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Author Topic: ATC GS determination question  (Read 5141 times)
beechsundowner
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« on: November 18, 2011, 05:21:32 PM »

Question for the ATC folks

Went playing today with fairly high winds aloft in a Cessna 172.  While poking along, I decided how slow could I go in GS.

Attached is what I got from KJAN approach showing 2 knot ground speed.  My GPS on a forthcoming video I will post under this thread showed 14 knots yet when I panned out side the window, plane was hovering and ever so slightly backing up.  ASI was indicating 0 knots.  Winds aloft forecast seemed spot on at 30 knots.

My question would be is how is the GS computed if GPS is not where the data is derived from?

As you will see from the forthcoming video, the ATC reported GS seem to true out to what I saw out the window and the GPS in the plane seemed incorrect?
« Last Edit: November 18, 2011, 05:47:27 PM by beechsundowner » Logged

DhruvK
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« Reply #1 on: November 19, 2011, 12:42:33 AM »

Not to nitpick, but if your ASI was indicating zero, how were you maintaining lift?
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davolijj
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« Reply #2 on: November 19, 2011, 01:03:22 AM »

ATC groundspeed information is based on radar track data for the aircraft receiving radar service.

I'm not sure about the technical specs for airport surveillance radar (ASR) like the one they have at KJAN, but for the EnRoute radar automation system the groundspeed is derived by an average of a track's last three to five radar hits, depending on speed variation.

ATC radar data is the only source of speed and direction for a given aircraft.  Altitude information is transmitted to the radar system via mode-C transponder.  No information is transmitted to the ground by your GPS equipment under the current system.  Supposedly ADS-B and NextGen are going to change all of that but as you can imagine the programs are a long way away for the majority of the flying public.
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beechsundowner
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« Reply #3 on: November 19, 2011, 10:44:09 AM »

ATC groundspeed information is based on radar track data for the aircraft receiving radar service.

I'm not sure about the technical specs for airport surveillance radar (ASR) like the one they have at KJAN, but for the EnRoute radar automation system the groundspeed is derived by an average of a track's last three to five radar hits, depending on speed variation.

ATC radar data is the only source of speed and direction for a given aircraft.  Altitude information is transmitted to the radar system via mode-C transponder.  No information is transmitted to the ground by your GPS equipment under the current system.  Supposedly ADS-B and NextGen are going to change all of that but as you can imagine the programs are a long way away for the majority of the flying public.

Thanks Dave,

I didn't realize the GS is based on an averaged value.   Understanding as you say it depends on speed variation, do you know the time span between radar hits for the system comes up with an average?  And would that average be a rolling average covering the last 5 hits or would it span the time from the first hit on the flight to the latest hit? 

Probably in the full scheme of things not that much difference but interesting to learn these little behind the scene details.
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beechsundowner
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« Reply #4 on: November 19, 2011, 12:07:23 PM »

Not to nitpick, but if your ASI was indicating zero, how were you maintaining lift?

Not an unreasonable nit pick.   I believe that CAS (Calibrated Air Speed) needs to be considered of which I'm not sure it's a reported value below V values of a particular airplane.

You can see the flight video and all the speed values I brought up in this thread as I just uploaded it to You Tube

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davolijj
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« Reply #5 on: November 19, 2011, 11:32:13 PM »

Thanks Dave,

I didn't realize the GS is based on an averaged value.   Understanding as you say it depends on speed variation, do you know the time span between radar hits for the system comes up with an average?  And would that average be a rolling average covering the last 5 hits or would it span the time from the first hit on the flight to the latest hit? 

Probably in the full scheme of things not that much difference but interesting to learn these little behind the scene details.

As far as the time span goes I can tell you that most ASRS Enroute radar sensors sweep 6 times per minute or once every 10 seconds.  So regardless of an aircraft's speed the hits will come 10 seconds apart.

As for the second part of your question regarding the average, I'll have to do some checking.  I'll get back to you on that.

And by the way, the name's Jim.
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JD
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« Reply #6 on: November 20, 2011, 02:38:02 PM »

Not to nitpick, but if your ASI was indicating zero, how were you maintaining lift?

I've done it a few times in slow flight, with just me in the airplane and light fuel load.

The orientation of the end of the pitot tube is fixed relative to the aircraft.  When you're trimmed for high AoA, the pitot tube is no longer aligned with the relative wind, and is so far out of alignment, that at low airspeeds, there's not much wind being forced into it.  No ram air pressure in the tube: no airspeed measurement.

--Carlos V.
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derekm1986
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« Reply #7 on: November 20, 2011, 04:40:30 PM »

It is my understanding that the radar determines groundspeed using the doppler effect...fresh with every sweep.  Also, the approach controller said "2".  The controller was looking at a display where speeds are shown in tens of knots, therefore, you were actually showing 20 knots of groundspeed.  An actual speed of 2 knots would not register on such a display.
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flyflyfly
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« Reply #8 on: November 20, 2011, 07:40:21 PM »

The doppler-effect only accounts for an object's speed component that's directed towards the observer. So, it's not enough to compute true groundspeed, unless the a/c was flying straight towards (or from) the radar station. Successive scans are still required to get all the data.
Clever radars use both effects - such as military target tracking radars: doppler-radar provides immediate data on an object's approach speed, while quick successive scans monitor the object's direction of movement - and true speed.
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notaperfectpilot
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« Reply #9 on: November 21, 2011, 08:00:59 AM »

What happened to your attitude indicator? Like, what caused it to be messed up like that?
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beechsundowner
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« Reply #10 on: November 21, 2011, 08:30:45 AM »

What happened to your attitude indicator? Like, what caused it to be messed up like that?

My guess is that it's getting close to meeting it's usage time and overhaul will be next in order.

Also, the approach controller said "2".  The controller was looking at a display where speeds are shown in tens of knots, therefore, you were actually showing 20 knots of groundspeed.  An actual speed of 2 knots would not register on such a display.

Being a user from the other side of the microphone and not sure myself, I always thought the ATC display similar to what flight aware displays?

http://flightaware.com/live/aircrafttype/C172

Highlight a plane and it does show the actual speeds down to the knot
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davolijj
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« Reply #11 on: November 21, 2011, 07:47:34 PM »

ATC displays are like flightaware in that both use datablocks, but that's about where the similarities end.

As far as speed goes the Enroute automation platform does display speed to the nearest knot, terminal systems display speed to the nearest ten knots.  Here's a pretty cool webpage with an explanation of both systems.

http://virtualskies.arc.nasa.gov/communication/10.html
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JD
beechsundowner
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« Reply #12 on: November 21, 2011, 07:54:46 PM »

ATC displays are like flightaware in that both use datablocks, but that's about where the similarities end.

As far as speed goes the Enroute automation platform does display speed to the nearest knot, terminal systems display speed to the nearest ten knots.  Here's a pretty cool webpage with an explanation of both systems.

http://virtualskies.arc.nasa.gov/communication/10.html

Very cool, thanks Scott!

Hey, it's the lack of O2 at a mere 2500 feet that made me do it.

Seriously though, really appreciate your input Jim along with everybody else. 

So, what KJAN approach reported 2 was close to what GPS was, it just rounded up.  Gotta luv learning!
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notaperfectpilot
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« Reply #13 on: November 21, 2011, 10:45:32 PM »

Gotta luv learning!

got to do it some time right? smiley
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