I have a dish. I have several GPSDOs, some I built myself. I just think it
would be a fun thing to try. It will not beat the performance of my GPSDOs
or rubidium oscillators. Someone with a micrometer still has use for a yard
stick. Yesterday, I worked on the dish feed. I checked the polarization
sense and figured out a way to mount it to the dish. Next week or early
August, I hope to have made an adjustable elevation/azimuth mount for the
dish. Maybe by fall I could have some interesting results to share.
Just having fun with time science. Heck, I even toy with tuning fork
oscillators and I hope to build a nice pendulum clock myself someday!
John WA4WDL
--------------------------------------------------
From: "Bob Camp" <[email protected]>
Sent: Saturday, July 06, 2013 10:23 PM
To: "Discussion of precise time and frequency measurement"
<[email protected]>
Subject: Re: [time-nuts] Speaking of Costas loops (WAAS)
Hi
Ok, lets *assume* there is some uber secret gizmo in the sat that makes
the unsupervised signal absolutely perfect when transmitted from the sat.
The sat still moves relative to the ground. It's speed is a vector in
three dimensions (up / down , north / south, east / west). Depending on
your location relative to the sat, the doppler will be different.
A cheap GPSDO will give you 1x10^-11 all day long, pretty much forever.
It'll do much better over long time spans. At 1.5 GHz, that would be 0.015
Hz.
If doppler is in the 50 to 100 Hz range, you need to cancel it by > 1000:1
simply to get the carrier as good as a simple GPSDO. That's going to
require accurate position data on the sat, it's velocity (all real time),
and your location.
-----------------
Next you need data on the rest of the constellation. They fly in the same
space as the WAAS birds, and transmit on the same frequencies. As they
pass within the capture area of your antenna you will need a way to figure
out which is the GPS and which is the WAAS sat.
The easy way to do that would be to run a GPS to get all the data and then
process it…..
----------------
Dish costs something
Downconverter costs something
Signal processing the received signal costs something
You still need a GPS
You still need a good local OCXO as a flywheel
It's going to be tough to convince me that's any cheaper than a GPSDO
-------------
Lots of things to slog through. I suspect there are other sat signals that
are better candidates.
Bob
On Jul 6, 2013, at 2:23 PM, jmfranke <[email protected]> wrote:
A lot of the changes from "bent pipe" to the new system including C-band
uplink is explained here:
http://www.insidegnss.com/node/697
While there, downlink the extended PDF version.
John WA4WDL
--------------------------------------------------
From: "Magnus Danielson" <[email protected]>
Sent: Saturday, July 06, 2013 12:57 PM
To: <[email protected]>
Subject: Re: [time-nuts] Speaking of Costas loops (WAAS)
On 07/06/2013 06:29 PM, Joseph Gwinn wrote:
Code/Carrier Frequency Coherence: The lack of coherence between the
broadcast carrier phase and the code phase shall be limited. The
short
term (<10sec) fractional frequency difference between the code phase
rate and the carrier frequency shall be less than 5x10-11 (one
sigma).
Over the long term (<100 sec), the difference between the change in
the
broadcast code phase (convert to carrier cycles) and the change in
the
broadcast carrier phase shall be within one carrier cycle (one
sigma).
This is interesting. Does it imply that they regenerate the code on
board?
Very unlikely, because then the bird would have to understand every
possible code, including those not invented when the bird was launched.
If it is within the Gold codes being used for GPS and WAAS, they only
need to alter the 10 bit reset-value of the G2 PRN code. See the WAAS
specification, as this method is being recommended for receivers.
Within that limit, it is relatively cheap to provide code tunability.
Cheers,
Magnus
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