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Sam

Joined: 18 May 2008
Posts: 135

Posted: Mon Dec 14, 2009 1:55 pm Post subject: Kind of off topic. Speed sensor needed.

Aeroelectric listers,



We're looking for a bright idea.  Preferably an inexpensive one.



The Sport Air Racing League, http://www.sportairrace.org/ is trying to develop an accurate and reliable way to time the speed of  an aircraft zooming down a runway, at 50 feet or so.  They are looking for some kind of trigger that will start the clock at one end, and stop it at the far end.

  

We don't really want to erect pylons to fly through to break a beam, that would be unwieldy. Some of the guys have looked at airborne systems, such as GPS tracking, but it's not accurate enough.



The SARL group is having a hard time coming up with something.  Anybody have any thoughts how we might accomplish this?

 

Thanks for any ideas.



Sam Hoskins

Race 22!

  [quote][b]

_________________
Sam Hoskins
www.samhoskins.blogspot.com

skywagon

Joined: 11 Feb 2006
Posts: 184

Posted: Mon Dec 14, 2009 2:48 pm Post subject: Kind of off topic. Speed sensor needed.

Hi Sam,

  

 One of the old fashion systems might be considered.   

 Mounting a gun sight device on a tripod and having some  young person, with good tracking reflexes man it.  Maybe a skeet and trap  gunner.  He sights on the aircraft nose and as it comes over the finish  line, the swinging and tracking of the gun sight rides over a micro switch with  a very fast make or break.  This swt. action starts and/or stops a  clock.  If the operator has trouble tracking the nose of the aircraft, then  all bets are off of course.

  

 You are sure right about gps.  All but, maybe some  military designs do not update fast enough.  You would probably need one  that updates and calculates at a minimum of 1 millisec. or  faster.

 David

 [quote]   ---

mrspudandcompany(at)veriz
Guest

Posted: Mon Dec 14, 2009 3:25 pm Post subject: Kind of off topic. Speed sensor needed.

We're looking for a bright idea.  Preferably an inexpensive one.

 

 The Sport Air Racing League, http://www.sportairrace.org/ is trying to develop an accurate and reliable way to time the speed of  an aircraft zooming down a runway, at 50 feet or so.  They are looking for some kind of trigger that will start the clock at one end, and stop it at the far end.

 

 We don't really want to erect pylons to fly through to break a beam, that would be unwieldy. Some of the guys have looked at airborne systems, such as GPS tracking, but it's not accurate enough.

 

 The SARL group is having a hard time coming up with something.  Anybody have any thoughts how we might accomplish this?

 

 Thanks for any ideas.

 

 Sam Hoskins

 Race 22!  [quote]  Sam, Have you considered a hand held radar gun, similar to the ones police departments use for traffic speed?? Roger[b]

Robert Reed

Joined: 22 Oct 2009
Posts: 331
Location: Dallas/Ft.Worth

Posted: Mon Dec 14, 2009 3:36 pm Post subject: Kind of off topic. Speed sensor needed.

You didn't indicate rather the speed sensor could be mounted in the plane or needed to be on the ground.  If it could be mounted on the plane I would say that a low powered laser similar to those use in a grocery check out could be used with ground based reflectors.  I can't see any problem with using the same system with a ground based unit since the lights are of brief duration and low power.  They are widely used by police and highway patrols.  The accuracy would be great.

   From: Sam Hoskins <sam.hoskins(at)gmail.com>

To: Aerolectric List <aeroelectric-list(at)matronics.com>

Sent: Mon, December 14, 2009 3:01:20 PM

Subject: Kind of off topic. Speed sensor needed.

 Aeroelectric listers,

We're looking for a bright idea.  Preferably an inexpensive one.

The Sport Air Racing League, http://www.sportairrace.org/ is trying to develop an accurate and reliable way to time the speed of  an aircraft zooming down a runway, at 50 feet or so.  They are looking for some kind of trigger that will start the clock at one end, and stop it at the far end.

We don't really want to erect pylons to fly through to break a beam, that would be unwieldy. Some of the guys have looked at airborne systems, such as GPS tracking, but it's not accurate enough.

The SARL group is having a hard time coming up with something.  Anybody have any thoughts how we might accomplish this?

Thanks for any ideas.

Sam Hoskins

Race 22!

  	  Quote: 	
		  [b] 	

 [quote][b]

FLYDAD57(at)neo.rr.com
Guest

Posted: Mon Dec 14, 2009 4:19 pm Post subject: Kind of off topic. Speed sensor needed.

Would a radar gun, like the cops use, work??

  

  

 [quote]   ---

Sam

Joined: 18 May 2008
Posts: 135

Posted: Mon Dec 14, 2009 6:05 pm Post subject: Kind of off topic. Speed sensor needed.

Radar guns were used with the AirVenture Cup racers several years ago and I think they didn't work too well.  The SARL guys are looking for some precise timing.



They currently are using sighting rods and triangulation, but it is leaving too much judgment to the timing person.  I think that would also apply to David's suggestion.

 

They want to capture a full runway length of time, to have a more representative view of true speed, rather than just a snapshot that a radar gun might provide.



An aircraft mounted lasar system might to it, but how.where would you mount it to many different kinds of aircraft, and in such a way that it would not create drag?

 

These are the problems the SARL guys are trying to work around.



Thanks for the suggestions.



Sam



On Mon, Dec 14, 2009 at 6:16 PM, Robert D. Taylor <FLYDAD57(at)neo.rr.com (FLYDAD57(at)neo.rr.com)> wrote:

 [quote]       Would a radar gun, like the cops use, work??

  

  

 [quote]   ---

_________________
Sam Hoskins
www.samhoskins.blogspot.com

william_slaughter(at)att.
Guest

Posted: Mon Dec 14, 2009 6:45 pm Post subject: Kind of off topic. Speed sensor needed.

Google “race car timing systems” and you will get lots of hits for transponder type systems. Don’t know how they would relate to your budget, but they are widely used, and not just at the professional level, so hopefully there would be one that would work for you.  

   

Will Slaughter  

     

From: owner-aeroelectric-list-server(at)matronics.com [mailto:owner-aeroelectric-list-server(at)matronics.com] On Behalf Of Sam Hoskins

 Sent: Monday, December 14, 2009 3:01 PM

 To: Aerolectric List

 Subject: Kind of off topic. Speed sensor needed.  

  

   

Aeroelectric listers,

 

 We're looking for a bright idea.  Preferably an inexpensive one.

 

 The Sport Air Racing League, http://www.sportairrace.org/ is trying to develop an accurate and reliable way to time the speed of  an aircraft zooming down a runway, at 50 feet or so.  They are looking for some kind of trigger that will start the clock at one end, and stop it at the far end.

 

 We don't really want to erect pylons to fly through to break a beam, that would be unwieldy. Some of the guys have looked at airborne systems, such as GPS tracking, but it's not accurate enough.

 

 The SARL group is having a hard time coming up with something.  Anybody have any thoughts how we might accomplish this?

 

 Thanks for any ideas.

 

 Sam Hoskins

 Race 22!  

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        [quote][b]

nuckolls.bob(at)aeroelect
Guest

Posted: Mon Dec 14, 2009 6:48 pm Post subject: Kind of off topic. Speed sensor needed.

At 03:01 PM 12/14/2009, you wrote:

  	  Quote: 	
		  Aeroelectric listers,



 We're looking for a bright idea.  Preferably an inexpensive one.



 The Sport Air Racing League, http://www.sportairrace.org/ is trying to develop an accurate and reliable way to time the speed of  an aircraft zooming down a runway, at 50 feet or so.  They are looking for some kind of trigger that will start the clock at one end, and stop it at the far end.



 We don't really want to erect pylons to fly through to break a beam, that would be unwieldy. Some of the guys have looked at airborne systems, such as GPS tracking, but it's not accurate enough.



 The SARL group is having a hard time coming up with something.  Anybody have any thoughts how we might accomplish this? 	


   Yes, photo cells looking straight up though a tube of 2 - 4"

   and length adjusted so that subtended angle of view is sufficiently

   small that the "shadow" of any part of the aircraft blocks the

   skylight. There are some self-calibrating level sensor circuits

   that will adjust for slow variability in sky light levels.



 

        Bob . . .

                    //// 

                   (o o) 

    ===========o00o=(_)=o00o========= 

    < Go ahead, make my day . . .   > 

    < show me where I'm wrong.      > 

    =================================



     [quote][b]

FLYDAD57(at)neo.rr.com
Guest

Posted: Mon Dec 14, 2009 7:22 pm Post subject: Kind of off topic. Speed sensor needed.

College cross country teams use transponders to track  runners and their positions crossing the finish line.  Maybe some  adaptation of this equipment could be employed.

  

 [quote]   ---

ainut(at)hiwaay.net
Guest

Posted: Mon Dec 14, 2009 8:25 pm Post subject: Kind of off topic. Speed sensor needed.

etienne.phillips(at)gmail
Guest

Posted: Mon Dec 14, 2009 10:11 pm Post subject: Kind of off topic. Speed sensor needed.

On 15 Dec 2009, at 4:46 AM, Robert L. Nuckolls, III wrote:

 	  Quote: 	
		  



   Yes, photo cells looking straight up though a tube of 2 - 4"

   and length adjusted so that subtended angle of view is sufficiently

   small that the "shadow" of any part of the aircraft blocks the

   skylight. There are some self-calibrating level sensor circuits

   that will adjust for slow variability in sky light levels.



 

        Bob . . .



	


This would work well - even if the light sensor wasn't in a tube, calibrated correctly, the change from direct sunlight to shadow should be enough to trigger a timing device of some description. I would suggest two or three sensors in a line across the runway. Then use the second or third sensor that is triggered so that you know it's the wing that's causing the shadow both times, and not the wing on the first end, and the spinner on the second (which would skew results).



Even a very simple microprocessor would be able to do the timing accurately enough, and could display the result on an LCD screen without too much effort.

There is one downside to this, if you're keen on using either end of a runway, I guess you're looking for about 1km between sensors. That's a lot of wire to wind up at the end of each competition! There may be some simple radio interfaces that could be substituted...?

Etienne

 [quote][b]

Robert Reed

Joined: 22 Oct 2009
Posts: 331
Location: Dallas/Ft.Worth

Posted: Tue Dec 15, 2009 7:03 am Post subject: Kind of off topic. Speed sensor needed.

If the laser were ground mounted the only thing required on the plane might be a couple of identifiable stripes of paint on the underside of the aircraft.  It would not require a high powered laser and shouldn't pose any danger to the pilot since the directionality of the beam would be vertical and not directed at the pilot.

  

 If the laser were mounted inside the plane it wouldn't require more than a dime sized opening that could be covered with glass.  The laser beam would be setup to oscillate back and forth with a ground based sensor to detect the beam as the plane crossed the detection points.  The size of current low power lasers are so small that there should be no problem with either weight or drag.  

 

Bob

 

   From: Sam Hoskins <sam.hoskins(at)gmail.com>

To: aeroelectric-list <aeroelectric-list(at)matronics.com>

Sent: Mon, December 14, 2009 8:01:49 PM

Subject: Re: AeroElectric-List: Kind of off topic. Speed sensor needed.



 Radar guns were used with the AirVenture Cup racers several years ago and I think they didn't work too well.  The SARL guys are looking for some precise timing.



They currently are using sighting rods and triangulation, but it is leaving too much judgment to the timing person.  I think that would also apply to David's suggestion.



They want to capture a full runway length of time, to have a more representative view of true speed, rather than just a snapshot that a radar gun might provide.



An aircraft mounted lasar system might to it, but how.where would you mount it to many different kinds of aircraft, and in such a way that it would not create drag?



These are the problems the SARL guys are trying to work around.



Thanks for the suggestions.



Sam



 On Mon, Dec 14, 2009 at 6:16 PM, Robert D. Taylor <FLYDAD57(at)neo.rr.com (FLYDAD57(at)neo.rr.com)> wrote:

 [quote]  Would a radar gun, like the cops use, work??

  

  

 [quote]  ---

nuckolls.bob(at)aeroelect
Guest

Posted: Tue Dec 15, 2009 7:56 am Post subject: Kind of off topic. Speed sensor needed.

On 15 Dec 2009, at 4:46 AM, Robert L. Nuckolls, III wrote:



   Yes, photo cells looking straight up though a tube of 2 - 4"

   and length adjusted so that subtended angle of view is sufficiently

   small that the "shadow" of any part of the aircraft blocks the

   skylight. There are some self-calibrating level sensor circuits

   that will adjust for slow variability in sky light levels.



E: This would work well - even if the light sensor wasn't in a tube, 

calibrated correctly, the change from direct sunlight to shadow 

should be enough to trigger a timing device of some description.



    You need to constrain the view angle . . .preferably

    without the use of optics. Long tubes seem the simplest

    approach.



E: I would suggest two or three sensors in a line across the runway. 

Then use the second or third sensor that is triggered so that you 

know it's the wing that's causing the shadow both times, and not the 

wing on the first end, and the spinner on the second (which would 

skew results).



    That would be useful . . . depending on how "loose"

    the rules are for alignment with the runway ceneterline,

    an array of senors of any number could be used.



E: Even a very simple microprocessor would be able to do the timing 

accurately enough, and could display the result on an LCD screen 

without too much effort.



    Sure, the jellybean PIC products offer one set of

    solutions.



E: There is one downside to this, if you're keen on using either end 

of a runway, I guess you're looking for about 1km between sensors. 

That's a lot of wire to wind up at the end of each competition! There 

may be some simple radio interfaces that could be substituted...?



    Ten years ago, I helped some RC racers craft a timing system

    that required communication over a many acres of event

    venue. Off the shelf, 433 MHz transmiter and receiver

    modules provided links. Controlling atency and jitter in

    detecting event-edges would be a critical design goal.



    Assuming 300 mph (440 f/s) over 5000 feet of runway

    (11.3 seconds) a .1% accuracy in timing would call for

    control of variability to less than 10 milliseconds.

    If shorter runs are anticipated, the number gets tighter.

    +/-1 millisecond would probably be a practical design goal.



    Sam. Define "accurate". What are the design goals for

    the reduction of uncertainty?



    Bob . . .

david.nelson(at)pobox.com
Guest

Posted: Tue Dec 15, 2009 8:05 am Post subject: Kind of off topic. Speed sensor needed.

Hi Sam,



Are you wanting something that displays/announces the avg speed in near real-time 

(ie after the plane passes the end of the runway) or this something that is 

announced at the end of show?



 	 /\/elson



On Mon, 14 Dec 2009, Sam Hoskins wrote:



 	  Quote: 	
		   Aeroelectric listers,



 We're looking for a bright idea.  Preferably an inexpensive one.



 The Sport Air Racing League, http://www.sportairrace.org/ is trying to

 develop an accurate and reliable way to time the speed of  an aircraft

 zooming down a runway, at 50 feet or so.  They are looking for some kind of

 trigger that will start the clock at one end, and stop it at the far end.



 We don't really want to erect pylons to fly through to break a beam, that

 would be unwieldy. Some of the guys have looked at airborne systems, such as

 GPS tracking, but it's not accurate enough.



 The SARL group is having a hard time coming up with something.  Anybody have

 any thoughts how we might accomplish this?



 Thanks for any ideas.



 Sam Hoskins

 Race 22!

user9253

Joined: 28 Mar 2008
Posts: 1927
Location: Riley TWP Michigan

Posted: Tue Dec 15, 2009 8:31 am Post subject: Re: Kind of off topic. Speed sensor needed.

Sam,

How about using two clocks and two cameras?  Synchronize two very accurate clocks or stop watches and locate them at opposite ends of the runway.  Take fast pictures of the plane (including the clock) as it passes starting and ending points.  Review the pictures and note the times on the clocks and subtract.

Joe

_________________
Joe Gores

raymondj(at)frontiernet.n
Guest

Posted: Tue Dec 15, 2009 10:57 am Post subject: Kind of off topic. Speed sensor needed.

The NASA system could be turned upside down with the sweeping laser 

fixed and the retroreflective tape on the underside of the aircraft.



Someone suggested the laser from a grocery store scanner as a painting 

laser.  The retroreflective tape is available from SAR equipment suppliers.



NASA system link:

http://searchandrescue.gsfc.nasa.gov/techdevelopment/l_sar.html



Just my thoughts.



Raymond Julian

Kettle River, MN



do not archive

Robert Reed wrote:

[quote] If the laser were ground mounted the only thing required on the plane 

 might be a couple of identifiable stripes of paint on the underside of 

 the aircraft.  It would not require a high powered laser and shouldn't 

 pose any danger to the pilot since the directionality of the beam would 

 be vertical and not directed at the pilot.

  

 If the laser were mounted inside the plane it wouldn't require more than 

 a dime sized opening that could be covered with glass.  The laser beam 

 would be setup to oscillate back and forth with a ground based sensor to 

 detect the beam as the plane crossed the detection points.  The size of 

 current low power lasers are so small that there should be no problem 

 with either weight or drag. 

 

 Bob

 

 ------------------------------------------------------------------------

 *From:* Sam Hoskins <sam.hoskins(at)gmail.com>

 *To:* aeroelectric-list <aeroelectric-list(at)matronics.com>

 *Sent:* Mon, December 14, 2009 8:01:49 PM

 *Subject:* Re: Kind of off topic. Speed sensor needed.

 

 Radar guns were used with the AirVenture Cup racers several years ago 

 and I think they didn't work too well.  The SARL guys are looking for 

 some precise timing.

 

 They currently are using sighting rods and triangulation, but it is 

 leaving too much judgment to the timing person.  I think that would also 

 apply to David's suggestion.

 

 They want to capture a full runway length of time, to have a more 

 representative view of true speed, rather than just a snapshot that a 

 radar gun might provide.

 

 An aircraft mounted lasar system might to it, but how.where would you 

 mount it to many different kinds of aircraft, and in such a way that it 

 would not create drag?

 

 These are the problems the SARL guys are trying to work around.

 

 Thanks for the suggestions.

 

 Sam

 

 On Mon, Dec 14, 2009 at 6:16 PM, Robert D. Taylor <FLYDAD57(at)neo.rr.com 

 <mailto:FLYDAD57(at)neo.rr.com>> wrote:

 

     Would a radar gun, like the cops use, work??

      

      

 

         ---

peter(at)sportingaero.com
Guest

Posted: Wed Dec 16, 2009 10:15 am Post subject: Kind of off topic. Speed sensor needed.

That reminds me! I saw this system used in a rowing race (over several 

miles), 2 cameras at the start, and 2 at the finish (in case of failure 

- 400 boats competing), all cameras filmed a 'master clock' to 

synchronise their own clocks before the start. Does require some effort 

after the speed runs, but only cost is 2 digital camcorders.



Peter



user9253 wrote:

 	  Quote: 	
		   



 Sam,

 How about using two clocks and two cameras?  Synchronize two very accurate clocks or stop watches and locate them at opposite ends of the runway.  Take fast pictures of the plane (including the clock) as it passes starting and ending points.  Review the pictures and note the times on the clocks and subtract.

 Joe



 --------

 Joe Gores





 Read this topic online here:



 http://forums.matronics.com/viewtopic.php?p 77622#277622

Sam

Joined: 18 May 2008
Posts: 135

Posted: Thu Dec 17, 2009 5:50 am Post subject: Kind of off topic. Speed sensor needed.

Bob, I think that 1 millisecond accuracy would be fine.  I think, just my opinion, if we could determine speed to one digit, it would be fine, eg. 234.6 mph.  Of course, two digits would be better.



Thanks to all who contributed suggestions.  I have complied and posted all of them to the SARL racers group.

 

For anyone who may be interested and wish to join, the e-mail list is at: http://groups.yahoo.com/group/SARL-Racers/

 	  Quote: 	
		  	
 Sam



On Tue, Dec 15, 2009 at 9:54 AM, Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)> wrote:

 [quote]--> AeroElectric-List message posted by: "Robert L. Nuckolls, III" <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)> 

 

 On 15 Dec 2009, at 4:46 AM, Robert L. Nuckolls, III wrote:

 

   Yes, photo cells looking straight up though a tube of 2 - 4"

   and length adjusted so that subtended angle of view is sufficiently

   small that the "shadow" of any part of the aircraft blocks the

   skylight. There are some self-calibrating level sensor circuits

   that will adjust for slow variability in sky light levels.

 

 E: This would work well - even if the light sensor wasn't in a tube, calibrated correctly, the change from direct sunlight to shadow should be enough to trigger a timing device of some description.

 

    You need to constrain the view angle . . .preferably

    without the use of optics. Long tubes seem the simplest

    approach.

 

 E: I would suggest two or three sensors in a line across the runway. Then use the second or third sensor that is triggered so that you know it's the wing that's causing the shadow both times, and not the wing on the first end, and the spinner on the second (which would skew results).

  

    That would be useful . . . depending on how "loose"

    the rules are for alignment with the runway ceneterline,

    an array of senors of any number could be used.

 

 E: Even a very simple microprocessor would be able to do the timing accurately enough, and could display the result on an LCD screen without too much effort.

 

    Sure, the jellybean PIC products offer one set of

    solutions.

 

 E: There is one downside to this, if you're keen on using either end of a runway, I guess you're looking for about 1km between sensors. That's a lot of wire to wind up at the end of each competition! There may be some simple radio interfaces that could be substituted...?

  

    Ten years ago, I helped some RC racers craft a timing system

    that required communication over a many acres of event

    venue. Off the shelf, 433 MHz transmiter and receiver

    modules provided links. Controlling atency and jitter in

    detecting event-edges would be a critical design goal.

 

    Assuming 300 mph (440 f/s) over 5000 feet of runway

    (11.3 seconds) a .1% accuracy in timing would call for

    control of variability to less than 10 milliseconds.

    If shorter runs are anticipated, the number gets tighter.

    +/-1 millisecond would probably be a practical design goal.

 

    Sam. Define "accurate". What are the design goals for

    the reduction of uncertainty?

 

    Bob . . . 

 



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 le, List Admin.

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[b]

_________________
Sam Hoskins
www.samhoskins.blogspot.com

nuckolls.bob(at)aeroelect
Guest

Posted: Thu Dec 17, 2009 7:09 am Post subject: Kind of off topic. Speed sensor needed.

At 07:28 AM 12/17/2009, you wrote:

 	  Quote: 	
		  Bob, I think that 1 millisecond accuracy would be fine.  I think, 

just my opinion, if we could determine speed to one digit, it would 

be fine, eg. 234.6 mph.  Of course, two digits would be better.


	


   It's technically feasible to get about any resolution

   you want but the costs of producing a user-friendly

   system can go up really fast as you add those decimal

   points!



   I did a drag track clocking system design many many

   moons ago wherein we used telescopes looking across

   the track at a strong light source. Magnification was

   so high that the entire illumination disk (6" roundel)

   filled the field of view in the telescope eyepiece.



   This system was vacuum tube sensor systems driving

   an HP counter/timer that was also vacuum tube and

   used columns of neon bulbs to display measured

   result.



   In this instance, ambient light had no measurable

   effect on triggering accuracy. It worked equally

   well at night or bright sunlight. Further, uncertainty

   of trigger timing was limited to the time it took

   for the vehicle to cross the 6" field of view. So

   back in 1974 or thereabouts, we had 0.1 mS resolution

   and 1 mS accuracy of timing over the distance between

   sensors. I think they installed them 10' apart at the

   end of the track. At 150 f/s, the interval of interest

   was about 66 milliseconds so we could calculate speed

   with an accuracy of about 1.5%  Folks were impressed

   by that kind of capability back then.



   Photo-detectors looking at sky-light through tubes

   (or even cheap telescopes) offers a means by which

   uncertainty in  trigger timing (dd/dt) can be very small.

   But as others pointed out, very small apertures,

   require resolution of the differences between a prop

   spinner and leading edge of wing. So an array of

   sensors driving a discriminator calling for two

   or more sensors to be triggered before the hack

   is marked seems to offer a solution.



   The use of radio to tie the ends of the system together

   is quite practical. There are inexpensive, off the shelf

   timers available with micro-second resolution/accuracy.

   The proof of the pudding is in the sensor array.

        Bob . . .



         ---------------------------------------

        ( . . .  a long habit of not thinking   )

        ( a thing wrong, gives it a superficial )

        ( appearance of being right . . .       )

        (                                       )

        (                  -Thomas Paine 1776-  )

         ---------------------------------------

nuckolls.bob(at)aeroelect
Guest

Posted: Thu Dec 17, 2009 8:44 am Post subject: Kind of off topic. Speed sensor needed.

 	  Quote: 	
		     The use of radio to tie the ends of the system together

   is quite practical. There are inexpensive, off the shelf

   timers available with micro-second resolution/accuracy.

   The proof of the pudding is in the sensor array.


	


    What is the size of the "box" that the aircraft must stay inside

    and what height above the ground is the lower edge of that box?



    I occurs to me that if you have a 100' wide by 100' high

    box with lower edge at 100' agl, the number of sensors

    required to cover that up-look area with any resolution

    could be pretty big. Fortunately, they're cheap and easy

    to build. So is the electronics for resolving the presence

    of an airplane "shadow"

        Bob . . .

                    ////

                   (o o)

    ===========o00o=(_)=o00o=========

    < Go ahead, make my day . . .   >

    < show me where I'm wrong.      >

    =================================

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