AOS Microwave Radiometer Target Parameter List.pdf

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MICROWAVE RADIOMETER PERFORMANCE

Passive microwave radiometers provide vital information on surface and atmospheric phenomena of the Earth. Over this portion of the electromagnetic spectrum, these sensors view thermal emission from the surface and atmosphere that is modified through surface reflection and atmospheric absorption and scattering. Radiometers are particularly effective for cloud and precipitation sensing, with microwave heritage, including millimeter-wave (mmWave; 30–300 GHz) bands, in the Defense Meteorological Satellite Program’s (DMSP) Special Sensor Microwave Imager (SSM/I) series and NASA’s Tropical Rainfall Measurement Mission (TRMM) Microwave Imager (TMI) and Global Precipitation Measurement (GPM) Microwave Imager (GMI). Bands at mmWave and submillimeter (submm; 300+ GHz conventionally using microwave technology) wavelengths are sensitive to ice- and mixed-phase precipitation and clouds while still providing information at considerably higher optical depths than infrared or visible wavelengths.

Given the applicability to clouds and precipitation, the 2017 Decadal Survey (DS) recommended the use of microwave radiometry, specifically submm sensor, for achieve the science goals of the Clouds, Convection, and Precipitation Designated Observable.

The subsequent NASA study for the combined Aerosols and Clouds, Convection, and Precipitation (ACCP) Designated Observables solicited, through an initial request for information (RFI), hardware concepts for assessing potential architectures to address the science and application objectives defined by ACCP. Based on the responses, feasible accommodations, and the DS guidance, mmWave and submm sensors with bands at 89 GHz and higher were selected for further study. The architecture evaluations demonstrated significant information content regarding ice water path (IWP) over three orders of magnitude, ranging from moderately thin cirrus (10 g m–2) to precipitating convective cloud (104 g m–2). The radiometers also provide contextual swath for radars that only have either narrow or nadir-only fields of regard, specifically for cloud and total liquid water path (CLWP, TLWP), ice water content profiles (IWC.z), and surface precipitation rate (PR2D), albeit at coarser horizontal (and vertical) resolution. Thus, the microwave radiometers were determined to be core sensors by the ACCP study for AOS.

Importantly, through the architecture evaluations, the ACCP Science Impact Team (SIT) identified the radiometer characteristics necessary to meet the desired capabilities detailed in the science and applications traceability matrix (SATM). Given the wide scope of geophysical variables relevant to ACCP/AOS, the Science and Applications Leadership Team (SALT) prioritized observations of aerosols, cloud, precipitation, and their related motions. Other geophysical variables, such as temperature and humidity profiles, are available from the program of record (PoR), the existing and planned remote sensing measurements that can be leveraged to complement the mission-specific sensors selected by the ACCP study. Thus, the minimum set of mandatory channels have been chosen because they directly target cloud and precipitation geophysical variables. Additional channels have been prioritized to, first, enhance the cloud and precipitation capabilities and, second, provide supporting environmental information for reducing uncertainties relative to the PoR. The capabilities for each radiometer channel, or set of channels, trace back to the desired capabilities listed in the SATM, as detailed in Table 2. System capabilities and resource allocations are available in Table

3. A listing of definitions follows the requirements to ensure clear interpretation of this document. The targets here are provided as general guidance and are not requirements. The AOS team welcomes information on system capabilities that may not achieve the targeted performance. While instruments that provide the full set of mandatory channels are preferred, the AOS team is also interested in compact instruments or receivers that may provide a more limited channel set that may be combined to provide the necessary observations. Likewise, innovative concepts that include channels and/ or capabilities other than those listed below are welcome, but such channels and/or capabilities will be considered at the lowest priority level.

Table 1 Radiometer Target and Desired Capabilities

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Channel Definition Frequencies IFOV Sampling Radiometric

Resolution

Long-Term Calibration Stability

SATM

Driver(s)

Orbit Priority

Surface Channel

1 channel with center frequency or sideband offset within 89–113 GHz

Desired:

≤10 km

Target:

≤20 km

Nyquist

Nyquist along scan, contiguous along track

0.5 K

1.0 K

0.5 K TLWP

(O4)

CLWP (O1,

O8)

PR2D (O6,

O3, O4)

Polar:

Important

Inclined:

Mandatory

G-Band Water Vapor Channels

3 channels, DSB or SSB, with offsets between 1 and 11 GHz from 183.31 GHz scan, contiguous along track

1.5 K

0.5 K IWP

(O3)

PR2D

Low Submm Water Vapor Channels

3 channels offset from 325.15

GHz++, similar weighting to 183 GHz bands scan, contiguous along track

2.0 K

0.5 K IWP (O2,

O4)

IWC.z (O2)

Ice Cloud Channel

1 channel centered at atmospheric window within 640– GHz scan, contiguous along track

Dual-Pol Ice Cloud Channel

Matched frequency (640–700 GHz), orthogonal polarization to required ice cloud channel along scan, contiguous along track

Particl e shape (O4)

G-band Window Channel(s)

1 (or 2 orthogonal) channel(s), centered at atmospheric window between 150–170 GHz or 210– GHz along scan, contiguous along track

TLWP

Helpful

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Dual-Pol Surface Channel

Matched frequency (89–113 GHz), orthogonal polarization to required surface channel along scan, contiguous along track

0.5 K TLWP

CLWP (O1,

O8)

(O6, O3, O4) mmWave Oxygen Channels

3 DSB

channels, centered at

118.75 GHz

with offsets at ±1, ±1.5, and ±2 GHz along scan, contiguous along track

Inclined: Low

High Ice Cloud Channel

1 channel centered at atmospheric window within 820– GHz along scan, contiguous along track

Low

Inclined: Low

++If necessary as a descope option, two of the three 325.15 GHz channels closest to the line can be moved to the 380.2 GHz water vapor line, as long as the weighting functions are matched to the analogous 183.31 GHz bands, and contamination by the oxygen transition at 368.5 GHz is avoided. To achieve this, the 380.2 GHz bands must be SSB on the lower frequency shoulder. These matching 183/325 GHz bands are necessary for profiling ice phase clouds and precipitation, specifically for separating snow and cloud ice contributions while minimizing the influence of water vapor on retrievals of AOS observables.

MICROWAVE RADIOMETER RESOURCE ALLOCATION TARGETS

The AOS team has developed target spacecraft resource allocations for the Microwave Radiometer based on information gathered during the ACCP Mission Concept Study Phase, including information gathered from an instrumentation Request for Information submitted during that period. From this information the mission systems team developed spacecraft concepts commensurate with allocations as found in Table 3. The Respondent should provide both their Current Best Estimate and Maximum Expected Value resource needs in the attached spreadsheet under tab labeled ‘Spacecraft Accommodation.’ Note: The values in the table below are not requirements but rather for informational purposes to provide the respondent with the notional resources needs currently envisioned by the AOS team. Exceedance of these values are acceptable and expected, especially in the event of enhanced performance capability.

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Table 2 Microwave Radiometer Target Resource Allocations (with Scan Type and Swath Coverage)

Radiometer Specification

Inclined Targets

Polar Targets

Swath >750 km >750 km

Scan Type Conical or cross-track Conical or cross-track

Data Rate (bps)^ 1.6x105 1.6x105

Power (W) 45 20

Mass (kg) 40 10

Envelope Dimensions in Operational Configuration: LxWxH (cm)

80 x 45 x 45 35 x 20 x 40

Please provide both the current best estimate (CBE) and the maximum expected value (MEV) for these resources. MEV = [(100 + XX)/100] CBE where XX is contingency in percent.

^Radiometer data rate will not be driving any mission resource, and exceedance from the target is welcome if it enables improved science.

Figure 1 Instrument reference coordinate system.

DEFINITIONS AND DISCUSSION

Channel definition describes the radiometer channel, or set of channels, based on location in the electromagnetic spectrum and surface or atmospheric sensitivity.

Frequency states specific details of the channel spectral location, including acceptable frequency range. For channels targeting molecular transition lines (i.e., water vapor or oxygen transitions), offsets are provided based on analysis by the SIT that determined optimal sub-band center frequency for cloud and precipitation sensing.

Instantaneous field of view (IFOV) is the diameter of a circle with area equal to that of the ellipse defined by the 3 dB contour of the antenna pattern projected on the Earth’s surface:

IFOV = (𝐴𝐴𝐴𝐴)

2 (1) where A and B are the length of the major and minor axes of the 3 dB ellipse of the antenna footprint on the Earth surface. A smaller-than-required IFOV is desirable to the extent that it does not reduce the sample spacing below the approximate Nyquist coverage. For cross-track scanning radiometers, to encourage wide swath coverage, the IFOV should be calculated using (1) at the edge of target swath listed in Table 3 , allowing footprint growth for wider swaths.

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Sampling is defined independently of averaging and noise equivalent differential temperature (NEΔT). Radiometric brightness temperature of the scene should be sampled at minimum twice per 3 dB beamwidth (approximately Nyquist) in the cross-track dimension. Brightness temperature should also be sampled twice per 3 dB beamwidth in the along-track dimension. In the case that scanning mechanisms cannot achieve sufficient scanning speed to achieve two samples per 3 dB beamwidth in the along-track direction, sampling should not be less than contiguous (one sample per 3 dB beamwidth). Nyquist sampling is a higher priority than finer horizontal resolution (i.e. a narrower IFOV) such that underillumination of a reflector at higher frequencies to achieve Nyquist sampling could be desirable.

If channels have differing IFOVs, Nyquist sampling is desired at the most narrow beam, with wider-beam channels being oversampled and integrated over multiple scans to improve radiometric resolution as discussed below.

Radiometric resolution (i.e, NEΔT) is defined as

NE∆T = 𝑇𝑇𝑠𝑠𝑠𝑠𝑠𝑠 �

𝐴𝐴𝐵𝐵

∆𝐺𝐺

𝐺𝐺

(2) where Tsys is the system brightness temperature including an estimated scene brightness temperature of 300 K Kelvin, B is the channel bandwidth, τ is the integration time, and ΔG/G is the ratio of the system gain fluctuation to the overall gain. Knowledge of NEΔT with and without the ΔG/G factor, then, is vital for understanding the stability of the radiometer.

The integration time τ may include multiple scans, and is defined here as the total integration time associated with scanning over the IFOV requirement. With this definition, a scanning total power radiometer would have an integration time of

𝐵𝐵 =

𝜋𝜋IFOV2

4𝑣𝑣𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 𝜙𝜙 𝑣𝑣𝑠𝑠𝑠𝑠

(3) where IFOV is defined in (1), vscan is the magnitude of the scan velocity at the surface in m s–1, φ is the scan rate s–1, and vsc is the magnitude of the spacecraft ground velocity (assume 7200 m s–1). This definition effectively normalizes the integration time across multiple beam sizes and scan geometries, while allowing for along-track averaging with rapid Nyquist scanning.

Long-term calibration stability is the calibration drift over the lifetime of the mission.

SATM drivers are the geophysical variables from the SATM to which the requirements can be traced, including the ACCP objective(s) for which the radiometer channel is relevant. Objective 1 (O1) encapsulates low clouds, including boundary layer clouds; objective 2 (O2) addresses high clouds, namely anvil and cirrus clouds; objective 3 (O3) is focused on convection from shallow to deep; objective 4 (O4) covers cold precipitation, including snowfall and high-lati- tude precipitation.

Orbit priority provides the priority (mandatory/important/helpful/low) for both inclined and polar orbit segments.

Mandatory refers to any channel that is necessary to meet the threshold science and applications requirements as detailed by the current SATM.

Important refers to any channel that, while needed to achieve baseline AOS science and applications requirements, can be descoped without impacting the ability to meet threshold science and applications requirements.

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Helpful refers to any capability that can improve or enhance AOS science and applications, e.g., reduces uncertainties in essential GVs, but is not necessary to meet requirements.

Low refers to the minimum priority for a capability. If the capability is offered without impacting the ability to accommodate the radiometer, then it will be accepted. Low priority capabilities will be the first to be descoped, assuming such a descope does not impact threshold capabilities.

Target refers to minimum capabilities needed to meet the threshold science and applications requirements as listed in the current SATM.

Desired refers to enhance capabilities that can enable baseline science and applications requirements.

All radiometer channels should be linearly polarized. For conical scanning sensors, the electric field vector should be oriented either parallel (vertical polarization) to the plane of incidence or perpendicular (horizontal polarization) to the plane of incidence. For cross-track scanning sensors, fixed-polarization basis is preferred, with the linear polarization either parallel or perpendicular to the scan plane. If the polarization cannot be fixed across the scan, for a single polarization band the electric field vector for any one channel should be parallel or perpendicular to the scan plane at nadir. For bands with dual polarization, the electric field vectors should be either perpendicular or parallel to the scan plane at ±45º relative to nadir with the two polarization, where the sign of the angle, relative to the flight direction, is arbitrary.

Surface channel provides sensitivity to the entire atmospheric column and to the surface, and it is important for surface precipitation estimates. AOS requires one surface channel centered within the range of 89 to 113 GHz. Lower frequencies are preferred as long as they do not compromise the IFOV requirement. A double sideband (DSB) 118 GHz ± 5 GHz channel is acceptable for this purpose.

*An open trade exists for the inclined orbit to decide priority of surface and ice cloud channels if both cannot be accommodated.

G-band water vapor channels provide information on falling snow and graupel. Additionally, when paired with similarly-weighted bands near the 325.15-GHz transition, these channels will help with separation of water vapor, ice-phase precipitation, and ice-phase clouds. AOS requires at least three single-sideband (SSB) or DSB channels around the 183.31 GHz water vapor transition with offsets between 1 and 11 GHz. If SSB, the channels should be offset to frequencies lower that 183.31 GHz.

Low submm water vapor channels provide information on falling snow, graupel and cloud ice. Pairing with bands at

183.31 GHz provides additional advantage in discriminating habits and separating water vapor. AOS requires at least three SSB or DSB around the 325.15-GHz water vapor transition. Ideally, these channels should be tuned to provide comparable clear sky weighting functions to the G-band water vapor channels.

++If necessary as a descope option, two of the three 325.15 GHz channels closest to the line can be moved to the 380.2 GHz water vapor line, as long as the weighting functions are matched to the analogous 183.31 GHz bands, and contamination by the oxygen transition at 368.5 GHz is avoided.

Ice cloud channel provides sensitivity to smaller ice cloud particles and IWP >10 g m-2. AOS requires a single RF channel centered between 640 and 700 GHz. The polarization should be either horizontal or vertical for a conical-scanning radiometer.

*An open trade exists for the inclined orbit to decide priority of surface and ice cloud channels if both cannot be accommodated.

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Dual polarization ice cloud channel is the highest priority additional radiometer capability desired by AOS. This channel should match the center frequency and have orthogonal polarization (horizontal or vertical) to the required ice cloud channel. Collocated beams are not required as long as beamwidths are matched and can be combined in post-processing.

G-band window channel(s) provide additional information over the full atmospheric column, particularly for ice-phase precipitation and mixed-phase clouds. A number of implementations would be acceptable, including a single channel centered within 150–170 GHz or 210–240 GHz. Alternatively, a DSB channel located at 183 GHz ± 11 GHz would be acceptable. Dual polarization is preferred.

Dual polarization surface channel provides additional information regarding precipitation and for separation of surface and atmospheric contributions to the signal. This channel should match the center frequency and have orthogonal polarization (horizontal or vertical) to the required ice cloud channel. Collocated beams are not required as long as beamwidths are matched and can be combined in post-processing.

mmWave oxygen channels provide additional information on ice-phase precipitation at or near the surface.

Additionally, these bands provide thermodynamic profile information that can reduce the uncertainty from relying on ancillary data for atmospheric temperature. AOS desires three DSB channels with center frequency of 118.75 GHz with offsets at ±1, ±1.5, and ±2 GHz.

High ice cloud channel will increase the sensitivity to thinner cirrus clouds with IWP ranging from 10 to 50 g m–2. AOS desires one channel with center frequency between 820 and 890 GHz.