(meteorobs) Fw: [baa-rag] Radio (meteor?) Reflection 201301071248

Mon Jan 13 00:04:19 EST 2014

Hello Felix

Thank you for your observation.

Consider this. The very nature of Doppler RADAR is that the character of the
recordings and displays are described in ideal terms or concepts. In
reality, this is never the case, only approaching the ideal, never attaining
it. This is the case with Backscatter RADAR, it widely departs from the
ideal, never attaining precise velocity measurements. 

Let us imagine the transmitter/receiver are separated by 120 Km. On a flat
Earth, unless the transmitter and/or receiver are off the ground by 30 or 40
meters relative, there is no line of sight between them; the curvature of
the Earth places the Earth between them. Depending on certain physical
arrangements of the antennae, reflections from aircraft always above the
horizon between the transmitter and receiver which can occur directly over
the transmitter. The reflected energy from the Aircraft typically passes the
center frequency of the transmitter as it pass the zero vector between the
transmitter and receiver. This is characterized by the Aircraft's "S" shaped
return. 

There is always an angle of movement of the reflecting body between the
transmitter and receiver by its very geometry being out the line of sight.
Aircraft velocities from backscatter are then always lower than their actual
speed. In the case of Space RADAR reflecting from ground-to-sky-to-ground,
the angle is shallow so the cosine of the angle is near zero, causing the
Doppler frequency to be low; near the carrier frequency. Aircraft are near
the ground compared to METEOR trails, so the angle of incidence is
shallower, or the cosine of the angle is nearer zero so their tracks show
much lower speed than their actual speed across the ground. 

If an aircraft is moving 500 MPH, its Doppler frequency from a 50 MHz
transmitter is near 700 Hz. But this would only be if the angle of movement
is ideally parallel which it can never be such, for many reasons foremost of
which is the aircraft is always above the line of sight between transmitter
and receiver, never on the line of sight of the transmitter and receiver; a
line that passes through the Earth.

In your previous email you attached a screenshot from your receiver, of a
recording of Aircraft movement and the significant return of a meteor trail
reflection passing through the carrier frequency, indicating the trail was
moving across the plane of incidence between the transmitter and receiver.
The massive Doppler return is a broad reflection caused by many returned
frequencies from a large object, referred to as a smudge reflection. Smaller
objects like aircraft return more monotonic frequencies than larger objects
like Meteor ion trails.

With my higher frequency Backscatter RADARs (35 GHz) when recording
automobiles at close range, cars cause smudge reflections; the front has
passed through the line of sight before the rear of the car causing a
reflection of many objects passing the line of sight at different speeds.
Different parts of the car are simultaneously moving at different angles to
the line of incidence.

When my Backscatter RADAR is parallel to the ground while Rain falls through
the beam, Smudge reflections are returned similar in character to the Meteor
return in your recording. 

-----Original Message-----
From: meteorobs-bounces at meteorobs.org
[mailto:meteorobs-bounces at meteorobs.org] On Behalf Of Felix Verbelen
Sent: Sunday, January 12, 2014 2:40 AM
To: Meteor science and meteor observing
Subject: Re: (meteorobs) Fw: [baa-rag] Radio (meteor?) Reflection
201301071248

Hello Jay,

Thank you for your mail.
At first sight there seems to be at least a "scale" problem...

Observing our 49.99 and 49.97 MHz beacons from a distance of respectively
120 and 90 km, almost permanently reflections of numerous jet planes are
registered.
The planes can of course come/go in different directions, but taking into
account the beacon's radiation diagram, we know that best reflections are
obtained when the plane follows a course which brings it over the beacon.
For these commercial plane reflections, the SpecLab registrations show a
maximum doppler of some 80 Hz.
Since a commercial jet plane travels at a velocity of some 800 km/h (this is
222 meters/second), we would obtain very roughly a velocity of some
10/80*222 (m/s) = 27 m/s, thus some 100 km/h in the case of Jean-Louis'
"corkscrew shapes" with a 10 Hz doppler.

Kind regards.

Felix

Jay Salsburg schreef op 12/01/2014 00:28:
> Let me express some facts to help get our brains wrapped around what is
being recorded from forward scatter Meteor RADAR beacons.
>
>
>
> First; Forward scatter, by its very nature, is not back scatter. Back
Scatter RADAR retro-reflects RADAR energy directly off the moving object,
back to the transmitter/receiver. The Doppler frequency is generated by
mixing the returned RADAR energy with the Transmitter to produce a
difference velocity frequency. The received velocity is a product of the
RADAR Frequency, the speed of light, and the cosine of the angle of movement
in relation to the illuminating RADAR transmitter. Unless the object is
moving directly toward or away from the transmitter with zero angle, or, in
other words, is moving parallel, the velocity will always be less than
parallel movement, which will be times the cosine of the angle of movement.
>
>
>
> Second; RADAR Doppler velocity for parallel movement can be easily
computed. For a 50 MHz RADAR Beacon, the Velocity constant = 0.1491 Hz per
MPH.
>
>
>
> Third; The technical requirements for receiving Forward scatter RADAR
energy is, by its very nature, not receiving Doppler RADAR Energy anywhere
near parallel to the RADAR Transmitter. In fact the angle of incidence may
be quit shallow, rendering the Doppler Frequency offset from the transmitter
frequency very low. Together with the low frequency of the beacon, the
Doppler offset is very low; only a few Hertz.
>
>
>
> Forth; For Backscatter RADAR, the receiver must use Carrier Wave or
Sideband reception, necessitating an offset frequency for recording and
display purposes. By subtracting this offset frequency from the received
Doppler, an actual relative velocity is realized.
>
>
>
> Observing the first image in the Rauly.pdf document referenced below, the
offset Carrier is centered around 590 Hz. The higher frequency is near 600
Hz. 600-590 is 10. 10 times 0.1491 Hz per MPH = 1.5 MPH. It is entirely
possible to closely compute the actual angle of incident of the movement
from the transmitter. Let us arbitrarily choose 30 Degrees. This means the
reflection is 60 degrees or 0.5 of the recorded velocity which equals 3 MPH.
There may also be movement at near right angle to the transmitter further
reducing the relative velocity.
>
>
>
> This very low velocity of only a few MPH is what is seen in the corkscrew
recordings.
>
>
>
> Jay Salsburg
>
>
>
> From: Jean-L. RAULT [mailto:f6agr at orange.fr]
> Sent: Wednesday, January 08, 2014 1:43 AM
> To: radiometeoren at vvs.be
> Subject: [radiometeoren] Fwd: Re: Fw: [baa-rag] Radio (meteor?) 
> Reflection 201301071248 [1 Attachment]
>
>
> -------- Message original --------
>
>
> Sujet:
>
> Re: [radiometeoren] Fw: [baa-rag] Radio (meteor?) Reflection 
> 201301071248 [1 Attachment]
>
>
> Date :
>
> Tue, 07 Jan 2014 18:54:11 +0000
>
>
> De :
>
> Jean-L. RAULT  <mailto:f6agr at orange.fr> <f6agr at orange.fr>
>
>
> RÃ©pondre Ã  :
>
> f6agr at orange.fr
>
>
> Pour :
>
> g4csd at yahoo.co.uk
>
>
>
> Paul
>
> This is a typical "corkscrew shape" echo which appears from time to time.
See for example http://www.imo.net/imc2010/talks/Rault.pdf for a meteor
echoes signatures gallery.
>
> I personally don't know any good paper on the meteor echoes signatures.
This is a domain which does not seem to be investigated by professionnal
searchers at the moment.
>
> Amateurs have a lot to do in this domain !
>
> Regards
>
> Jean-Louis Rault
>
>
>
>
>
>
>
> _______________________________________________
> meteorobs mailing list
> meteorobs at meteorobs.org
> http://lists.meteorobs.org/mailman/listinfo/meteorobs
>
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