RE: [Paparazzi-devel] distance measurement for landing.

[Elden Crom]

Joe, your bumming me out on the altimeter route to precision landing....... 
Painfully you make a convencing argument....

Perhaps another route. No cammera, only light intensities needed.

Do you know how a/c land on an aircraft carrier (old style anyway)?
One of the systems that they use is to have a set of light and Fresnel lens on 
the deck of the carrier.
The pilot looks at the lights (the ball), if they are on the correct glide 
slope it appears green, too low or too high, left or right, it appears to them 
as a diffeent color. (not exactly right but for my purposes....)
(For a better explaination 
http://everything2.com/title/How+to+land+a+jet+plane+on+an+aircraft+carrier
 , http://answers.yahoo.com/question/index?qid=20071105145150AAYcwq8)

Anyway,
Perhaps a simmilar system with a box of lights on the ground and 6 light 
sensors on the A/C.  Three colors for up and down. Threee colors for left and 
right. (when I say 'colors of lights' I proabably mean different frequencies of 
infrared.)

Therory of Operation (only describing up and down for now,left and right will 
be similar):
The box of lights shines 3 beams of light. The beams diverge at ~3degrees.
The too high light is red.
The too low light is blue.
The just right light is green.
X RRRRRRRR
X      RRRRRRRR
X AC       RRRRRRRR
X GGGGGG       RRRRRRRR
X       GGGGGG     RRRRRRRR
X             GGGGGG   RRRRRRRR
X BBBBBBBBB         GGGGGG RRRRRRRR
X          BBBBBBBBB     GGGGGG RRRR[----Box---]
X                  BBBBBBBBB  GGGGGG[----of----]
X                          BBBBBBBBB[--Lights--]
The A/C approachs the landing site via GPS (+/-20meters=GPS + flight error?) at 
~1 kilometer out the A/C sees the green only.
The navigation system swithes to moth mode (ie follow the lights).  If it sees 
red, more elevator; if it sees blue, less elevator; if it sees green, do 
nothing.  As the thing gets closer to the box, it will see pairs and maybe all 
three colors because of bleed. So basically just fly the plane such that Red 
and Blue have the same intensity.

Come to think about it, the desire for the Green is more a human 
thing...perhaps just Red and Blue and fly the path that keeps them the same 
intensity.  (This will be all sorts of fun getting the PID loop constants 
adjusted!)

Back into 3D space,
So that would just require, 4 light sensors with different color filters on 
them to be on the AC. Less than 20 grams? They would all be aligned the same 
(downward at about 3 degrees) so they could be flat on one PCB.  The box of 
lights could just be 4 flash lights with lens....maybe even 1 flash lights with 
a four color lens.

Or even better, one ground light with a lens that is in thirds and only 3 light 
sensors on the AC.

It might even (maybe, perhaps, questionably?) be possible with just one color, 
but now the AC would be constantly drifting left/right and up/down  trying to 
cross the highest intensity portion of the beam, kinda like WiMax (WiFi's 
bigger bother) does in the time domain......but the SW algoithm gets a bit 
compilcated!

Enough babling for now.....

Elden






-----Original Message-----
From: gisela.noci [mailto:address@hidden 
Sent: Wednesday, November 11, 2009 12:58 PM
To: address@hidden
Subject: RE: [Paparazzi-devel] distance measurement for landing.

Hi Eldon.

Ok, had a look on the web site. Pity they do not show the other side of the 
PCB....Anyway, as I intimated, the only way they can achieve 140mm resolution 
is by amplifying the sensor, and limiting the range( as they say, from 50 to 
approx 2000meters)  to squash more bits per meter in. They certainly cannot 
have a 16 to 18bit A/D on that little PCB! The lower 4 to 5bits would be in the 
noise... And, most important, they talk of resolution, which is fine for 
maintaining an 'altitude' whatever that might be , but they do not mention the 
units accuracy and that is what is important in trying to use it for AGL 
determination for autoland. There is simply no unit accurate enough, and 
repeatable enough, to be able to use for autoland. In addition, the atmospheric 
model used for pressure- altitude calculation varies all over the world...

That is why Pilots are told by control tower to set their altimeter to a 
specific 'altitude' in that towers controlled airspace. It might not be an 
accurate altitude, but at least all other aircraft in the same airspace are 
referencing the same 'inaccurate' altitude. Resolution is important, to insure 
you do not drift up or down without knowing it, but accuracy is so poor, that 
it is not used in absolute terms.

It seems that sensor is 'read' by observing the led flash count, in which case 
the output rate is going to be VERY slow - seconds - rendering it useless even 
for altitude control.  

Our autopilot flies to pressure altitude, which is augmented in a kalman filter 
against GPS Alt to get some semblance of reasonable altitude. The sensor sample 
rate is at 60Hz, while the GPS is at 4Hz. The pressure Alt is used to determine 
a climb/descent rate, to feed the pitch/altitude loops.
So, sampling at a rapid rate is essential for good altitude control response. 

However, as to what sample rate is best for landing by means of pressure alt 
sensor, all I can say is - Don't.... The only way to land with a pressure 
sensor as your only reference is to sample  at least at a 50Hz rate, and to use 
that to derive a descent rate and then to approach from a long way, descend to  
a 'safe' altitude, say 30 or 40 meters AGL at least, and then just keep wings 
level, speed constant at 3 or 4meters/second above stall, and fly in at a 
controlled descent rate of around 1 meter/second, and wait till you reach 
terra-firma. It works, I have done this a number of times, but you need space, 
lots of it. For example, at 16meter/second approach, with a 1m/sec descent rate 
you will cover 40X16 meters = 640 meters. If the accuracy of the 'safe' 
altitude was such that is was actually 50 meters AGL, then you will cover 
800meters......No magic bullet here, I fear. A 1/4 mile is to short for the 
above scenario; no room for error. If your approach speed is 5meters/sec then 
maybe, but at that speed, just catch it in your hand as it passes by......

I am not sure what you mean by not wanting to use 'radar' because of sloping 
terrain; that is in fact the best reason to use sonar/radar, and then to fly at 
a fixed height above ground,ie, terrain following flight, and then descend when 
at the correct point, reducing height AGL at 1meter/second, flying an actual 
trajectory to the touchdown point. That is what we do, and we touch down at 
20m/s ( a HEAVY plane!!!), within plus/minus 15meters each time of the 
designated touchdown point. That requires better than 1 second accuracy in 
descent rate, speed and pitch control.

We do the same on a much bigger plane, 6meter wingspan and 145kg all up 
weight.. 

And if you want to use radar to detect the height AGL of the flat patch, just 
use it to land with......

I have watched a $300K UAV with a infrared camera in the nose try to land 
3meters below ground level, at 4meters/sec descent rate - no more landing gear 
and droopy wing resulted.......

But, don't give up. If you really want to try to do landing with a pressure 
sensor reference, I would suggest that you use a sensor such as the Honeywell 
ASDX015, a 4volt output for 100 to 1013millibar sensor that is temp compensated 
pretty well, amplify and offset the output to give you good resolution for your 
desired altitude range, make 2 identical units, keep one on ground and use any 
change on that one to correct the airborne one. This should give you a short 
term accuracy in the 1 to 3 meters range, and then land by approaching to 5 or 
so meters AGL, and then wings level, kill throttle and glide in. Works (almost) 
every time ..

Happy landings

 Joe

-----Original Message-----
From: address@hidden
[mailto:address@hidden
] On Behalf Of Elden Crom
Sent: Wednesday, November 11, 2009 7:44 PM
To: address@hidden
Subject: RE: [Paparazzi-devel] distance measurement for landing.



Thanks Joe, 

The link must have clipped, the long way is http://www.hobbycity.com On the 
left side choose DataRecording Then ShadowWing Then How High

http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=6776&Prod
uct_Name=WS_HowHigh_Altimeter_Feet_&_Meter_(New_Version)
 
I just go it in the mail, but I haven't had time to play with it much yet.
The output is already digital and they taught me something.
(off topic) LEDs work in reverse! (same band gaps as a solar cell, I should 
have already know this) The output led also a light sensor.  Try it yourself, 
just hook up a digital meter (ie HiZ) and shine a light at a random LED in your 
junk drawer, I got about 0.5 volt with a red LED and a flash light.

Anyway, they also have a 'reader', so I'm hoping to have TWOG read the already 
digitized data (from what I'm hearing, it will need to be as farway from the 
antennas as possible!)

Another thing I don't know is how slow the reading will be. (I'm guessing that 
after internal filtering it will be slowish, like 1 sec or worse, I've got my 
fingers crossed for 100milli-sec)

Any idea what sampling rate I must have to be able to use it as landing data?  
If I have a long approach (1/4 mile) does this sound plausable?  My issue for 
not using a radar/sonar range finder is very slopped terrain with a smallish 
flat spot to land on.


My worst case may be a radar/sonar first pass over flat spot to adjust the 
altemeter and a second pass to actually land.


-----Original Message-----
From: gisela.noci [mailto:address@hidden
Sent: Tuesday, November 10, 2009 12:27 AM
To: address@hidden
Subject: RE: [Paparazzi-devel] distance measurement for landing.

What 'Baro' sensor?

The maths does not lie.

We cannot generalize when categorizing sensors in this class of performance.

 The picture I painted below is typical of a fairly good quality MEMS type 
airdata sensor. For example, the Honeywell ASDX015 is a 0 to 103.4kPa sensor, 
fully temp compensated from 0 to 85degC with 4V output span, and 2%/V accuracy. 
This means that 4V output covers the full 10,000m altitude pressure range range 
and that means 4/10000 = 400uV per meter. To achieve 14cm, you need to be able 
to sample around 50uV. If you have ever tried this it is not easy, esp in the 
noisy environment of RF modems and video transmitters, etc

In addition, the inherent sensor accuracy is typically in the order of 1 to 2% 
per volt, ie, at sea level, lets say at a sensor o/p of 1volt  the error
could be 10mv, or 25 meters!!   

One can amplify the sensors output, and offset the output, to generate a large 
sensor voltage change for a smaller altitude change, thereby reducing the 
number of sampling bits of the A/D, althought then the sensor range is limited. 
This is often done, since most of our applications do not go to 10,000meters,(I 
suspect this is what they have done in the sensor in the
quadrotor) .....But, there is no free lunch here - you have to use amplifiers 
that have very low input and output noise, very low DC offset, and good temp 
stability. If you have access to a high accuracy pressure calibration system, 
you could put the sensor pack in the environmental chamber, and cycle the temp 
from 0 to whatever, and vary the pressure to the sensor at each temp, and 
generate a calibration curve to eliminate all the errors,ie, temp, offset, 
drift, etc but the cost.....(takes many hours to
do) 

In essence, obtaining repeatability, high accuracy, and temp stability, to 
achieve centimetric pressure alt accuracy, is what $10k sensors are all about. 

We do this type of work at our company ( we are a UAV manufacturer) and all 
this come from painfull experience..

Have fun all
Joe
PS - I could not access the sensor referenced by Eldon - seems the only one I 
find is a 1meter 'accuracy', and 400m or so range??


--Original Message-----
From: address@hidden
[mailto:address@hidden
] On Behalf Of antoine drouin
Sent: Monday, November 09, 2009 10:48 PM
To: address@hidden
Subject: Re: [Paparazzi-devel] distance measurement for landing.

this is done with a baro and a 10 bits ADC

http://poinix.org/video/booz2_vert_ctl_v0.2_perturb.mp4

On Mon, Nov 9, 2009 at 9:31 PM, gisela.noci <address@hidden> wrote:
> Baro_altitude sensors can certainly measure to that kind of accuracy. 
> The difficulty arises in digitizing the output ( via an A/D convertor) 
> and ensuring that the noise on the signal is very low. Some typical 
> sensors
give
> a 0-4v output for a pressure variation from 1013millibar (close to sea
> level) to around 200millibar ( a difference of 813mb, 81kPa, about 
> 8000meters alt change). This means to achieve 14cm resolution, you 
> need 8000m/0.014m = 57000 bits, or increments. This implies at least a 
> 16bit
A/D,
> and then remember that each bit is 4v/57000 = 70uV !!!   Couple this 
> with the sensor output variation over temp ( not insignificant!) and 
> the noise
on
> the signal, you will be VERY lucky to get 1meter accuracy, and that at 
> a specific temp.
>
> Ask any real pilot - land with your eyes, not your altimeter!!!
>
> Ground ranging (Radio-Altimeter, radar, ultrasound, etc is the only 
> way to go.
>
> Joe
>
> -----Original Message-----
> From: 
> address@hidden
>
[mailto:address@hidden
> ] On Behalf Of Elden Crom
> Sent: Monday, November 09, 2009 7:57 PM
> To: address@hidden
> Subject: RE: [Paparazzi-devel] distance measurement for landing.
>
>
> I was considering the pressure/altitude route
>
>
http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=6776&Prod
> uct_Name=WS_HowHigh_Altimeter_Feet_&_Meter_(New_Version)
>
> But I think this may require one on the ground station as well to deal
with
> a cold front coming in and changing the barometric pressure.
>
> Supposedly, this thing can measure altitude accurately to 14cm.
> Does anybody believe that? (I have on one order to see if it's 
> actually
that
> accurate)
> I wonder how to mount it such that it is not affected by wind speed.
>
>
>
>
> -----Original Message-----
> From: Dirk-Willem van Gulik [mailto:address@hidden
> Sent: Fri 11/6/2009 1:04 PM
> To: address@hidden
> Subject: Re: [Paparazzi-devel] distance measurement for landing.
>
> Roman Krashanitsa wrote:
>
>> The http://www.maxbotix.com/ founder did some comparison for 
>> performance of his sensors with Sharp sensors you are linking to. As 
>> far as I remember, there are some plots in the FAQ section and in 
>> "Preformance Data" section that might be useful for you.
>
> I found that even very simple DIY shop ultrasonic meters; like below:
>
>
http://www.sella.co.nz/general/building-renovation/tools/other-tools/503tx7/
>
> which can be had for 5 to 10 euro's at the local DIY market - are 
> easily hacked and not that unreliable.
>
>
> Thanks,
>
> Dw
>
>
>
>
>
>
>
>
> _______________________________________________
> Paparazzi-devel mailing list
> address@hidden
> http://lists.nongnu.org/mailman/listinfo/paparazzi-devel
>


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