On 6/9/11 10:36 PM, Henry Hallam wrote:
On Thu, Jun 9, 2011 at 2:42 PM, Jim Lux<jim...@earthlink.net> wrote:
GPS orbits are tough from a radiation standpoint too.
In particular, the orbits are considerably worse for radiation than
GEO, and photovoltaic panels are quite susceptible to radiation. Of
course you could put a GNSS in GSO but I think it's not as favorable
from a constellation design point of view, and the launches are more
expensive.
I think this is an interesting thing, especially from a time-nuts
perspective.. In order to transfer time from one place to another, you
need to know where those two places are, very accurately (i.e. 3ns
accuracy implies 1 m position uncertainty)
Transit and GPS both make use of the dual curse and blessing of having a
moving satellite so it has doppler (a pain for acq and track) but it
also means that visibility is good. you can cover the sky with moving
satellites, so pretty much anywhere you are, at some point you'll get
enough visibility to make it work. The Doppler actually makes it easier
to determine the accurate position of the satellites, too.
In GEO, the positions of the satellites aren't known to anywhere near 1
m accuracy (10s-100s of meters, if they're using GPS<grin>). And, as
pointed out, you don't have anywhere near the flexibility of orbits.
The MEO height of GPS was a deliberate choice (again, that GPSWorld
series is a fascinating history of how it came about). Don't forget
that one of the original reasons for GPS was for doing midcourse
correction on ICBMs.
The Japanese have a satellite with Nav signals up that is in a not quite
GEO height orbit that is inclined so its ground track sort of appears to
make a big figure 8 stretched N/S over Japan.
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