4 ms·

I'm having trouble understanding the practical consequences of this project. If I would be in Japan, with a regular, GPS-tuned/understanding device, would I ge

by ersii 13y ago

I'm having trouble understanding the practical consequences of this project. If I would be in Japan, with a regular, GPS-tuned/understanding device, would I get better service? Or would I need a special device, that'd be able to understand GPS+"JPS"? The part where they transmit correction data to the satellites also confuse me, if that's what they're doing; It's not the same as the United States Global Positioning Service if that'd be the case, right? For what I've understood, the GPS doesn't need a constant feeding of data from ground stations - it's ""simply"" a bunch of satellites sending out the current time from their on-board atomic clocks.

dfc 13y ago
> GPS doesn't need a constant feeding of data from ground stations This is not the case. USNO steers the clocks(4x) on the satellites. I think they do this via the AF but I have always been a little uncertain of the exact method: GPS time is automatically steered to UTC(USNO) on a daily basis to keep system time within one microsecond of UTC(USNO) modulo 1 second, but during the last several years has been within a few hundred nanoseconds. The rate of steer being applied is +/-1.0E-19 seconds per second squared.[^1] One of the reasons for JPS's need for direct contact with ground stations is that the newer satellites do not contain the any rubidium frequency standards and use an experimental time synching system. [^1]: http://tycho.usno.navy.mil/gpstt.html http://tycho.usno.navy.mil/gpstt.html
- icegreentea 13y ago
  GPS ground updates correct for clock skew and disturbances in orbits. The Japanese system also seems to upload the differential measurements of their entire ground reference points to rebroadcast to individual receivers. In some senses, the technique is just an augmentation/expansion of WAAS [0] to Japan (WAAS is strictly North American). [0] http://en.wikipedia.org/wiki/Wide_Area_Augmentation_System http://en.wikipedia.org/wiki/Wide_Area_Augmentation_System EDIT: And to the OP, you would need some sort of GPS+ device.
- rasz_pl 13y ago
  W T F??? Why wouldnt they include rubidion clock on sats? Its at most $10K expense on something costing tens of millions to launch. Does this mean GPS will stop working when Zombies/Pandemic/whatever strikes and wipes out US military?
  - cyanoacry 13y ago
    A really big problem with satellite technology are the harsh environments they go through. You're dealing with something like 10-20g's of vibration peak-to-peak on ascent, and then you subject it to the freezing colds and searing heat of space, all while doing this in vacuum. Since those clocks need to be super precise, it's easier to have an accurate reference on the ground where everything's not getting shaken and baked to bits.
  - dfc 13y ago
    The majority of GPS satellites do not have one rubidium clock on board, they have four. For a discussion of the experimental time sync system planned for QZSS see: http://web.archive.org/web/20130411122243/http://unsworks.unsw.edu.au/fapi/datastream/unsworks:4274/SOURCE02 http://web.archive.org/web/20130411122243/http://unsworks.un... I am curious before reading my comment did you think that GPS would continue to work without navy.mil and af.mil?
    - rm445 13y ago
      I would have done. Having once convinced myself that four satellites shouting out the (accurate) time is sufficient to locate oneself in three dimensions, I assumed that was all there is to it.
    - rasz_pl 13y ago
      Hell yes I did. Whole point of Rubidium clocks is you dont have to correct them. I was hoping GPS would stay up for at least 5-10 years - I assumed all they needed was periodic orbit corrections. Well, there goes my GPS to the rescue Zombie survival plan :/. Time to learn how to read stars, or better yet - find android app that does it automagically from a picture :)
7952 13y ago
The services web page has much more detail... http://www.qzs.jp/en/services/index.html http://www.qzs.jp/en/services/index.html This system add additional satellites that a normal GPS receiver can receive. By using different orbits they can improve reception. It also transmits on multiple frequencies that allow the effects of the atmosphere on the signal to be better calculated. It will also transmit augmentation information to improve accuracy further.
InclinedPlane 13y ago
> For what I've understood, the GPS doesn't need a constant feeding of data from ground stations - it's ""simply"" a bunch of satellites sending out the current time from their on-board atomic clocks. When you get a GPS fix the very first thing your handset does is download the ephemeris data. This is broadcast at regular intervals along with the location signals. This data tells you the orbital parameters of each satellite, without it you couldn't get a precise location fix because you wouldn't know where the satellites you were getting signals from were. The GPS satellites are incapable of calculating their own ephemeris data, it needs to be uploaded from the ground. The system cannot simply reuse the same data forever because the orbits of the satellites change over time, and because the system is sensitive to very small changes a lot of higher order effects (such as radiation pressure, the non-sphericity of the Earth's geoid, and so on) come into play which are not easily calculable. Over short periods the future ephemeris are predictable within reasonable bounds of precision, so the USAF uploads 30 days worth of ephemeris data to the GPS satellites on a regular basis. If it were no longer possible to upload that data the system would eventually become either non-functional (depending on how the satellites and individual handsets are designed) or increasingly inaccurate until no longer useful.
- avar 12y ago
  > If it were no longer possible to upload that data the system > would eventually become either non-functional (depending on how > the satellites and individual handsets are designed) or > increasingly inaccurate until no longer useful. If that were to happen you could just augment GPS receivers to use more accurate ephemeris data provided from other sources. It is, after all, now computed on the ground and uploaded to the GPS constellation, having it be provided by the satellites is just a convenience, not a necessity.

by ersii 13y ago

dfc 13y ago

> GPS doesn't need a constant feeding of data from ground stations This is not the case. USNO steers the clocks(4x) on the satellites. I think they do this via the AF but I have always been a little uncertain of the exact method: GPS time is automatically steered to UTC(USNO) on a daily basis to keep system time within one microsecond of UTC(USNO) modulo 1 second, but during the last several years has been within a few hundred nanoseconds. The rate of steer being applied is +/-1.0E-19 seconds per second squared.[^1] One of the reasons for JPS's need for direct contact with ground stations is that the newer satellites do not contain the any rubidium frequency standards and use an experimental time synching system. [^1]: http://tycho.usno.navy.mil/gpstt.html http://tycho.usno.navy.mil/gpstt.html

icegreentea 13y ago
GPS ground updates correct for clock skew and disturbances in orbits. The Japanese system also seems to upload the differential measurements of their entire ground reference points to rebroadcast to individual receivers. In some senses, the technique is just an augmentation/expansion of WAAS [0] to Japan (WAAS is strictly North American). [0] http://en.wikipedia.org/wiki/Wide_Area_Augmentation_System http://en.wikipedia.org/wiki/Wide_Area_Augmentation_System EDIT: And to the OP, you would need some sort of GPS+ device.
rasz_pl 13y ago
W T F??? Why wouldnt they include rubidion clock on sats? Its at most $10K expense on something costing tens of millions to launch. Does this mean GPS will stop working when Zombies/Pandemic/whatever strikes and wipes out US military?
- cyanoacry 13y ago
  A really big problem with satellite technology are the harsh environments they go through. You're dealing with something like 10-20g's of vibration peak-to-peak on ascent, and then you subject it to the freezing colds and searing heat of space, all while doing this in vacuum. Since those clocks need to be super precise, it's easier to have an accurate reference on the ground where everything's not getting shaken and baked to bits.
- dfc 13y ago
  The majority of GPS satellites do not have one rubidium clock on board, they have four. For a discussion of the experimental time sync system planned for QZSS see: http://web.archive.org/web/20130411122243/http://unsworks.unsw.edu.au/fapi/datastream/unsworks:4274/SOURCE02 http://web.archive.org/web/20130411122243/http://unsworks.un... I am curious before reading my comment did you think that GPS would continue to work without navy.mil and af.mil?
  - rm445 13y ago
    I would have done. Having once convinced myself that four satellites shouting out the (accurate) time is sufficient to locate oneself in three dimensions, I assumed that was all there is to it.
  - rasz_pl 13y ago
    Hell yes I did. Whole point of Rubidium clocks is you dont have to correct them. I was hoping GPS would stay up for at least 5-10 years - I assumed all they needed was periodic orbit corrections. Well, there goes my GPS to the rescue Zombie survival plan :/. Time to learn how to read stars, or better yet - find android app that does it automagically from a picture :)

7952 13y ago

The services web page has much more detail... http://www.qzs.jp/en/services/index.html http://www.qzs.jp/en/services/index.html This system add additional satellites that a normal GPS receiver can receive. By using different orbits they can improve reception. It also transmits on multiple frequencies that allow the effects of the atmosphere on the signal to be better calculated. It will also transmit augmentation information to improve accuracy further.

InclinedPlane 13y ago

> For what I've understood, the GPS doesn't need a constant feeding of data from ground stations - it's ""simply"" a bunch of satellites sending out the current time from their on-board atomic clocks. When you get a GPS fix the very first thing your handset does is download the ephemeris data. This is broadcast at regular intervals along with the location signals. This data tells you the orbital parameters of each satellite, without it you couldn't get a precise location fix because you wouldn't know where the satellites you were getting signals from were. The GPS satellites are incapable of calculating their own ephemeris data, it needs to be uploaded from the ground. The system cannot simply reuse the same data forever because the orbits of the satellites change over time, and because the system is sensitive to very small changes a lot of higher order effects (such as radiation pressure, the non-sphericity of the Earth's geoid, and so on) come into play which are not easily calculable. Over short periods the future ephemeris are predictable within reasonable bounds of precision, so the USAF uploads 30 days worth of ephemeris data to the GPS satellites on a regular basis. If it were no longer possible to upload that data the system would eventually become either non-functional (depending on how the satellites and individual handsets are designed) or increasingly inaccurate until no longer useful.

avar 12y ago
> If it were no longer possible to upload that data the system > would eventually become either non-functional (depending on how > the satellites and individual handsets are designed) or > increasingly inaccurate until no longer useful. If that were to happen you could just augment GPS receivers to use more accurate ephemeris data provided from other sources. It is, after all, now computed on the ground and uploaded to the GPS constellation, having it be provided by the satellites is just a convenience, not a necessity.