We developed a wideband radar to map near-surface internal layers in firn with fine resolution of about 3 cm to a depth of about 10 m. It is a frequency-modulated continuous-wave (FM-CW) radar that operates over the frequency range of 12-18 GHz with an antenna operated in the near field to obtain plane-wave illumination. The plane-wave illumination reduces off-vertical scattered signals from masking reflections caused by internal layers. To operate the radar on the snow surface, we designed and built a sled that includes a gimbaled mount and control system to ensure that the antenna points at nadir. In addition, the antenna is offset from the sled so it points at undisturbed snow. The radar features a fast transmit waveform synthesizer implemented using a voltage-controlled oscillator (VCO) and a phase-locked loop (PLL) with a linear digital chirp as a reference. The highly linear reference chirp is compared against the instantaneous VCO output to generate a highly linear 12 to 18 GHz transmit signal. The waveform synthesizer can be swept from 12 to 18 GHz in 1 millisecond. We tested the radar at both Summit, Greenland, and a field camp in West Antarctica in July 2005 and January 2006, respectively. We collected a large amount of data at both sites, and we have been able to follow internal layers over distances exceeding 10 km. We verified radar data by comparing radar echoes to visible wind crust and depth hoar layers observed in 2-m deep snow pits. We also measured snow and firn density with a resolution of 5 cm to determine the dielectric constant for estimating propagation velocity of the wave in snow and firn. We collected more than 200 sample traces at each pit location for comparison with visual observations. Each sample trace uses 10 sweeps that are coherently integrated to improve signal-to-noise ratio (SNR). We made measurements in stationary mode and by dragging the sled behind a snowmobile driven at a speed of about 2.5 km/hr. Results show an excellent agreement between the snow pit stratigraphy and echoes from our plane-wave radar.
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机译：我们开发了一个宽带雷达，以在FiRN中映射近表面内层，细分分离量约为3厘米至约10米的深度。它是一种频率调制的连续波（FM-CW）雷达，其在12-18GHz的频率范围内，通过在近场中操作的天线以获得平面波照明。平面波照明从内层引起的掩蔽反射减少了偏离垂直散射信号。为了在雪地表面上操作雷达，我们设计并构建了一个橇，包括万向安装和控制系统，以确保Nadir处的天线点。此外，天线偏离雪橇，所以它在不受干扰的雪处。雷达具有使用电压控制振荡器（VCO）和锁相环（PLL）实现的快速发射波形合成器，其具有线性数字啁啾作为参考。将高线性参考啁啾与瞬时VCO输出进行比较，以产生高线性12至18GHz发射信号。波形合成器可以在1毫秒内从12到18 GHz扫描。我们在2005年7月和2006年1月分别在西南极洲的峰会，格陵兰岛和野外营地测试了雷达。我们在两个站点收集了大量数据，我们已经能够遵循超过10公里的距离内部层。通过将雷达回声与2米深雪坑观察到的可见风地壳和深度树层，通过将雷达回波进行验证雷达数据。我们还测量了雪和FIRN密度，分辨率为5厘米，以确定用于估计雪和FIRN中波浪的传播速度的介电常数。我们在每个坑位置收集了200多个样本迹线，以便与视觉观测相比。每个样品迹线使用10次扫描，该扫描是完全集成的，以提高信噪比（SNR）。我们在静止模式下进行了测量，并通过以约2.5 km / hr的速度拖动雪地摩托车后面的雪橇。结果表明，雪坑地层与我们的飞机波雷达之间的回声之间的一致意见。
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