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Gorn
2015-Aug-16, 10:55 PM
Hello. I don't know this stuff..so forgive some of my questions. Number 1. When I see these 'Radar' movies of asteroids..I was wondering..is it possible when these radar returns get back to the Earth is it possible to 'encode' them..so that they can be recognized?

That's not required to make these movies..it's just the 'timing' of these signals that are important..but my thinking is if you tried to make a movie of a planet outside of the
solar system..you could tell the difference between the signal you sent out..and any interference you might get back from other sources of radio.

Hmm..one of the things I heard was that it is hard to tell the difference between a star's light and the reflected light off of a planet. But if the light that your getting back is encoded..there would be an obvious difference between the signal your looking for and something else

Does anyone think this is possible?

Any and all responses welcome
Bye
G

Jens
2015-Aug-17, 12:16 AM
I think I understand what you're asking, but it's not really easy to answer. First of all, I think the simple answer is well, no and yes. You can't encode a radar signal in the way you're thinking, but actually I think you are encoding them, as long as you know the distance to the object you are detecting. If you send out a radar signal, the signal from the moon will come back sooner than the reflection from Mars, so in fact you have an encoding, I suppose. So if you knew the distance to the planet and the distance to the star, you could know which reflection you're seeing. Though in any case, you can't use radar to look at stars. Plus radar isn't looking at light in the first place, so it's hard to understand your last question.

Jeff Root
2015-Aug-17, 04:36 AM
I seriously do not understand what you are asking, but I do
know a few bits and pieces of the answers...

Radar signals are always sent out in some kind of pattern.
They have a particular frequency, last a certain length of time,
and have a certain length of pause between pulses. That is
what you appear to mean by 'encoding', and it is done exactly
for the reason you suggest: so that they can be recognized
when they return. When laser pulses are reflected from the
Apollo retroreflectors on the Moon, the detectors are turned
on for a very short 'window' of time in which the returning
pulse is expected. I don't recall how long, but a few tens
of microseconds would be reasonable. If the detector is on
for 20 microseconds, that would allow a variation of almost
6 kilometres in the distance to the Moon, and the light would
still return during the 'open window'. If there are a thousand
pulses per second, each lasting one nanosecond, the detector
would be on for 20 μs and off for 980 μs, switching on and off
1000 times per second. That greatly reduces the amount of
'noise' the detector sees. The timing is adjusted to match the
expected delay, and the 'window' is wide enough that the
return is certain to be sometime within it.

That kind of thing is necessary because even with the
retroreflectors, and even with quite large telescopes to send
and receive the light signals, only a few photons are detected
from each pulse. That is because of the enormous distance
to the Moon, and the fact that even a laser beam spreads out
over such a distance.

Radar beams spread out more than light beams do, but it is
possible to make radio reflector dishes much larger than
telescope mirrors. So it is possible to send radar beams to
Venus from Earth and see the returns clearly enough to map
the surface under the clouds. But with nowhere near the
detail that the Magellan Venus orbiter could get.

I think radar has been bounced off of Jupiter and the Sun.
I'm not sure anything more distant has been done. Certainly
it will never be possible to bounce a radar signal off of a star
other than the Sun, or any exoplanet. They are much, much,
much too far away.

-- Jeff, in Minneapolis
.

Shaula
2015-Aug-17, 05:07 AM
Radar pulses are often chirped. This allows a pulse to be longer (to get more energy back) while keeping an acceptable range resolution. The returned signal is then matched against the reference waveform. Modern systems use other encoding schemes as well, I believe. So what you are talking about basically already happens for most systems.

BigDon
2015-Aug-17, 03:03 PM
I worked on airborne fire control radars powerful enough to kill a human being at more than half a mile. 130KW plus focused energies.

Why only one signal? Too easy for one signal to get absorbed or deflected. Fire a dozen. Doesn't take up that much more room. Especially with a government budget.

Even early SAM sites used multi-frequency radar. There was three levels of danger all with a different, more urgent tone on the warning system. I worked on those too. Being swept by search radar, the lock on and then the dreaded "triplets" of weapons guidance radars. The triplets giving the incoming missile a 3D picture of the target. The incoming missile alarm is a truly nightmarish warble I might add.

My radar emitted over two dozen coded frequencies simultaneously. We could knock down aircraft and nap of the Earth cruise missiles at a range of 110 nm.

A radar to observe exoplanets would have to use the gravitational lensing of the Sun to not require a transmitter powerful enough to sterilize the planet with its side lobes alone. That would require the transmitter to be off planet anyway, so it's all good.

The energies required, with the use of the Sun's gravity, are achievable now.

Jens
2015-Aug-18, 12:22 AM
My radar emitted over two dozen coded frequencies simultaneously. We could knock down aircraft and nap of the Earth cruise missiles at a range of 110 nm.

110 nanometers? :)


A radar to observe exoplanets would have to use the gravitational lensing of the Sun to not require a transmitter powerful enough to sterilize the planet with its side lobes alone. That would require the transmitter to be off planet anyway, so it's all good.

The energies required, with the use of the Sun's gravity, are achievable now.

I'm curious about what you mean. What would the purpose of the gravitational lensing be, to collimate the beam? I'm not an expert or anything, but it seems intuitive to me that the lensing would actually do the opposite, since the part of the beam closer to the sun would be more strongly deviated than the part further way. But maybe you're thinking of something else.

Shaula
2015-Aug-18, 02:48 AM
110 nanometers? :)
Nautical miles ;p

Next you'll be telling me that kiloyards isn't a standard unit. Pfff. Landlubbers.

Jeff Root
2015-Aug-18, 03:56 AM
Nautical miles was the easy part. I was waiting for you to
explain the bit about the part of the beam closer to the Sun
being more strongly deviated than the part farther away.
How do we get useful lensing of a point object despite that?
Is it that the star or planet *isn't* a point object, so the
gravitational lensing can concentrate light from across its
diameter? Meaning that we don't get improved spatial
resolution, but do get more light? (Radar return)

My guess is that the gravitational lensing would be done
only on the returning signal, not the outgoing signal. The
focal point would be very far from the Sun. Many AUs.
Maybe well past the orbit of Pluto.

-- Jeff, in Minneapolis