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Comment by Thaxll | Hacker News Reader

Is it actually legal to build radars?

It is legal to receive and process almost any electromagnetic radiation, yes. Transmission is licensed for nearly anything slightly below infrared, mostly because it would become a mess if everyone transmitted whatever they felt like. It is also restricted above UV, but for other reasons :)

It's not legal in the UK

as a UK person living in the US who recently passed the US (technician) Ham radio exam, it astonishes me how draconian the UK laws are about RF.

it's even illegal to listen to UK air traffic control radio? what? how can they even be serious

> it's even illegal to listen to UK air traffic control radio

How exactly would they know that you're listening?

Generally, they don't, and no-one cares. Airband scanners are freely available to buy online and in shops.

It really manifests itself when you look online to see "listen to ATC" live streams and YouTube videos. There aren't any from the UK.

https://www.liveatc.net/faq/

You can receive anything... sending is another issue.

This is discussing passive radar, where you use another emitter, like a FM station or 20 to be transmitters, and your "radar" just sits and listens for the bounces off other objects.

This is definitely not true for every legislation.

Yes, the United States has a you-can-recieve-anything philosophy (with some limits for old analog cellphone frequencies, and restrictions on using, e.g. a police scanner in a car) that isn't matched in many other countries, which is the legal basis of how Ireland and the U.K. require you to buy a license to watch over-the-air television.

The TV licence has been expanded to include any live broadcast, so watching sport on Amazon requires a TV licence.

I also think that in the UK it’s illegal to listen to ATC.

How did the UK ever get to be this way?

Centuries of legal scar tissue accumulated through continuous, unbroken political order.

Treaty of Rome. Maybe North-Korea did not sign it but that is nitpicking.

Can you give an example of a legislation where receiving a signal by multiple receivers is illegal?

Yes -- the UK. You need to sit an exam and get a radio operator's license to receive anything civil aviation related, for example.

Arguably everyone in the US who owns or constructs an SDR is a felon, by virtue of violating the Electronic Communications Privacy Act of 1986 [1,2]. Certainly everyone who sells one.

AFAIK this misguided law has never been repealed, despite being rendered obsolete by the demise of the old 800 MHz AMPS standard. No 800 MHz-capable SDRs I'm aware of make even the most casual attempt at blocking coverage of that range.

So, yes, it's legal to build a radar -- certainly a passive one -- but given that most SDRs cover the ECPA-prohibited frequency range, it would be hard to build a radar with off-the-shelf equipment without breaking other laws.

1: https://en.wikipedia.org/wiki/Electronic_Communications_Priv...

2: https://forums.radioreference.com/threads/cellular-blocked-s...

The ECPA makes it illegal to eavesdrop on all phone calls, it’s the 1992 amendment to the Federal Communications Act of 1934 that allows the FCC to restrict the part 15 authorization of anything that can receive cellular calls:

https://www.govinfo.gov/content/pkg/STATUTE-106/pdf/STATUTE-...

  SEC. 403. INTERCEPTION OF CELLULAR TELECOMMUNICATIONS.
  […]
 “(d)(1) Within 180 days after the date of enactment of this subsection, the Commission shall prescribe and make effective regulations denying equipment authorization (under part 15 of title 47, Code of Federal Regulations, or any other part of that title) for any scanning receiver that is capable of—
  "(A) receiving transmissions in the frequencies allocated to the domestic cellular radio telecommunications service,
  "(B) readily being altered by the user to receive trans- missions in such frequencies, or
  “(C) being equipped with decoders that convert digital cellular transmissions to analog voice audio.

Which is why wideband SDR's like hackRF are licenced as laboratory equipment. So the regulatory regime is the one for spectrum analyzers and signal generators.

I have always wondered if people in the old days used the FM demodulation option to listen in on phone calls.

That was what was so stupid about the mandatory cellular-blocking legislation. You didn't need test equipment or even a scanner to spy on AMPS phones... you just needed an old TV with a UHF tuner. The upper end of the UHF TV band overlapped the 800 MHz AMPS band nicely, and NTSC TV audio was FM, just like cellular (albeit WBFM rather than NBFM.)

It's probably lucky that no one ever brought a case to court (or testified in Congress) regarding 'civilian' use of a spectrum analyzer for cellular eavesdropping. Otherwise we'd be unable to buy anything even remotely resembling today's SDRs. They'd be driven out of the market like pirate radio transmitters were. The law should be brought up to date, but who wants to open that can of Pandora's worms?

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