| Paper: | TU-A1.3 |
| Session: | New Concepts in Radiometry I |
| Time: | Tuesday, March 27, 10:00 - 10:20 |
| Presentation: |
Oral
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| Topic: |
Current and future satellite missions: |
| Title: |
Enabling the Next Generation of Soil Moisture, Salinity, Sea Surface Temperature and Wind Measurements from Space: Instrument Challenges |
| Authors: |
Sidharth Misra; NASA Jet Propulsion Laboratory | | |
| | Javier Bosch-Lluis; NASA Jet Propulsion Laboratory | | |
| | Isaac Ramos; NASA Jet Propulsion Laboratory | | |
| | Simon Yueh; NASA Jet Propulsion Laboratory | | |
| | Tong Lee; NASA Jet Propulsion Laboratory | | |
| | Shannon Brown; NASA Jet Propulsion Laboratory | | |
| Abstract: |
NASA’s Aquarius mission demonstrated the capability of space-borne microwave active and passive sensors to successfully measure sea-surface salinity at a very high precision. Similarly the SMAP (Soil Moisture Active Passive) mission provided global soil-moisture maps with a similar instrument. The Aquarius and SMAP missions had an active radar at 1.2 GHz and radiometer operating at 1.4 GHz. The radiometer operation was within a narrow protected bandwidth of 24 MHz that limited the radiometer noise performance. For salinity a limited bandwidth reduced sensitivity to cold-water salinity retrievals and prevented concurrent retrievals of SST and WS. For soil moisture, the choice of RF frequencies and limited bandwidth prevented root-zone soil moisture detection as well as increased sensitivity to vegetation. In order to mitigate these limitations the spectrum of operation of the microwave sensors need to be expanded to lower frequencies, such as 500 MHz, with continuous coverage up to 1.5 GHz.
In order to achieve the stated objectives there are unique instrument design challenges that need to be addressed. The first challenge is developing a continuous ultra-wideband radiometer in order to be sensitive to different oceanographic geophysical phenomenon and be sensitive to cold-water salinity at lower frequencies. This involves a wideband digital backend capable of ingesting >1GHz bandwidth, RF electronics and wideband antenna. The second challenge is the instrument’s ability to perform on-board radio frequency interference (RFI) detection and mitigation. Even though the measurements are made over the ocean, most of it is made in an unprotected spectrum, requiring an advanced RFI mitigation capability. The instrument also presents a unique challenge of integrating an active radar spectrum allocation within the passive radiometer bandwidth requiring a common radar/radiometer receiver chain.
In order to ensure an RF subsystem capable of ingesting wide-band thermal emissions, and yet maintaining radiometric performance a unique instrument topology was adopted. This includes design for attaining front-end wide-band isolation, dynamic gain amplifier implementation, as well as selection of components with relatively stable spectral properties across 1 GHz. We will also discuss the inclusion of a combined radar system and steps taken to ensure isolation between the active and passive components of the design. We will also briefly discuss the wide-band 256 channel digital backend with on-board RFI detection and polarimetric capabilities. Though the digital backend can detect and mitigate RFI, for a wide-band system as this the RF electronics must also have the capability to deal with strong RF sources without saturating the front-end receiver. We will briefly present design decisions made to enable RF functionality.
In this presentation we will discuss the prototype active/passive microwave sensor airborne instrument developed by the Jet Propulsion Laboratory that currently operates from 500 MHz to 1.5 GHz with a fully polarimetric digital backend and on-board RFI detection and mitigation capabilities as well as an active channel for surface roughness correction. We will present the instrument performance achievable currently across with band with a receiver temperature of 200 to 400K. In addition to lab results we will also present field experiment results demonstrating the stability and sensitivity of the radiometer to various geophysical functions, including salinity. We will also discuss the difficult RFI environment observed by the instrument and it's impact on the received signal.
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