[time-nuts] New Japanese GPS accuracy

[Chris Albertson]

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.
>From the point of vie of someone on the ground they tend to hover at
about 63 degress N lat.     You can get continous coverage with only
two sats but with three you never have to have a view clear to the
horizon

-- 
=====
Chris Albertson
Redondo Beach, California