reflected signals.
Due to subsequent diffusion of the trail, the radius of overdense core inside the trail increases with time
(and that tends to increase reflected power). However, simultaneously, the central density of the trail
decreases, which will eventually make the radius of the overdense core shrink, until the overdense part
totally disappears and only a big underdense trail is left. As a result, a short overdense meteor echo shape
(Received Power vs. Time) is characterized by a fast rise, a slow increase (due to the diffusion ) with a
superimposed ripple, and then by a decrease (when the ion density gets too low). When the overdense
core of the trail has vanished, a typical undredense meteor echo fall (with exponential decay) will occur.
In case of enduring overdense echoes, strong oscillations in received power are often encountered. This is
caused by high atmosphere winds distortion of the meteor trail. As a consequence, multiple reflection
point can appear in the trail, leading several reflection contributes to interfere. As the reflection points are
not stationary due to wind shear, interference conditions change continuously, thus originating a deep
fading on the received signal.
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This phenomenon can only be observed on longer bursts. Shorter overdense meteors echoes are instead
characterized by a duration which, in first approximation, is proportional to the square of wawelength.
That means, for example, an overdense meteor capable of provide a 1 s Burst on 144 MHz, will have an
echo duration of about 10 sec on 50 MHz, but only about 0.1 sec on 432 MHz!
Anyway, also geometry of reflection must be taken into account to determine received power / duration of
the echo.
Geometry of Meteor Scattering
As we have seen, a meteor is capable of scattering an incident radio wave off its ionized trail.
Anyway, taking into account relative positions of trail with respect to transmitter and receiver, it must be
noted that a meteor does not always reflect radio waves to the receiver. Meteor trails must meet some
geometric requirements to be usable in a radio link.
Since specular reflections are governed by principle of Fermat (shortest optical path), the meteor can
scatter an incoming radio wave to the receiver, only if its ionized trail is tangent to an ellipsoid, having
transmitter and receiver as foci. Meteors that don’t meet this condition, will not be “seen” by the radio
system.
Depending on the relative position of scattering volume with respect to transmitter and receiver, Forward
Scatter, Back Scatter and Side Scatter echoes may be obteained. Projecting scattering volume on a
“horizontal” plane containing receiver and transmitter locations, if the angle β between transmitterreceiver path and direction of scattered ray to the receiver is close to 0°, we are under Forward Scatter
conditions. If the angle β is significant, but lower than 90°, the mechanism is named Side Scatter, while
Back Scatter is characterized by an angle β greater than 90°.
A generalized theory and a mathematical model, from which Forward / Side / Back scatter results could
be deduced as special cases, has been developed.
Forward scatter condition is considered to be the best for radio meteor observations. In comparison to
Back scatter tecnique, it allows to observe considerably fainter and higher meteors. In meteor burst
communications, that means two radio stations, on the average, yeld more and stronger meteor echoes, in
Forward-scatter modality.
It has to be noted that, for uniform radiant distribution (sporadic meteors) the relative duration density of
forward scatter echoes has two maxima, corresponding to two areas named “Hot Spots”.
They are located halfway between the stations, on either side of the midpoint of the path. A corresponding
antenna beaming offset from forward direction is required to “illuminate” a hot spot area. Anyway,
beaming offset is significant only if distance between two stations is rather short. Beaming a common hot
spot may be viewed as searching Side scatter optimal path, in this case.
If distance between two stations is longer, say, than 1000 Km, a typical antenna beamwidth is wide
enough to include both hot spot areas directions into Main Lobe, while beaming forward direction to the
other station. That means, in most cases beaming directly to the other station gives best results.
Meteors ablate in the ionosphere, mostly around 100 Km height. For each meteor shower, normalized
height distributions (Percentage of echoes vs. Height) curves can be defined.
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They are Gaussian-like curves, typically centered around 95-100 Km height. Quote of ablation, together
with other geometrical effects and radio station parameters (antenna radiation pattern first) affect the
distance limits in a MS radio contact. For example, in 144 MHz band, minimum distance for forward MS
is around 700 Km, while maximum distance is about 2200 Km. Optimal distance for forward MS in 144
MHz band is around 1300 Km.
The general condition for forward scattering is that the angle between the incident ray and the ionized
trail should equal the angle between the reflected ray and the trail. Reflected rays satisfying this condition
will lie on a cone with the same axis as the trail.
Keeping into account the scattering cross section of the meteor trail and the way it reradiates (a radiation
pattern, depending by polarization of incident wave, can be defined for trail reradiation), the scattered
wave rays intersect earth’s surface illuminating a specific area; this area is known as Meteor Echo
Footprint. The shape of this area depends by many factors (mostly geometric), while its extension is,
typically, relatively small (even less than 100 Km2).
This means that two different receiving stations may not be able to receive the same MS echo, even if
they are distant only few kilometers each other.
For meteor showers, radiant position plays a major role in determining expected meteor rates in forward
MS links. The system response to meteor streams is a function of radiant elevation and the relative
position of the radiant, with respect to the direction from transmitter to receiver. An Observability
Function can thus be defined, for a given shower and for a given path between transmitter and receiver.
Observability is a measure of shower ”efficiency” in providing meteors with right geometry (and thus
meteor echoes) for the given path, in function of time.
Observability function is a combination of several effects. It depends by the scattering area seen by both
receiver and transmitter and the illumination function, which is a combination of antenna radiation
patterns for receiver and transmitter, and also by latitude of path’s midpoint. Anyway, most important
effects are the radiant Elevation and Azimuth coordinate of radiant with respect to transmitter /receiver
locations.
Both Elevation and Azimuth coordinates of radiant vary with time in the observer’s sky, because in one
day, due to earth’s rotation around its axis, the radiant moves at a constant rate along a circle of constant
declination, around the north celestrial pole. Observability function values will consequently vary with
time during the day, having a 24 hours periodicity in first approximation.
Regarding radiant elevation above horizon, it has been denmostrated (both matematically and
experimentally) that maximum radio observability is obtained for radiant elevations between 40° and 50°
above horizon. Actual value of elevation for maximum observability is dependent by antenna patterns of
receiver and transmitter.
Observability of a given shower decreases when the radiant is too low (or even too high) above horizon;
of course, observability drops to zero value when radiant is below local horizon.
Concerning dependence by radiant’s Azimuth coordinate, in first approximation radio observability
reaches a maximum when radiant’s direction is orthogonal to transmitter-to-receiver path direction.
That means, in a plane projection, that meteor trail coming from radiant is orthogonal to the transmitter /
receiver path.
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In other words, direction of best radio-efficiency for a given shower is always shifted by 90 degrees with
respect to direction of its radiant. For example, a North to South (or viceversa) radio path will yeld better
efficiency when radiant is in West or East direction.
This radiant’s Azimuth effect is particularly important in modulating fainter meteors rates. Brighter
meteoroids, due to formation of glints and blobs in an overdense trail, lead to echoes that are somewhat
less geometric-sensitive, and are mostly affected only by radiant elevation.
Important: radio observability is not a measure of actual meteor shower activity! The activity is
determined only by meteoroids distribution in the stream. Observability is a measure of how much the
observer would be able to detect meteors of a given shower, at a given time, regardless how many
meteors of that shower are actually enetering the atmosphere.
Radio observability can thus be used to derive real activity curve of a given shower. Once the total radio
meteor rate is measured, and data reduction has been performed (“Dead Time”correction for
superimposed echoes, subtraction of expected sporadic meteor rates), the resulting “shower echoes” can
be multiplied to the inverse of observability function at that time, to get an estimation of actual shower
activity in the same time interval.
References / Sources
McKinley D.W.R, “Meteor Science and Engineering”, McGraw-Hill 1961
Yrjola I., Jenniskens P., “Meteor Stream Activity. A survey of annual meteor activity by means of forward meteor
scattering”, Astronomy and Astrophysics 330, 739-752 (1998)
Wislez J.M, "Forward scattering of radio waves off meteor trails", Proceedings of the International Meteor
Conference, Brandenburg 1995, edited by Paul Roggemans and André Knöfel, International Meteor Organization,
1995, pp. 99-117.
Lunsford R, “Sporadic Meteors” http://www.spaceweather.com/meteoroutlook/sporadics.html
IMO Website: http://www.imo.net
RMOB Website: http://visualrmob.free.fr/index.php
http://WWW.MMMonVHF.DE
Kind regards, Massimo, IV3NDC
on behalf of the Team of MMMonVHF
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