[time-nuts] Thunderbolt - trouble locking with some types of antennas

Magnus Danielson magnus at rubidium.dyndns.org
Wed Aug 2 08:16:05 EDT 2017

On 08/02/2017 01:20 AM, jimlux wrote:
> On 8/1/17 12:11 PM, Graham / KE9H wrote:
>> Dider:
>> This is a CDMA signal. (With a 'chip' rate that far exceeds the
>> information
>> rate.)
>> If you put a different correlator on every multipath signal, which are
>> each
>> differently delayed in time, then they can be independently demodulated.
>> (Or time shifted and added back together with some quality indicator for
>> weighting.)
>> So, in CDMA, multipath is used as a form of (time) diversity reception
>> and
>> will improve the signal to noise of the combined signal.
>> By definition, the signal with the least time delay either is, or is
>> closest to, the most direct path.
>> You don't have to necessarily fully demodulate this early signal by
>> itself,
>> just know what its timing is. (And remember where it was, if fading in
>> and
>> out.)
>> Since multi-path is a destructive mechanism in most narrow band radio
>> systems, the above may not be intuitive to people not familiar with CDMA
>> and "rake receiver" based systems.
>> Multipath helps, not hurts, these systems, as long as the multipath
>> delays
>> are most of one chip apart, or more.
> For GPS raw (off the air), when they post process at JPL, they use a
> fairly sophisticated correlation process, incorporating an estimator of
> the underlying time delay trajectory: you can use a later "big peak" to
> help recover the early "first peak", for instance.
> Rake receivers (under the name "adaptive equalizers") are also why
> digital TV works fairly well in high multipath environments - with
> modern receivers that do this.

The real increase in precision comes from the higher sampling rate, i.e. 
sample per chip, which allows for more elaborate schemes to be used. 
Narrow correlation, RAKE etc. is all just different approaches to that, 
but the core comes from the oversampling and increased bandwidth.
Just doing increased bandwidth with traditional correlator is know to 
improve precision as narrow-range multi-path can be better suppressed.
Novatel patented an extension to that using a par of narrow correlators 
to further improve on that aspect.

Regardless how you do it, improved bandwidth and related oversampling 
factor is the enabler, then the weapon of choice to make use of it is 
the next thing to look at.

BTW, I saw that the JPL receiver at ISS got upgraded to support Galileo. 
Cool stuff.


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