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Patrick

Wed Mar 11 2015, 07:53AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Im investigating the use of radar altimetry for my drone, as were supposed to stay below 400 feet AGL. are there particular frequencies of low power that can be used to see "soft" (meaning non-metallic) objects ? (meaning dirt, trees and such) from a mostly directional cone would be desirable. perhaps +/- 15 feet in resolution.

Of course i dont want to use time of flight pico second crap. i geuss itd need to be phase difference type. Is any of this possible, and still light enough to fly? or do i need 12 million $ ... ?

Dr. Slack

Wed Mar 11 2015, 09:08AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

You don't need to use pico second crap, the speed of light in air is 1ns per foot, which means 2ns per foot for a there and back bounce.

With a +/- 15ft tolerance, you could aim for a personal ceiling of 385ft nominal, with +/- 30nS timing resolution.

You need to find a frequency, or a processing regime, that can see the ground but can't see trees, otherwise you may go soaring way over height as you overfly trees.

Chirp frequency difference is probably the easiest to use, as long as your local emissions regulations permit that form of modulation.

Sulaiman

Wed Mar 11 2015, 10:38AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

an optical range finder may be simpler,
could be as simple as a laser module and the on board camera + some digital processing?

Trees, tall buildings etc. would give false height above ground though.

Shrad

Wed Mar 11 2015, 12:43PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

can't you do GPS? relying on ground would not be ideal as there are too much material types to discriminate...

if you want to get through trees but still want to bounce on water, that will not be simple either, so I guess the most reliable reference is GPS (and I assume you already have GPS somewhere in your copter)

Sulaiman

Wed Mar 11 2015, 02:39PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

GPS would only give absolute altitude, or altitude relative to starting position,
if the copter flies over a valley for example, it would be much more than maximum allowed height above ground.

P.S this ruling means that if you want privacy you should live more than 400 ft. above the ground

Ash Small

Wed Mar 11 2015, 02:42PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Is the regulation 400 feet above sea level, or 400 feet above the ground?

Where I live, at the top of a hill in a military training area, we have 'copters and jets flying past the windows, and down the valleys at much lower altitudes than 400 feet. Even had a couple of typhoons 'dog fighting' up here the other day, that was LOUD

Sulaiman

Wed Mar 11 2015, 02:44PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

If you live more than 400 feet above sea level
that ruling would ruin your hobby !

A long time ago I was rock climbing in Wales,
a military jet flew past BELOW me !

I think that the military have their own rules.

Dr. Slack

Wed Mar 11 2015, 03:26PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Shrad wrote ...

can't you do GPS? relying on ground would not be ideal as there are too much material types to discriminate...

if you want to get through trees but still want to bounce on water, that will not be simple either, so I guess the most reliable reference is GPS (and I assume you already have GPS somewhere in your copter)

GPS accuracy is horrible for altitude, something to do with not seeing satellites below ground, which is why most walking GPS thingies also have baro altimeters, then you'll have to reference it to what the local ground elevation is. You can do this by querying a site that gives altitude versus lat/lon, but you'll need a live internet connection to fly.

<edit> Better would be DGPS with respect to a receiver on the ground, perhaps at your launch/control point, as long as the ground level didn't vary too much over the operational area. </edit>

radiotech

Mon Mar 16 2015, 07:21AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Your propellers might be re crafted some how so they make a +1 mach
sonic pulse(s) in sequence and geometrically aimed down. It possibly
could return a timed echo.

wiki
"designed as a ... part of the whip exceeds the speed of soundâ€”thereby creating a small sonic boom"

Uspring

Mon Mar 16 2015, 10:08AM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

Dr. Slack wrote:

GPS accuracy is horrible for altitude, something to do with not seeing satellites below ground...

Sounds plausible. GPS relies on time of flight differences between signals from different satellites. A time of flight difference between 2 signals from 2 satellites above you defines a vertically oriented plane of possible positions. More satellites will define several such planes, whose intersection gives you the current position. Intersections of nearly vertically oriented planes are prone to altitude error if not located precisely. Satellites near the horizon will help, but there might be additional error coming from a long passage of the signal through the atmosphere, since the speed of light depends on its density.

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