Björn,
On 01/15/2015 07:55 PM, "Björn Gabrielsson" wrote:
Brooke,
The traditional GPS has C/A and P(Y) on L1 and P(Y) on L2.
Most Civilian GPSes only uses C/A.
Advanced receivers can also use P(Y) code, since the P-code is known,
the hand-off to P code is known and the way that P-code is encrypted
into Y-code is known (XOR with another code, called A-code or W-code in
different sources). Modern receivers is able to do both code and carrier
phase observations on the P(Y) code signals.
Read up on semi-codeless tracking, I dont think (pure) codeless is used
anymore.
http://www.colorado.edu/ASEN/asen6090/ztracking.html
https://books.google.se/books?id=-sPXPuOW7ggC&pg=PA240&lpg=PA240
This is a better reading:
http://www.navcomtech.com/navcom_en_US/docs/download_center/white_papers/current/optimum_semi_codeless_carrier_phase_tracking_of_l2.pdf
The Ashtech literature referred to 13 dB better efficiency of their Z12
Z-tracking compared to the cross-correlation technique of the then
competitor Trimble 4000 receiver. The above paper indicate 14 dB
difference between the methods. All semi-codeless receivers will
experience "squaring loss", and it shifts with the C/N.
This have made the original code-less squaring approach completely
useless in comparison to the more modern approaches. The squaring method
assumes that what-ever magic code there is, is encoded as multiplying
the carrier by +1 or -1 and squaring it makes it go +1 and +1.
Cross-correlation gains 3 dB, but real gain comes only when applying the
known P-code.
Since processing at these rates is relatively cheap these days, there is
no point in wasting implementation on code-less.
The military goal of this "break-in" is not lost, as those receivers
still rely on the C/A code and that is easy to jam. Also, the "break-in"
comes at a signal quality loss and the advancement of methods have
reduced this loss.
The benefit of dual frequency observation is that ionspheric shift can
be almost completely taken out of the error budget, adjusting both code
and carrier phase observations. Then working on the integer ambiguity
you can get carrier phase observations with accurate pseudo-ranges.
Carrier-phase observations has a much higher precision to them, so that
gives a very high precision and using a good reference network
corrections can be adjusted to give good absolute position.
If civilian receivers where to implement L2C and L5 which now is
becoming common, they would gain quite a bit of precision in a similar
fashion. For car navigation, the GPS would know which lane you are in.
There ARE civilian receivers doing this, and has been for quite some
years. And its not from only a few vendors - all the big ones have it -
Trimble, Novatel, Topcon, Javad, Leica, Septentrio and a few more. There
are now receivers tracking "GPS L1/L2/L2C/L5, Galileo
E1/E5A/E5B/AltBoc/E6, GLONASS L1/L2/L3, BeiDou B1/B2/B3, QZSS L1/L2/L5"
The price exceeds my home hobby budget, but so does a replacement CS-tube
a factory new OCXO based GPSDO and many other things you can sometime find
at reasonable cost used/recycled.
I naturally meant with a reasonable price-tag, sorry for being sloppy on
that detail, and I do know that there is vendors for those signals.
If we had dual or tripple frequency receivers below 500 USD things would
start to be interesting. If high-volume kits would be just twice as
expensive, it would be possible to consider for more luxury models.
Cheers,
Magnus
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