Two Antennas (was RE: "Blunkett u-turn on data privacy plans")

[Brian Gladman]

----- Original Message -----
From: "Ian G Batten" <I.G.Batten@ftel.co.uk>
To: <ukcrypto@chiark.greenend.org.uk>
Sent: Tuesday, March 04, 2003 11:02 AM
Subject: Re: Two Antennas (was RE: "Blunkett u-turn on data privacy plans")


> On Tue, 04 Mar 2003, Owen Lewis wrote:
> > between the antennas. I missed the start of this thread but 100 microns
> > seems a significant distance in comparison to the wavelength of light and
> > therefore should pose no great problem in the accuracy of calculation from a
> > detected phase shift. However, if the wavelength of one's chosen radiation
> > is, say, 1500 metres then detecting a phase shift with sufficient accuracy
> > to make 100 micron measurements possible may be implausible. Or do I
> > misunderstand?
>
> Quite.  The discussion is with regard to GPS L1, at about 20cm
> wavelength.

As you say, differential GPS techniques can be used to measure changes in the distance between two antennas down to cm resolution
for small distances.

Since 1mm will result in a phase change of about 2 degrees, my suspicion is that this could certainly be measured if the signal to
noise ratio is good and a relatively long integration time is used. I have certainly made phase measurements to better than this
accuracy at these frequencies in other contexts.

I hence suspect that mm accuracy is possible for distance changes, but I am doubtful that this could be extended to 100 micrometers.

However these measurements would require a laboratory environment - as you suggest, it does not seem likely that sub-cm resolution
would be possible in differential GPS measurements in a field environment.

   Brian Gladman