[time-nuts] Sinlge ADC multi-band receiver
attila at kinali.ch
Mon Apr 10 13:58:44 EDT 2017
On Mon, 10 Apr 2017 09:00:17 -0400
Bob kb8tq <kb8tq at n1k.org> wrote:
> > Only if you *need* the Galileo E5.
> The other point with E5 is the nature of the data on the various sub signals.
> Galileo has three classes of service and only one of them is free (open).
Yes. Thats why we do not talk about E6, or E1 PRS.
On E5 there are OS signals and CS signals. More accurately, the F/NAV data
which is part of the OS signal, is on E5a, and the I/NAV data, which is
both part of OS and CS is on E5b. Additionally, there is a dataless pilot
on both E5a and E5b.
As far as I am aware of, neither the CS nor the PRS specifications are
public yet. If someone has any information on those, I would be interested.
> As with traditional L1 / L2 survey receivers, you don’t *have* to recover
> full data from a signal for it to be useful.
Yes. But semi-codeless tracking only works because:
1) A lot of the signal structure is known and it is actually such
that you can correlate quite a bit of it without knowing the P(Y) code.
2) The P(Y) code is send out on both L1 and L2, which allows correlating
both signals possible.
Without these two points, the use of L2 would not have been possible.
And yes, the US military learned from this and made the M code without
the strucutre that helped correlating it in 1). And they also learned
that not documenting it is the best protection against people using it.
Though I wonder how long it will take until someone figures out what
the signal structure is.
> That said, the free (open) service is only on one of the two sub signals.
No. See above.
> If you are building a L1 / L2 / L5 GNSS receiver, you might
> well opt to only grab the lower part of the E5 signal.
E5a overlaps with L5, The center frequncy of L2 is a mere 20MHz
from the E5b center frequency. So, if you are building an L1/L2/L5
receiver, there is very little point in not investing a little bit
in getting E5b as well.
> You might also decide on a setup that only used two of the three bands.
> That would give you all the data and ionospheric correction. It is a bit
> unclear what the third band would add other than a “cool factor”
E5 allowes, due its large bandwidth, a supperior multi-path supression.
But Galileo is not yet fully functional, so using L1 C/A & L2C for now is
the best option. Supporting L5 is a good idea, to make the receiver
future proof (again supporting a large bandwith for multi-path supresion)
but it is not yet known, when L5 will reach full constellation (there
only 12 satellites transmitting, yet). Also L2 was only recently marked
as GNSS band and thus there are still radar systems working in this band,
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