[time-nuts] GPS discipline oscillator vs phase lock

[jimlux]

On 6/20/17 4:22 PM, Hal Murray wrote:
>
> jimlux at earthlink.net said:
>> sequential tone ranging: by putting a "ranging tone" at, say, 1 MHz,  on the
>> carrier
>
> Thanks.  The part that attracted my attention was your "spectrally pure
> signal" for the VCO.


Typically a maser at the ground station - that probably gets you fairly 
good close in phase noise characteristics.


Deep space uses very simple modulations (none of this fancy 64QAM)  - 
BPSK direct modulation or BPSK on a 25 or 32 kHz subcarrier.  They 
adjust the mod index of the data and ranging signals to leave 
significant power in the carrier (unless they're in a very good SNR 
situation).

But you can see why people would like PN ranging - it takes NO power 
from the carrier, so you can get a really good doppler estimate.

In reality, of course, you can post process to remove the BPSK data 
(just like removing the PN ranging) and essentially get all the power 
back in the carrier.  But you can really only do that on the downlink. 
On the other hand, the uplink usually has much better SNR than the 
downlink (80kW transmitter into a 34 or 70 m dish gives you a *healthy* 
EIRP), and the data rate on uplink is usually fairly low (nobody uplinks 
at megabits yet)