Primary and Secondary Radar?

[ATCO1987]

Hello,
I was wondering what exactly is the difference between primary and secondary radar? I THINK the phrase "unknown primary contact" means a dot on radar with no details etc, is this correct? So if I could have some explanation as to the difference between primary and secondary radar I would be greatful.
Thanks a lot.
Dan.

[EGCC Rwy 24]

Well, I've had enough answers from the chaps on this forum, perhaps I can save them some typing on this one (then again, I might get shot down!).....

Primary, as you say, works on the reflection of the RADAR signal from the aircraft.

Secondary surveilance RADAR (SSR) is a bit cleverer and relies on kit in the aircraft. When the aircraft's transponder picks up the RADAR scanning past, it leaps into life and transmits its identity (the squawk - a number between 0000 and 7777).

This squawk code can be displayed on the RADAR screen next to the blip. With the right computing power, the squawk can be translated into a call sign.

You'll also hear "mode C". This means that the transponder is also telling the receiving RADAR the aircraft's altitude/height/flight level (I await correction, but I think always a flight level in reality). I don't think this is trusted by the controller until he has checked the reading with an altitude read out to him by the pilot.

You can get a primary return only if the aircraft has no transponder. I think you can also get a secondary only if the primary cover is not good enough (too near the ground, etc) but the transponder still shouts out its identity when triggered.

Hope that helps a bit??

Cheers,

Nick
Nick Locke's Reporting Points (http://www.nicklocke.co.uk/Points)

[Timothy]

In principle it's very simple.

Primary Radar
Radio waves are broadcast and then their reflections are recorded. This depends upon the reflectivity of the aircraft to radio waves and other objects (flocks of birds, traffic on the motorway, the surface of the ground) also reflect, thus causing spurious returns.

Secondary Radar
The radio signal is picked up by a box of electronics in the aircraft (transponder), information is added (a unique code and the aircraft's altidude...and gradually more information as new technologies arise) and then rebroadcast. Thus the returning signal is considerably stronger than one that is only reflected.

Some units only use primary (Southend for example) and some only secondary, but many must see both before they can rely on the radar picture.

W

[chiglet]

My twopenn'rth
As has been said, Radar contact [Primary] is just that. "Proper" Radar :hmm: [analogue] paints a "return" onto a CRT[Cathode Ray Tube] basically a TV screen. SSR [Secondary Surveilence Radar] (Speeling) can give a "return" of a "bright bit" or numbers or "Callsign Conversion"........ eg
./ .7000 .EZE407
Also, "older" radars will paint clouds etc, where "Modern" Digital Raster Scan TV Tabulated Displays only show "Computor Generated Images". Primary + Secondary x Both * [Were you paying attention Nick?]
Radar is "Line of Sight" and is rather limited, but SSR being a tx/rx system has a "lot" more range.
For ATC purposes, a "primary" contact is "Called". An "Unverified" SSR squawk [eg 7000] with a height difference of [I think 5000'] is "Avoided" and a "Verified" SSR is given "Standard Separation"
Rather a long diatribe, but it's been a long day and I've had the odd G'n'T but I'm not [too] far out
we aim to please, it keeps the cleaners happy

[Bern Oulli]

You now only need 3,000ft against "unknown" squawkers in class G airspace (in the UK). Don't ask me why the change.

[ATCO1987]

Hello,
Wow thanks for all the info, greatly appreciated. So the unverified contact (another phrase Ive heard from the Bristol lads) is that just a dot on the radar? When an aircraft squawks standby is it just a dot? Same goes for if they do not have a transponder. As far as I know if an unknown prim contact pops up on radar they have to vector other traffic around it just incase, is this correct?
Thanks.
Dan.

[Legs11]

Unverified: The indicated level has not been verified and therefore cannot be assumed to be correct.:(
The controller can 'verify' the level themselves by speaking to the aircraft concerned (if possible), speaking to another controller who is in contact with that aircraft or if the aircraft has a recognised squawk then it can under certain circumstances be assumed to be verified.

Squawk standby: Depending on the radar display, some sort of SSR label will be visible, be it a 'z' or whatever.

No transponder: only a primary return will be seen.


As for vectoring round unknown primary contacts, oh Dan:oh: now you're starting to get into the dreaded differences in service provision, ie Radar Control, Radar Advisory, Radar Information and Flight Information.:{ :} Far too long and complicated to go into here.

Hope that's been of some help.
:ok:


Oh and Chiglet, just cos it's verified doesn't mean it gets separated and unverified doesn't necessarily get avoided - service provision old chap...better revise on that one.:ugh:

[spekesoftly]

No transponder: only a primary return will be seen.

Depending, of course, on primary radar performance, and target size/speed/heading/position/level. ;)

[ATCO1987]

Legs, that brings me to another point. What is the difference between RIS and FIS? I understand RAS, and from what Ive heard when listening to EGGD they vector aircraft around the unverified contacts. I sometimes hear ATCOs at EGGD say to an aircraft that they are too low for RIS and its FIS, or very limited FIS due to low altitude, some explanation please?
So if the aircraft has no transponder or whatever, does it just appear as a blip on radar? just a dot so to speak?
Sorry to be a pain, information greatly appreciated.
Thanks a lot.
Dan.

[Eira]

Dan,
I would suggest it may be a good idea to go to the following site, it will give you the Manual of Air Traffic services Part 1 on line. So many of the questions you ask can be answered here clearly and concisely
http://www.caa.co.uk/docs/33/CAP493Part1.pdf

[ATCO1987]

I have that PDF, Ive only looked through the FPS bit lol.
I'll read more, thanks!
Dan.

[LostThePicture]

FIS: Flight Information Service
RIS: Radar Information Service
RAS: Radar Advisory Service

Aircraft operating outside controlled airspace can be receiving any or none of these services. FIS is the bare minimum that a unit will give an aircraft, and will basically only provide MET and airfield information. If two aircraft report estimates that put them at more-or-less the same place at the same time, traffic information can be given. But there is no radar, so the FISO/ATCO has no real way of knowing whether any two aircraft are close to each other or not.

RIS is much more comforting if you're a pilot. For no extra charge you get everything included in a FIS, plus the controller has radar to help look after you and provide up-to-date traffic information on potential conflicts. However, this is where the controller's responsibility ends - the pilot is ultimately responsible for effecting his own separation by acquiring any conflicting traffic visually. The controller may continue to provide traffic info if the confliction appears to be getting worse, but should not provide radar vectors.

RAS is a real hot potato. Controllers agreeing to provide a RAS will aim to separate the pilot and his aircraft from all other unknown traffic by 5nm horizontally or 3000' vertically. So if a radar return pops up with no height information, the controller is obliged to provide radar vectors to the pilot to allow him/her to miss it by 5nm. As unknowns can pop up anywhere, this can sometimes mean fairly drastic turns. The pilot CAN opt to try and maintain visual separation from the conflict, but in doing so he effectively reduces his service to a RIS, and becomes responsible for his own separation again. Needless to say, there can be few controllers who enjoy providing a RAS, especially when they can be busy with higher priority traffic, ie commercial traffic inside CAS.

A service can be limited by the ATCO if he/she feels that it is necessary. I've never heard of a FIS being limited - it's a fairly basic service anyway! RAS and RIS are usually "limited" if the subject aircraft is flying very low or is near the limit of solid radar cover (ie a long way away from the radar being used by the ATCO). The pilot may be cautioned, "little or no warning of traffic from below / from the right" due to the risk of pop-up traffic appearing at the edge of radar cover. Services can also be limited due to excess radar clutter or weather, although this should be less of a problem with today's processed radars.

So if the aircraft has no transponder or whatever, does it just appear as a blip on radar? just a dot so to speak?
Yes, it does. More or less. Although again, with modern processed radars, the raw dot may be changed to a symbol which denotes "primary return only" - there may be another symbol for aircraft picked up solely on secondary radar, and another for aircraft that have been picked up by both.

LTP

[ATCO1987]

Thank you very much that is extremely helpful.
Clears things up much more :).
Dan.

[Iron City]

Everything very correct in response to the basic question of the thread. Please also note that what the ATCO sees on his/her display in modern systems is NOT directly connected to the primary or secondary radar systems. That is, the raw primary and secondary information is correlated and the targets that are consistently recognized used to create one or more track files which are fed to a video generator to generate the picture the ATCO sees. Uncorrelated targets if they are consistent and persistent enough or secondary only matching a clearance etc will also be displayed but random primary targets and other clutter will generally not be displayed.

Thats right, what the ATCO sees on the display has more in common with a high end video game than you thought.

And the best (?) call I ever get is "Traffic 12 oclock, speed and altitude unknown" Is that just a U.S. thing?

[Spitoon]

Dan, I don't understand - what's funny about the FPS bit?

[ATCO1987]

Nothing, interpret my "lol"'s as a chuckle ;).
Dan.

[Roger Dodge]

"A very limited FIS" - wow that's really covering your arse:O

[ATCO1987]

LOL Nice one RD!
Ive only heard it mentioned once or twice at EGGD though. Not too often, I hear limited RIS more.
Dan.

[Chilli Monster]

Oi Legs - are we using the same kit?
Squawk standby: Depending on the radar display, some sort of SSR label will be visible, be it a 'z' or whatever
Whenever I tell people to squawk standby they put the transponder onto standby mode (normally the switch position before 'off' - keeping it warm and ready to go) and nothing gets displayed on the screen at all.

Hope all is well at the funny farm ;)

[Legs11]

Ah Chilli, to err is human:O

:{ :{ :{ :{ :{

[ATCO Two]

And lastly my favourite - FAS!

:D

[Spank me baby!!!]

With secondary radar, the distance of the transponder (aircraft) is calculated from the time difference between the radar sending out a signal and the radar receiving a response from the transponder on board the aircraft. This is the distance in a straight line from the radar site to the aircraft. But the aircraft could be over 40000FT above the ground, so this distance wouldn't be a true measurement of the "along ground" distance from the radar site to the aircraft. The computer looks at the Flight Level that the transponder has included in it's response and makes an adjustment (Slant Range Correction) before displaying a symbol on the ATCO's screen. The symbol on the screen is displayed at the "along ground" distance from the radar site, not the straight line distance up to the aircraft.

[Legs11]

Is that the sweet FAS?:p

[2 sheds]

Some slightly incorrect use of terminology in some of the above posts which could confuse. The term "contact" or "radar contact" actually implies that the particular aircraft is IDENTIFIED. Hence, "unknown..." or "unidentified contact" is a contradiction in terms.

[Spank me baby!!!]

...we use the phrase "unidentified traffic".