Phased array radar basics
Radar systems emit STRONG radio waves, usually as tone-bursts of the RF carrier.

The target reflects the radio wave, usually in all directions, with only a tiny bit of energy returning to the Antenna.

If you receive -100 dBm reflected energy (with 0 dBm across 50 ohms being 0.623 Vpp), you have 0.632 Cpp/105 or 632,000 uVpp/100,000 = 6.32 uVpp.

To detect that, after just one pulse, you need a rather clean signal, good signal-to-noise ratio, such as 20 dB SNR, thus noise needs to be -100, -20,
or -120 dB noise power. Some people will balk at a 20 dB SNR, but you'll like the cleanliness of the signal.

Now...............the key..........noise power increases with bandwidth.

1 hertz produces -174 dBm (comes from 10*log10(KT))
10 hertz produces -164 dBm
100 hertz produces -154 dBm
1 kHz produces -144 dBm
10 kHz produces -134 dBm
100 kHz produces -124 dBm.
200 kHz produces -121 dBm. 


Thus we know 1/200 kHz, or 5 microsecond observation time, is what we can afford.

For range accuracy, you need narrow pulses (far as I know). Pulse Compression, using SAW devices, is one way. You need some sort of correlation.

To map the terrain, you could use a LIDAR.