[geeks] GPS on planes

[wa2egp at att.net]

> The problem with using GPS on an airplane is that the pilots also use
> GPS. Your handheld receiver is very likely to prevent theirs from working.
> How?
> 
> <technical explanation>
> All GPS receivers are of the superhetrodyne type. That means that (using
> round numbers), let's say the GPS sattelites transmit at 11.123 gHz.
> Some receivers will use an oscilator (radio signal generator) at 10.000 gHz,
> mix it with the GPS signal and produce a new signal at 1.123 gHz.
> 
> Depending upon how well sheilded your receiver is from "leaking" signal,
> it could interfere with the airplanes GPS receiver. The leaking signal
> since it would be so close would not only cover the GPS signal from the
> sattelite, but it would prevent the recevier from hearing it at all.

Boy those GPS units have a crappy front end on them were they cannot knock a 
signal 10% away in frequency.  A decent crystal radio could to that.

> Strong signals not only "capture" a receiver, put they desesitise the 
> front end. The signals don't have to be close in frequencey, just near,
> and at those frequencies, it's near enough.

True, but how stong is that 10 GHz signal?  Usually take a signal stronger
than the original one if it off frequency, many times stronger (>80 db for a
well designed receiver).  Simple to make good filters at that frequency.

> Now imagine if the receiver used a local oscilator frequency of 11.000 gHz.
> The local oscilator would not only interfere with their GPS, but with a
> product of .123 gHz, the mixed signal at 123mHz, right in the middle of
> the aircraft band, interfeering with their short range ground communications.

Wouldn't that be true with theirs?  Proper shielding of any receiver is a must
to prevent radiation and to prevent signals at the IF frequency from being
received.  In your example, the short range ground communication would screw
up the GPS.

Bob