# Presentation: 537407

## Calibration Plans for the Global

- _[bidwell@agnes.gsfc.nasa.gov](mailto:bidwell@agnes.gsfc.nasa.gov)_
***(301) 614-5639***

**Calibration Plans for the Global**
**Precipitation Measurement (GPM)**

**S. W. Bidwell, J. Turk*, G. M. Flaming,**
**C. R. Mendelsohn, D. F. Everett, **
**W. J. Adams, and E. A. Smith**
- NASA Goddard Space Flight Center
- Greenbelt, MD  20771 U.S.A
- *Naval Research Laboratory,
- Monterey, CA  93943 U.S.A

***μCal 2002 Workshop, ***
***Barcelona, October 9 – 11, 2002***

## GPM Calibration Plans Outline

- GPM Overview
- GMI Instrument
- GPM Calibration
- IV.	Conclusions

- _Outline_

## I. GPM Overview

- GPM Overview

## GPM Concept

- _Core Satellite_
- TRMM-like spacecraft (NASA)
- H2-A rocket launch (NASDA)
- Non-sun-synchronous orbit
-     ~ 65° inclination
-     ~400 km altitude
- Dual frequency radar  (NASDA)
-     Ku-Ka Bands (14-35 GHz)
-     ~ 5 km horizontal resolution
-     ~250 m vertical resolution
- Multifrequency radiometer (NASA)
-     10, 19, 23, 36, 89, (150/183 ?) GHz V&H

- _Constellation Satellites_
- Pre-existing operational-experimental & dedicated satellites with microwave / millimeter-wave radiometers
- Revisit time
-     3-hour goal at ~90% of time
- Sun-synch & non-sun- synch orbits
-     600-900 km altitudes

- _Precipitation Processing System_
- Produces global precipitation products
- Products defined by GPM partners

- _Precipitation Validation Sites for Error Characterization  _
- Select/globally distributed ground validation “Supersites” (research quality radar, up looking radiometer-radar-profiler system, raingage-disdrometer network, & T-q soundings)
-  Dense & frequently reporting regional raingage networks*** ***

**Core**

**Constellation**

## GPM Core Satellite Instruments

- 13.6 GHz radar
- (Similar to TRMM PR phased array)

- 35.55 GHz radar
- (phased array)

- GMI Microwave Radiometer

- +  Possible “Instrument of Opportunity”

## Core Satellite Observations

**PR-U Swath Width= 245 km**

**DPR**
**Dual-frequency**
**Precipitation Radar**
**Ku and Ka Bands**

**GMI **
**Microwave**
**Radiometer**

**Flight Direction**
**Surface Track**
**    ****Speed = 7.2 km/s**

**5 km**

**Altitude = 400 km**

**GMI Swath**
**Width = 890 km**

**PR-A Swath **
**Width = 100 km**

## GPM Sampling Goal

**Constant Area Pixels**

**GPM Era**
**2007-2010**

**EOS Era**
**2002-2007**

- Percentage of 3-Hour Intervals Sampled
- in 7-Day Period (Simulation)

- TRMM, DMSP F13, F14, F15,
-  AQUA, and ADEOS-II

- GPM Core, GPM Constellation, DMSP F18,
- DMSP F19, GCOM-B1, Megha-Tropiques,
-  Euro-GPM, and Additional Partner

## II. GMI Instrument

- GMI Instrument

## Conical Scanning Radiometers

*** ***

- ?

## GMI View Geometry

## GMI Parameters

| Chan. | Center Freq. fc
[GHz] | CFS
(Stab+/)
[MHz] | Pass-band Bandwidth
B [MHz] (Maximum) | Pol. | NEDT [K] (Maximum) | Antenna
3dB beam width
θ3dB (degrees) (Maximum) |
| --- | --- | --- | --- | --- | --- | --- |
| 1 | 10.65 | 10 | 100 | V | 0.55 | 2.05 |
| 2 | 10.65 | 10 | 100 | H | 0.55 | 2.05 |
| 3 | 18.70 
(19.35) | 20 | 200
(500) | V | 0.65 | 1.20 |
| 4 | 18.70 | 20 | 200 | H | 0.65 | 1.20 |
| 5 | 23.80
(21.30) | 20 | 200 | V
(H)* | 0.85 | 0.95 |
| 6 | 36.50
(37.00) | 50 | 1000
(2000) | V | 0.40 | 0.90 |
| 7 | 36.50 | 50 | 1000 | H | 0.40 | 0.90 |
| 8 | 89.0
(85.5) | 100 | 4000
(3000) | V | 0.50 | 0.40 |
| 9 | 89.0 | 100 | 4000 | H | 0.50 | 0.40 |

**Highlights represent some changes from TRMM (TMI Requirements in parentheses)**

## GMI Performance Enhancements

**1)****	****Improved surface spatial resolution on low frequency channels (10.65 GHz, 18.7 GHz, 23.8 GHz)**
**	****Implies:  an antenna diameter increase to 1.2 meters**** **
**Improved capabilities for the measurement of light rain and snow**
**	****Specifically:****	****150 GHz or 166 GHz (V and H)**
**	****	****	****	****183 GHz ±1, ±3, ±6, ±9 GHz (V or H)**
**Complete surface scene sampling on all channels**
**	****Implies:  multiple 89 GHz feedhorns to ensure swath contiguity**

_**GMI Enhancements in Order of Preference:**_

## High Frequency Channels

- Snowfall Over Ocean

- Suggested primary channels for snow detection are 163 GHz at V-pol & H-pol.
- Dual polarization allows use of PCT which is more sensitive to snowfall than any
- single polarization.
- If only one polarization is possible, use V-pol instead of H-pol, because dynamic
- range is much greater at V-pol (i.e., background is much warmer).

**Courtesy of: **
**Prof. Guosheng Liu/FSU**

## High Frequency Channels

- Suggested primary channels for snow detection are 183±3 & 183±9, both at V-pol.
- First channel is for low water vapor densities while second is for high water vapor densities.

- Snowfall Over Land

**Courtesy of: **
**Prof. Guosheng Liu/FSU**

## Radiometer Altitude / Resolution Comparisons

**SSMI (0.61 m)**

**GMI Core**
**(1.0 m) **

**TMI (0.61 m)**

**AMSR-E **
**(1.6 m)**

**GMI Const.**
**(1.0 m)**

**CMIS**
**(2.2 m)**

**Orbital Altitude (km)**

**Resolution (km)**

**30 km**

**10.65 GHz**

**17 km**

**18.7 GHz / 19.35 GHz**

## GMI Down-Track (Cross-Scan) Sampling

**GMI will have increased resolution than TRMM at 10, 18, and 21 GHz**
**GMI will have similar resolution to TRMM at 36 and 89 GHz**
**Contiguity is required at 36 GHz**
**Rotational rates greater than 40 rpm viewed with caution**

## GMI Along-Scan Sampling

## III. GPM Calibration

- GPM Calibration

## GPM Transfer Standard

- _What is meant by a Transfer Standard?_
- Two distinct things:
- GMI as the Radiometric _Calibration_ Standard
- Core Satellite Retrievals as the _Precipitation_ Standard
- 	___________________________________
- (1)  GMI as the Radiometric Calibration Standard:
- _For GPM purposes_, the GMI aboard the Core satellite will act as a calibration reference for the conically scanning member radiometers of GPM, e.g.:
- 	SSMI, AMSR, CMIS, MADRAS, GPM Const. GMI,
- 	E-GPM, etc.
- As a routine GPM procedure, prior to processing and ‘merging’ of data, calibrations of other sensors will be adjusted for consistency with the GMI.

## GMI Radiometric Calibration Standard

- _Why the Core Satellite GMI  as the Calibration Reference?_
-   The Core GMI is NASA-managed throughout mission life
-     (i.e. its calibration will be well-understood by GPM)
-   The Core GMI will have an excellent on-board calibration system
-   The Core GMI will have the highest spatial resolution amongst
-     sensors
-   One instrument needs to be designated as the standard

## Precipitation Standard

- (2)  Core Satellite Retrievals as the Precipitation Standard:
- _What is the Precipitation Transfer Standard?_
- The Core Satellite combined radar / radiometer retrievals will create a data base ‘standard’ of observed brightness temperatures, precipitation systems, and  precipitation structures
- Retrievals using GPM Constellation Radiometers will access the data base standard to choose optimal precipitation parameters consistent with its brightness temperature observations (similar to Goddard Profiling Algorithm GPROF currently in use)
- _Why the Core Satellite as the Precipitation Transfer Standard?_
- The Core Satellite is a unique platform with the combined Dual-frequency Precipitation Radar (DPR) and GMI Radiometer

## External Calibration

_**External Calibration Methods**_
**(1)****	****Radiometer Calibrations with an Absolute Reference at the Ground Validation (GV) Sites:**
- Tropical Oceanic Site
- Tropical Continental Site (uniform vegetation)
- 	Under clear sky conditions, with GV measurement of:
- water vapor and temperature profiles
- surface state conditions
**Inter-Sensor Comparison Events:**
- 	Orbital intersections (Core with Constellation Members) with swath overlap within specified time delay and spatial separation

## GPM Ground Validation

- Japan

- South Korea

- EC

- Brazil

- England

- NASA KSC

- NASA Land

- Canada

- Taiwan

- ARM/UOK

- Supersite

- Regional Raingauge
- Network

- Supersite & Regional
- Raingauge Network

- North Europe BALTEX

- NASA Ocean

_**Potential Supersites and Regional Raingauge Networks**_

## GPM Ground Validation

- 150 km

**Triple Gauge Site**
- (3 Economy Scientific Gauges)

**Single Disdrometer/**
**Triple Gauge Site**
- (1 High Quality-Large Aperture/
- 2 Economy Scientific Gauges)

- 150 km

**Low Resolution Domain **
- 100-Gauge Site, Centered on Multi-Parameter Radar

- 5 km

**High Resolution Domain**
- 50-Gauge Site, Center-Displaced with
-    Matched Radar / Radiometer                                                                        	[ 14, 35 / 10, 19, 22, 37, 85 GHz ]
-    S-/X-band Doppler Radar Profilers
-    Cloud Radar Profiler

**DELIVERY**

**Supersite Template**

**GV Products**
- (1)  Error Biases & Bias Uncertainties
- (2)  Error Structures / Error Covariances
**GV Product Customers**
- (1)  Data Assimilation Specialists
- (2)  Climate Diagnosticians
- (3)  Algorithm Specialists

- Data Analysis Facility

- Multi-Parameter Radar

-  Matched Dual Freq. Radar &
-  Multi-Channel Radiometer
-  S-/X- Band Radars / Profilers
- Cloud Radar Profiler

- Meteorological Tower &
- Sounding System

- Site Scientist (3)

- Technician (3)

- {

## Azimuth Viewing Differences

***Example*****:  **An Aqua (AMSR-E swath in red) overpass occurs, then a GPM core (GMI swath in blue, simulated by the TMI instrument) follows Δt minutes later.

- The Δt offset is complicated by the fact that although the satellite zenith angles are designed to be the same, their azimuth angles are different.

**GMI on-Earth**
**beam projection**

**AMSR on-Earth**
**beam projection**

## Inter-Sensor Comparisons / No Rain

- Narrower beamwidth 85 GHz channels (red) sense primarily along-path water vapor and cloud, and some of the surface (more so for drier atmospheres)

- Over land, variations arise from variable soil conditions

- (not drawn to any scale)

- Wider beamwidth, lower frequency channels (green) sense along-path cloud and azimuthal variations of surface wind speed and direction (+/-3 K)

## Inter-Sensor Comparisons / No Rain

**Baseline No-Rain Conditions, Coincident TMI-SSMI **

**water**

**land**

- T=3 months   Δt=1 minute    Δd=10 km   TMI=no-rain, 3x3 ave

## Inter–Sensor Comparisons / No Rain

**Baseline No-Rain Conditions, Coincident TMI-SSMI (binned)**

**water**

**land**

- T=3 months  Δt=1 minute   Δd=10 km   TMI=no-rain, 3x3 ave, _then binned to 1-degree_

## Inter-Sensor Comparisons / Raining Conditions

- Narrower beamwidth 85 GHz channels (red) sense higher in the cloud, but horizontal asymmetries give rise to large TB variations depending upon azimuth view

- The 3-D effects should average-out with many events and with spatial averaging

- (not drawn to any scale)

- Wider beamwidth, lower frequency channels (green) sense lower in the cloud where horizontal asymmetries are smaller

## Inter-Sensor Comparisons / Raining Conditions

**Raining Conditions, Coincident TMI-SSMI**** **

**water**

**land**

- Δt=1 minute  Δd=10 km  TMI rainflag=rain, 3x3 average  Nocean=13433  Nland=1320

**3-D effects**

## Inter-Sensor Comparisons / Raining Conditions

**Rainy Conditions, Coincident TMI-SSMI **
**TMI-SSMI  Difference vs. TMI 2A12 Rain Rate**

- Δt=1 minute  Δd=10 km  TMI rainflag=rain, 3x3 average  Nocean=13433  Nland=1320

## Inter-Sensor Comparisons / Raining Conditions

- Rainy Conditions, Coincident TMI-SSMI
- TMI-SSMI  Difference vs. Relative Azimuth

- Δt=1 minute    Δd=10 km    TMI rainflag=rain, 3x3 ave  N=5336** **

## IV. Conclusions

-      NASA will procure two radiometers, GPM Microwave 	Imagers (GMI), for the GPM.
-      Core GMI Calibration will be used as a Reference 	Standard 	for the Constellation Member Radiometers.
-      GPM plans External Calibration Techniques for 	Calibrating the Core GMI:
- 	(1) Ground Validation System and
- 	(2) Inter-Sensor Comparisons from Swath 	              	Intersection Events.

- _Conclusions_