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Hi Paul

Firstly, I reckon the vast [ I reckon way way beyond 90% ] of all ???¡é?¡é?????¡­?¡°radar detections???¡é?¡é???????? are not really radar at all, but signals that are interpreted as such [ If anyone has any data on this, please let me know ]. So why is this ?

If you go to any airport, you will see a bunch of folks sitting around with their laptops in a Wi-Fi zone. There are several types of radar there, including weather and air traffic control. A big panic went on in Europe about the possibility that Wi-Fi systems could cause problems with interference for those radars. There are a whole bunch of issues here, including the generally highly limited range of AP???¡é?¡é?????¡é???¡és and laptop cards [ basically just in the ???¡é?¡é?????¡­?¡°coffee shop area???¡é?¡é???????? ]. Is it possible that some of these signals could interfere???¡é?¡é???????| yes, in most cases it really is just overkill. It???¡é?¡é?????¡é???¡és usually more of a case of the radar affecting the Wi-Fi than vice versa.

Apart from police traffic radars [ which should operate on non wi-fi bands ], the average user is highly unlikley to be in an area where an active radar is going to be. I think this was just the European Aviation Administrations and then the FAA being extra extra cautious. Could mutual interference occur ? Yes, but in very rare circumstances. That being said, we have to live with the ???¡é?¡é?????¡­?¡°new???¡é?¡é???????? rules.

There are two main types of satellites [ geostationary and non-geostationary ]. The geos appear to be in a fixed point in the sky [ e.g. Intelsat satellites ]. Non-geos e.g. Irridium appear to move from horizon to horizon. These guys usually work at C-band or Ku band [ 6/4 Ghz, 14/12/11 Ghz???¡é?¡é???????| 4 and 12/11 on the downlink to earth, 6 and 4 up to the ???¡é?¡é?????¡­?¡°bird???¡é?¡é????????]. There are very strict regulations about the amount of power that satellites can put out to prevent inteference with existing microwave systems, but none that I know of affect Wi-Fi. In the Falklands, we had a very unusual situation???¡é?¡é???????|.a satellite antenna at a very low elevation angle right beside the water. The fighters flew very very close to us and went right through the beam. If they had been far away, we wouldn???¡é?¡é?????¡é???¡ét have noticed it. The aircraft was in what is called the ???¡é?¡é?????¡­?¡°near field???¡é?¡é????????of the antenna and all sorts of weird effects can occur there???¡é?¡é???????|the radio wave has not fully developed and obeys laws different from the usual ones when fully developed [ called far-field ].

Provided that you are not near a military base or airport, it is most likely that you are seeing ???¡é?¡é?????¡­?¡°false positives???¡é?¡é?????¡é???¡é???¡é?¡é???????|.in other words general RF or noise.

In 802.11 we have two main types of detection [ for general CSMA/CA signals ] ???¡é?¡é?????¡é?€?? ???¡é?¡é?????¡­?¡°true signal detect???¡é?¡é????????and ???¡é?¡é?????¡­?¡°energy detection???¡é?¡é????????. With ED, the radio basically says ???¡é?¡é?????¡­?¡°hey, I???¡é?¡é?????¡é???¡éve detected some sort of a signal above a threshold level???¡é?¡é???????|maybe it???¡é?¡é?????¡é???¡és a genuine radio signal, maybe it???¡é?¡é?????¡é???¡és just some old rubbish, but I???¡é?¡é?????¡é???¡ém going to tell the arbitration mechanism to hold off trying to access the channel just now???¡é?¡é????????.

The other one looks for particular patterns in what is called the ???¡é?¡é?????¡­?¡°pre-amble???¡é?¡é???????? of the signal. This is added to all 802.11 signals by the PLCP layer [ part of the physical layer ]. This usually requires a higher amount of energy for detection. When this is recognized, the radio says ???¡é?¡é?????¡­?¡°hey, it looks like there is a genuine radio signal here, so I???¡é?¡é?????¡é???¡éll just hold off???¡é?¡é????????.



Radar is different. In general, there is no preamble [ in other words, most radars are not modulated with data, but are rather just pulses of RF energy ]. Because of this, it is very hard for a cost-effective [ to keep Wi-Fi from becoming a rich-man???¡é?¡é?????¡é???¡és toy ] detection system to trully be able to discriminate between real radar and just plain old interference.

Usually if you have radar present, it will ???¡é?¡é?????¡­?¡°sweep???¡é?¡é???????? i.e. appear and disapear in a regular pattern.
In my first job overseas, we were getting massive interference on our international satellite link [ an entire nation???¡é?¡é?????¡é???¡és communications ]. Lots of flapping [ panicking ] going on. I looked at the spectrum analyzer and said that looked like a military radar. Everyone laughed and reminded me we were in the middle of a desert with nothing for miles. The problem went on and teams were sent from the UK to no avail. I repeated my comment and again was shot down in flames. A lot of money was being lost. One day, one of the drivers came in shouting that he saw two men ???¡é?¡é?????¡­?¡°miraculously appear???¡é?¡é???????? out of a hole in the desert. Turned out our neighbor was preparing to invade the little island and people from another [ invited ] country had come in the middle of the night, dug holes, put in low-flying aircraft detection radar with diesel generator, the works. All beautifully camouflaged. They were not happy when we banged on the hatch. Calls were made to the appropriate embassy [ who denied they were there !! ]. A few days later, interference gone. They changed to another band.


Dave

thanks Dave for the excellent reply.

You???¡é?¡é?????¡é???¡ére welcome. Interference is a big problem for some satellite ground stations. In the following case, the source was the AWACS system. Note how one guy [ psuee [ vistor ] - about two thirds of the way down the post ] used the rotational time of the big radome on top of the plane as a clue as to where the inteference came from:

http://images.google.com/images?sourceid=navclient&rlz=1T4ADBS_enUS249US249&q=awacs&um=1&ie=UTF-8&sa=N&hl=en&tab=wi

http://www.eng-tips.com/viewthread.cfm?qid=15231&page=4

http://www.globalsecurity.org/military/systems/aircraft/e-767.htm

http://www.satsig.net/satellite/reducing-interference-satellite-tv.htm

Dave