4 ms·
All terrestrial base stations (1) require GPS signal for time synchronization. Most of them can run for 4 hours - 24 hours without GPS, but that's it, after tha
by timdux 6y ago
All terrestrial base stations (1) require GPS signal for time synchronization. Most of them can run for 4 hours - 24 hours without GPS, but that's it, after that holdover period it's dead.
There is no GPS signal on the moon. That's a problem.
(1) except micro base stations
- kitteh 6y agoThey could use their own timing source, though? I recall back in the day being able to pull timing source off a circuit.
- timdux 6y agoYes, that's the clock holdover circuit, usually built using a OCXO or double OCXO or rubidium clock. It's good for 4 to 24 hours. Example: [0] https://www.rakon.com/19-rak-product/rak-product-ocxo https://www.rakon.com/19-rak-product/rak-product-ocxo
- snypher 6y agoIt's a problem being worked on, but it should be possible to use the existing GPS satellites on Moon. Also, given it is just being used as a time source and not for navigation, it should be possible to make a moon-based time signal with less hardware units? https://www.nasa.gov/feature/goddard/2019/nasa-eyes-gps-at-the-moon-for-artemis-missions https://www.nasa.gov/feature/goddard/2019/nasa-eyes-gps-at-t...
- anticensor 6y agoLow lunar orbits are unfortunately too unstable.
- cozzyd 6y agoThere's only like an extra 25 dB of path loss to the moon than Earth since GPS satellites are at orbits of 20,000 km. Of course the GPS antennas are probably oriented facing the Earth, but you'd still see GPS satellies on the far side (with a bit more path loss). Since GPS has a 43 dB processing gain from the modulation scheme, and thermal noise levels on the moon are similar to Earth (I think?), that means you need -150 dBm or so at the receiver. I believe newer GPS satellites have an EIRP of 50 dBm or so, so receiving GPS signals on the moon might work without even using a high gain antenna. Use of a high gain antenna would probably completely compensate for the increased path loss to the moon.
- 213123124123 6y agoWhat exactly requires time sync? I think it's mainly stuff like inter-cell handover, MIMO with multiple cells streaming downlink to a single phone, these kind of things. It should work as long as the base stations are synced well -- no matter what reference. Or they just drop the features that require good sync. [Edit: maybe TDD mode? but they could settle for FDD...]
- bewaretheirs 6y agoThere are indications that signals from the US GPS constellation and other GNSS constellations can be received on at least part of the moon; coverage gaps could be closed with just a couple of satellites near the moon. https://spectrum.ieee.org/tech-talk/aerospace/satellites/can-astronauts-use-gps-to-navigate-on-the-moon-nasa-scientists-say-yes https://spectrum.ieee.org/tech-talk/aerospace/satellites/can...
- nitrogen 6y agoDoes the GPS protocol or another protocol already handle satellites in arbitrary and arbitrarily large orbits?
- karteum 6y agoTDD base stations require synchronization (around 3µs) to avoid interfering with each others, but this does of course not apply if there is only one BS or if there is any other way to sync them (e.g. SyncE + IEEE-1588v2, G.8275...). See https://docdb.cept.org/download/220ac21f-b44b/ECCREP216.PDF https://docdb.cept.org/download/220ac21f-b44b/ECCREP216.PDF As far as I know 3GPP2 cdma2000/EV-DO also need GPS because the codes are synchronous (although I am not familiar with those technologies). Apart from that, 3GPP FDD base stations only require a stable frequency reference, not a time reference (LTE/WCDMA base stations are not connected to GPS !)
- Nokinside 6y agoAs there will be only one cellular network, inference coordination and timing is relatively easy to solve. You just use PTP. No need to GPSS Everywhere.