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>1: Japan has their own Just Japan system, QZSS, which I have vague memories is different, but I don't remember enough to explain it. Japan's QZSS is complemen

>1: Japan has their own Just Japan system, QZSS, which I have vague memories is different, but I don't remember enough to explain it. Japan's QZSS is complementary to America's GPS and also covers Australia and everywhere in-between due to its orbit. For generally practical purposes, it can be considered part of GPS. Any GPS receiver in range can receive its signals.

mandevil 3y ago
On the receiver side, yeah. On the satellite side, I had vague memories they were weird when I wrote that. From some quick googling I think they are doing a trick with clock synchronization from ground stations to avoid needing an atomic clock in space, since over Japan and Australia they will always have LOS to a ground-station (which they can use in place of the atomic clock as the master time source for each satellite)[1]. At least, that's what the 2008 Ph.D thesis "Remote Synchronization Method for the Quasi-Zenith Satellite System: study of a novel satellite timekeeping system which does not require on-board atomic clocks"[2] suggests that they were doing. That was published before the QZSS system actually flew, so I still am not sure if they actually went that way or stuck with the conventional atomic-clocks-in-space approach. So maybe they are unique in not needing so much hardware in space, perhaps? 1: GPS, designed to deal with at least the opening stages of nuclear war, had as a design goal the ability to operate without ground stations at least for a while, and there are large swathes of the world that don't have suitable ground stations, so they had to go with the atomic-clocks-in-space route to provide global coverage. By focusing on just a small area and ignoring the 'must operate for several days during a nuclear war' case, QZSS maybe could get away with something cheaper. 2: https://unsworks.unsw.edu.au/server/api/core/bitstreams/93862270-6598-4a9f-be97-db54ad62ef12/content https://unsworks.unsw.edu.au/server/api/core/bitstreams/9386...

mandevil 3y ago

On the receiver side, yeah. On the satellite side, I had vague memories they were weird when I wrote that. From some quick googling I think they are doing a trick with clock synchronization from ground stations to avoid needing an atomic clock in space, since over Japan and Australia they will always have LOS to a ground-station (which they can use in place of the atomic clock as the master time source for each satellite)[1]. At least, that's what the 2008 Ph.D thesis "Remote Synchronization Method for the Quasi-Zenith Satellite System: study of a novel satellite timekeeping system which does not require on-board atomic clocks"[2] suggests that they were doing. That was published before the QZSS system actually flew, so I still am not sure if they actually went that way or stuck with the conventional atomic-clocks-in-space approach. So maybe they are unique in not needing so much hardware in space, perhaps? 1: GPS, designed to deal with at least the opening stages of nuclear war, had as a design goal the ability to operate without ground stations at least for a while, and there are large swathes of the world that don't have suitable ground stations, so they had to go with the atomic-clocks-in-space route to provide global coverage. By focusing on just a small area and ignoring the 'must operate for several days during a nuclear war' case, QZSS maybe could get away with something cheaper. 2: https://unsworks.unsw.edu.au/server/api/core/bitstreams/93862270-6598-4a9f-be97-db54ad62ef12/content https://unsworks.unsw.edu.au/server/api/core/bitstreams/9386...