Re: Euro GPS system

Dale DePriest ha scritto:

Holger Issle wrote:

On Thu, 27 Jul 2006 13:27:22 -0700, "Wolfgang S. Rupprecht" wrote:


Both units will send a satellite status report packet with only
12-channels


How do you know that the output format used is indeed reflecting the
internal processing in the unit? As others have stated already, the
extra channels can be used to correlate signals and to get around
multipath issues. Maybe they just selected to ever report 12 sats to
be sure not mess up receiving applications?

The 20 channel capability is adjustable with firmware and only
represents the full channel bandwidth of the DSP implementation. The
design of the unit does not have dedicated channels like most earlier
designs. It is not being used in any of the computation of Garmin units
and is unlikely to be used in any but very specialized units unless
competitive pressure forces it. There are several true 16 channel units
out there today however in the consumer space. In the professional units
going up to 24 channels by including Glassnos capability.

Dale

Hi to everybody,
I hope the following (taken from various discussions in the
www.gpspassion.com forum) can help to clarify the SIRFIII 20
channels/12 satellites issue:

"You asked an interesting question about the difference between a
satellite and a channel. A channel typically represents a subdivision
of the hardware dedicated to tracking a satellite, but that definition
goes away in SiRFstarIII. In reality, SiRFstarIII has no channels.
Rather it has a tracking engine that tracks whatever it is told to
track at any given instant. Most receivers track in real time, meaning
as the stream of 1s and 0s come in from the RF section, the hardware
assigned to track a satellite compares that incoming stream with
another stream created inside the receiver and adjusted to match what
that satellite should be sending. When the streams match sufficiently,
we have a track. SiRFstarIII operates in faster than real time, much
faster in fact. But of course that means it must work in the past. It
captures a snapshot of the streams of 1s and 0s in a buffer. When the
buffer is full the receiving hardware switches to another buffer to
collect the next samples while the tracking hardware starts processing
the filled buffer. That tracking hardware repeatedly processes the
buffer for each satellite it is tracking -- if it is tracking 8
satellites it processes the buffer 8 times, once for each satellite.
The tracking hardware is fast enough that it could process the buffer
up to 22 times before the other buffer was full and the receiving
hardware would need to switch and start filling this buffer again.

Since there are only enough satellites visible at one time to justify
12 "channels" (areas in memory to collect processing results, really),
and since currently defined SiRF messages only support 12 satellites,
SiRF chose at this time to only implement 12 parallel tracks. When
circumstances justify it, we will expand things. Note also, that as
processes improve so that we could speed up the chip, that 22 satellite
limit will also go up.

If there were more than 12 satellites visible, the software would
generally track the 12 highest in the sky, since lower satellites are
generally more prone to multipath. If the user selects to use SBAS as
well, we would sacrifice only one tracking slot for an SBAS satellite.
Note that when we lock onto an SBAS satellite, we listen to the data
from it and learn which other SBAS satellites are available in that
constellation (WAAS, EGNOS, MSAS). Then, we select the one that is
closest to our current longitude and if necessary switch to that one.

Those of you who have seen SiRFstarIII reported as 20 or maybe 22
channels may wonder what the benefit is. As noted above, you can only
see 12 or 13 satellites at most at any one time. However you can also
usually see a WAAS or EGNOS satellite, and you need a channel for that.
So theoretically you only need 13 or 14 channels, yet some receivers
have 12, some 16 and some more. What is the benefit?

In the SiRFstarIII receiver, there really are no channels in the
hardware, only a very flexible hardware architecture that can be
reconfigured as necessary for the job at hand. So if we are tracking 6
satellites, we only have 6 (plus a couple "equivalent" channels) in
use, whereas if we are tracking 12 satellites we have 12 in use. What
is really happening is we are allocating some memory for each "channel"
and repeatedly processing the same data for each channel's purposes. In
addition to tracking satellites, we are constantly searching for any
satellites we didn't know were there, we are monitoring the background
noise so we can give a true carrier to noise density reading, and we
are constantly validating the signals we are tracking to be sure they
aren't cross correlations or autocorreclations. When we say we have 20
or 22 channels, what we really mean is that we have the ability to
process the data memory for that many times before it is time to go on
to the next block of memory. Memory is filled in real time as we
collect data from the RF sampling, but is processed after a buffer is
full. Once we finish processing the current buffer, if the next buffer
is not yet full, we stop processing so that we save power.

Carl - SiRF Customer Support"

francesco

.

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