The objective of this study is to cross-validate observations of the Temporal Experiment for Storms and Tropical Systems Demonstration (TEMPEST-D) CubeSat mission with those observed by the Global Precipitation Measurement (GPM) Microwave Imager (GMI) [1] over precipitating systems. The purpose of this paper is twofold: first, to show consistency between TEMPEST-D and GPM/GMI, and second, if the measurements are consistent, to demonstrate the potential to enhance temporal sampling when the TEMPEST-D and GPM/GMI observations are merged. This paper demonstrates both objectives and shows good agreement between TEMPEST-D and GPM/GMI observations.

Observations from the Special Sensor Microwave Imager/Sounder (SSMIS) onboard the Defense Meteorological Satellite Program F16, F17, F18, and F19 spacecrafts provide a significant portion of the microwave radiometer data within the Global Precipitation Measurement (GPM) mission constellation. In preparation for the GPM Version 7 (V07) data release, SSMIS corrections developed over a decade ago and incorporated in GPM Version 5 (V05) are reexamined and updated. The calibration updates presented here include pointing parameters affecting geolocation and viewing geometry, along-scan bias adjustments to account for scan edge falloffs and sun angle corrections to account for heating anomalies including an emissive reflector. To address errors in the V05 geolocation, the sensor roll, pitch, yaw, half cone angle, and timing offsets are reanalyzed and updated. The along-scan bias adjustment is updated in a manner consistent with recent Special Sensor Microwave Imager updates to account for variations in scene temperature. Finally, significant improvements to the SSMIS sun angle correction are made by using data from the entire mission, extending the corrections to include the higher frequency channels, and deriving a more consistent channel-to-channel correction. From V05 to V07, the geolocation adjustment is approximately 5–10 km, the earth incidence angle difference is 0–0.4°, and the average brightness temperature change is 0–2 K, but individual pixels may be up to several kelvin difference depending on sensor and channel. The result of these updates is a significant improvement in the quality and long-term consistency of the SSMIS data that are included in the GPM V07 dataset.

This paper presents some of the first nearly simultaneous observations between TEMPEST-D and RainCube, two CubeSat missions supported by NASA for on-orbit validation of technology for studying the Earth's atmosphere. This paper presents simultaneous observations by a Ka-band radar and multi-frequency millimeter-wave radiometers over precipitation systems, in three widely dispersed locations over the globe. The first storm was near Mexico's Pacific coast, whereas the second storm was over the South Pacific Ocean near the Solomon Islands, and the third storm was near Houston, Texas, USA. The comparisons showed good physical consistency between the TEMPEST-D and RainCube observations.

The Special Sensor Microwave Imager/Sounder (SSMIS) provides vital long-term microwave radiometer observations as a member of the Global Precipitation Measurement (GPM) mission constellation. There are four SSMIS sensors that have been launched with three still in operation. In preparation for the GPM Version 7 (V07) data release, previous SSMIS calibration corrections are reexamined and updated using the latest data and recent knowledge gained from calibrating similar microwave radiometers. These calibration updates are presented here and include pointing parameters that affect geolocation, along-scan bias adjustments to correct for fall-off at the edge of the scan, and thermal heating issues caused by an emissive reflector and solar intrusions. The new updates greatly improve the accuracy of the SSMIS observations that will be released as part of the GPM V07 dataset.

This paper presents a machine learning model to estimate surface rainfall from TEMPEST-D observations. An artificial neural network (ANN) was chosen to build the rainfall estimation model from TEMPEST-D measurements. TEMPEST-D brightness temperature (TB) observations performed at five frequencies (i.e. 87, 164, 174, 178 and 181 GHz) were used as inputs, and the Multi-Radar/Multi-Sensor System (MRMS) radar-only rain rate product at the surface was used as ground truth and target to train the ANN model. A spatial alignment algorithm was developed to align the TEMPEST-D observed storm with the storm measurement from ground radar. The training data set was generated from 14 storm events observed simultaneously by the ground radar network and TEMPEST-D over the continental U.S. Two storm events were used for independent testing. The testing showed that estimated rainfall matched well with the MRMS surface rainfall product in terms of rainfall intensity, area, and precipitation system pattern. The structural similarity index measure scores for the two independent test cases are 0.72 and 0.81.

Temporal Experiment for Storms and Tropical Systems-Demonstration (TEMPEST-D) is a 6U CubeSat satellite with a cross-track scanning millimeter-wave radiometer measuring at five frequencies from 87 to 181 GHz. It employs a direct-detection architecture with InP HEMT monolithic microwave integrated circuit (MMIC) low-noise amplifiers and related new technologies. An end-to-end two-point external calibration is performed every 2-s rotation of the scanning mirror, based on observations of the cosmic microwave background and an internal blackbody calibration target, with three thermistors to monitor the target physical temperature. Corrections for antenna pattern effects and cross-scan biases based on prelaunch measured values were updated using data from an on-orbit calibration pitch maneuver. Validation of the observed brightness temperatures ( TB) is performed by comparing to coincident nonprecipitating ocean observations from five well-calibrated on-orbit instruments, including Global Precipitation Measurement (GPM) mission Microwave Imager (GMI) and four Microwave Humidity Sounder (MHS) sensors on board NOAA-19, MetOp-A, MetOp-B, and MetOp-C satellites. Absolute calibration accuracy is within 1 K for all channels, well within the 4-K requirement. Calibration precision, or stability over time, is within 0.6 K for all channels, also well within the 2-K requirement. The intrinsic noise of TEMPEST-D is lower than MHS, resulting in similar on-orbit noise equivalent differential temperatures (NEDTs), even though TEMPEST-D has a much shorter integration time of 5 ms as compared to 18 ms for MHS. As a result, although the TEMPEST-D radiometer is substantially smaller, lower power, and lower cost than similar current operational radiometers, it has comparable or better performance in terms of instrument noise, calibration accuracy, and calibration stability or precision.

TEMPEST-D is a 6U CubeSat with a payload of a 5-channel millimeter wave cross-track scanning radiometer. It is a technology demonstration mission with requirements of 2 K precision and 4 K absolute calibration. Since its deployment from the International Space Station in July of 2018, the TEMPEST-D team has focused efforts on validating the calibration of the instrument by comparing with similar well-calibrated operational sensors. Such comparisons have shown the instrument to be very well calibrated and stable, with very low noise, well within the requirements. Efforts have subsequently focused on demonstrating that the data can be used for various science applications, including water vapor and cloud water/ice retrievals and data assimilation.

While spaceborne microwave radiometers have been providing Earth observations for many decades, more recently, applications have focused on the use of constellations of well-calibrated sensors. Perhaps the best known of these is the Global Precipitation Measurement (GPM) mission, which involves state-of-the-art radars and a radiometer to measure precipitation, but also acts as the calibration reference for a constellation of research and operational radiometers. This involves challenges related to the development of a high-quality stable calibration reference, intercalibration of a diverse suite of radiometers, and revisiting calibration issues in older instruments that are no longer operational. Lessons from this effort include the importance of instrument design, on-orbit calibration maneuvers, and detailed knowledge of factors such as instrument pointing, thermal conditions, spectral response functions, antenna patterns, RFI, etc. Even more recently, the development of miniaturized radiometer components to be deployed in constellations of low-cost CubeSat satellites has raised new calibration challenges.

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 −45ºto +45º, 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 Temporal Experiment for Storms and Tropical Systems (TEMPEST) mission concept consists of a constellation of five identical 6U-Class nanosatellites observing at five millimeter-wave frequencies with five-minute temporal sampling to observe the time evolution of clouds and their transition to precipitation. The TEMPEST concept is intended to improve understanding of cloud processes, by providing critical information on the temporal development of cloud and precipitation microphysics and by improving our understanding of some of the largest sources of uncertainty in cloud process models. TEMPEST millimeter-wave radiometers are able to perform observations inside the cloud to observe changes as the cloud begins to precipitate or ice accumulates inside the storm. The TEMPEST Technology Demonstration (TEMPEST-D) mission is intended to reduce risk and demonstrate measurement capabilities for 6U-Class satellite constellations for Earth Science. The capabilities to be demonstrated include differential drag maneuvers to provide desired time separation in a 6U-Class satellites constellation. In addition, TEMPEST-D millimeter-wave radiometers will be cross-calibrated with space-borne radiometers with similar frequency channels. TEMPEST-D will provide radiometric observations at five millimeterwave frequencies from 89 to 183 GHz using a low-power, compact instrument that is highly suitable for deployment on 6U-Class satellites.

The Global Precipitation Measurement (GPM) mission is a constellation-based satellite mission designed to produce unified precipitation retrievals from a constellation of available microwave radiometers [1]. The core observatory builds on the successes of the Tropical Rainfall Measuring Mission (TRMM) providing advances in satellite precipitation monitoring including a dual-frequency radar, increased sensitivity to frozen precipitation, and coverage over higher latitudes. The constellation approach, however, is a unique and challenging aspect of the mission that has implications for science applications far beyond global precipitation monitoring. The success of the GPM mission has led to plans to extend the constellation concept back to the launch of TRMM [2]. Advances in radiometer calibration and improved intercalibration techniques are subsequently being used to improve the quality and stability of the TRMM Microwave Imager (TMI) data record and the associated radiometer constellation for an extended high-quality radiometer data record approaching two decades.

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° to +45°, 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 millimeter-wave 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 Temporal Experiment for Storms and Tropical Systems (TEMPEST) mission concept consists of a constellation of five identical 6U-Class nanosatellites observing at five millimeter-wave frequencies with five-minute temporal sampling to observe the time evolution of clouds and their transition to precipitation. The TEMPEST concept is designed to improve the understanding of cloud processes, by providing critical information on the time evolution of cloud and precipitation microphysics and by improving our understanding of the largest sources of uncertainty in cloud models. TEMPEST millimeter-wave radiometers are able to perform observations inside the cloud to observe changes as the cloud begins to precipitate or ice accumulates inside the storm. The TEMPEST Technology Demonstration (TEMPEST-D) mission will be deployed to demonstrate measurement capabilities required for a constellation of 6U-Class nanosatellites to directly observe the temporal development of clouds to understand the conditions that control their transition from non-precipitating to precipitating clouds. TEMPEST-D will provide observations at five millimeter-wave frequencies from 89 to 183 GHz using a single compact instrument that is well suited for the 6U-Class architecture.

TEMPEST-D will demonstrate technology for 6U-Class nanosatellites to advance NASA's Earth Science Goals. It will also reduce risk, cost, and development time for future constellations of small satellites to perform Earth Science measurements. It will raise the TRL of a millimeter-wave radiometer instrument from 6 to 7, representing the first on-orbit demonstration of 35-nm InP HEMT-based millimeter-wave radiometer front ends.

A constellation of disparate radiometers is inherent to the Global Precipitation Measurement (GPM) mission concept. The task of the Intersatellite Calibration Working group is to generate adjustments to make the measurements of all these radiometers physically consistent. A key role of the GPM Microwave Imager (GMI) on the GPM Core satellite is to serve as a transfer standard among the constellation radiometers. The TRMM Microwave Imager (TMI) has served this role during the development phase and for interim corrections early in the GPM mission. The stability of GMI appears to be very good and a physically based calibration has been generated that appears to be accurate at the 1K level or better.

The long-term data record of microwave imager data from the series of six Special Sensor Microwave/Imagers (SSM/Is) on board the Defense Meteorological Satellite Program (DMSP) spacecraft has been used to produce global multidecadal time series of a number of geophysical parameters, including precipitation, total precipitable water, ocean surface wind speed, and sea ice extent. As part of an effort to produce an intercalibrated fundamental climate data record (CDR) of the brightness temperature (Tb) data from the SSM/I, an examination of geolocation errors and the subsequent impact on the view angle [or the Earth incidence angle (EIA)] is performed. Using a combination of techniques, estimates of changes in the sensor/spacecraft attitude, including deviations in roll, pitch, and yaw, have been computed for the life of each of the SSM/I sensors. Applying these corrections results in an improved pixel geolocation, but more importantly, it provides accurate estimates of the EIA across the scan and throughout each orbit. An analysis of uncertainties in the calculation of EIA shows mean errors within 0.1°, which translates to errors in the calibration of less than 0.2 K for all channels. The availability of these precise estimates of EIA is extremely important for producing CDRs since the mean EIA decreases over time due to the decay in the DMSP orbits, which will lead to an artificial climate trend if not properly accounted for by the geophysical retrieval algorithms.

Multiple independent intercalibration techniques are used to derive calibration adjustments for the development of a fundamental climate data record of physically consistent brightness temperature data from the series of six special sensor microwave/imagers (SSM/Is). The techniques include direct polar matchups, double differencing against model simulations from reanalysis profile data, double differencing against matchups with the Tropical Rainfall Measuring Mission Microwave Imager, vicarious cold calibration, and an Amazon warm calibration. Multiple realizations of three of the five techniques have been applied using different reanalysis data and retrieval techniques to account for Earth incidence angle-dependent differences between sensors. Excellent agreement has been achieved between each of the techniques with typical spread within 0.5 K at the cold end, with slightly higher spread when the warm end estimate is included. A strategy for estimating mean intercalibration values is described with justification for the use of a simple offset based on error characteristics. Intercalibration offsets are smaller for the more recent SSM/I (<; 1 K for F14 and F15 compared with F13) and slightly larger for the older satellites (<; 2 K for F08, F10, and F11 when compared to F13).

The Global Precipitation Measurment (GPM) mission requires a high degree of consistency among the microwave radiometers in the constellation which, in turn, demands a standard against which all the sensors can be compared. Ultimately this standard will be the GPM Microwave Imager, but for the present the TRMM Microwave Imager (TMI) fills this need. Since its calibration leaves much to be desired, a refinement using Windsat has been developed. This article defines the Consensus Calibration 1.1 which is applied to the TMI. In turn the TMI serves as a transfer standard to other satellite radiometers.

An important aspect of the GPM mission is the merging of precipitation data from multiple radiometers on different satellites. This requires that each radiometer be consistently calibrated and that each be intercalibrated with a mission reference standard. For GPM the reference standard is to be the core satellite carrying a dual frequency precipitation radar and a well calibrated conically scanning radiometer. This paper describes a common format for representing these intercalibrated brightness temperatures which will be used for all radiometer products from GPM partner satellites. The use of common formats ensures that users obtain all the required information and also facilitates the rain retrieval algorithm code preparation as it can always except to have the data that it needs for the retrieval.