[time-nuts] Boeing 787 GPS reception trouble
Jim Lux
jimlux at earthlink.net
Mon Jun 2 09:57:12 EDT 2014
On 6/2/14, 2:27 AM, Poul-Henning Kamp wrote:
>
> It would be trivial to add a passive GPS repeater to the plane, but
> the airtraffic industry has never been happy about people being
> able to receive navigation signals inside planes, worrying that
> somebody might try to blow up the plane at some specific place
> (or non-place), so that ain't gonna happen.
>
>
I don't know that it's that reasoning. It's more about the innate
conservatism of people who make things that fly.
The reason for "radio receiver ban" originally was fear that Local
Oscillator leakage would adversely affect cockpit instrumentation:
particularly things like low frequency beacon receivers, which were none
too selective, and since navigation using the ADF and tuning to a AM
broadcast station wasn't unusual. I had to learn how to do it when
taking flying lessons: it was widely acknowledged ( in 1980) to be
nearly useless, but, hey, if all the other radios fail, any port in a
storm, etc. About the only older radio nav technology is A-N ranges (if
you believe Wikipedia, they were gone by 1980 "mostly disappearing by
the 1970s")
Birdies in a consumer radio in your living room or car aren't a big
problem. Birdies in a navigation instrument are a potentially big problem.
Even in the 1980s, there were a lot of planes flying with fairly archaic
radios, although I suspect no commercial jet was using a VFO tuned
radio: they'd be using "banks of crystals" or PLL tuning. In general
aviation, the first non VFO radios were from King in the 60s, and I
think synthesizers came in around 1970 (King KX 170 and 175). I was
astounded at the number of crystals in one of my Narco radios when I
took it out of the plane to fix it (a 1973-74 vintage radio). Half that
box was basically a big rotary switch and dozens of crystals.
Typical spurious responses in a COM or NAV receiver would be something
like -60dB down, but a few milliwatts leaking from some guy's FM radio
on board would easily be bigger than than that, since the receiver
threshold is about 1 microvolt into 50 ohms (-110 dBm).
Once the rule is in place, it's very, very hard to get it removed,
because of the "if we allow X, and a plane has a problem, everyone is
going to say "it was because of X" even if it wasn't, so let's just keep
things the same."
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