[Laser] Laser retroreflectors on Sats

[James Whitfield]

Patrick

I have not worked with satellites as you asked in your post, so please 
consider this as opinion and comment.

You described the task as confirmation that a drag device had deployed using 
the difference in laser reflectivity between the device stowed and the 
device deployed.  My immediate impression is that you would not need a laser 
( ground based light source ) to accomplish the task.  With the satellite 
tumbling at an unknown rate and orientation ( May I suggest that is a better 
term than "randomly" which to me implies it will change its rate and / or 
orientation. ) any flat shiny surface should reflect sunlight that could be 
observed by the tracking telescope you descibed.  The reflective surface 
could be used for another purpose, a small solar panel perhaps.  If observed 
once, the deployment of the drag device would be confirmed, presuming that 
the drag device does not have such a reflective surface.  Even if it did, it 
should be possible to track the satellite over time to gather sufficient 
data that reflections from the drag device could be identified separately 
from the main satellite.  Such data could be used to infer the tumbling rate 
and orientation of the system for predictability and decay.

A laser system such as you had suggested would be useful for studying the 
path parameters such as loss, scatter, selective absorbtion, and dependence 
on atmospheric conditions.  All of those would be influenced by the path of 
the system overhead and any data would need to be corelated to tumbling 
influences.

In designing such a laser system, it may be necessary to take into 
consideration the finite travel time for the beam and rotation of the Earth. 
It is one thing to aim the laser at the point in the sky where the satellite 
will be when the beam arrives.  It is yet another to know where the return 
beam will arrive.  The power requirements will also depend on the beam 
divergence, which may be well controled from the ground up, but will 
certainly degrade by the retroreflector technology used.  A broad footprint 
on the return beam might ease the time of flight problems, but at the cost 
of a much higher power requirement.

I hope that my comments have some value for you.

James
 n5gui





----- Original Message ----- 
From: "Patrick Barthelow" <apolloeme at live.com>
To: <laser at mailman.qth.net>
Sent: Sunday, August 23, 2009 10:40 PM
Subject: [Laser] Laser retroreflectors on Sats




A question from a newbie:

Anyone out there done any laser stuff with satellites, that are equipped 
with retro prisms on them?

There may be an interesting app for such involving a cubesat.
If it works out, I may have access to a tracking optical telescope that can 
point a laser into space, and track a satellite with Az-El Accuracy 
approaching a few seconds of arc.

I have been musing about ways to check for deployment of a drag device on 
the Cubesat that is spring loaded, for deployment.  I am  trying to figure 
out a way to confirm that the drag device has indeed deployed.

The drag device when deployed, exposes new cubesat surfaces that are 
normally covered when not deployed.  If you used some tiny retroprisms (or 
scotchlight tape,or other glass bead retro reflective surface treatment), 
that were uncovered when deployed then you could theoretically illuminate 
the cubesat and look for a strong return laser signal to the ground.
The sat will be tumbling, randomly at a slow angular rate.

  I would have to research the required intensities, and laser power needed 
to get a detected signal back to the ground.

Hmmm I bet there are some regs limiting scanning the skies with a laser, 
both wrt aircraft, and wrt possibly blinding unintended satellite optics by 
inadvertantly hitting them with a laser.....

I used to use Electronic Distance meters in surveying, decades ago, and we 
used to substitute sometimes highway reflectors for the expensive glass 
retroprisms used as targets for the laser and before them, natural light 
EDMs.
You could use scotchlight reflective tape, or the round traffic reflectors 
if you could accept a decrease in range over the glass retroprisms.

Someone told me that the GPS sats have laser retroprisms on them for 
precision ranging from the ground, so there might be some expertise out 
there in this cubesat application..

Best Regards,    73,
Pat Barthelow   AA6EG
apolloeme at live.com
"Echoes of Apollo"



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