[time-nuts] GNSS beam forming

jimlux jimlux at earthlink.net
Fri Aug 31 13:15:15 EDT 2018

On 8/31/18 9:38 AM, Gregory Maxwell wrote:
> On Fri, Aug 31, 2018 at 2:56 PM Attila Kinali <attila at kinali.ch> wrote:
>> "Just DSP work" is a tad bit more than you think. You are dealing
>> with sevaral 1Msps of data, even for a simple L1 C/A receiver.
>> If you are going multi-band-multi-GNSS you are usually in the 50MHz BW
>> at L1 and 80MHz BW at L2/L5 range, which means you are dealing with
>> something in the order of 100Msps of data per channel (either as
>> a single stream of sample or two streams of samples with half rate).
> I have done beamforming in software for wifi signals, which is quite
> similar a task.
> You are probably underestimating somewhat how fast modern systems have
> become.  There are now people using GPUs for SDR now as well, which is
> perhaps more fitting to a GNSS timestamper than many other SDR
> applications, since that application doesn't need to be low latency.
> There are now consumer GPUs that do 14 trillion single precision
> multiply-adds per second.
>> Then you add to it that you will need at least 4bit ADCs to get
>> somewhat jaming proof, probably even 10bit or more and suddenly
> There I agree.  It seems to me that a interesting architecture would
> be to run a sinusoid killer on a FPGA immediately behind the DACs,
> then spit out a 2 or 4 bit stream.

<snip of discussion>

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.

The literature has been around since the 60s on how to do this in a 
variety of cases and signals, with plenty of hardware demonstrations 
over the decades ranging from analog combiners to very sophisticated 
digital approaches using algorithms like ESPRIT or MUSIC.

There's also extensive work on AJ techniques for PN signals, although I 
don't know how much is in the open literature.

One wants to think about the jamming/interference signal

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.

The jamming signal is likely to be somewhat broadband = pulsed, or 
harmonics of some lower frequency signal, which is none too clean - most 
power supply designers do not lock their PWM rate to a maser, after all.

If you're talking about a sophisticated jammer - typically they'll 
attack some design feature - For instance, way back (more than 50 
years), a strategy for radars was to transmit two tones separated by 
60MHz (Radar IF chains were at 60 MHz) and pulse it, in hopes that you'd 
hose up the AGC, or create intermods. Similarly, transmitting on some 
ham bands in the  50s would get into the TV receiver IF.

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.

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.

So now it comes down to the classic tradeoff - what is the value of the 
thing you are protecting.

On time-nuts, we're used to "better performance for its own sake" - it's 
cool, it's nifty, it's educational, for most of us, we don't necessarily 
have to justify our desire to get that next digit. We're also often very 
thrifty - as part of the challenge - I know I can get accuracy X by 
spending $100k, but can I get there spending only $200.

The commercial world, which has to consider the slings and arrows of 
these threats does put a value on it, and that drives the budgets for 
the fixes.  I would venture that *in most cases*, the cost of a fancy 
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.

Sure, if you've got 1000 sites, and you need to upgrade them all at 
$10k/each, that's a $10M hit you need to explain to the shareholders, 
but, it *is* a cost of doing business.

And, realistically, it's much more likely your network failure will be 
because some idiot dug in the wrong place, or someone rolled out a 
software update with a bug, rather than a freak ionization event 
blocking GPS.

Check out the telephone switch bug that shutdown phone service for a day 
in 1990 -

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