GPS first LO need to be locked?
On Mon, 10 Apr 2017 08:08:10 +0100
"David J Taylor" david-taylor@blueyonder.co.uk wrote:
Thanks for that pointer! Most interesting. I wonder whether anyone has an
updated document detailing the different systems and their current state, as
that information is approaching 10 years old?
As far as I am aware of, that is the current state.
The GPS/Galileo was signed slightly before that thesis, IIRC.
The L2C and L5C signals are "old-school" LFSR generated PSRN BPSK
signals. The only "special" signals are L1C, E1 OS and E5.
QZSS uses GPS L1 C/A signals, IIRC
Beidu and IRNS I don't know.
If I am not mistaken, navipedia is up to date with everything,
but I have not read everything, nor checked against the standards
what I have read there.
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, Neil Stephenson
On Mon, 10 Apr 2017 09:00:17 -0400
Bob kb8tq kb8tq@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:
- 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. - 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,
causing interferences.
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, Neil Stephenson
This approach is known as “security through obscurity”, and is deprecated in the professional of information security. What one invents, another can discover.
The most secure systems use well-documented algorithms with open-source software — widely scrutinized for bugs or implants, and therefore with well-understood performance limitations. The secrecy comes from good crypto key management.
— Eric
On 2017 Apr 10, at 13:58 , Attila Kinali attila@kinali.ch wrote:
And they also learned
that not documenting it is the best protection against people using it.
depend how much in-band loss could you afford it is relative easy to
make cavity filters if you have a network analyzer available
73
Alex
On 4/10/2017 9:11 AM, Attila Kinali wrote:
On Sun, 9 Apr 2017 18:13:48 -0400
Bob kb8tq kb8tq@n1k.org wrote:
The beauty of the system would be that you don't need a SAW filter
at all. If the input stage (LNA + mixer) has a high enough dynamic
range, then the (first) IF filer alone can remove all those out of
band interference.
Why would you substitute an expensive IF filter for a cheap front end filter?
Availability: Although there are L1/L2 filters available, they are not
easy to get unless you buy them in bulk. The standard L1 filters you
can buy are rather narrow band (just 2-4MHz) and don't allow the
reception of the modern signals. L5 filters are very rare and E5 filters
simply do not exist yet.
And keep in mind that the IF filter does not need to be a special
ultra-steep filter. The high sampling rate of the ADC allows to place
the input signal such, that the stop band can be quite far from the
pass band. Also, the filter is only really necessary to filter out
narrow band interference, which is hopefully far from the signal anyways.
Having a bit of noise fold in is, as Jim noted earlier, not a problem at all.
Attila Kinali
On 4/10/17 2:08 PM, Eric Scace wrote:
This approach is known as “security through obscurity”, and is deprecated in the professional of information security. What one invents, another can discover.
The most secure systems use well-documented algorithms with open-source software — widely scrutinized for bugs or implants, and therefore with well-understood performance limitations. The secrecy comes from good crypto key management.
The M-code is described in a fair amount of detail here:
www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA456656
On Mon, 10 Apr 2017 16:49:24 -0700
jimlux jimlux@earthlink.net wrote:
The M-code is described in a fair amount of detail here:
www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA456656
Not really. All it says that it's a BOC(10,5) signal using some
code that allows direct aquisition. It doesn't even mention what
the rate of the data on the new signal is. From [1] one can see
that the modulation is a BOC_sin (and not a BOC_cos). And that
is all I could find out about the M code so far.
Unfortunately, all the other papers concerning the M code are
behind the ION paywal, to which I do not have access.
But guessing what else I have seen on the M code, I do not think
they contain much technical detail on the spreading code itself
or the data transmitted or the encryption.
Attila Kinali
[1] "Design and Performance of Code Tracking for the GPS M Code Signal"
by John Betz, 2000
http://www.dtic.mil/get-tr-doc/pdf?AD=ADA460257
--
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, Neil Stephenson