[time-nuts] Speaking of Costas loops (WAAS)

jmfranke jmfranke at cox.net
Sun Jul 7 13:19:19 UTC 2013


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" <lists at rtty.us>
Sent: Saturday, July 06, 2013 10:23 PM
To: "Discussion of precise time and frequency measurement" 
<time-nuts at febo.com>
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 <jmfranke at cox.net> 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" <magnus at rubidium.dyndns.org>
>> Sent: Saturday, July 06, 2013 12:57 PM
>> To: <time-nuts at febo.com>
>> 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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