laser detection and ranging deep space

What is the range of laser detection and range? Can it track Voyager?




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In article ,
Lynndel Humphreys wrote:
What is the range of laser detection and range? Can it track Voyager?

With considerable difficulty, it can track the retroreflectors left on the
Moon by the Apollo expeditions. Beyond that, forget it unless you have
active cooperation from the target.

Lidar suffers from the same problem as radar: the strength of the return
signal scales with the inverse fourth power of distance. And it's much
easier to build high-powered radar transmitters than high-powered lasers.
--
MOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer
pointing, 10 Sept; first science, early Oct; all well. |

Lynndel Humphreys wrote:
What is the range of laser detection and range? Can it track Voyager?

No.
In principle, lasers work quite well for ranging and detection, if you have
a good idea where to look, and you can generate a beam that's significantly
stronger (after any filters) than sunlight.
Even the largest optical telescope on earth (around 5m) could only
(neglecting the atmosphere) generate a beam that diverges by one
part in ten million or so.
Voyager is around 20 light hours out (plus or minus a factor of 5) which
works out to 72000 light seconds.
Divide by 10 million to get a beam 7ms wide, or in meters around 2000Km.
Sunlight is significantly weaker at that distance, but to cover an area
of 2000Km diameter, even the brightest earth lasers are utterly lacking.
So, you'd be better trying to pick it up by reflected light from the sun.
But even then, it's so dim that it's a practical impossibility.

Given that there is a working radio transmitter on it, there are varios
ways to use that to get a fairly accurate position.

Henry Spencer wrote:
Lidar suffers from the same problem as radar: the strength of the
return signal scales with the inverse fourth power of distance.
And it's much easier to build high-powered radar transmitters than
high-powered lasers.

Right. Also, it's much easier to build extremely sensitive radar
receivers than extremely sensitive light receivers. There's a
fundamental limit of sensitivity to light, since you can't detect
less than one photon. (True also of radar, but radar photons are
enormously smaller in energy.)

On the other hand, it's far easier to make extremely directional
transmitters and receivers for light than for radar. This may be more
than enough to compensate. I've heard arguments that interstellar
communications would use light rather than radio for this reason.
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Henry Spencer wrote:
In article ,
Lynndel Humphreys wrote:
What is the range of laser detection and range? Can it track Voyager?

With considerable difficulty, it can track the retroreflectors left on the
Moon by the Apollo expeditions. Beyond that, forget it unless you have
active cooperation from the target.

Lidar suffers from the same problem as radar: the strength of the return
signal scales with the inverse fourth power of distance. And it's much
easier to build high-powered radar transmitters than high-powered lasers.

A second benefit can be that photons of light are much bigger particles
of energy than photons of radio waves.
Radio is also a much easier frequency to work with in some ways.
Electronics works well for radio, you can do things with it easily that are
nearly impossible, or cutting-edge with optics.

The one real benefit of light is its short wavelength.
In a few cases, this can mean that you can match the beam size to the target
size.
If you can focus the beam tight enough that it is around the same size
as the target, then you go back from inverse hypercube to inverse square.
The downside is that you require a much, much better knowledge of where it
is, and a tighter beam means slower searches.

"Keith F. Lynch" writes:

Henry Spencer wrote:
Lidar suffers from the same problem as radar: the strength of the
return signal scales with the inverse fourth power of distance.

Right. Also, it's much easier to build extremely sensitive radar
receivers than extremely sensitive light receivers. There's a
fundamental limit of sensitivity to light, since you can't detect
less than one photon. [...]

On the other hand, it's far easier to make extremely directional
transmitters and receivers for light than for radar.

Both are *completely* hopeless with today's technology, though light is
slightly less hopeless. Here are some numbers:

The world's longest range radar is Arecibo. Its web page states it can detect
a (metallic) golf ball at the distance of the moon. A golf ball is about 4cm
across, Pioneer was 2.3 meters IIRC, so we could detect pioneer at about
7.5 times further away ((230cm/4cm)^2)^0.25 = 7.5 . That's about 3M km.
40 AU is about 1866 times further away. With the r^4 behavior, you need about
a factor of 12 *trillion* times more power to detect Pioneer at 40 AU.
Arecibo already transmits a MW, so a factor of 12 trillion will be hard to get.

Light is theoretically a little better, but not enough to matter. A big
mirror is about 10 million wavelengths across, as opposed to about 2000 for
Arecibo. For the same transmitter power, that gives about 25,000,000 times
more power on target. You lose about a factor of 800 due to the quantized
photons, since the noise temperature of Arecibo is about 30K, and each
light photon has about 800 times the energy of a 30K black-body photon.
So the optical system is about 30,000 times better, which is still nowhere
near good enough (still about a factor of a billion short). And this is not
even considering the atmosphere.

Lou Scheffer

It seems I read Voyageur was reaching its transmitting limit (either no
power or distance?). Not sure what a laser, weak or otherwise , would do to
the instruments. However, a light souce of known frequency might make it
trackable. Voyageur, lost in space in that lonely void, seems lonely to me.
What would it be like from its prospective?


Lidar suffers from the same problem as radar: the strength of the
return signal scales with the inverse fourth power of distance.

Right. Also, it's much easier to build extremely sensitive radar
receivers than extremely sensitive light receivers. There's a
fundamental limit of sensitivity to light, since you can't detect
less than one photon. [...]

On the other hand, it's far easier to make extremely directional
transmitters and receivers for light than for radar.

Both are *completely* hopeless with today's technology, though light is
slightly less hopeless. Here are some numbers:

The world's longest range radar is Arecibo. Its web page states it can
detect
a (metallic) golf ball at the distance of the moon. A golf ball is about
4cm
across, Pioneer was 2.3 meters IIRC, so we could detect pioneer at about
7.5 times further away ((230cm/4cm)^2)^0.25 = 7.5 . That's about 3M km.
40 AU is about 1866 times further away. With the r^4 behavior, you need
about
a factor of 12 *trillion* times more power to detect Pioneer at 40 AU.
Arecibo already transmits a MW, so a factor of 12 trillion will be hard to
get.

Light is theoretically a little better, but not enough to matter. A big
mirror is about 10 million wavelengths across, as opposed to about 2000
for
Arecibo. For the same transmitter power, that gives about 25,000,000
times
more power on target. You lose about a factor of 800 due to the quantized
photons, since the noise temperature of Arecibo is about 30K, and each
light photon has about 800 times the energy of a 30K black-body photon.
So the optical system is about 30,000 times better, which is still nowhere
near good enough (still about a factor of a billion short). And this is
not
even considering the atmosphere.

Lou Scheffer




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It seems I read Voyageur was reaching its transmitting limit (either no
power or distance?).

Nope, the Voyagers are expected to last until 2020 or so, if nothing else
breaks. One of the Pioneers recently - about one year ago - was too far
away for its signal to be decoded (it was still detectable, IIRC).

Jan

In article ,
Lynndel Humphreys wrote:
It seems I read Voyageur was reaching its transmitting limit (either no
power or distance?).

No, DSN is still talking to the Voyagers fine, and will be for quite a
while yet (assuming they continue to function). The last of the Pioneers
died last winter, but that was a matter of insufficient power to run its
hardware, not inability to make itself heard. Improvements to DSN more
than kept pace with the weakening of its signal due to distance.

There's no U in the name, by the way.

Not sure what a laser, weak or otherwise , would do to the instruments.

At that distance, weak or otherwise, nothing.

...[Voyager], lost in space in that lonely void, seems lonely to me.
What would it be like from its prospective?

The view from either of the Voyagers is much the same: a starry night sky
in all directions, with one especially bright star in the direction the
antenna is pointed. Nothing moves, except that once in a long while, one
of the Voyager thrusters burps for a fraction of a second, reversing an
imperceptible attitude drift. The view never changes, except that every
year the bright star gets a little dimmer. Just silent, clear, cold night
going on forever.
--
MOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer
pointing, 10 Sept; first science, early Oct; all well. |