[time-nuts] Re: Looking at ultra precise 50 km-100 km Electronic Distance Measurement.

[Lux, Jim]

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.