[time-nuts] Spacecraft Timekeeping

Kevin Watson time-nuts at enuuf.com
Tue Mar 8 17:57:05 UTC 2011


Hi Jim,

As part of my research into keeping time on rockets and spacecraft, I joined
this list to see what I could learn from the masters. Of course I'm a
knuckle-head for not assuming that you'd be one of the resident masters
<grin>. Anyway, as my accuracy needs are modest (~10uS across many onboard
computers), have access to GPS most of the time and don't really need to
worry about relativistic effects (yet, anyway <grin>) or radiation effects
(due to redundancy), I thought I'd use a GPSDO that can handle a decent
amount of holdover and then use PTP to distribute time across the vehicle.
Do you, or anyone else, have a recomendation for the GPSDO? Jackson Labs'
(http://jackson-labs.com/) DROR seems like it might work, but I wonder if
there might be better alternatives.

-Kevin Watson


----- Original Message ----- 
From: "jimlux" <jimlux at earthlink.net>
To: <time-nuts at febo.com>
Sent: Tuesday, March 08, 2011 6:20 AM
Subject: Re: [time-nuts] GPS Filter


> On 3/8/11 4:24 AM, Pieter ten Pierick wrote:
>> Hi,
>>
>>> GPS phased arrays aren't new, nor is it necessary to physically steer
>> the antennae within the aray:
>>> http://www.navsys.com/papers/0005004.pdf
>>
>> But would such a system help with the LNA overload due to a local
>> transmitter?
>> I would expect that using separately steered antennas with good
>> directivity could prevent this out-of-band LNA overload?
>>
>
>
> As long as your phasing is done before the one-bit ADC, you can place a 
> null on the offending interference source.  This has been used for 
> adaptive interference cancellers for decades (for cosited transmitters and 
> receivers).  A friend worked for American Nucleonics back in the 80s when 
> they were starting to implement digital control loops (but the 
> cancellation was still done with analog techniques) for this application.
>
> You *might* be able to do it even after the sampler, if the interfering 
> signal isn't too strong.  At a guess, if the total noise power at the 
> input to the sampler were within about 10dB of the interfering signal, 
> you'd probably be ok.  When the interfering signal gets up to 10-20 dB 
> over the noise power, it starts to really dominate.
>
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