Receive Antenna Mixer L I R LO Driver Amplifier LNA Transmitter Transmit Antenna Coupler Limiting Amplifier Normalized Power (dB) 0 Simulation Delay Line -10 -20 -30 -40 -50 -60 13 13.5 14 14.5 15 15.5 Distance (m) 16 16.5 17 fIgURE 7. A comparison between the measured radar-system Receiver ADC ADC Storage Digital System LPF DDS 5-20 MHz, 10 ms 10-MHz Reference Clock june 2017 ieee Geoscience and remote sensing magazine fIgURE 6. The system block diagram of the 2-8-GHz YIG oscillator-based Snow Radar. IF Amplifier Amplifier Amplifier PLL Highpass Filter Frequency Divider (f/8) D/M several major modifications to improve its performance for long-range operation. Phase-Locked Synthesizer Loop Filter Amplifier YIG Oscillator 2-8 GHz LPF Coupler response using a 15-m optical delay line and the simulated response [20]. YIG OSCILLATOR-BASED SYSTEM The very first version of the Snow Radar built upon the 2-8-GHz surface-based FM-CW system was reported in [18] and [19]. Figure 6 shows the system block diagram of the radar. The 2-8-GHz FM chirp is synthesized using a 2-8-GHz yttrium iron garnet (YIG) oscillator, a frequency divider, a phase-locked loop (PLL), and a low-frequency direct digital synthesizer (DDS). A sample of the YIG oscillator output is frequency divided to generate a 100-400-kHz signal, which is then phasecompared with a DDS-generated reference chirp from 5 to 20 MHz. The phase-error signal is then filtered, amplified, and fed to the main coil of the YIG oscillator to generate an extremely linear 2-8-GHz chirp with 10-ms sweep time. The resultant signal is then amplified and passed to the transmit antenna. The backscattered signal is collected by the receive antenna and passed through an LNA before mixing with a sample of the transmitted signal that has been amplified by a local-oscillator (LO) driver amplifier. The purpose of the LNA is to set the noise floor of the system and minimize the leakage of the LO signal to the receive antenna. The beat-frequency signal output from the IF port of the mixer is then filtered and amplified before feeding into a 12-b, 50-MS/s analog-todigital converter (ADC) for data storage and viewing. Figure 7 shows the measured system response of the radar using a 15-m optical delay line as compared to the corresponding simulated response [20]. The mainlobe of the system response is very clean and the nearest sidelobes are lower than −30 dB. The measured response shows two symmetric lobes of −30 dB on both sides of the mainlobe and an additional lobe of -25 dB on the right side. This indicates that the 2-8-GHz chirp has small amplitude 63