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Primary Vs. Secondary Radar?

kevinb
post Sep 3 2007, 03:05 PM
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Hi everyone! I am a new guy, so I apologize if I put this in the wrong section, or if it has already been discussed etc.....

There is something about 9/11 I have been curious about. I just recently thought of this, so I haven't really done any research on it yet, but maybe you can help.

I remember specifically hearing on the news that the "hijackers" had turned off the transponders in the aircraft so the planes had become invisible? But in flight school we were taught that only "Secondary Radar"(SSR) requires a transponder onboard an aircraft to report position information.

We we re also taught that "Primary Radar"(PSR) does not require a transponder onboard for position identification. So if the area was covered by Primary Radar the transponders would not be an issue. Their position would still be known. I would expect the radar around the Pentagon to be the best of the best.

I live on the west coast, and all of my flying has been here. I have never flown on the east coast, or in any of the airspace where the events of 9/11 took place. I would like to know if any of the airspace in the associated areas are in fact covered by "Primary Radar".
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andrewkornkven
post Sep 4 2007, 12:58 PM
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Three out of the four planes were in areas covered by primary radar when the transponders were turned off. For these three planes, it was no problem for FAA ATC controllers to use primary radar to track the planes all the way to their final crash sites.

The only plane to be in an area of primary non-coverage was AAL77. I'm not sure if the primary radar was down temporarily for maintenance that day, or if it is an area of permanent non-coverage. A primary target did appear, however, eight and a half minutes later tracking toward the Washington D.C. area. That target was presumed to be AAL77, but was never positively identified as such.
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Robin Hordon
post Sep 5 2007, 03:23 AM
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I would like to point out to this questioner that the "data" showing the primary target that andrewkornkven alludes to as being "appearing only 8 minutes" after AA77 was lost to positive radar contact, was actually garnered from data in a radar RECONSTRUCTION some time AFTER 9/11/2001, and that it was not seen in real time 8 minutes after AA77 was lost to positive radar contact. The existence of this primary target was only firmly developed several days AFTER 9/11/2001. There was no such real time "appearance" as suggested.

Therefore, the statement that the primary target "appear[ed]" eight minutes later is not accurate in the sense that it was NOT observable to the air traffic controller who was last working, and thus responsible for AA77.

In real time, A primary target in question, ie: the one seen by Danielle O'Brien west of Dulles airport heading towards DC, was noticed by a now "engaged" controller [O'Brien had been "alerted" by her supervisor to look for such aircraft just a few minutes earlier], ie: over 25 minutes after positive radar contact had been lost on AA77 during its DESCENDING left turn in eastern Ohio.

[AA77 lost on radar 08:56...primary target seen approaching Dulles at 09:24]

However, it is true that nobody ever positively re-identified the eastbound target noticed west of Dulles by O'Brien as being AA77, it has only been presumed as such.

There are reports that other controllers working peripheral sectors in FAA facilities along the "assumed route of AA77" may have also seen an eastbound high speed primary target at some time before 09:24. Or, the delayed radar reconstruction may have shown that such a target was "available" to be seen along this routing.

Love, Peace and Progress with Clean Elections using PAPER BALLOTS

Robin Hordon
former air traffic controller
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andrewkornkven
post Sep 6 2007, 10:39 PM
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QUOTE (Robin Hordon @ Sep 5 2007, 07:23 AM)
I would like to point out to this questioner that the "data" showing the primary target that andrewkornkven alludes to as being "appearing only 8 minutes" after AA77 was lost to positive radar contact, was actually garnered from data in a radar RECONSTRUCTION some time AFTER 9/11/2001, and that it was not seen in real time 8 minutes after AA77 was lost to positive radar contact. The existence of this primary target was only firmly developed several days AFTER 9/11/2001. There was no such real time "appearance" as suggested.

Therefore, the statement that the primary target "appear[ed]" eight minutes later is not accurate in the sense that it was NOT observable to the air traffic controller who was last working, and thus responsible for AA77.

Robin,

I realize that the 9/11 Commission Report says that the target was reconstructed, not observed in real time. But there is at least one report that it was indeed observed:

QUOTE
Washington flight control notices a new eastbound plane entering its radar with no radio contact and no transponder identification. They do not realize it is Flight 77. They are aware of the hijackings and crashes of Flights 11 and 175, yet they apparently fail to notify anyone about the unidentified plane. [Newsday, 9/23/2001; 9/11 Commission, 6/17/2004]
-Terror Timeline


The Newsday article has this paragraph:

QUOTE
Back in Leesburg, air traffic controllers knew at about 9:05 a.m. that they had a new eastbound plane on their radar, but they didn't know it was Flight 77. The aircraft had entered their airspace with no radio contact and no transponder identification.


I don't trust the 9/11 Commission Report any farther than I can spit. Put yourself in the shoes of the controllers at Washington Center. I believe they knew about the hijackings in NYC, and I would bet every controller was sitting in front of their scopes, with their primaries on, with their eyes peeled for anything unusual. How could they not see a speeding primary moving through their airspace?

(I guess I have to ask, what do you mean the primary was only "developed" several days later? Do you mean they reconstructed the radar, and saw the primary, but that no one observed it in real time? Or do you mean they somehow reconstructed data that was not observeable in the first place? I have no experience with such radar reconstructions, etc.)

Anyway, it's not hard to imagine these Washington controllers seeing this primary target, telling their supervisors, and being told in turn-- correctly-- that the military was already aware of the situation.
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amazed!
post Sep 9 2007, 06:39 PM
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Well for this civilian a$$, I just wonder how thoroughly, to what degree, can the radar displays be spoofed?

If they can spoof them at all, it seems to me that makes most of it worthless as to exactly what happened with aircraft tracks.
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dMz
post Jan 2 2008, 02:37 AM
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Some radar and aviation navigation information for the laypersons and the not-so-laypersons out there.

My work with the USAF 84 RADES data sent me looking for latitude/longitude uncertainty estimates- I'll post that here after some calculation. On the way, I found some very good information about radar and navigation systems.

Start with wikipedia (these articles looked accurate):
http://en.wikipedia.org/wiki/Radar

http://en.wikipedia.org/wiki/Air_traffic_c...#Radar_Coverage

Secondary Surveillance Radar (SSR)
http://en.wikipedia.org/wiki/Secondary_surveillance_radar

They show how reflected multipath "ghosts" can show up on PRIMARY or "ground based" PASSIVE radar (like RADES "Height" at 102,000 feet Above Mean Sea Level [AMSL] and at negative altitudes):
http://en.wikipedia.org/wiki/Image:Multichemin.svg

As Turbofan stated, primary/passive radar is like a policeman's radar gun "clocking" an automobile, although the car has no transponder equipment.

Probably the best site overall that I found (in English, French, German, etc.) was at:
http://www.radartutorial.eu

BE SURE to check the Secondary Radar (SSR) diagram (RADES Mode C-type airplane-based "transponder" system) there- it is excellent:
http://www.radartutorial.eu/13.ssr/sr04.en.html
http://www.radartutorial.eu/02.basics/rp05.en.html

And the increasingly technical links:
http://www.vectorsite.net/ttradar.html

http://earth.esa.int/applications/data_uti...ar_equation.htm

Primary vs. Secondary Radar
http://www.bbc.co.uk/dna/h2g2/A588684

Radar Range Equation
http://www.bbc.co.uk/dna/h2g2/A743861

Meterological Radars (some common information with aviation)
http://apollo.lsc.vsc.edu/classes/remote/l...onal/index.html

Some good related aviation navigation links are:
Distance Measuring Equipment (DME)
http://en.wikipedia.org/wiki/Distance_Measuring_Equipment

VHF Omnidirectional Range (VOR)
http://en.wikipedia.org/wiki/VHF_omnidirectional_range
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dMz
post Jan 2 2008, 03:44 AM
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Now for the RADES specific information. I have put together a file with some maps and diagrams of the Northeastern US RADES radar sites (with grid in lat/lon degrees and radar range circles in nautical miles):

http://www.orbitfiles.com/download/id2414427832.html

NOTE: The "ghost/phantom" primary radar diagram I posted above does NOT explain "curious" Mode C secondary radar (SSR) altitudes, or the transponder working for about 15 minutes after the RADES primary radar went "stealth" several minutes before the respective aircraft transponders (AA77/UA93) were "turned off."

RADES site codes
-------------------
OCA J01 Oceana, VA
QFF J02 Ft. Fisher, NC
PLA J50 The Plains, VA
GIB J51 Gibbsboro, NJ
RIV J52 Riverhead, NY
NOR J53 North Truro, MA
BUC J54 Bucks Harbor, ME
REM Remsen, NY
DAN Dansville, NY
EMP J58 Empire, MI
NSH J60 Nashwauk, MN
DTW J62 Detroit, MI
CAR J63 Caribou, ME
BAR R51 Barrington, CANADA
OM4 J01 Mode 4 XMit (OCA?)
GM4 J51 Mode 4 XMit (GIB?)
RM4 J52 Mode 4 XMit (RIV?)
NM4 J53 Mode 4 XMit (NOR?)
BM4 J54 Mode 4 XMit (BUC?)
WSD Wheeler/Sack AAF
-----
Regarding altitude accuracy, the USAF RADES "readme.doc" states:
------
2. Radar Capabilities. (page 2, para. 2)
"In general, the most reliable height information comes from aircraft transponder systems turned ON, responding to mode C interrogations. Mode C height accuracy is limited to +/-100 feet (assuming standard barometric pressure), the value of the least significant bit in the mode C altitude report. Because mode C height is always based on a standard barometric pressure setting, it is not corrected for local pressure conditions, although an approximate correction can be made based on local atmospheric data (D-value). Note, aircraft true height is found by adding local D-value to the mode C reported height. The D-value generally varies +/-1000 feet. The ARSR-4 3-D height data is generally accurate to within +/-2000 feet when the aircraft is within 175 nmi of the radar site. The primary range accuracy limitation for both primary and secondary radar systems is +/-1/8 nmi due to the target reporting format employed by the radar system. Azimuth accuracy is limited to approximately 0.2 degrees for both primary and secondary radar systems."

"5. Data Adjustments. The accuracy of radar data projection provided by RS3 is limited by factors discussed in paragraph 2. The RS3 uses the Earth-Centered Earth-Fixed (ECEF) method to calculate the latitude and longitude positions of a radar return based on the target height, range and azimuth from the radar site."
-----
Earth Centered Earth Fixed (ECEF) geodesy coordinates:
http://www.colorado.edu/geography/gcraft/n...um/datum_f.html

I have also used Spherical Law of Cosines and Vicenty geodesy/mapping formulae in my newest velocity calculations. The more spreadsheet-friendly Law of Cosines that I used is the least accurate (especially for short distances), but my calculations in Excel should be accurate to within +/-1 meter based on the "accurate enough for USAF/FBI" RADES latitude/longitude .XLS numbers, and should be accurate enough for my velocity calculation purposes.

More on geodesy (finding distances from GPS or lat/lon):
http://williams.best.vwh.net/gccalc.htm

http://www.movable-type.co.uk/scripts/latlong.html
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JackD
post Jan 2 2008, 04:48 PM
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Note that the FAA & NORAD both employ air route surveillance radar-4 systems
ARSR-4

see
ARSR-4

and these more sophisticated radars can estimate altitude based on primary radar return, without need of secondary (transponder) ID.

the RADES squad based out of Utah's Hill AFB keeps personnel staffing the NEADS station at Rome NY. they can access realtime radar as well as other detailed taped info from the actual radar site, after the fact.

this was used in locating JFK jr's plane, the Egypt Air crash, etc.

for TRAINING and EXERCISE purposes, radar blips or targets can be inserted onto the digitized scope images of NORAD's radar, from onsite or remote location. it's called a "target generator"
(think about it... you have to TRAIN ATC on some system.. you cant use 12 live planes... you use 12 computer or human-controlled targets...etc)

it is alleged that the FAA radar systems are independent of NORADs. hard to confirm.

one thing remains odd -- the 'hijackers' did not need to turn off transponder... that would only red-flag the plane even more than NORDO and off-course status...

the only thing that changes by going transponder-off is that you can't confirm with 100% certainty that the re-acquired radar target is actually the same physical plane as when you lost it.

this is the crux of the 'plane-swap' hypothesis, for what it's worth.

what is unbelievable is that hijackers would KNOW where the poor primary radar coverage holes (slivers really!) were located, and turn off transponder while inside the hole.

how would they know such esoteric detail, or be able to coordinate the timing of their hijack (unpredictible) to put them in poor-primary-coverage area just at the right moment?
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Omega892R09
post Jan 2 2008, 05:56 PM
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QUOTE (JackD @ Jan 2 2008, 03:48 PM)
one thing remains odd -- the 'hijackers' did not need to turn off transponder... that would only red-flag the plane even more than NORDO and off-course status...


Precisely my thinking all along, the lack of transponder information should have been like a beacon.

QUOTE
the only thing that changes by going transponder-off is that you can't confirm with 100% certainty that the re-acquired radar target is actually the same physical plane as when you lost it.


Exactly.

QUOTE
this is the crux of the 'plane-swap' hypothesis, for what it's worth.

what is unbelievable is that hijackers would KNOW where the poor primary radar coverage holes (slivers really!) were located, and turn off transponder while inside the hole.


Absolutely

Why is it that more cannot see this?

This stuff should be shouted at all those who demean us by calling us 'conspiracy nuts' and put on placquards.
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dMz
post Mar 8 2009, 07:51 AM
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Here are some FAA regulations on radar facilities:

http://www.faa.gov/airports_airtraffic/air.../Ch3/s0307.html
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