[time-nuts] GNSS beam forming (was: NIST)

Attila Kinali attila at kinali.ch
Fri Aug 31 10:55:12 EDT 2018

On Thu, 30 Aug 2018 23:05:48 +0000
Gregory Maxwell <gmaxwell at gmail.com> wrote:

> Seeing some open source software implementing beam-forming was one of
> the things I hoped to see result from the open hardware multi-band
> GNSS receivers like the GNSS firehose project (
> http://pmonta.com/blog/2017/05/05/gnss-firehose-update/ ) since once
> you're going through the trouble of running three coherent receivers
> for three bands, stacking three more of them and locking them to the
> same clock doesn't seem like a big engineering challenge... and the
> rest is just DSP work.

"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).
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
you are dealing with 200-400Mbyte/s data per antenna. Constantly.
To be able to do reasonable beam forming, you probably need a 4 by 4
grid at least, that makes 16 antennas which brings us into the
3Gbyte/s to 6Gbyte/s region. And that's just the raw _input_ datarate 
you have to handle....  A modern GNSS receiver has something in the
order of 50-100 correlators per band, each of which needs to receive 
the full data rate mentioned above. So inside the chip, the data rate
gets multiplied as well.

Now take into considerations that beside running the correlators,
after you phase shifted and weighted the inputs correctly, you
have to run some fancy algorithms (on the raw data) to figure
out what these phase shifts and weights are. For each satellite
you are tracking individually. All this toghether means you have
run some pretty heavy computation, that is very likely not going
to fit into an FPGA, so you need to build a custom ASIC.

> Even absent fancy beam forming, for GNSS timing with a surveyed
> position except at high latitudes it should be possible to use a
> relatively high gain antenna pointed straight up and by doing so blind
> yourself to terrestrial jammers at a cost of fewer SVs being
> available. But I've never tried it.

You still need the lower satellites to survey your position accurately.
Besides that you lose a lot of in terms of timing accuracy, if you
have only a limited number of satellites. So you cannot decrease the
lower elevation angles too much. Going below -10dB is probably not
agood idea. Also, going from ~10 birds in view down to ~5 means that
your PPS jitter just increased by a factor of 5 to 10. 
(source: experiment i've done here some time ago)

Besides, a narrow band jammer trips up most of the commercial receivers
badly (adds a correlation peak where it does not belong).
And for that to be effective you only need to be 5-10dB above the noise
level. Which is pretty easy to achieve, even if you have very directive
antenna (the sidelobes are usually only 10-30dB down from the main lobe)

Fortunately, narrow band jammers are also pretty easy to mask, given
you have enough bits in your ADC.

> As others have noted intermittent jamming is pretty benign to a GPSDO.
> Spoofing, OTOH, can trivially mess up the timing.  It's my view that
> if you need timing for a security critical purpose there isn't really
> any GNSS based solution commercially available to the general public
> right now, the best bet is a local atomic reference with a GPSDO used
> to monitor and initially set it.

There is a reason why Microsemi is building more 5071 these days than
ever before (rumors have it that they are are 3-4 devices per week).

			Attila Kinali
It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
                 -- Miss Matheson, The Diamond Age, Neal Stephenson

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