1. Introduction
Table 1. 3CPR Key Parameters
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Parameter
| Value (Ku/Ka/W)
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Reflector Size | 5 m x 3 m |
Feed Array Length | 2.5 / 2.87 / 2.87 m |
Feed elements (each for TX / RX) | 160 / 480 / 1152 |
Transmit Power (peak) | 3200 / 1600 / 1267 W |
Pulse length | 1.5 µs |
Scan angle (+/-) | 4.5 / 12 / 3.5 degrees |
The Three-band Cloud and Precipitation Radar (3CPR) is an instrument that provides capabilities for spaceborne three-dimensional cloud and precipitation measurements using a versatile architecture that enables a wide range of instrument performance vs. resource allocation trades. 3CPR enables a) simultaneous and collocated transmission of the three radar bands, b) cross-track scanning at all bands, and c) use of advanced waveform generation and signal processing methods to maximize Doppler performance vs. resource demands.
2. RADAR SYSTEM DESIGN
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Figure 1. Exploded diagram of 3CPR's ACPRA showing key elements and dimensions (for clarity, only W-band ALAF is shown). (a) Cylindrical parabolic reflector, (b) Active Linear Array Feed (ALAF), (c) Scanning Array Tile (SAT), (d) Interlaced array unit cell with four metal patch radiators, (e) MMICs (1-W GaN power amplifier shown)[3].
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The key operating parameters of 3CPR are given in Table 1. The 3CPR antenna configuration uses a parabolic cylinder is illuminated by a linear dual-frequency active linear array feed (ALAF) to enable electronic beam scanning in the cross-track direction. The array-fed parabolic cylindrical reflector has several attractive characteristics: 1) it provides a large aperture that would not be feasible using a two-dimensional active array, 2) it provides the cross-track scanning and beam agility required to maximize science return, 3) the solid-state array
transmitters provide significantly lower phase noise than high-power vacuum electron devices used current and planned spaceborne W-band radars. The key enabling component is a trio of ALAF (for 14, 35 and 94 GHz) located along the focal line of the parabolic cylinder. The 94 GHz array is centered on the focal line while the 14 and 35 GHz arrays are slightly offset.
2.1. Antenna System Design
Antenna system preliminary design was performed using a combination of commercial and custom computer codes to perform three-dimensional electromagnetic simulation of antenna element patterns, synthesis of array patterns and calculation of array/reflector patterns using physical optics and physical theory of diffraction. The antenna system trade space was explored, trading performance vs. area, aspect ratio and focal length. We have also studied the effects of displacing the Ka-band feed array from the focal line of the parabolic cylinder while accommodating the W-band array on the focal line. For the required displacements, degradation of Ka-band antenna pattern was insignificant. Displacing the Ka-band feed causes small along-track beam shift (relative to W-band). However, it is simple to correct the registration of the Ka- and W-band data by applying a very small time shift to the data. Due to the challenging array spacing and performance constraints of the W-band feed, new technology is required. We have developed a preliminary feed design that incorporates high-power and efficiency GaN MMIC power amplifiers, GaN LNA, SiGe beamforming MMICs and microfabricated radiators and interconnects.
Our point-design for the W-band feed is 2.88 m long with 2x1152 radiating elements. The 2304-element array is composed of separate transmit and receive arrays that are interlaced on a triangular grid. This approach eliminates the need for circulators (which are impractically large) or switches (which introduce substantial front-end losses). However, this approach requires careful design to minimize element-to-element coupling over the full range of scan angles. Additionally, the receiver, LNA MMIC must be able to tolerate significantly higher leakage power than a typical LNA. Gallium Nitride (GaN) LNAs are used to provide high leakage power tolerance. While the noise figure may be degraded (compared to an Indium Phosphide LNA), the degradation is less than the losses that would be incurred by using switches.
The ALAF is composed of scanning array tiles (SAT). The elements are fabricated in a microfabrication process called PolyStrata® (Nuvotronics, LLC). The PolyStrata® process permits precise fabrication of dual-
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Figure 2. Four patch unit cell (with US dime coin for scale) (top), measured vs. predicted input return loss (middle), measured vs. predicted (HFSS) radiation pattern (bottom) | |
polarized, probe-fed patch radiators with no dielectric. The lack of dielectric improves radiation efficiency and also improves our ability to accurately model the element performance using 3D electromagnetic simulations. Figure 2 shows a photograph of a prototype 4-element unit cell fabricated in the PolyStrata® process.
The PolyStrata® radiator assembly routes the transmit and receive elements to carriers that contain GaN PA and LNA MMIC. A single-sided carrier on one side of a central heat-spreader holds eight PA MMIC each with a transmit peak power of greater than 1 W. On the opposite side of the heat spreader is double-sided carrier with sixteen LNA MMICs. Behind the GaN MMICs are multi-channel Silicon Germanium (SiGe) integrated beamformer MMICs. SiGe bipolar technology enables the integration of many phase-shifter channels along with digital control circuitry on to a single die only a few millimeters on a side. With integrated serial control circuitry, the SiGe beamformer eliminates thousands of interconnects compared to standard phase shifter ICs with parallel control.
3. ConCLUSION
The proposed instrument concept uses a combination of new technologies that have matured over the last five years to provide dual-frequency (35, 94 GHz) scanning radar capabilities for the future spaceborne cloud and precipitation radars. This approach will substantially improve the science benefits of the mission by providing three-dimensional data at three radar frequencies. The current state-of-the-art and ongoing technology development supports an early 2020's launch date for the 3CPR radar as part of the proposed ACE, CaPPM or other mission.
Acknowledgement - This work was performed at the Jet Propulsion Laboratory / California Institute of Technology under contract with the National Aeronautics and Space Administration.
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