On 1/22/22 9:48 AM, Patrick Barthelow wrote:
Looking to increase accuracy of a microwave transceiver pair designed to
measure very long distances (*up to even 100km, Line of sight)
They send a microwave carrier, modulated by HF radio modulation sine waves.
To an identical Transceiver at the far end. Which sends the modulated
carrier back. and phase comparisons of emitted and returned sigs are
compared and precisely quantified. Most instrument specs from the old
days are still pretty good, +- a centimeter or so, and a ppm variation
with total distance. --In 40-60km High accuracy stuff needs good
meteorology data along the line measured, which affects c = speed of radio
waves at the time of measurement.
The Modulation frequencies need to be either monitored continuously and/or
need to be stable to assigned values. Trying to modify internal frequency
reference of a pretty old system to GPSDO accuracy.
Anyone here that has done EDM work in surveying or Geodesy, etc?
Isn't this basically the same as the ranging we do with deep space
transponders and DSN?
For that, we send a signal to the other end, track it with a PLL and
send a signal back with a precise ratio (880/749 for X band). Then, as
you describe, phase comparison between outgoing and incoming signals. To
disambiguate, we'll phase modulate either a PN or sine or square wave on
the carrier (leaving a lot of carrier). The ranging tone (or sequence)
is either just reflected back around, or is decoded and regenerated
(regenerative ranging).
Doing a coherent turnaround at the far end allows you to cancel out any
frequency variations at the far end, so all the "work" is at the one
station. The oscillator on board the spacecraft is a not particularly
special TCXO.
1 cm in 100km is 1E-7, right?
Typically, with deep space, and long averaging, we can get down into the
1E-14 or 1E-15 range (at 1000 second averaging time). - that's cm in
range and mm/s in Doppler at ranges of 1E9 km. Run of the mill
performance is ~1 meter.
The challenge would be finding some way to measure the difference in
speed of light. For ionized materials, there's a frequency dispersion,
so using two frequencies allows you to correct for it. (I don't know how
well you can do using the ranging sidebands.. is 10 MHz enough separation?)
But I don't know if air density and humidity have a strong enough
frequency dispersion effect. For DSN, we measure humidity along the path
with radiometry.
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