Eliminating range folding error in doppler radar returns 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.



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.
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.