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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