Re: DIY Radar.. tip?

From: Klave (Klave_at_myrealbox.com)
Date: 10/15/04 

Date: 15 Oct 2004 06:20:07 -0700

Mac <foo@bar.net> wrote in message news:<pan.2004.10.15.01.47.13.712660@bar.net>...
> On Thu, 14 Oct 2004 06:42:08 -0700, Klave wrote:
>
> > Mac <foo@bar.net> wrote in message news:<pan.2004.10.14.04.56.36.696296@bar.net>...
> >> On Wed, 13 Oct 2004 09:07:04 +0000, pbdelete wrote:
> >>
> >> >>There is an equation for radar resolution. It is BW=C/(2*R). This can be
> >> >>found in any radar text book.
>
> >> >>R is resolution, C is the speed of light in the medium, and BW is the
> >> >>bandwidth of the transmit chirp.
>
> >> >>So if C is 300 Mega meters per second, then to get 0.01 M resolution, you
> >> >>need 15 GHz of bandwidth.
> >> >
> >> > R=C/(2*BW) so:
> >> >
> >> > c=2.997*10**8
> >> > bw=14.9*10**9
> >> > c/(2*bw) = .01 (m/Hz*s ?)
> >> >
> >
> > This describes the width of the received burst of radar energy not the
> > accuracy to which the measurement can be made. If there is only one
> > reflector, eg a flat surface, the peak of the wide pulse can be found
> > very accurately. This is how tank gauging instruments achieve 1.5 to
> > 5mm accuracy with only 200-500MHz of bandwidth (called beamsplitting
> > in the spatial sense).
>
> There is no pulse. We are talking about a CW radar. The resolution is the
> size of each range cell after you do an fft on the sampled IF. Nothing
> more, nothing less.

Good point, somewhat nitpickey but accurately picked. I work with both
and the effect is the same on both. Pulse radar also has bandwidth
limitations and the resultant pulse broadening (width - 1/T where T is
pulse length) also needs to be "split" for higher resolution.
Depending on the type of sampler, the edge of the pulse may be used or
the peak.

So, seeing as we're nitpicking, the "_range_ resolution" is the
correct term to use for CW radar, refering to discriminating between 2
reflectors. Again, make no mistake and concede what you want, much
higher resolution (detection of change in position of the single
target, not range resolution) can be achieved by looking at the peak
within the range cell. This is usually achieved by zero padding the
time response before FFT.

>
> I am willing to concede that the range can be estimated to greater
> precision than one range cell.
> [snip]
>
> --Mac

As to an accurate DIY radar, I suggest pulse radar is simpler to
implement if a sampled system is used to reduce the speed of light to
something much lower, the range information is already in a form where
very little digital processing is required. The antenna is equally
complicated for both although the S/N can be better for CW.

GK

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