[time-nuts] What position is measured?

[jimlux]

Pierpaolo Bernardi wrote:
> On Wed, Sep 8, 2010 at 02:16, jimlux <jimlux at earthlink.net> wrote:
>> Mark J. Blair wrote:
>>> On Sep 7, 2010, at 6:30 AM, jimlux wrote:
> 
>> Yes.. except that the cable's physical and electrical length *do* vary with
>> temperature, so if you're looking at the gnat's eyelash sort of thing, you
>> need to take that into account.  Maybe 10 ppm/degree, so a 20 meter run will
>> change a bit less than a millimeter.  That's down in the fractional
>> picoseconds time-wise.
>>
>> It's an issue if you're doing things like interferometry at higher
>> frequencies..
> 
> Would be possible for the receiver to take into account automatically
> the delay of the antenna cable, by measuring the delay of an echo of
> a signal it sends towards the antenna?  Do such receivers exists?
> 

Not for GPS, to my knowledge, but in other time distribution systems, 
certainly. It's also used in antenna ranges when you need phase 
information (as in a near field range).  It's also been done "over the 
air" in radio telescope arrays (e.g. VLA).

At JPL, we navigate spacecraft in deep space by very accurately 
measuring the time delay of a round trip to the spacecraft from earth 
and back. These days, position uncertainties are in the cm range and 
velocity in the mm/s, implying measurements of picoseconds in a round 
trip time of 10,000 seconds.  All of this implies that the entire 
measurement chain (including the cables carrying the maser reference 
signal) are carefully characterized and controlled.