[time-nuts] GNSS beam forming

Gregory Maxwell gmaxwell at gmail.com
Fri Aug 31 13:38:22 EDT 2018

On Fri, Aug 31, 2018 at 5:16 PM jimlux <jimlux at earthlink.net> wrote:
> AJ is a slightly different problem than straight up beamforming
> You need N+1 receivers to suppress N point source jammers - it's more of
> an adaptive canceller than a beamformer.

One nice property of AJ though is that the nulls it forms can be very
deep, so the result is much more suppression than you might have
expected from looking at the lobe gains from beamforming.  This is for
the same reason that normal radio direction finding uses e.g. the null
in a loop antenna.

It's also an argument as to why few antennas are sufficient for
blocking jammers at a single location.

> In general, you're probably not going to design to "notch" a sinusoid -
> sure that might be a common jammer, but the sinewave generated by the
> jammer might be very noisy and unstable in frequency - sort of the
> opposite of the sinusoids desired by list members :) - the spurious
> oscillation of the TV antenna amplifier was in this bucket.

Hm. My belief that a sinusoid notch was interesting in part because of
how badly a loud one trashes a bit-depth reduced signal compared to
e.g. pulse interference with an equivalent amount of energy.

With infinite bit depth I wouldn't expect a narrowband jammer to be
particularly bothersome after despreading (at least not more than any
other jamming signal that isn't correlated with the spreading code).

[As an aside, the sinusoidal estimation paper I linked can also model
higher order parameters like amplitude and frequency modulation,
including 2nd order and higher modulation, assuming you have enough
SNR for those terms to be anything but noise.]

> Then you get into sophisticated approaches which are more like spoofing
> - transmit a PN modulated signal that replicates the desired signal,
> then pull it away and turn off, forcing the receiver to be in
> "acquisition" mode all the time.

Indeed, and with SDR so inexpensive, making a jammer that produces a
spoof GPS signal is something a bored teenager can do.

> Can you build a receiver which is immune to all of these and sell it for
> $10?  Probably not.  Can you do it for $100-1000, almost certainly.

A robust timing receiver with a parts cost of $1000 many would buy
now, but I think the issue is that the first unit costs $10m ... it's
only the nth thousandth one built that costs $1000. :P

> holdover GPSDO is a tiny, tiny part of the cost of a cell site -
> Installation of the concrete pad for the tower probably costs more.

Indeed, but people are now using GPS based timing for security
critical applications where skewing someones clock lets you steal
millions in an electronic bank heist.   We don't actually see any of
these attacks right now in part because it's still much easier to
attack by social engineering humans or leaving malware on USB sticks
outside an office... but also because people who are aware of these
risks put in a local atomic clock instead of getting their timing from
a GPSDO, but many aren't aware of the risks.

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