[time-nuts] New Japanese GPS accuracy

Chris Albertson albertson.chris at gmail.com
Tue Jan 4 22:14:52 UTC 2011

On Tue, Jan 4, 2011 at 12:53 PM, Hal Murray <hmurray at megapathdsl.net> wrote:
> jimlux at earthlink.net said:
>> Similar in concept to waas or tass, the satellite provides a nav signal and
>> differential corrections.
>> One of the goals is to make a nav system that performs well (sub meter) in
>> urban canyons, which conventional gps does not
> I thought the idea with waas was to correct for the delays through the
> ionosphere by measuring the error at a known (nearby?) location and
> broadcasting the correction.  The idea is that a nearby location would have
> similar delays and similar errors.
> I thought the problem with urban canyons was multi-path and blocked signal.
> How is a correction for ionospheric delays going to help that?
> I must be missing something interesting.
> phk at phk.freebsd.dk said:
>> >As far as I know, it is a geo-synchronous polar orbiting D-GPS system.
>> Duh!  "Sun-synchronous" of course.
> The original crunchgear article said they needed 3 satellites to get 24 hour
> coverage.
> I can't picture an orbit pattern that's going to use 3 satellites.  Geosync
> would work with one satellite, but Japan is fairly far north.  Are they doing
> something like picking the orbit height and inclination angle so that the
> satellite period is 24 hours and over Japan rather than the equator at the
> right time?

I think 12 hour period

My guess is it is a "Molniya orbit"  It is very highly eliptic with a
12 hour period.  You can cover all of Japan with only two satellites.

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