We would like to thank the authors for their work that will help to improve the time stability of the Aura MLS observations of H2O and N2O. We also would like to advertise them that the N2O drift of the MLS v4 has already been discussed in the evaluation paper of the BASCOE Reanalysis of Aura MLS, version 2 (BRAM2, Errera et al., 2019) using observations from ACEFTS v3.6, MIPAS IMK v5 and MLS N2O from the 640 GHz radiometer. The N2O drift is clearly visible on the Fig. 6 of Errera et al. (reproduced here below) which shows the time series of the monthly mean bias and standard deviation of the differences between BRAM2 and the above mentioned satellite data for several pressure levels and latitude bands. Also, Errera et al. wrote that “analyses of the deseasonalized time series of the biases [of BRAM2] reveal a significant drift of 5, 7 and 5% per decade against ACE-FTS, MIPAS and MLS_N2O_640 for the period 2005–2012 and 10% against ACE-FTS for 2005–2017” (see on their page 14). We thus believe that Errera et al. could be cited in your paper.

The Microwave Limb Sounder (MLS), launched on NASA's Aura spacecraft in 2004, measures vertical profiles of the abundances of key atmospheric species from the upper troposphere to the mesosphere with daily near-global coverage. We review the first 15 years of the record of H2O and N2O measurements from the MLS 190 GHz subsystem (along with other 190 GHz information), with a focus on their long-term stability, largely based on comparisons with measurements from other sensors. These comparisons generally show signs of an increasing drift in the MLS “version 4” (v4) H2O record starting around 2010. Specifically, comparisons with v4.1 measurements from the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) indicate a ∼ 2 %–3 % per decade drift over much of the stratosphere, increasing to as much as ∼ 7 % per decade around 46 hPa. Larger drifts, of around 7 %–11 % per decade, are seen in comparisons to balloon-borne frost point hygrometer measurements in the lower stratosphere. Microphysical calculations considering the formation of polar stratospheric clouds in the Antarctic winter stratosphere corroborate a drift in MLS v4 water vapor measurements in that region and season. In contrast, comparisons with the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on NASA's Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) mission, and with ground-based Water Vapor Millimeter-wave Spectrometer (WVMS) instruments, do not show statistically significant drifts. However, the uncertainty in these comparisons is large enough to encompass most of the drifts identified in other comparisons. In parallel, the MLS v4 N2O product is shown to be generally decreasing over the same period (when an increase in stratospheric N2O is expected, reflecting a secular growth in emissions), with a more pronounced drift in the lower stratosphere than that found for H2O. Comparisons to ACE-FTS and to MLS N2O observations in a different spectral region, with the latter available from 2004 to 2013, indicate an altitude-dependent drift, growing from 5 % per decade or less in the mid-stratosphere to as much as 15 % per decade in the lower stratosphere. Detailed investigations of the behavior of the MLS 190 GHz subsystem reveal a drift in its “sideband fraction” (the relative sensitivity of the 190 GHz receiver to the two different parts of the microwave spectrum that it observes). Our studies indicate that sideband fraction drift accounts for much of the observed changes in the MLS H2O product and some portion of the changes seen in N2O. The 190 GHz sideband fraction drift has been corrected in the new “version 5” (v5) MLS algorithms, which have now been used to reprocess the entire MLS record. As a result of this correction, the MLS v5 H2O record shows no statistically significant drifts compared to ACE-FTS. However, statistically significant drifts remain between MLS v5 and frost point measurements, although they are reduced. Drifts in v5 N2O are about half the size of those in v4 but remain statistically significant. Scientists are advised to use MLS v5 data in all future studies. Quantification of interregional and seasonal to annual changes in MLS H2O and N2O will not be affected by the drift. However, caution is advised in studies using the MLS record to examine long-term (multiyear) variability and trends in either of these species, especially N2O; such studies should only be undertaken in consultation with the MLS team. Importantly, this drift does not affect any of the MLS observations made in other spectral regions such as O3, HCl, CO, ClO, or temperature.

The Cubesat radiometer radio frequency interference technology validation mission (CubeRRT) was developed to demonstrate real-time onboard detection and filtering of radio frequency interference (RFI) for wide bandwidth microwave radiometers. CubeRRT's key technology is its radiometer digital backend (RDB) that is capable of measuring an instantaneous bandwidth of 1 GHz and of filtering the input signal into an estimated total power with and without RFI contributions. CubeRRT's onboard RFI processing capability dramatically reduces the volume of data that must be downlinked to the ground and eliminates the need for ground-based RFI processing. RFI detection is performed by resolving the input bandwidth into 128 frequency subchannels, with the kurtosis of each subchannel and the variations in power across frequency used to detect nonthermal contributions. RFI filtering is performed by removing corrupted frequency subchannels prior to the computation of the total channel power. The 1 GHz bandwidth input signals processed by the RDB are obtained from the payload's antenna (ANT) and radiometer front end (RFE) subsystems that are capable of tuning across RF center frequencies from 6 to 40 GHz. The CubeRRT payload was installed into a 6U spacecraft bus provided by Blue Canyon Technologies that provides spacecraft power, communications, data management, and navigation functions. The design, development, integration and test, and on-orbit operations of CubeRRT are described in this article. The spacecraft was delivered on March 22nd, 2018 for launch to the International Space Station (ISS) on May 21st, 2018. Since its deployment from the ISS on July 13th, 2018, the CubeRRT RDB has completed more than 5000 h of operation successfully, validating its robustness as an RFI processor. Although CubeRRT's RFE subsystem ceased operating on September 8th, 2018, causing the RDB input thereafter to consist only of internally generated noise, CubeRRT's key RDB technology continues to operate without issue and has demonstrated its capabilities as a valuable subsystem for future radiometry missions.

Earth's ionosphere responds dynamically over a wide range of temporal and spatial scales to changes in the magnetosphere and solar wind from above, and to neutral atmospheric dynamics from below. Determining the dynamics and coupling of Earth's magnetosphere, ionosphere, and atmosphere by vector magnetic field measurements at all altitudes is essential, as the field plays a major role in controlling the distribution of ionospheric plasma. It is difficult to measure the magnetic B-field, either locally or globally, at the altitudes of the upper mesosphere and lower thermosphere (UMLT) where the transfer of energy and momentum between the plasma and neutral components of the system occur. The 118-GHz imaging magnetometer will measure all four Stokes parameters as a function of frequency about the Zeeman-split center of the 118-GHz molecular oxygen line. We are developing an array of fully-polarimetric millimeter-wave radiometer/spectrometers operating near 118 GHz. These low cost, low power, low mass polarimetric 118 GHz millimeter-wave array receivers do not require cooling to meet the sensitivity requirements. With digital spectrometers we will be able to integrate the complete end-to-end system. This work will enable a second generation small satellite mission with an off-nadir viewing instrument comprised of 120 such receivers (arranged so that each spot on the Earth is observed simultaneously by four receivers each) that will globally map the magnetic field in the mesosphere at 42-76 km altitude at a horizontal resolution similar to 100 km with 1-sigma error of 40-120 nT, in typical mesospheric temperature conditions.

Abstract. Hyperspectral infrared sounding in a CubeSat will provide a new dimension to the current suite of IR sounders by allowing measurements at multiple times of day and enabling formation flying of IR sounders for new data products such as atmospheric motion vector winds. We focus on technology development during the CubeSat Infrared Atmospheric Sounder (CIRAS) project sponsored by the NASA Earth Science Technology Office (ESTO) and coincident studies by the National Oceanic and Atmospheric Administration (NOAA) Office of Projects, Planning, and Analysis (OPPA). The CIRAS approach incorporates key instrument technologies, developed at the Microdevices Lab (MDL) at Jet Propulsion Laboratory (JPL), including a two-dimensional array of High Operating Temperature Barrier Infrared Detector (HOT-BIRD) material, selected for its high uniformity, low cost, low noise, and higher operating temperatures than traditional materials. The second key technology is a mid-wavelength infrared grating spectrometer designed by Ball Aerospace with a JPL MDL slit and immersion grating to provide hyperspectral infrared imaging in a CubeSat volume. The third key technology is a blackbody calibration target fabricated with MDL’s black silicon to have very high emissivity in a flat plate construction. JPL has completed design and breadboard of the mechanical, electronic, and thermal subsystems for the CIRAS payload including a HOT-BIRD focal plane assembly, with filters in a dewar and a breadboard of the electronics and scan mirror assembly. Blue Canyon Technologies, developer of the CIRAS 6U CubeSat, completed the Final Design Review for the spacecraft. NOAA is sponsoring the continued development of the CIRAS Proto-Flight Model (PFM) instrument at JPL using many of the existing subsystems. Completion of the PFM is expected in mid 2021, with launch no earlier than 2022.

This work presents a Chisel ASIC spectrometer generator, which supports a wide array of applications through a modular and parameterized hierarchical design. Customizable features include filter coefficients, FFT bins, automatic pipelining, bitwidth selection with automatic bitwidth growth, and overall topology. An instance of the generator was selected, verified, and taped-out in a 28nm UTBB-FDSOI process to demonstrate its efficacy. Custom serial links bring high-speed data from a 3-bit external ADC to the digital spectrometer, which achieves a sample rate of 17 GS/s with an 8192-point FFT. Such a generator greatly simplifies ASIC spectrometer development, paving the way for future low-cost ASIC systems.

In this paper we discuss the necessary technology required to enable the future of spectrum resource constrained missions. We discuss the CubeSat Radiometer Radio Frequency Interference Technology (CubeRRT) validation mission and the development of its digital backend, necessary for performing on-board RFI detection and filtering for wideband high frequency radiometry. The CubeRRT mission will validate the on-board RFI filtering technology solving technological challenges such as bandwidth, data downlink volume, and RFI types. We present a few initial results of the backend spectrometer leading to full-system integration and test.

Hyperspectral infrared sounding in a CubeSat will provide a new dimension to the current suite of IR sounders by allowing measurements at multiple times of day and enabling formation flying of IR sounders for new data products such as Atmospheric Motion Vector (AMV) winds. This paper focuses on technology development during the CubeSat Infrared Atmospheric Sounder (CIRAS) project sponsored by the NASA Earth Science Technology Office (ESTO), and coincident studies by the NOAA Office of Projects, Planning, and Analysis (OPPA). The CIRAS approach incorporates key new instrument technologies developed at JPL's Microdevices Lab (MDL) including a 2D array of High Operating Temperature Barrier Infrared Detector (HOT-BIRD) material, selected for its high uniformity, low cost, low noise and higher operating temperatures than traditional materials. The second key technology is an MWIR Grating Spectrometer (MGS) designed by Ball Aerospace with a JPL MDL slit and immersion grating to provide hyperspectral infrared imaging in a CubeSat volume. The third key technology is a blackbody calibration target fabricated with MDL's black silicon to have very high emissivity in a flat plate construction. JPL has completed design and breadboard of the mechanical, electronic and thermal subsystems for CIRAS payload including a HOT-BIRD FPA, with filters in a dewar and a breadboard of the electronics and scan mirror assembly. Blue Canyon Technologies, developer of the CIRAS 6U CubeSat, completed the Final Design Review for the spacecraft.

The CubeSat Radiometer Radio Frequency Interference Technology Validation (CubeRRT) mission is developing a 6U CubeSat system to demonstrate radio frequency interference (RFI) detection and filtering technologies for future microwave radiometer remote sensing missions. CubeRRT will perform observations of Earth brightness temperatures from 6-40 GHz using a 1 GHz bandwidth tuned channel and will demonstrate on-board real-time RFI processing. The system is currently under development, with an expected launch date in mid-2018 followed by a one year period of on-orbit operations. CubeRRT spacecraft and radiometer instrument testing as well as the mission concept of operations are described in this paper.

The demand for microwave spectrum for commercial and industrial use has been increasing rapidly over the last decade, putting stress on the limited spectral resources for passive microwave remote sensing. Radio frequency interference from man-made sources is expected to become worse over the coming years. At 1.4 GHz, the SMAP mission has implemented and demonstrated advanced interference detection algorithms for its microwave radiometer. This scheme will not be feasible at higher microwave frequencies (above 6 GHz) due to much larger radiometer bandwidths used and the limited downlink data volume available to implement RFI filtering algorithms in the ground processing. In this paper, we present the design, development, and test of an advanced on-board interference detection and RFI filtering digital back-end that is capable of operation for a 1 GHz-radiometer bandwidth. We describe the combined RFI detection algorithms implemented in the digital backend’s firmware and the on-board RFI filtering of interference-corrupted data that will be necessary to limit downlink rate requirements for future high-frequency microwave missions.

In this paper we discuss the design and development of the Radiometer Digital Backend (RDB) of the CubeSat Radiometer Radio Frequency Interference Technology (CubeRRT) validation mission. We present a brief introduction of the mission and the Radio Frequency Interference (RFI) detection and mitigation algorithm. The digital backend developed for the CubeSat is presented. The digital backend has unique capabilities of taking in wide bandwidths of up to 1GHz and can perform on-board complex operations to detect and filter out RFI in real-time. We present a few initial results of the backend spectrometer leading to full-system integration and test.

The CubeSat Infrared Atmospheric Sounder (CIRAS) is a NASA Earth Science Technology Office (ESTO) sponsored mission to demonstrate key technologies used in very high spectral resolution infrared remote sensing of Earth's atmosphere from space. CIRAS was awarded under the ESTO In-flight Validation of Earth Science Technologies (InVEST) program in 2015 and is currently under development at NASA JPL with key subsystems being developed by industry. CIRAS incorporates key new instrument technologies including a 2D array of High Operating Temperature Barrier Infrared Detector (HOT-BIRD) material, selected for its high uniformity, low cost, low noise and higher operating temperatures than traditional materials. The second key technology is an MWIR Grating Spectrometer (MGS) designed to provide imaging spectroscopy for atmospheric sounding in a CubeSat volume. The MGS is under development by Ball Aerospace with the grating and slit developed by JPL. The third key technology is a blackbody fabricated with JPL's black silicon to have very high emissivity in a flat plate construction. JPL will also develop the mechanical, electronic and thermal subsystems for CIRAS, while the spacecraft will be a 6U CubeSat developed by Blue Canyon Technologies. This paper provides an overview of the design and acquisition approach, and provides a status of the current development.

The Temporal Experiment for Storms and Tropical Systems Technology Demonstration (TEMPEST-D) instrument is a five-frequency millimeter-wave radiometer capable of observing thermal radiation from the Earth at 89, 165, 176, 180, and 182 GHz. The direct-detection architecture of the radiometer reduces its power consumption and eliminates the need for a local oscillator and mixer, reducing complexity. The instrument includes an ambient blackbody calibration target and a scanning reflector. The reflector rotates to scan the antenna beams in the cross-track direction so that the TEMPEST-D feed horn and receiver view first the blackbody calibration target, then the Earth over a range of nadir angles from −45 º to +45 º , and finally the cosmic microwave background radiation at 2.73 K. This enables precision end-to-end calibration of the millimeter-wave receivers every scan period. The TEMPEST-D millimeterwave radiometers are based on 35-nm InP HEMT MMIC low-noise amplifiers and related technology developed under extensive investment by the NASA Earth Science Technology Office (ESTO).

The operating frequency of InP high electron mobility transistor (HEMT) based amplifiers has moved well in the submillimeter-wave frequencies over the last couple of years. Working amplifiers with substantial gain in waveguide packages has been reported beyond 700 GHz. When cooled cryogenically, they have shown significant improvement in their noise temperature. This has opened up the real possibility of cryogenic amplifier based heterodyne receivers at submillimeter wavelengths for ground-based, air-borne, and space-based instruments for astrophysics, planetary, and Earth science applications. This paper provides details of a cryogenically cooled 340 GHz sideband separating spectrometer with InP HEMT receiver front-end for atmospheric science applications. We will show that these cooled amplifiers are now mature enough to be used for air-borne and space-based missions.

Millimeter and submillimeter indium phosphide (InP) microwave monolithic integrated circuits (MMICs) are increasingly used in applications spanning Earth science, astrophysics, and defense. In this paper, we characterize direct detection and heterodyne gain fluctuations of 35-, 30-, and 25-nm gate-length InP MMIC low-noise amplifiers (LNAs) designed for the 200-670-GHz frequency range. Of the twelve MMIC LNAs, five pairs have also been measured in multistage or cascaded configuration. In direct detection mode, the MMICs room temperature (RT) 1/f noise spectrum and responsivity were measured. From these the power spectral density, the noise equivalent temperature difference (NETD), equivalent system noise temperature (T-D (D)(sys)), and low-frequency normalized gain fluctuations (Delta G/ G) are derived. On the same set of MMIC LNAs, using a heterodyne down conversion technique, the Allan variance method is applied to obtain the Allan stability time and normalized 4-8 GHz gain fluctuation noise at both RT and two cryogenic temperatures. We find in the case of 35-, 30-, and 25-nm gate-length InP MMIC LNAs that the derived direct detection and heterodyne gain stability is highly process dependent with only a secondary dependence on gate periphery, the number of gate fingers, and/or gain stages. This observation confirms the underlying solid-state physics understanding that gain fluctuation noise is the result of a temporal distribution of the generation and recombination of electron free carriers due to lattice defects and surface impurities. Upon cooling below similar to 66 K, it is observed that on average gain fluctuations increase by greater than or similar to 2.2x and the Allan stability time decreases by similar to 2.5x. The presented measurement results compare favorably to the ALMA system gain specification of Delta G/G <= 1.4E-4 from 0.05-100 s, and offers guidance for application of InP LNAs for RT and cryogenic direct detection and heterodyne systems.

The CubeSat Radiometer Radio Frequency Interference Technology Validation (CubeRRT) mission is developing a 6U CubeSat system to demonstrate radio frequency interference (RFI) detection and filtering technologies for future microwave radiometer remote sensing missions. CubeRRT will perform observations of Earth brightness temperatures from 6-40 GHz using a 1 GHz bandwidth tuned channel and will demonstrate on-board real-time RFIS processing. The system is currently under development, with an expected launch date in mid-2018 followed by a one year period of on-orbit operations. Development of the CubeRRT spacecraft, radiometer instrument, and concepts of operation are described in this paper.

The CubeSat Radiometer Radio Frequency Interference Technology Validation (CubeRRT) mission is developing a 6U CubeSat system to demonstrate radio frequency interference (RFI) detection and mitigation technologies for future microwave radiometer remote sensing missions. CubeRRT will perform observations of Earth brightness temperatures from 6-40 GHz using a 1 GHz bandwidth tuned channel, and will demonstrate on-board real-time RFI processing. The system is currently under development, with launch readiness expected in 2018 followed by a one year period of on-orbit operations. Project plans and status are reported in this paper.