Spaceborne Ka-band Radar Prototype Design for Equatorial Precipitation Measurement
Abstract
Life on Earth is fundamentally linked to reservoirs of freshwater. In the tropics, freshwater is replenished by the vertical transport of water and air in deep convective storms. This transport in the tropics, hereafter referred to as the convective mass flux (CMF), drives the large-scale overturning of the troposphere that controls regional and global climates, impacts precipitation rates and hence our supply of fresh water, determines the intensity of severe weather and hence the destruction of life and property, and governs the production of high clouds in the form of "anvils" that are a principal component of the Earth's greenhouse effect. While CMF is fundamental to weather and climate, we have no global observational data to develop even an elementary understanding of CMF, nor to constrain its representation in models. At present, this transport can differ by more than 200% across our most advanced models. Any observations of CMF with even modest capabilities offer genuine potential to make significant advances in our understanding. The Jet Propulsion Laboratory (JPL) flew a small rain mapping Radar on a 6U satellite (Rain Cube) to demonstrate the technology for mapping tropical storms (convective mass flux, CMF) and is currently studying a three small satellite mission concept in low Earth orbit to do global equatorial region mapping of the frequency and dynamics of these CMFs. The Satellite Research Centre (SaRC) at Nanyang Technological University (NTU), Singapore is studying the possibility of adding one or two satellites to that mission to allow more frequent spatial and temporal coverage. The payload would consist of a highly miniaturized down looking 5-beams Ka-band Radar. Before developing the space qualified radar payload sub-system, a ground based Ka-band radar prototype demonstrator is useful for proof of concept. There are two main changes between the new radar payload and the RainCube radar payload: 1) A Multi-Beam Front-End Switch Assembly (FESA-MB) is inserted to activate sequentially operated 5 feed horns. Both mechanical & ferrite switches and relevant control protocol are investigated. The beam forming approaches are also are being evaluated. 2) The ultra-low sidelobe waveform is being studied to replace the linear frequency modulation (LFM) signal used in the RainCube. For spaceborne radars the echo from the ground is considerably stronger than that from the precipitation, so the sidelobes of the ground echo must be sufficiently low that the ground echo does not corrupt the wanted echo from the precipitation. Furthermore, this performance must be maintained over the full Doppler bandwidth associated with the antenna footprint. The Ka-Band radar is being designed to have a fixed antenna of 1.6m diameter. The dish will be mounted on a spacecraft platform of size 27U (30x30x30 cm). The spacecraft will have heritage from the INSPIRE series of spacecrafts. Currently SaRC is developing two 27U spacecrafts for launch in 2021. The 27U spacecraft will have downlink capability in X-band and is expected to be launched into an equatorial orbit. For downlink, the mission will utilize the 6.1m X-band downlink station at SaRC.
- Publication:
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43rd COSPAR Scientific Assembly. Held 28 January - 4 February
- Pub Date:
- January 2021
- Bibcode:
- 2021cosp...43E..24L