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

[Owen Lewis]

> -----Original Message-----
> From: ukcrypto-admin@chiark.greenend.org.uk
> [mailto:ukcrypto-admin@chiark.greenend.org.uk]On Behalf Of Roland Perry
> Sent: 04 March 2003 09:51
> To: ukcrypto@chiark.greenend.org.uk
> Subject: Re: "Blunkett u-turn on data privacy plans"
>
>
> In message <1165392191-7086274@espace.net>, Fearghas McKay
> <fm-lists@st-kilda.org> writes
>
> >no not normal GPS receivers - two antenna and a specialised box that
> >compares the difference.

> >
> >>And you can detect a difference of 100 microns? Surely that would
> >>require each receiver to have a readout significant to 100 microns, even
> >>assuming they drifted and wobbled absolutely in time with one another.

Not so. The common method of 'measurement' used with a two-antenna systems
compares the relative time of arrival/phase of arrival of a selected signal.
This one uses electronic manipulations to extrapolate distance from
knowledge of the speed of light, the phase shift found and the distance
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?

However, to do well, such systems get a bit expensive and complex. The usual
purpose of dual antennas on 'consumer electronics' type receivers is that of
dual diversity reception. The reason for using such a system is as follows.

All electromagnetic energy exhibits some reflective properties. As a rule of
thumb and up to a certain point only in the e-m spectrum of radiation, the
higher the frequency, the more readily reflective patterns occur. In terms
of radio transmission, signals at upper UHF and at SHF reflect very readily
indeed. For most communication systems where the patterns of reflection
cannot be predicted and controlled, this is an unwanted characteristic since
the reflected wave front(s) are very likely to arrive at an receiver's
antenna out of phase with the wavefront received directly from the transmit
antenna. Thus, a partial or even total signal cancellation can occur at the
receive antenna. This phenomenon can be extreme where the receive antenna is
within a box with several corners reflectors(e.g. a normal room).

In a dual diversity receiver system, two antennas are placed in a fixed
relationship to each other in a way that they are separated by a significant
part of a wavelength of the signal band it is wished to receive. Where a
strong cancellation effect exists, it will not exist at both antennas
because of the physical separation that is a significant part of a
wavelength. The receiver then selects to use input from whichever antenna
momentarily offers the greater signal input.

> >>From the conversations I have had with the marine engineers on
> this I think
> >that it is not impossible but I don't have source material to verify it.
>
> There's another list I'm on that recently carried the observation:
>
>         "I am confused" in ITU-T speak means "You are talking bull****".
>
> I have to say that I'm still *very* confused about the 100 micron
> claims.
>
> However, my practical experience and what I've read says that yes, I
> would believe that a suitable receiver could easily detect relative
> yaw/pitch movements of a ship of the order of a few cm (and back) at
> relative speeds of a few cm per second.

Exploitation of the Doppler shift phenomenon does not necessitate two
receive antennas and, yes, it has been used to detect movement for many
years now. Thirty years ago, it was good enough to be able to discriminate
between the movement of, say, a man's legs and the movement of his body.
Here, the frequency of a source signal is accurately known and compared with
the phase shifted signal returned from a moving object. What you propose
requires only that the reflector stays still and the T/R position is in
motion relative to it. It is, of course also possible to exploit Doppler
shift where radiation is received directly at one object from another with
on reflection being involved. All that is required is that one body is in
motion relative to the other.

Owen