GPS L2C transmissions?

On the chance that there are GPS folks here that are
familiar with the status of the new L2C signals: is there
a currently-maintained list of which satellite is emitting
what? In particular, PRN 29 seems to have no L2C (or
at least none that I can acquire), while PRN 31 has a nice
strong carrier (tracked with L2CM) but no NAV or CNAV data
modulation—it’s apparently sending only carrier. I’d
like to find a signal with CNAV data.

While I haven’t tried it yet, the USRP should be capable
of saving to disk both L1 and L2C using a pair of dbsrx
daughtercards. That should permit dual-frequency
receiver software. It would be nice to have a
waveform-pair-to-RINEX tool to allow use of the various
web services for dual-frequency solutions.

Cheers,
Peter

Hi Peter,

maybe you could be interested on this: at www.gps-sdr.com there is a C
driver for the USRP that manages a couple of DBSRX daughterboards, one
centered at L1 and the other one at L2, just what you are mentioning for
building a dual-frequency GPS software receiver.

Cheers,
Carles.

I can verify that there is NO DATA on any of the L2C capable SVs.

Ah. I’d thought from the ICD that the only valid options were legacy
data or
new data. While I understand that this is all pre-operational, it would
be
nice to have some rough guidelines from the GPS people about what to
expect with the on-orbit L2C signals over the next year or two.

Check out www.gps-sdr.com for more info.

Very interesting—thanks!

Would you know if there are any open implementations of P/Y
semicodeless?

Cheers,
Peter

Peter:

I’ve been tracking L2C this summer also. I can verify that there is NO
DATA on any of the L2C capable SVs. The US Coast Guard has a great GPS
status site at http://www.navcen.uscg.gov/. The GPS constellation status
is listed at:

http://www.navcen.uscg.gov/navinfo/Gps/ActiveNanu.aspx

Block IIR-M SVs are L2C capable. Right now in my software receiver the
USRP client can record L1 and L2C using a USRP with 2 DBS-RXs. Check out
www.gps-sdr.com for more info. The L2C tracking capability will be
included in the code after a couple more bugs have been ironed out. Feel
free to email me directly if you need more help or info.


[email protected]
NASA Goddard Space Flight Center
AETD - Mission Engineering and Systems Analysis
Component and Hardware Systems Branch
Mail Code 596, Greenbelt Rd.
Greenbelt, MD 20771
(301) 286-6565
(301) 286-3823 (fax)

From my knowledge all the semi-codeless tracking algorithms are
heavily patented, and hence cannot be found in textbooks or in any
open forum.

But the patents themselves contain pretty good descriptions of the
techniques;
also there is this paper:

http://www.navcomtech.com/Support/Download/Optimum%20Semi-Codeless%20Carrier%20Phase%20Tracking%20of%20L2.pdf

which summarizes all the options and adds a soft-decision refinement to
Z-tracking. Some of the patents will expire pretty soon, probably
before
L2C is fully usable.

There ought to be a fully open-source, geodetic-quality, dual-frequency
(or
tri-frequency once we have L5, or quad- or quint-frequency once we have
Galileo and Glonass and accurate IGS orbits and clock ties for these
other
GNSS systems), software receiver. I have no real connection with any of
this (just a hobbyist), but I’d enjoy having access to state-of-the-art
timing
and survey solutions.

Finally snagged a dual-frequency antenna on Ebay a few weeks ago, so
only software sits between me and centimeter positioning and
subnanosecond
UTC (or UTC(GPS) or UTC(IGS) anyway).

Cheers,
Peter

Peter:

Yeah, it really sucks. If you look at IS-GPS-200D at table 20-VIII the 6
bit SV health message has a value that indicates “L2C Signal Has No Data
Modulation”, unfortunately the Air Force is not following their own ICD.
From my knowledge all the semi-codeless tracking algorithms are heavily
patented, and hence cannot be found in textbooks or in any open forum.

Maybe little bit off-topic in this list, but any of you have tried gpstk
(
http://www.gpstk.org/bin/view/Documentation/WebHome)?

I’m trying to use it in combination with gps-sdr. I’ll keep you updated
on
my progress!

BR,
Carles

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