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With weather radar, it's not uncommon to see large areas of the image blocked out due to detected range folding. It's my understanding that this occurs when a reflection outside of range comes back after the next pulse has been transmitted, making the receiver think that a late reflection from the first pulse is actually for the second pulse and near by.

Range Folding

Graphic Source: http://www.wdtb.noaa.gov/courses/dloc/topic3/lesson1/Section4/Section4-5.html

It seems to me that it if we could differentiate between pulses, we could eliminate the possibility of range folding error.

Frequency

My first thought was that each pulse could be sent at different frequencies. They would have to be close enough to not affect the amount of energy reflected, but far apart enough to be easily detected and not assumed to be part of doppler shift. Even something as wide as 1MHz seems to fit this characteristic. Is there a reason we don't use this method today?

Polarization

If we have to use the same frequency for both pulses, could we instead mount two antennas on the transmitting rig, one polarized clockwise and the other polarized counter-clockwise? In this configuration, the exact same equipment could be used for transmitting with something that just switches between antennas for each pulse.

I'm curious if either method would work, and what the possible pitfalls are.

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  • $\begingroup$ Hmm... I don't know if I can help or not (I'm going to do some reading and such); however, this is really interesting to me because i have often wondered the same thing! $\endgroup$
    – L.B.
    Jun 21 '15 at 16:39
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Frequency

Most high-power radio-frequency amplifiers (klystrons, cavity magnetrons) have a very limited frequency range. A dual-frequency radar would essentially be two radars hooked up to the same antenna.

Polarization

Horizontally-polarized radar waves reflect more strongly from the ground than vertically-polarized waves, while circularly-polarized waves reflect only weakly from rain. Consequently, vertical polarization gives the best results for weather radar. A dual-polarization radar would sacrifice detail to eliminate range error.

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  • $\begingroup$ Circularly polarized waves reflecting weakly... is that because the energy is absorbed or because it passes right through? I'm wondering if for fixed point-to-point microwave installations if it makes sense to use circular polarization to avoid rain fade. $\endgroup$
    – Brad
    Jun 24 '15 at 1:04
  • $\begingroup$ You could clarify that you are talking about a CW/CCW dual polarization radar sacrificing detail to avoid ambiguity with the vertical/horizontal dual-pol we now have on the WSR88D network. $\endgroup$
    – casey
    Jun 24 '15 at 1:43
  • $\begingroup$ @casey Can you elaborate on how the NEXRAD radar works today? Are you saying there is dual polarity, and we use both to image the precipitation? $\endgroup$
    – Brad
    Jun 24 '15 at 14:54
  • $\begingroup$ @Brad that might be better asked as a new question. In short, horizontal/vertical dual-pol has been used in research radars for a while and last year that started upgrading the US national 88D network to dual-pol (adding the vertical polarization). There are now products available like differental reflectivity (ZDR), correlation coefficient (CC), specific differential phase (KDP), melting layer, hydrometeor classification and a few others (14 new products in total). wdtb.noaa.gov/courses/dualpol/Outreach/#mets $\endgroup$
    – casey
    Jun 24 '15 at 15:10

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