PERFORMANCE

A multi-port vector network analyzer was used to test the transmitter and receiver. Transmitter performance was verified by measuring its scattering parameters. Figure 5a shows the measured transmitter gain, indicating an increase of about 10 dB compared to the connectorized system. This was expected, because the modular transmitter was designed to have 10x more output power for the same input drive. The gain variations across the band were also expected, largely due to the power amplifier’s frequency response.

For the receiver, a frequency offset mode with a fixed IF was used to simulate FMCW operation. The RF range was set to cover 1 to 19 GHz with four different IF frequencies within the four bands supported by the radar’s digitizer: DC to 125, 125 to 250, 250 to 375 and 375 to 500 MHz. Figure 5b shows the measured conversion gain of the receiver with an IF frequency of 100 MHz. Responses for the other IF frequencies are nearly identical. The response of the connectorized testbed receiver is included for reference. Slight differences in roll-off at the lower and upper ends of the band are due to the marginally different filter responses; the impact of these variations on radar performance is negligible.

Bench Top Performance

The modules integrated into the radar system were tested using an electro-optical transceiver with a 1.72 μs fiber-optic delay line. The delay line between the transmitter and receiver simulates a single target at a fixed range of 850 ft., used to assess the radar’s impulse response and sensitivity. Coaxial attenuators in the signal path emulate the large power loss experienced by a long-range signal, totaling 90 dB. The receiver’s IF output, after onboard coherent averaging, was digitized by the radar’s data acquisition system, and a Hanning windowing function was applied to the recorded time-domain samples before the FFT to obtain a set of power versus range profiles (see Figure 6).

Figure 6 Measured radar responses for modular and connectorized receivers with the same low-power transmitter (a) and modular receiver with a higher power modular transmitter (b).

The two traces in Figure 6a show the modular and connectorized receivers with the same low power transmitter. The range profiles were normalized both in range and amplitude to facilitate comparisons. The radar echoes in both cases were detected approximately 60 dB above the noise, achieving a loop sensitivity of 150 dB. The main lobe widths in the responses correspond to approximately 0.55 in. (1.4 cm) in both cases and agree with the range resolution expected from a 16 GHz bandwidth with windowing. The range sidelobes were low in both cases, as expected using Hanning smoothing. Note the improvement in the “skirt” around the main response of the modular receiver, which indicates outstanding linearity and phase noise performance. This is expected with a higher level of integration and using an ultra-linear chirp generator and LO driver amplifier with low phase noise.

Figure 6b shows the performance with a higher power modular transmitter. Here, the system had a 10 dB improvement in signal-to-noise ratio, resulting in a total loop sensitivity of 160 dB. The width of the main lobe - and, thus, the range resolution - and low leading-edge sidelobes was preserved. The asymmetry in the response and the slightly larger trailing edge sidelobes are attributed to the transmitter gain variations previously noted. While such variations have an adverse effect on the radar’s system response, this is outweighed by the benefit of having additional sensitivity from the increased transmit power. Systematic gain fluctuations can typically be corrected in post-processing.

Figure 7 Field results with fine vertical resolution from a flight over the North Pole, showing thick (a) and thin (b) snow cover on Arctic sea ice.

FIELD TESTS

Figure 8 Echogram data from the Greenland ice sheet, showing snow buildup to 20 m below the surface.

Field tests with the complete system onboard a NASA P-3 were conducted in 2018, as part of NASA Operation IceBridge. The transmitter and receiver modules were installed inside a small rack-mounted enclosure and operated with the rest of the system, covering 2 to 18 GHz. Antennas in the aircraft’s “bomb bay” were connected to the radar via low loss coaxial cables. Range profiles were recorded at a nominal altitude of 1500 ft. above ground level. Figure 7 shows sample radar images from data collected over two 5 km flight segments at different sea ice transects near the North Pole, demonstrating the radar’s capability to resolve the air-snow and snow-ice interfaces on thick (greater than 50 cm) and thin (less than 5 cm) snow-covered sea ice, respectively. Figure 8 is an echogram produced from data collected over the Greenland ice sheet, showing the yearly snow buildup.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge Dr. J. Paden at the University of Kansas for generating the radar images and J. Richardson at X-Microwave for his valuable technical input.

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