[time-nuts] L1 and L2 frequencies

Magnus Danielson magnus at rubidium.dyndns.org
Thu Jan 15 16:29:50 EST 2015


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:

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.


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