[time-nuts] Spacecraft Timekeeping

Magnus Danielson magnus at rubidium.dyndns.org
Wed Mar 9 18:03:58 UTC 2011


On 03/09/2011 03:09 PM, jimlux wrote:
> On 3/8/11 1:45 PM, Magnus Danielson wrote:
>> Kevin,
>>
>> On 03/08/2011 06:57 PM, Kevin Watson wrote:
>>> Hi Jim,
> <snip>
>>> 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.
>>
>> First thing to consider is that standard GPSes will not meet your needs,
>> since they have to obey the height and speed limits for export rules.
>
>
> OTOH, if you're building a rocket that's big enough to need something
> like this, you can likely get the needed export licenses, or at least,
> comply with the export control laws. But yes, the vanilla off the shelf
> GPS probably has the "don't report over 60,000 ft or 1000 km/hr" lockouts.

Considering the linked GPS, I just wanted to make the point that a 
standard off the shelf civilian GPS won't cut it, unless it's for launch 
pad use only.

>> The side-effect is that doppler frequencies may be much higher and both
>> tracking and acquisition needs to include these more extremer doppler
>> frequencies.
>
> That would be my concern with GPS... the so called "high dynamic"
> environment. LEO orbit is 7km/sec, so you'd think the Doppler would be
> huge, but actually, that's not a big problem, since you already have to
> deal with an even higher Doppler from the GPS SVs already. Whether your
> receivers nav solution can work with a fast moving platform is another
> story. It may assume that nothing can go that fast, and so it doesn't
> track.
>
> OTOH, if you're buying a GPS module from someone like Trimble or
> Motorola or whoever, you can probably ask them.

I was trying to point out issues beyond that of export limitations where 
a normal civilian GPS would not quite cut it. There is many such issues.

>> Use of PTP within a rocket or spacecraft may or may not be a good thing.
>> NMEA + PPS may suffice and be less power-hungry. IRIG may also be an
>> option.
>
> I would agree.. unless you're trying to minimize wire count and you
> already have Ethernet. Spacecraft designs are very mass and pin count
> sensitive (every pin in the connector needs to be tested, which costs
> money, etc.)

If you only run fast ethernet, you can use spare pins for whatever 
signal you like.

Cheers,
Magnus




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