3 ms·

Absolute positioning is not that important as long as the relative positions are constant to within a small fraction of the wavelength, in this case to within a

Absolute positioning is not that important as long as the relative positions are constant to within a small fraction of the wavelength, in this case to within a millimeter. Which is feasible if you mount it to the side of a building and there isn't too much wind. The much bigger problem is that you need a very accurate clock. Much better than what you can get from a GPS receiver. And lose the blockchain bullshit.

toomuchtodo 6y ago
Are there any crystal or photonic oscillators that are up to the task for this time keeping? Or is RF interferometry bound to something like a chip atomic clock [1] (~$4k in low quantities)? [1] https://coverclock.blogspot.com/2017/05/my-stratum-0-atomic-clock_9.html https://coverclock.blogspot.com/2017/05/my-stratum-0-atomic-...
- ajford 6y ago
  It's been almost ten years since I last done any of the back end math for this kind of thing, but you could probably get within the ballpark using COTS rubidium oscillators like those used for cell towers. They often show up on auction sites at affordable prices. Use the pulse output to lock a station clock for your local time needs (like syncing your data recording to wall time) and the sine output to lock your mixing oscillators.
- scionthefly 6y ago
  I suspect that you can get a start by looking at how cosmic ray studies conducted at different sites are coordinated: https://ieeexplore.ieee.org/document/1351816 https://ieeexplore.ieee.org/document/1351816
  - petschge 6y ago
    Except you need closer to 0.01ns for this use case instead of the 55ns you get out of the GPS receivers. And half your data would have more, possibly much more, deviation than that anyway. If you don't know which half, that will lead to loss of sensitivity and possibly artifacts in the reconstructed signal. Interferometry is actually hard.
    - brandmeyer 6y ago
      GPS receiver accuracy relative to the constellation's time base is much much better than 55n. I'm working with a cheap (< 100 USD in prototype qty) OCXO that's good to a few parts in 10^-12 over 10 second periods. The GPS timebase varies from the TAI time base by up to a couple dozen nanoseconds, but it is very slowly varying. A good GPS-disciplined OCXO can hold that accuracy over much longer intervals. To take advantage of that accuracy you have to share the master reference oscillator between the telescope's clock tree and GPS receiver's clock tree. A commercial chipset's PPS output isn't going to cut it, but a dedicated amateur could definitely build a GPS-tagged radio telescope off of a common crystal like that.
      - petschge 6y ago
        Something like that might work. I am surprise that the temperature stabilized local oscillator is so cheap. Care to share a link? The other thing that can help is to put receivers not too far apart. Say 25 dishes in an area with 100 meters diameter. That improves sensitivity by an order or magnitude, resolution by two orders of magnitude and the station can either be connected with a clock network, or (if using GPS) will at least see correlated clock errors that don't matter much in the correlation. The 1 part in 10^-7 accuracy of the GPS receivers is plenty accurate for the phase folding.
        brandmeyer 6y ago
        We've measured Milliren 220 series to be about 5e-12 at 10 seconds, but that's not fair since it turns out that part is "call for quote", even if we only bought it in small qty. Conner-Winfield OH320 advertises an Allan Deviation of 2.7e-12 at one second. Its a very similar part, without all the hirel and space qualifications. 98 USD/ea at Digi-Key. I agree wholeheartedly on the low-baseline setup case. An offline carrier phase position-time solution could stretch you out to much wider baselines, though.

toomuchtodo 6y ago

Are there any crystal or photonic oscillators that are up to the task for this time keeping? Or is RF interferometry bound to something like a chip atomic clock [1] (~$4k in low quantities)? [1] https://coverclock.blogspot.com/2017/05/my-stratum-0-atomic-clock_9.html https://coverclock.blogspot.com/2017/05/my-stratum-0-atomic-...

ajford 6y ago
It's been almost ten years since I last done any of the back end math for this kind of thing, but you could probably get within the ballpark using COTS rubidium oscillators like those used for cell towers. They often show up on auction sites at affordable prices. Use the pulse output to lock a station clock for your local time needs (like syncing your data recording to wall time) and the sine output to lock your mixing oscillators.
scionthefly 6y ago
I suspect that you can get a start by looking at how cosmic ray studies conducted at different sites are coordinated: https://ieeexplore.ieee.org/document/1351816 https://ieeexplore.ieee.org/document/1351816
- petschge 6y ago
  Except you need closer to 0.01ns for this use case instead of the 55ns you get out of the GPS receivers. And half your data would have more, possibly much more, deviation than that anyway. If you don't know which half, that will lead to loss of sensitivity and possibly artifacts in the reconstructed signal. Interferometry is actually hard.
  - brandmeyer 6y ago
    GPS receiver accuracy relative to the constellation's time base is much much better than 55n. I'm working with a cheap (< 100 USD in prototype qty) OCXO that's good to a few parts in 10^-12 over 10 second periods. The GPS timebase varies from the TAI time base by up to a couple dozen nanoseconds, but it is very slowly varying. A good GPS-disciplined OCXO can hold that accuracy over much longer intervals. To take advantage of that accuracy you have to share the master reference oscillator between the telescope's clock tree and GPS receiver's clock tree. A commercial chipset's PPS output isn't going to cut it, but a dedicated amateur could definitely build a GPS-tagged radio telescope off of a common crystal like that.
    - petschge 6y ago
      Something like that might work. I am surprise that the temperature stabilized local oscillator is so cheap. Care to share a link? The other thing that can help is to put receivers not too far apart. Say 25 dishes in an area with 100 meters diameter. That improves sensitivity by an order or magnitude, resolution by two orders of magnitude and the station can either be connected with a clock network, or (if using GPS) will at least see correlated clock errors that don't matter much in the correlation. The 1 part in 10^-7 accuracy of the GPS receivers is plenty accurate for the phase folding.
      - brandmeyer 6y ago
        We've measured Milliren 220 series to be about 5e-12 at 10 seconds, but that's not fair since it turns out that part is "call for quote", even if we only bought it in small qty. Conner-Winfield OH320 advertises an Allan Deviation of 2.7e-12 at one second. Its a very similar part, without all the hirel and space qualifications. 98 USD/ea at Digi-Key. I agree wholeheartedly on the low-baseline setup case. An offline carrier phase position-time solution could stretch you out to much wider baselines, though.