Here is probably more than you wanted to know about how ATC “sees” you on RADAR, but I couldn’t resist.
The only RADAR return that controllers use to determine aircraft position is primary radar, which is sometimes known as “skin paint”. This does not require any response from the airborne transponder. In all Centers and in most Terminal facilities this return is digitized and depicted on our scopes as a small rectangle. The transponder reply adds a secondary radar return “slash” through the primary target, and this is used for position correlation and computer tracking. A RADAR alignment check is performed each time a new controller assumes the operating position–kind of like how a blackjack dealer checks over the table when they change dealers.
Mode C altitude is transmitted by the transponder in Pressure Altitude. The ATC controller/supervisor keeps the ATC computer updated with the current altimeter setting appropriate for the area of control, the goal being that the computer converts each target’s PA to true altitude and accurately depicts TA on each target. This is more of a challenge than it sounds, believe me.
Strong transponder replies do cause ATC problems, most notably false targets showing up 180° in azimuth from the actual aircraft and at the same distance. Some very strong replies near the RADAR antenna cause “ring around”, which is dozens of targets appearing in a circle around the antenna, all showing the same squwak code. Weak transponder replies usually just fail to show up at all.
PAR approaches require a special, precise RADAR installation dedicated to this one specific purpose. As far as I know, the FAA no longer has any PAR systems. The DOD still conducts PAR approaches on a very limited scale, and civilian access is difficult (but not impossible!) ASR approaches can be accomplised with the very same RADAR in use at all FAA terminal facilities, but controller training/certification is the issue here. Unless an airport under their jurisdiction publishes an ASR approach for general use, it is highly unlikely that the controllers are current to perform that service. If you frequent the airspace of a TRACON that still conducts ASR approaches, I think you’ll find them very willing to give practice ASR approaches during slower traffic periods.
Kelly Rudy SR22 #178
Just for the record there are very few PAR approaches available. There may be some at military facilities but they’re virtually extinct at civil fields in the US. ASR approaches are also becomming much less common although in a pinch I suppose that a controller could improvise one.
It’s probably easier to show your wife how to use the autopilot for an approach.
J. Seckler SR22 #063
Above started me thinking and checking. As you said it seems the PAR approach is a thing of the past. However a call to my local ATC facility revealed that ASR approaches are alive and well.
They said all class “C” and larger, and in fact almost any airport that could be covered with radar could be offered an ASR approach, even some without a tower on the field.
The controler advised they are even required to stay current with these and that if a pilot wants to practice one the best time is near the end of a quarter.
When my airplane comes the use of the autopilot will be at the top of the list for her but any option I can offer her is one more she has to chose from.
Mike
Sorry for the off-topic post but I have been thinking about the Skywatch unit recently and have a pretty basic question about transponders.
How does ATC radar get distance information from the transponder?
Seems like one way might be to measure the time it takes from from when sending an interrogation until when receiving a reply. But this relies on even a 25-year old Cessna-brand transponder being able to respond to an interrogation in a VERY precise amount of time. (i.e. it needs to respond to an interrogation in EXACTLY X ms).
Another way would be to use the signal strength of the reply as a distance estimation. But that’s obviously troublesome when different transponders have different transmit power.
I can’t see how either of these mechanisms gives ATC radar enough information to allow them to say “you’re 3 and a half outside the marker” for example… (meaning that I can’t see how either method would be good enough for 1/2 mile resolution).
Does anyone know how this really works?
(The thought occured to me while pondering why they say that TCAD distance info is unreliable since it paints powerful transponders closer than weaker transponders. If anyone knows whether the “active” interrogation Ryan 9900BX TCAD or BFG Skywatch is able to get more reliable distance information than the passive TCAD units, that would be helpful too!)
Thanks,
Steve
Opps, hit enter by mistake.
Anyway, I’am not positive but I think ATC relies on the primary radar return for direction and distance but has to depend on transponder returns for correct altitude.
Mike
I forgot about Precision approach radar available at larger airports. This does it all. In fact that is what I tell my wife (a pinch hitter pilot) to ask for if it is up to her to get down through the clouds in an emergency. PAR approach. Another, ASR (air surveillance approach) can be given by ATC. It gives only azimuth navigation but they can tell you when to start your decent and when you are at the missed approach point. Some of the instructors on this site may comment pro-or-con about this but to me it would definitely lighten the workload for a non-instrument pilot or in our case a pinch hitter in an emergency.
It must be my day to ramble off-topic.
Mike