[Laser] Re: Laser Digest, Vol 3, Issue 12

[wa4qal at ix.netcom.com]

> Message: 1
> Date: Tue, 20 Jul 2004 23:03:18 EDT
> From: TWOSIG at aol.com
> Subject: [Laser] light beacon on balloon
> To: laser at mailman.qth.net
> Message-ID: <88.f71c179.2e2f36f6 at aol.com>
> Content-Type: text/plain; charset="US-ASCII"
> 
> This is a project that is not quite a laser project, at least not yet.  

Close enough in my opinion...
  
> There is a local group that sends up balloon packages for fun and education.  
> They have sent payloads for radio tracking, telemetry beacons, APRS tracking, 
> Amateur TV, and cut down control uplink.  
 
There was a pretty active group up in Indianapolis, IN a number of years
ago, but they seem to have disappeared.  I went up and watched them
launch one of their balloons.  They had an incredibly extensive (and 
expensive) payload "chain", including ATV, GPS/packet/telemetry,  and a
dual cross-band repeater.

> I spoke to one of the group about a light communication related package.  He 
> said that they had tried a strobe light that was unsuccessful in that it did 
> not improve the ability to see the balloon with the naked eye.  The group is 
> always looking for another experiment to send up so they have another reason to 
> have a launch event.  So he encouraged me to come up with a "package design", 
> and suggested that it start with a simple beacon to demonstrate light band 
> down link and perhaps optical tracking.  They have a microcontroler board that 
> could provide a data stream to it.  
> 
> I invite suggestions from you for this first beacon project, or for future 
> missions.  I was thinking of a hemisphere of bright LEDs flashing at audio 
> frequency.  I could use some suggestions on this.  I thought of modulating the 
> light beacon with an audio "carrier" modulated with MCW alternating with BPSK 
> signals to compare the two modes.  MCW because it could be decoded electronically 
> and by eyeball as a flashing light.  PSK31 or PSK64 might be superior for 
> electronic decoding.  I still need to select an audio frequency -- any 
> recommendations from you?    I am thinking of sendin call sign and a time code that could 
> be corelated to the APRS altitude and location tracking that will be the 
> primary payload (and used to recover the package).  
>
> A typical simple mission profile would be a launch before 10 AM near 
> Hutchinson, Kansas, a two hour climb to about 100,000 feet for balloon burst, then a 1 
> hour to return to the ground by parachute.  Ground track would be 30 to 200 
> miles depending on winds aloft, eastward or southeastward.  The horizon at peak 
> altitude should be about 300 miles away.  
> The balloons tend to spin and swing the payload ( ATV pictures often show the 
> horizon instead of the Earth below, or full rotations in a few seconds.) so a 
> wide field of view is needed.  It will also be in constant motion, vertically 
> and horizontally.  

One thing you might consider is that infrared LEDs are typically more
efficient than visible LEDs.  Of course, you can't visibly see infrared LEDs.

You also need to consider the efficiency of the receiver, since you're not
going to have much received power to work with, and you're going to have
an incredible amount of ambient light to overcome.  Any type of narrow
band filters that you can use should help.  How does the infrared compoent
of ambient light compare with the visible component?

Something that will be working in your favour is that the temperature
at those altitudes is pretty cold (-40F?), so that may increase the 
efficiency of LEDs somewhat.  On the other hand, any battery package
will also be cooled, decreasing the available power output, although,
for short duration flights, it is possible to insulate the battery pack,
and use its thermal mass to keep it warm (Would preheating the 
battery pack before launch help even more?).

One of the problems you're going to be facing is that the motion of the
package will make the received signal fluctuate, probably pretty wildly.
I've seen the images, and those packages swing extremely wildly due
to the variability of the intense winds at altitude.  Even received VHF/UHF
signals reveal the extent of the motion as the signals being sent from
a dipole peak and dip wildly.

You're facing two opposing design parameters because of this.  On the 
one hand, you want as wide of an optical beamwidth as you can get to 
minimize problems with the motion instability.  On the other hand, you
want as tight of a beam as you can get to get the maximum amount of
transmitted power sent in your direction.

In the same vein, you're facing the same problem with the modulation
rate.  To get the maximum sensitivity, you want a low modulation rate.
However, to get the maximum information through as the transmitted
beam swings around, you want a high modulation rate.

Unfortunately, there really isn't a good answer, at least, none that I've
found.  
 
> Future experiements that I have though about are: a night launch for improved 
> optical tracking, passive retro-reflector,  voice signal instead of digital 
> data (AM or 40 kHz subcarrier FM),  radio uplink with the light beacon 
> repeating (uplink could be data or voice), a light uplink (requires a wide field of 
> view detector array) with a radio repeating downlink, distance measurement (with 
> light beam as uplink, downlink, or both), a servo alt-az system to point a 
> "narrow" field of view sensor or emmiter.

I'm aware that there are some pretty severe requirements for night launches,
including a stobe that's visible for a certain number of miles (10?) (It's been 
over five years since I've looked at the FAA web-pages, so this may have 
changed, probably for the worse.).  

You have to be a little careful shooting lasers into the air due to concerns
about blinding airline pilots (That's probably not a big concern for the low 
powered stuff, but the high powered stuff is a definite no-no.).

In any case, I'm afraid that the motion is going to present problems either way.

One idea you may want to look at is to use a triggered strobe, where the
time delay between flashes contains information.  A strobe, even a small one, 
can have an instaneous power in the megawatt category, over a period of
microseconds.  The information rate would be pretty low, but it's an avenue
you may want to think about.
  
> I am still working on how many LEDs and what pattern would give a useable 
> transmission pattern, or the duty cycle on the light emitter.  Also if anyone can 
> suggest a method of dampening the balloon payload oscillations that does not 
> weigh much, it would help.

Probably as many as you can, consistent with adequate battery life.

As for the duty cycle, LEDs tend to be more efficient in a pulsed mode,
so it may make sense to operate them at a duty cycle of 10 percent or
less.  Maybe.

Concerning stabilization, there have been a couple of ideas for that.  Some of
the work has been done as part of the Strategic Defense Initiative as part of
the High Altitude Balloon Experiments (HABE).  

Some additional work has been done by other amateur balloon launches.  
One approach involves remotely controlled airfoils attached to the package:

http://www.tapr.org/tapr/list-archive/aprssig/9508/msg00316.html

I've seen another approach using static airfoils as dampers.

Another approach uses a rate gyro that's typically used in remote controlled
model helicopters:

http://www.eoss.org/pubs/str9406.htm#12

Yet another approach might be to use a plain gyroscope.  

Or, you could use a combination of all of these.

In any case, as a first step, I'd recommend building something and ground
testing it.  There's no sense of sending something up to 100,000 feet on a 
balloon if you can't see it 1000 feet across a field (Try it suspended on a long
pole on a windy day to recreate the conditions it'll be experiencing at altitude.
Also, place it on a hilltop, so that you're viewing it with the full ambient skylight
behind it.).  When you have something that works in that situation, then next 
step might be to try a small tethered balloon that just hoists your package.  If 
all goes well, then you should be ready to try the real launch.

Something else you might think about is, instead of using an active emitter
on the launch package, use a passive reflector.  After all, the sun pumps out
a fairly intense amount of optical energy, so what would it take to modulate
a mirror in order to be able to see the reflections on the ground?  Of course,
you'd have to take into account the position of the sun, and this might favour
a sunrise launch, where the sun is on the horizon instead of nearly overhead.
Anyway, it's something to think about.

> 
> James
> N5GUI

Dave
WA4QAL