[Laser] Laser retroreflectors on Sats

[James Whitfield]

John

As you have real experience, I will defer to your statement that it is hard 
to see big LEO satellites and smaller ones are harder.

That being the case, I submit my opinion that it would be even harder to 
point an amateur accessible laser system at a cubesat and get meaningful 
data from the return.  I would be satisfied if consideration is given of the 
relative merits of two different methods to accomplish the stated task, that 
is to verify the deployment of the drag device.  I would be even more 
pleased to think engineering calculations were used to support any program 
decision.

If, as you suggest, the probability of seeing sunlight reflected from a flat 
surface is too small for practical use, then perhaps it would be possible to 
compare the general brightness of the satellite before and after the 
deployment event.  That is, unless you are suggesting that a cubesat cannot 
be observed optically at all.

I confess to a personal shortcoming that my first reaction to your post was 
a rather sarcastic supposition that Iridium flares must be myth and that the 
Vangard satellite ( approximately the size of a cubesat ) was never seen. 
Perhaps you could enlighten me in some small way without it being too far 
afield from the topic of this post.  Please forgive my unkind thoughts about 
your post.




The remainder of this post is a general comentary which I hope will be of 
some value to those interested in Patrick's original posted message.  It is 
not intended as a rebuttal of John's post, but simply my thoughts.  I 
strongly caution the reader that this does contain mathematics and some 
educational content is intended.



A cubesat has six flat surfaces.  I would estimate the shiny part, say of 
solar cells to be roughly equivalent in area to a 2 inch diameter mirror.


#############( Oooops.  In checking for errors before I post this, I 
realized I meant 2 inch radius, which makes all the following off by a 
factor of 4.  Rather than correct this mistake, I ask the reader to proceed. 
If it makes any difference to you the observed brightness of the mirror 
would be four times brighter than stated.  In stellar magnitude the 
difference is about 1.5 which would not allow daylight observation.  Sorry 
for any confusion.  --- James )#############


The solar cells would not reflect as much light as the mirror, but ignore 
that for the moment ( The calculation will be refined slightly later. ) and 
consider the heliograph rule of thumb that a mirror flash can be seen in 
daylight for ten miles for each inch of diameter.  At a distance of 20 
miles, the mirror would reflect as much light as it would cover of the sun's 
disk.

Forgive me if I don't show the math, but that represents one part in 
30,000,000.  Expressed as stellar magnitude, that is 18.71 down ( up? ) from 
the Sun at -26.73, or -8.02, which would certainly be visible in daylight, 
but not as bright as the claimed for a mythical Iridium flare ( -9.0 see the 
Wikipedia article on apparent magnitude ).

OK it is a long way around to the point.  Such a 2 inch diameter reflector 
on a satellite 300 miles away ( and for the record, seen through less 
atmosphere than a terrestrial twin 20 miles away ) would have a magnitude 
about -2.13, or 1.8 times brighter than any star in the regular night sky. 
Even with a reflectivity of one percent, it could be seen in a rural night 
sky with the naked eye.  It may not be possible to observe the satellite 
with a 36 inch telescope under normal lighting conditions, but if that rare 
alignment did happen, it could be seen with no more equipment needed than is 
required for meteor watching.

Now for the math on "...probability of ...almost zero. "  The surface of a 
sphere is its diameter squared times pi.  The beam cone from the Sun 
reflected off a flat mirror is the same as the Sun's apparent diameter, 
approximately a half degree.  ( Skip the math!!! ) There is about one chance 
in 210,000 if the satellite is in your field of view the mirror will be at 
the right orientation for the Sun to reflect off it to you.  If you put 
reflectors on all six sides, the odds improve to one in 35,000, not good, 
but better than the lottery.  Boost the reflectivity to 50 percent and 
dedicate each surface to this task by dividing it into 50 mirrors with 
orientation at least a half degree from each other.  The odds improve to one 
in 700.

Give me the information ( OK, not me since I am a grumpy old guy, but to 
someone with the training and tools ) about the satellite orbit and probable 
tumbling characteristics, and it should be a simple and straight forward 
task to calculate probable brightness and likelyhood of seeing a flash on 
any given satellite pass from any location.  Put the formula on a website 
and invite school kids all over the world to help you by looking for the 
flashes on predicted passes, sending in reports of their location, observed 
time and yes/no on flashes seen ( it would also help to know if the flash 
was overhead or at some direction N, NE, E, SE, S, SW, W, NW ).  Then 
collect the data for a month or so.  Run the data to see if it provides 
enough of a pattern to predict which sightings were "real" and which were 
flawed.  If you continue to collect data and refine the predictions, there 
is an increasing confidence that the drag device did, or did not deploy, 
which was the described task.

Of course, you might generate a following like those who go out looking for 
mythical Iridium flares.

My point is not that this is a good way to do the job, or that laser 
retroreflector system is or is not worth the investigator's time and effort. 
My point is that once you have defined the task and the conditions and 
limitions that you must operate within, consider options for how to do the 
task and decide which by an orderly process.  Sometimes that process is 
simple economics.  Sometimes the process is a lack of imagination by those 
given the authority to decide.  I prefer engineering prediction and 
calculations, even when they must include economics and imaginationless 
leadership.

I know it is a long post.  I just hope that I have not discouraged Patrick 
from building a laser system if that is what he wants to do.

Best wishes to all that read this.

James
 n5gui




----- Original Message ----- 
From: "J. Forster" <jfor at quik.com>
To: "Free Space LASER Communications" <laser at mailman.qth.net>
Sent: Monday, August 24, 2009 10:32 AM
Subject: Re: [Laser] Laser retroreflectors on Sats


You say "any flat shiny surface should reflect sunlight that could
be observed by the tracking telescope you descibed"

You are describing a specular reflector, a mirror like thing. There is
essentially only one bright source in the solar system and in a mirror the
angle of incidence equals the angle of reflection. This means that the
mirror must be exactly aligned so that the reflected sun hits your
eyeball. The probability of this happning is almost zero.

I've observed LEO satellites, BIG ones, through a 36" telescope. They are
HARD to seem and many of them are barrel shaped so reflect more of the
time. Small ones are harder.

-John