David Newell

The Global Precipitation Measurement (GPM) Microwave Imager (GMI): Instrument Overview and Early On-Orbit Performance

The Global Precipitation Measurement (GPM) mission is an international satellite mission that uses measurements from an advanced radar/radiometer system on a core observatory as reference standards to unify and advance precipitation estimates made by a constellation of research and operational microwave sensors. The GPM core observatory was launched on February 27, 2014 at 18:37 UT in a 65° inclination nonsun-synchronous orbit. GPM focuses on precipitation as a key component of the Earths water and energy cycle, and has the capability to provide near-real-time observations for tracking severe weather events, monitoring freshwater resources, and other societal applications. The GPM microwave imager (GMI) on the core observatory provides the direct link to the constellation radiometer sensors, which fly mainly in polar orbits. The GMI sensitivity, accuracy, and stability play a crucial role in unifying the measurements from the GPM constellation of satellites. The instrument has exhibited highly stable operations through the duration of the calibration/validation period. This paper provides an overview of the GMI instrument and a report of early on-orbit commissioning activities. It discusses the on-orbit radiometric sensitivity, absolute calibration accuracy, and stability for each radiometric channel.

Assessing Calibration Stability Using the Global Precipitation Measurement (GPM) Microwave Imager (GMI) Noise Diodes

With rising demand for smaller, lower mass microwave instruments, internal calibration using noise diodes is becoming increasingly more attractive for space-borne radiometer applications. Since noise diodes can exhibit on-orbit excess temperature drift, internally calibrated systems typically require vicarious on-orbit recharacterization. The GMI is the first instrument of its kind to include both internal (noise diodes) and external (hot load/cold sky) calibration systems. The dual-calibration system provides the unprecedented capability to directly measure transient behaviors in the hot load, cold sky view, and receiver nonlinearity. Furthermore, the behavior of the noise diodes can be directly evaluated, which may shed light on improvements to internal calibration for future missions. This paper directly examines the behavior of the GMI noise diodes using the hot load and cold sky views for the first 6 months of operations. Two of the seven channels with noise diodes have exhibited on-orbit noise diode excess temperature drift of about 1 K. The other noise diodes have remained exceptionally stable. The noise diodes are used to evaluate transient behaviors in the GMI hot load, cold sky view, and nonlinearity. The hot-load brightness temperature variation due to gradients is re-evaluated and shown to be smaller at the lower frequencies than at preflight calibration. Radio frequency interference (RFI) in the cold view is evaluated using the noise diode backup calibration. The on-orbit nonlinearity is trended over the first 6 months and shown to be stable over that time period.

Global Precipitation Measurement Microwave Imager Prelaunch Hot Load Calibration

For typical scanning microwave radiometers, a significant source of calibration error arises from thermal gradients on the hot load. Even when direct or reflected solar illumination is blocked, hot load gradients arise from thermal coupling between the target and the surface facing the target which is heated and cooled as the instrument orbits the earth. For the GlobalL Precipitation Measurement (GPM) Microwave Imager (GMI), a rotating metal annular ring called the “hot load tray” serves to guard the hot load against solar intrusion, and is the surface immediately facing the hot load during the majority of the scan. The planned GMI calibration algorithm corrects for the target gradients induced by thermal coupling between the hot load tray and hot load. The correction uses an empirically derived relationship between the target gradient and the temperature differential between the target and the tray. The correction is derived using target-level and GMI system-level calibration testing. The dual calibration of GMI, in connection with thermal vacuum calibration measurements, is a key aid to determining and correcting the hot load gradients.

GPM microwave imager instrument design and predicted performance

The Global Precipitation Measurement (GPM) Microwave Imager (GMI) Instrument is being developed by Ball Aerospace and Technology Corporation (BATC) for the GPM program at NASA Goddard. We describe the instrument and predict the performance of the GMI instrument. The GMI instrument is a conical-scanned passive microwave radiometer used to make calibrated, radiometric measurements from space at multiple microwave frequencies and polarizations from 10 GHz to 190 GHz. The instrument has a 1.22 meter offset parabolic reflector for high spatial resolution and high beam efficiency. Receivers from 10 to 190 GHz frequency receive, amplify and detect the microwave radiation with different detection approaches used at each frequency to maximize performance while minimizing cost and risk. Mechanical features include a deployment assembly for the main reflector and slip rings to transfer power and signals to the rotating portion of the instrument. The core GPM spacecraft, including GMI, will be used to develop a retrieval transfer standard for the purpose of calibrating precipitation retrieval algorithms. For this reason calibration is critical for GMI and a number of features have been included in the GMI design to allow GMI to meet the 1.35 K calibration uncertainty requirement.

The ozone mapping and profiler suite (OMPS): on-orbit calibration design

The Ozone Mapping and Profiler Suite (OMPS) will collect total column and vertical profile ozone data and continue the daily global data produced by the current operational satellite monitoring systems, the Solar Backscatter Ultraviolet radiometer (SBUV/2) and the Total Ozone Mapping Spectrometer (TOMS), but with higher fidelity. The collection of this data will contribute to fulfilling US treaty obligations to monitor ozone depletion for the Montreal Protocol. OMPS has been selected to fly on the National Polar-Orbiting Operational Satellite System (NPOESS) spacecraft - the next generation of polar orbiting environmental satellites. The first OMPS flight unit will fly on the NPOESS Preparatory Project (NPP) spacecraft. On-orbit calibration of the OMPS instruments is critical to maintaining quality data products. A number of signal corrections and calibrations are applied on-board the sensor and in ground processing to account for instrument non-idealities and to convert measured digital signals to calibrated radiances and irradiances. Three fundamental on-orbit calibration measurements are made to provide the required data to perform the radiometric calibration and trending.

GPM Microwave Imager design, predicted performance and status

The Global Precipitation Measurement (GPM) Microwave Imager (GMI) Instrument is being developed by Ball Aerospace and Technologies Corporation (Ball) for the GPM program at NASA Goddard.

A Space-Based Observational Strategy for Characterizing the First Stars and Galaxies Using the Redshifted 21-cm Global Spectrum

Author(s): Burns, Jack O; Bradley, Richard; Tauscher, Keith; Furlanetto, Steven; Mirocha, Jordan; Monsalve, Raul; Rapetti, David; Purcell, William; Newell, David; Draper, David; MacDowall, Robert; Bowman, Judd; Nhan, Bang; Wollack, Edward J; Fialkov, Anastasia; Jones, Dayton; Kasper, Justin C; Loeb, Abraham; Datta, Abhirup; Pritchard, Jonathan; Switzer, Eric; Bicay, Michael

Global Precipitation Measurement (GPM) Microwave Imager (GMI) calibration features and predicted performance

The GMI, scheduled to launch in 2013, includes innovative calibration features designed to mitigate or eliminate sources of calibration error that have plagued past microwave radiometers. Solar radiation illuminating the surface of the flight calibration hot load is a common calibration problem. Tests at BATC show that hot load solar intrusion can cause a calibration error on the order of twice GMFs required uncertainty. To reduce the likelihood of hot load solar intrusion, the GMI includes stray light shrouding features around the hot load. The GMI contains a dual-calibration system with noise diodes to provide a backup calibration method in the unlikely case the hot load experiences solar intrusion. Also, the GMI program is working with coating vendors to avoid reflector emissivity problems on-orbit. GMI includes temperature sensors on the main and cold-sky reflectors to correct possible nonzero emissivity. The GMI instrument design meets the required calibration uncertainty.

An assessment of radio frequency interference using the GPM Microwave Imager

A simple data-driven algorithm is used to assess radio frequency interference (RFI) in Global Precipitation Measurement (GPM) Microwave Imager (GMI) data. RFI originates from land-based and space-based sources, affecting the GMI 10 and 18 GHz bands. Land-based RFI demonstrably impacts the GMI 10 GHz data over Europe, China, Japan, and Mexico. Land-based RFI also affects the 18 GHz channels to a lesser degree over particular countries such as Belarus, Libya and Chile. Reflected RFI from the earth surface at 18 GHz from direct broadcast satellites is observed around the continental United States and Hawaii. Geosynchronous direct-broadcast satellites also provide a source of RFI in the GMI cold view. The cold-view RFI is detected and removed in the operational algorithm.

GPM microwave imager key performance and calibration results

The Global Precipitation Measurement (GPM) Microwave Imager (GMI) instrument was launched onboard the GPM core spacecraft in February 2014. The instrument has exhibited highly stable operations through the duration of the calibration/validation period. This paper provides an overview of the GMI instrument and a report of early on-orbit commissioning activities. It discusses the on-orbit radiometric sensitivity and stability for each channel, hot load performance, noise diode stability and early indicators of absolute calibration performance.