Primary (Corresponding) Author: Keith Raney

Organization of Primary Author: Applied Physics Lab

Co-Authors: Carl Leuschen, Marshall Jose

Abstract: In July 2005, the Johns Hopkins University, Applied Physics Laboratory began ìPathfinder Airborne Radar Ice Sounder (PARIS)î funded under the NASA Instrument Incubator Program (IIP). The primary objective of this project was to demonstrate successful radar sounding of ice sheet layering and bottom topography from a high-altitude platform. This objective was met; examples will be presented. Major contributing factors included a high-fidelity 150-MHz radar, supported by along-track partially-coherent processing. 'High-fidelity' implies very wide dynamic range, extreme linearity, and very low sidelobes generated by the transmitted pulse. This paper describes the radar design, including in particular those critical features that contributed to the observed 90-dB dynamic range. ìPartially-coherent processingî implies the delay-Doppler technique, previously proven in airborne radar altimeter and low-altitude radar ice sounding embodiments. The radar was mounted on the NASA P-3, and deployed on a mission over the Greenland ice sheet in the spring of 2007. Data were recorded on board as well as displayed in flight on a quick-look processor. The data subsequently were processed in the laboratory to quantify performance characteristics, including dynamic range, sidelobe level control, and contrast improvement from the delay-Doppler algorithm.

The transmit waveform is a 5-MHz bandwidth chirp at a 150-MHz operating frequency with a trapezoidal envelope. Such severe weighting is essential to reduce the ringing commonly associated with the initial on-off transition of weakly-weighted waveforms. The 300-W (peak) linear-FM pulse has ~6 MHz bandwidth. The amplifier is class AB to help ensure the high linearity needed to suppress the internal clutter (sidelobes) generated by the chirp waveform. Laboratory measurements of the driver and power amplifier show excellent linearity with a two-tone third-order inter-modulation of at least -26 dBc at 311 W peak power.

There is no down conversion or IF signal within the receiver, greatly simplifying the design, and eliminating most potential sources of distortion and intermodulation. Upon reception, the radar samples the RF signal directly. The sample rate is well below Nyquist, but is chosen so that the resulting spectra shift an alias of the main signal to baseband in a clear channel. The receiver includes variable attenuators to adjust the voltage range of the signal input to the analog-to-digital converter as well as sensitivity time control (STC) to increase the effective dynamic range of the response as a function of depth of penetration. The overall noise figure of the receiver is less than 5.5 dB with a gain of over 60 dB and a 45 dBm third-order intercept point.

The digital components consist of a field programmable gate array (FPGA) radar synchronizer, a direct digital synthesizer (DDS), and an under-sampling analog-to-digital converter (ADC). All components of the digital subsection are clocked by a stable 66.6 MHz reference oscillator. The radar data are time-tagged by reference to GPS.

The flights included passes over the summit ridge, from which results show internal layering, and the bottom profile at several km depth.