I. Osaretin

Radiometer Calibration Using Colocated GPS Radio Occultation Measurements

We present a new high-fidelity method of calibrating a cross-track scanning microwave radiometer using Global Positioning System (GPS) radio occultation (GPSRO) measurements. The radiometer and GPSRO receiver periodically observe the same volume of atmosphere near the Earths limb, and these overlapping measurements are used to calibrate the radiometer. Performance analyses show that absolute calibration accuracy better than 0.25 K is achievable for temperature sounding channels in the 50-60-GHz band for a total-power radiometer using a weakly coupled noise diode for frequent calibration and proximal GPSRO measurements for infrequent (approximately daily) calibration. The method requires GPSRO penetration depth only down to the stratosphere, thus permitting the use of a relatively small GPS antenna. Furthermore, only coarse spacecraft angular knowledge (approximately one degree rms) is required for the technique, as more precise angular knowledge can be retrieved directly from the combined radiometer and GPSRO data, assuming that the radiometer angular sampling is uniform. These features make the technique particularly well suited for implementation on a low-cost CubeSat hosting both radiometer and GPSRO receiver systems on the same spacecraft. We describe a validation platform for this calibration method, the Microwave Radiometer Technology Acceleration (MiRaTA) CubeSat, currently in development for the National Aeronautics and Space Administration (NASA) Earth Science Technology Office. MiRaTA will fly a multiband radiometer and the Compact TEC/Atmosphere GPS Sensor in 2015.

Assessment of Radiometer Calibration With GPS Radio Occultation for the MiRaTA CubeSat Mission

The microwave radiometer technology acceleration (MiRaTA) is a 3U CubeSat mission sponsored by the NASA Earth Science Technology Office. The science payload on MiRaTA consists of a triband microwave radiometer and global positioning system (GPS) radio occultation (GPSRO) sensor. The microwave radiometer takes measurements of all-weather temperature (V-band, 50-57 GHz), water vapor (G-band, 175-191 GHz), and cloud ice (G-band, 205 GHz) to provide observations used to improve weather forecasting. The Aerospace Corporations GPSRO experiment, called the compact total electron content and atmospheric GPS sensor (CTAGS), measures profiles of temperature and pressure in the upper troposphere/lower stratosphere (~20 km) and electron density in the ionosphere (over 100 km). The MiRaTA mission will validate new technologies in both passive microwave radiometry and GPSRO: 1) new ultracompact and low-power technology for multichannel and multiband passive microwave radiometers, 2) the application of a commercial off-the-shelf GPS receiver and custom patch antenna array technology to obtain neutral atmospheric GPSRO retrieval from a nanosatellite, and 3) a new approach to space-borne microwave radiometer calibration using adjacent GPSRO measurements. In this paper, we focus on objective 3, developing operational models to meet a mission goal of 100 concurrent radiometer and GPSRO measurements, and estimating the temperature measurement precision for the CTAGS instrument based on thermal noise Based on an analysis of thermal noise of the CTAGS instrument, the expected temperature retrieval precision is between 0.17 and 1.4 K, which supports the improvement of radiometric calibration to 0.25 K.

A Compact 118-GHz Radiometer Antenna for the Micro-Sized Microwave Atmospheric Satellite

A linear polarized 118-GHz antenna is designed for a radiometer payload hosted aboard a 3U atmospheric CubeSat. The radiometer antenna is a horn-fed offset parabolic reflector. The antenna has a maximum +37 dBi realized gain, 2.4° half-power beamwidth, and a minimum 95% beam efficiency within the operational bandwidth. The antenna is compact, meeting the MicroMAS mission requirement for a highly integrated and ultra-compact radiometer. CubeSats present stringent size/volume and weight constraints on overall component design. We present our design, along with simulated and measured results, that meet the size/volume, weight, and electrical requirements for the radiometer antenna and comply with CubeSat standards.

Earth limb calibration of scanning spaceborne microwave radiometers

We introduce a new technique for absolute “through-theantenna” calibration of cross-track-scanning passive microwave radiometers viewing earth from a low-earth orbit. This method offers significant advantages, in that neither internal calibration targets nor noise diodes are needed to calibrate the radiometer. The algorithm does require periodic updates of the atmospheric state, which can be readily provided by GPS radio occultation observations, for example. An iterative algorithm retrieves the radiometer gain given a sequence of observations of the earths limb. The algorithm uses a parameterized radiative transfer model of a spherically-stratified atmosphere. The algorithm works best for opaque temperature sounding channels. This method, when used on idealized radiometer measurements (impulse response functions in frequency and space), yields calibration accuracies similar to those that could be obtained with ideal internal calibration targets. This analysis is based on global Monte Carlo simulations using the NOAA88b profile set. An analysis will also be presented showing how calibration performance degrades as the radiometer characteristics deviate from the ideal case. Among the factors considered are: 1) antenna pattern, 2) spectral passband, 3) pointing errors, 4) atmospheric state variability, 5) the number of limb observations required, and 6) sensitivity to sensor noise.

Calibration and validation of small satellite passive microwave radiometers: MicroMAS-2A and TROPICS

Miniaturized microwave radiometers deployed on nanosatellites in Low Earth Orbit are now demonstrating cost-effective weather monitoring capability, with increased temporal and spatial resolution compared to larger weather satellites. MicroMAS-2A is a 3U CubeSat that launched on January 11, 2018 with a 1U 10-channel passive microwave radiometer with channels near 90, 118, 183, and 206 GHz for moisture and temperature profiling and precipitation imaging. The Time-Resolved Observations of Precipitation structure and storm Intensity with a Constellation of Smallsats (TROPICS) mission is projected to launch in 2020, and its 1U 12-channel passive microwave radiometer is based on the current CubeSat mission MicroMAS-2A. TROPICS will provide rapid-refresh measurements over the tropics and measure environmental and inner-core conditions for tropical cyclones. In order to effectively use small satellites such as MicroMAS-2A and TROPICS as a weather monitoring platform, calibration must ensure consistency with state of the art measurements, such as the Advanced Technology Microwave Sounder (ATMS), which has a noise equivalent delta temperature (NEDT) at 300 K of 0.5 - 3.0 K. In this work, we present initial analysis from the MicroMAS-2A radiometric bias validation, which compares MicroMAS-2A measured brightness temperatures to simulated brightness temperatures calculated by the Community Radiative Transfer Model (CRTM) using input from GPS radio occultation (GPSRO), radiosonde, and numerical weather prediction (NWP) atmospheric profiles. We also model solar and lunar intrusions for TROPICS, and show that the frequency of intrusions with a scanning payload allows for the novel opportunity of using the solar and lunar intrusions as a calibration source.

A compact 118GHz radiometer for the Micro-sized Microwave Atmospheric Satellite (MicroMAS)1

Summary form only given. A novel compact radiometer observing nine channels near the 118.75GHz oxygen absorption line is introduced. The radiometer is designed as the payload for the Micro-sized Microwave Atmospheric Satellite (MicroMAS). MicroMAS is a dual-spinning 3U CubeSat that aims to address the need for low-cost, mission-flexible, and rapidly deployable spaceborne sensors. The focus of the current MicroMAS mission is to observe convective thunderstorms, tropical cyclones, and hurricanes from a near-equatorial orbit. As a low cost platform, MicroMAS offers the potential to deploy multiple satellites, in a constellation, that can provide near-continuous views of severe weather. The existing architecture of few, high-cost platforms, infrequently view the same earth area which can miss rapid changes in the strength and direction of evolving storms thus degrading forecast accuracy. MicroMAS is a scalable CubeSat-based system that will pave the path towards improved revisit rates over critical earth regions, and achieve state-of-the-art performance relative to current systems with respect to spatial, spectral, and radiometric resolution. The current MicroMAS mission will demonstrate the viability of CubeSats for high-fidelity environmental monitoring and space control that would provide profound advances by reducing costs, by at least an order of magnitude, while increasing robustness to launch and sensor failures. This discourse focuses on the compact radiometer designed for this CubeSat mission. The radiometer is housed in a 1U (10 × 10 × 10 cm) payload section of the 3U (10 × 10 × 30 cm) MicroMAS CubeSat. The payload is scanned about the spacecrafts velocity vector as the spacecraft orbits the earth, creating crosstrack scans across the earths surface. The first portion of the radiometer comprises a horn-fed reflector antenna, with a full-width at half-maximum (FWHM) beamwidth of 2.4°. Hence, the scanned beam has an approximate footprint diameter of 20Km at nadir incidence from a nominal altitude of 500Km. The antenna system is designed for a minimum 95% beam efficiency. The next stage of the radiometer consists of superheterodyne front-end receiver electronics with single sideband (SSB) operation. The front-end electronics includes an RF preamplifier module, a mixer module, and a local oscillator (LO). The RF preamplifier module contains a low noise RF amplifier and a weakly coupled noise diode for radiometric calibration. The mixer module comprises a HEMT diode mixer and an IF preamplifier MMIC. The LO is obtained using a 30GHz dielectric resonant oscillator (DRO) and a resistive diode tripler to obtain a 90GHz LO frequency. A key technology development in the MicroMAS radiometer system is the ultra-compact intermediate frequency processor (IFP) module for channelization, detection, and analog-to-digital conversion. The antenna system, RF front-end electronics, and backend IF electronics are highly integrated, miniaturized, and optimized for low-power operation. The payload also contains microcontrollers, with one of such being in the payload interface module (PIM), to package and transmit radiometric and housekeeping data to the spacecraft bus. A voltage regulator module (VRM) was also designed for the payload to convert the input bus voltage to the required voltages for the payload electronics. The payload requires 3W (average) of power. The MicroMAS payload flight unit is currently being developed by MIT Lincoln Laboratory, and the spacecraft bus flight unit being developed by the MIT Space Systems Laboratory and the MIT Department of Earth and Planetary Sciences for a 2014 launch to be provided by the NASA CubeSat Launch Initiative program.

Microwave receiver prototype development for the Hyperspectral Microwave Atmospheric Sounder (HyMAS)1

Recent technology advances have significantly changed the landscape of modern radiometry by enabling miniaturized, low-power, and low-noise radio-frequency receivers operating at frequencies up to 200 GHz. These advances enable the practical use of receiver arrays to multiplex multiple broad frequency bands into many spectral channels. We use the term “hyperspectral microwave” to refer generically to microwave sounding systems with approximately 50 spectral channels or more. We present the design and analysis of the receiver subsystem for the Hyperspectral Microwave Atmospheric Sounder (HyMAS), with focus on the ultra compact Intermediate Frequency (IF) processor module. HyMAS comprises multiple receivers operating near the oxygen absorption line at 118.75GHz and the water vapor absorption line at 183.31GHz. The hyperspectral microwave receiver system will be integrated into a scanhead compatible with the NASA GSFC Conical Scanning Microwave Imaging Radiometer (CoSMIR) airborne system to facilitate demonstration and performance characterization. HyMAS is designed to have a 52-channel hyperspectral microwave receiver subsystem with four temperature sounding bands (two antennas) near 118.75GHz and two moisture sounding bands (one antenna) near 183.31GHz. Both polarizations are measured (although at slightly different IF passbands) to increase the total channel count. Subharmonic mixers will be pumped by phase-locked oscillators, and single-sideband operation will be achieved by waveguide filtering of the lower sideband. Size/volume constraints on the receiver subsystem led to a relatively high IF frequency (18 - 29GHz) to facilitate miniaturization of the IF processor module. Broadband operation over such a relatively high intermediate frequency range is a technical challenge for the front-end receiver sys

Preparations for the MicroMAS CubeSat mission

The Micro-sized Microwave Atmospheric Satellite (MicroMAS) is a 3U CubeSat (10×10×34 cm, ~4 kg) hosting a passive microwave spectrometer operating near the 118.75-GHz oxygen absorption line. MicroMAS is a dual-spinning 3U CubeSat that aims to address the need for low-cost, mission-flexible, and rapidly deployable spaceborne sensors. The focus of the current MicroMAS mission is to observe convective thunderstorms, tropical cyclones, and hurricanes from a near-equatorial orbit.