[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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