[time-nuts] Boeing 787 GPS reception trouble

[Brian Lloyd]

On Mon, Jun 2, 2014 at 8:57 AM, Jim Lux <jimlux at earthlink.net> wrote:

> 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.


Well, it is quite unusual for IFR (instrument flight rules) operation. But
VFR pilots would sometimes use an AM broadcast station for navigation
assistance.


> I had to learn how to do it when taking flying lessons: it was widely
> acknowledged ( in 1980) to be nearly useless,


Not entirely. I still make sure my planes are equipped with ADF (LF/MF
direction finding) due to my experience with GPS outages over the Caribbean
and Atlantic. I have experienced outages of over an hour where both my
panel-mount and hand-held GPS receivers stopped working. ADF was all I had.
I suspect that since I was flying a plane popular with drug-smugglers (a
Piper Aztec), I was being tracked, followed, and GPS jammed. (I lived in
the Virgin Islands, traveling to Florida on a regular basis. I would stop
in the Turks and Caicos or Bahamas to refuel.)


> 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.
>

They could be and they are. Interestingly enough, the only radios that ever
interfered with my VHF nav receivers were my own VHF comm and nav
receivers. LO leakage from one radio would show up on one of the others. I
have never experienced that problem with any consumer device.

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.
>

A "crystalplexer" radio that was dual-conversion with both LOs using
switched crystals. CB radios used the same thing back in the late 1960s and
early 1970s. PLL LOs came later.

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).
>

That may be true but I have never experienced it, even when I tried. The
only time I have ever experienced interference with my comm or nav radios
it was from another comm or nav radio in the plane. Most use a 10.7MHz IF
which means, for the spectrum from 108MHz-138MHz, you are very likely to
have a lot of overlap between LO and desired receive frequency.

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."


Amen.

-- 
Brian Lloyd
Lloyd Aviation
706 Flightline Drive
Spring Branch, TX 78070
brian at lloyd.com
+1.916.877.5067