[time-nuts] GPS down converter question

Attila Kinali attila at kinali.ch
Tue Dec 1 23:17:58 UTC 2015


On Mon, 30 Nov 2015 21:00:30 -0800
Peter Monta <pmonta at gmail.com> wrote:

> For GPS C/A with wide correlator, about 2 MHz; if you want Galileo BOC and
> (eventually) GPS L1C, or legacy C/A with narrow correlator, about 8 MHz;
> for GPS P code about 20 MHz.  Books on GNSS software receivers will detail
> the many tradeoffs available---if you're starting out with a
> proof-of-concept lab receiver, go for 8 MHz.


Galileo E1 and GPS L1C are not BOC(1,1). They both use a variant
of the MBOC(6,1), CBOC(6,1,3/11) for E1 and TMBOC(6,1,4/33) for L1C.
You can view those as a weighted combination of a BOC(1,1) signal with
a BOC(6,1). This gives an additional peak at 7MHz offset, compared to
a simple BOC(1,1) and helps to improve multipath. For a good description
on the signals, see [1]. I.e. you'll need at least a 16MHz wide bandwidth
to make use of those signals. Although it's possible to track both
of these signals with just a BOC(1,1) decoder (and thus a narrower bandwidth),
this will induce a slight bias into the tracking loop and thus an error
in the PVT solution.


Little sidenote here: The final definition of the GPS E1 signal and
the GPS L1C are quite new (E1 was defined around 2006 and officially
published in 2008, L1C even later). I have found publications from
2011 that still talk about E1 being BOC(1,1). So if you read anything
on the Galileo E1 or GPS L1C signals, check whether they got the
right signal or not :-)

			Attila Kinali

[1] "The MBOC Modulation - A Final Touch for the Galileo Frequeny
and Signal Plan", by Avila-Rodrigez et al., 2007
http://www.insidegnss.com/node/174
http://www.insidegnss.com/auto/1007-AvilaRodriguez-final-hires2.pdf

-- 
Reading can seriously damage your ignorance.
		-- unknown



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