CGMS INTERNATIONAL PRECIPITATION CGMS/IPWG-1/Doc. 16

WORKING GROUP (12.V1.2001)

FIRST SESSION

FORT COLLINS, COLORADO, USA ITEM: 4

20-22 JUNE 2001

**NASA PERSPECTIVE ON**

**GLOBAL PRECIPITATION MEASUREMENT (GPM) MISSION**

*(Submitted by Dr. Eric A. Smith, NASA/Goddard Space Flight Center)*

# 1.0 Overview

## 1.1 Project Definition

The GPM mission will measure precipitation on a global basis with
sufficient quality, Earth coverage, and sampling to improve prediction
of the Earth\'s climate, weather, and specific components of the global
water cycle (GWC).

## 1.2 Science Objectives

The GPM mission is a science-based R & D program with the fundamental
objective of advancing prediction skill in the aforementioned
geophysical disciplines. This will be accomplished by making substantive
improvements in global precipitation observations, specifically
improvements in measurement accuracy, precision, sampling frequency,
spatial coverage, and spatial resolution. The GPM mission shall also
pursue a long term goal of moving toward a space-based global
precipitation observing system motivated by its identity under the NASA
Earth Science Division\'s post-EOS Systematic Measuring Mission program.

Ongoing scientific research within the Tropical Rainfall Measuring
Mission (TRMM) and interpretation of results from that mission by
affiliated scientists have determined that a more comprehensive
precipitation measuring program utilizing various advanced
radar-radiometer instrument technologies will lead to such improved
predictability.

The mainstream scientific objectives of the GPM mission are as follows:

> **For Climate**, to accurately measure the global-regional variability
> of rainfall, relate those variations to concomitant variations in
> global-regional temperature, detect the presence or absence of a
> speculated acceleration in the global water cycle due to global
> temperature change, and improve global climate datasets and climate
> prediction through data assimilation of global rainfall measurements
> into global climate models (i.e., global climate reanalyses and
> simulation experiments);
>
> **For Weather,** to improve the accuracy of global and regional
> numerical weather prediction models through data assimilation of
> precipitation measurements, with emphasis on improving predictability
> of hurricanes and severe local storms, and verification of such models
> with globally continuous and consistent rainfall measurements;
>
> **For Global Water Cycle,** to improve the understanding and
> predictability of relevant components of the Earth's water cycle \--
> which includes water in the atmosphere, within the land surface, in
> the oceans, and in the cryosphere \-- by achieving substantive
> accuracy improvement in basin-scale water balance across the relevant
> space-time scales, with particular emphasis on improving the
> prediction of damaging floods and the availability of fresh water
> resources.

## 1.3 Mission Strategy

The science objectives and framework for research will develop from a
rationale projection of global water cycle-based science strategies
articulated in the 2001 NASA Earth Science Enterprise Strategic Plan and
NASDA\'s Science Implementation Plan for their ATMOS-A1 mission which is
how they identify the core satellite \-- onto the GPM mission\'s Science
Implementation Plan (the latter due out in February-2002). The GPM
program will be recognized as the satellite mission centerpiece of
NASA\'s initiative in the Global Water & Energy Cycle (GWEC) program.

In order to achieve measurement accuracy, consistency, and coverage
across the Earth, including the oceans, continents, and snow-ice fields,
a space-based measuring system is required made up of a constellation of
low orbiting satellites carrying passive and active microwave measuring
instruments. One of these satellites (referred to as the \"GPM core
satellite\") will be similar to the TRMM observatory insofar as its
carrying an advanced combined radar-radiometer instrument payload. This
enables the highest quality rain measurements and thus a source of
transfer calibration information to the rest of the constellation
members (referred to as \"drone satellites\").

The constellation system must be technically and financially feasible.
Because TRMM demonstrated much of the technical capacity needed by the
GPM mission, development of space hardware will be low risk by
definition. In order to be financially responsible, the GPM mission will
use the vehicles of international participation and international
partnership.

The central spacecraft of the constellation will be the \"GPM core
satellite\" to be developed under the primary GPM partnership involving
NASA and the National Aeronautics and Space Development Agency (NASDA)
of Japan. The core spacecraft will carry a NASA-provided passive
microwave rain radiometer and a NASDA-provided dual-frequency rain
radar.

The drone satellites will carry a variety of partner-contributed passive
microwave multichannel radiometers. These satellites will consist of a
collection of ongoing operational/experimental platforms provided by GPM
partners (including NASDA and DOD/IPO) and new dedicated \"GPM drone
satellites\". NASA will be required to provide one or more new dedicated
constellation members. Other national and international partnerships
will be developed under flexible arrangements to increase the population
of the constellation and to provide ground-based calibration-validation
sites to ensure high quality space measurements and an understanding of
their uncertainties.

Scientific research is to be conducted by a competed international
science team. Whereas individual international partners are expected to
develop their own sovereign science teams, a Joint GPM Science Team made
up of selected members of the individual partner teams will act as a
governance body for program-level decisions affecting the scientific
research.

Whereas precipitation is the major unresolved term in the water budget
equations whose improved measurement will guarantee improvement in
atmospheric and hydrologic predictability, and thus must be given high
priority, this mission recognizes that global water cycle research is
highly multi-disciplinary and inter-disciplinary. Thus the GPM mission
endorses complimentary missions and data gathering programs focused on
other key elements of the global water cycle. Of particular additional
interest to GPM are improved global datasets of tropospheric wind,
atmospheric water vapor, non-precipitating cloud physics, soil moisture,
upper layer ocean salinity, freeze-thaw cycling, snow-ice accumulation,
river runoff, and lake-sea levels. The GPM program also recognizes that
auxiliary payloads may be required on both core and constellation
spacecraft to fulfill some of the more difficult science requirements.

## 1.4 Mission Components

In summary, the GPM mission is made up of the following components:

> \(1\) a consortium of partner-based science teams and a down-selected
> Joint GPM Science Team largely dedicated to use of a global
> precipitation measuring system and additional synergistic global
> measurements to better understand hydrological processes and to
> advance predictability of climate, weather, and the GWC itself,
>
> \(2\) a core spacecraft mission based on a partnership between NASDA,
>
> \(3\) a scalable partnership-based constellation of radiometers that
> provide precipitation data streams,
>
> \(4\) a multi-mission operations system,
>
> \(5\) various GPM-centric high quality ground calibration and
> validation sites sponsored by NASA and additional partners, and,
>
> \(6\) a science data processing system with multiple centers which
> coordinate to produce near-real-time \"broadcast quality\" rainfall
> products and a final time series of \"climate quality\" global
> precipitation measurements plus associated accuracy and precision
> uncertainties.

# 2.0 Mission Segments

## 2.1 Core Spacecraft

The core spacecraft will produce measurements which improve our
understanding of the microphysics of precipitation and the relationship
between precipitation and the vertical structure of latent heating.
Improved understanding of precipitation microphysics and its
relationship to diabatic heating is critical to the accuracy and
precision requirements of the mission. The measurement requirements
needed to produce this understanding and fulfill the level 1 GPM science
requirements have been stipulated in preliminary form and will be under
continuous refinement until the core satellite\'s Mission Confirmation
Review in the 2nd quarter of 2003.

### *2.1.1 Sensitivity to Liquid/Ice Water Content*

The core satellite shall carry a multichannel-polarized passive
microwave radiometer system supplied by the NASA/Goddard Space Flight
Center (GSFC) to provide measurements of the liquid/ice water content of
precipitating clouds. The main radiometer shall be of the conical
scanning type to ensure consistency with the scanning modes to be used
by most radiometers in the constellation. The radiometer(s) will measure
brightness temperatures at notional rain frequencies selected across the
10-150 GHz cm-mm radio spectrum.

Based on past analysis of Special Sensor Microwave Imager (SSM/I)
measurements from the Defense Military Satellite Program (DMSP) and from
TRMM Microwave Imager (TMI) measurements from the TRMM satellite, the
nominal required frequencies are 10.7, 19, 22, 37, and 85 GHz with a 150
GHz frequency now deemed \"highly desirable\" for detection of snow as
determined from the AMSU-B sensors flown on the current generation of
NOAA polar orbiting satellites. The lower frequency measurements (10-37
GHz) detect the presence and amount of liquid water mass in the
atmosphere because they are a measure of liquid water
absorption-attenuation, while the higher frequency measurements (37-150)
detect the presence and amount of ice mass through sensitivity to ice
scattering-attenuation.

### *2.1.2 Sensitivity to Drop Size Distribution*

The core satellite shall carry a dual-frequency (Ku-Ka band)
non-coherent rain radar system supplied by NASDA and its affiliate
partner, the Communications Research Laboratory (CRL) of Japan, to
provide measurements of the rainfall drop size distribution (DSD) within
focused areas of precipitation. The radars will measure attenuated radar
reflectivities at 13.6 and 35 GHz within the radio spectrum. The DSD is
one of the two fundamental measures of precipitation (the other being
fall velocity which the DSD adequately predicts), in which a
dual-frequency bore-sighted radar is the first order means by which to
characterize the DSD properties of precipitating clouds. This is
possible because a dual-frequency radar measures differential
reflectivity (not possible on the single frequency Ku-band radar of
TRMM) which is in turn sensitive to variations in the drop size spectra
of precipitation-size particles.

### *2.1.3 Discrimination of Storm Type*

The core spacecraft shall make measurements which enable the
discrimination between convective and stratiform precipitation and \"GPM
core satellite\" retrieval algorithms will provide explicit
classification of precipitation type. This information is critical to
numerical weather and climate prediction models which formulate
convective and stratiform cloud processes separately and distinctly.
This can be done because first, the radar can detect the presence or
absence of the bright band (melting layer) which is generally considered
as a feature which differentiates between convective-stage and
stratiform-stage of precipitation (a pronounced bright band indicating
stratiform rain), and second because the radiometer can detect the
degree of heterogeneity in the rain field (convective rain generally
being more heterogeneous).

### *2.1.4 Measurement of Precipitation Over Land*

The core spacecraft shall improve the measurement of precipitation over
land by taking advantage of its measurements to sensitivity to the DSD
spectrum, and the fundamental difference between DSD properties over
land and ocean environments.

### *2.1.5 Calibration of Radiometer Measurements*

The core spacecraft shall make precipitation measurements which enable
the calibration of measurements produced by the constellation
radiometers. The core spacecraft will be able to synthesize radiometer
measurements, which produce liquid/ice water content factors, with radar
measurements, which produce DSD and vertical structure factors, ensuring
the highest quality rain measurements for which the accuracy can be
passed on to the remainder of the constellation through \"calibration
transfer\". This process adjusts \"raw\" measurements from the
constellation radiometers to \"normalized\" measurements free of bias
relative to the core spacecraft, thus achieving spatial and temporal
consistency in the global rain datasets. In this process, it would be
highly desirable that a portion of the radar and radiometer measurements
be bore-sighted within a common space-time framework.

### *2.1.6 Detection of Snow*

The core spacecraft shall be capable of detecting dry snow over water
and wet snow over land. The more variable properties of a land
background makes it more difficult to detect dry snow falling above it,
except for intense events in which large snow flakes and large
crystal-flake aggregates are present. The measurement of atmospheric
snowfall rate and its follow up accumulation will be considered as a
forefront topic in new research.

### *2.1.7 Mission Lifetime*

The core spacecraft shall be designed to operate for three (3) years
with limited-life items and expendables, including the battery and solar
array, sized to a five (5) year goal.

### *2.1.8 Launch*

The core spacecraft shall be designed for launch on a NASDA provided
H2-A launch vehicle.

### *2.1.9 Data Delivery*

Level 0 radiometer and radar data streams will be available to
GPM-designated processing centers in a near-real time mode, a 3-hour
mode, and as daily quality controlled rain products. All level 0 data
will be archived. GPM will process the level 0 near-real time data to a
"broadcast quality" continuously updated global rain image and the
3-hour data to "broadcast quality" sequential 3-hour global rain maps.
Designated instrument processing centers will convert the daily products
to level 1 "climate quality" products. The GPM data processing center
will then process the level 1 "climate quality" products to final level
2 and 3 products as the required input data become available. All level
1, 2, and 3 products will be archived. The "climate quality" products
will undergo multiple reprocessing throughout mission operations to
ensure continuous refinement of the accuracy and precision of the
measurements.

### *2.1.10 End-of-Life Reentry Requirements*

The design and operation of the NASA-NASDA core spacecraft will comply
with the reentry policies of NASA.

### *2.1.11 Radio Spectrum Requirements*

Active radio frequency (RF) emitters shall use assigned portions of the
RF spectrum. Passive microwave radiometers shall use portions of the RF
spectrum to the greatest extent possible reserved for exclusive use by
Earth Remote Sensing (first priority), or for shared use (second
priority). Whereas the use of shared and non-protected bands is
required, bandwidths and channel center frequencies shall be selected so
as to minimize potential interference.

## 2.2 Constellation Spacecraft

### *2.2.1 Precipitation Sampling*

The constellation spacecraft are to provide sufficient sampling of
precipitation to enable resolving hydrologic processes taking place at
the diurnal time scale (i.e., the daily solar cycle). The
geophysicist\'s convention is that three (3) hours is a minimal
allowable time step to resolve diurnal scale processes and thus the goal
of the GPM mission is to develop a constellation count through the tool
of international partnering that enables no worse than 3-hour sampling
at any point on the Earth. Under an orthodox constellation orbit
architecture, this sampling capability could be achieved with one core
spacecraft and 7-8 constellation members. Given that the orbit profiles
will be heterogeneous because the individual partners operate under
individual orbit profile constraints, it is likely that 9-10
constellation members will be needed to achieve the 3-hour sampling
goal.

### *2.2.2 Calibration of Radiometer Measurements*

The constellation spacecraft shall make \"raw\" precipitation
measurements which will later be bias-adjusted into \"normalized\"
measurements on the basis of calibration-quality measurements provided
by the core spacecraft. The calibration adjustments will be determined
by dataset time series specific to individual constellation satellite
members gathered during times when a given constellation member and the
core satellite overlap in coverage. This procedure ensures spatial and
temporal consistency in the final global rain datasets.

### *2.2.3 Data Delivery*

Level 0 radiometer data streams will be available to GPM-designated
processing centers in a near-real time mode, a 3-hour mode, and as daily
quality controlled rain products. All level 0 data will be archived. GPM
will process level 0 near-real time data to a "broadcast quality"
continuously updated global rain image and the 3-hour data to
\"broadcast quality\" 3-hour global rain maps integrating data from all
GPM partners. Designated instrument processing centers will convert the
daily products to level 1 "climate quality" products. The GPM data
processing center will then process the level 1 \"climate quality
products to final level 2 and 3 products as the required input data
become available. All GPM partners will have open access to both
\"broadcast quality\" products and \"climate quality\" products. All
level 1, 2, and 3 products will be archived. The "climate quality"
products will undergo multiple reprocessing throughout mission
operations to ensure continuous refinement of the accuracy and precision
of the measurements.

### *2.2.4 NASA Contributed Constellation Spacecraft*

####  *Mission Lifetime*

Any NASA constellation spacecraft shall be designed to operate for three
(3) years with limited-life items and expendables, including the battery
and solar array, sized to a five (5) year goal.

####  *Launch*

Any NASA constellation spacecraft shall be designed for launch on a
commercial launch vehicle sanctioned by use by NASA.

####  *End-of-Life Reentry Requirements*

The design and operation of any NASA constellation spacecraft will
comply with the reentry policies of NASA.

####  *Radio Spectrum Requirements*

All NASA constellation spacecraft passive microwave radiometers shall
use portions of the RF spectrum to the greatest extent possible reserved
for exclusive use by Earth Remote Sensing (first priority), or for
shared use (second priority). Whereas the use of shared and
non-protected bands is required, bandwidths and channel center
frequencies shall be selected so as to minimize potential interference.

## 2.3 Mission Operations System

The GPM mission shall include the operations of the core spacecraft and
any NASA-provided constellation spacecraft. These spacecraft will be
operated by NASA, with the operations costs considered in mission design
trades and included within the scope of the program.

## 2.4 Ground Calibration & Validation Sites

### *2.4.1 Purpose*

The GPM program will require 8-10 local-area and regional validation
sites which will provide independent measurements of rainfall reaching
the surface to be used to assess the accuracy and precision
uncertainties of the satellite measurements. These sites must remain in
operation for a minimum of five (5) years commencing at least two (2)
years before launch of the core spacecraft and continuing for at least
three (3) years after launch of the core spacecraft. The foremost users
of these measurement uncertainty factors are specialists in NWP data
assimilation who require what are called \"error covariances\"
determined by the uncertainty factors, and climate specialists who must
carefully understand the accuracy uncertainties of the space-based
precipitation measurements before drawing conclusions regarding climate
trends and amplitudes of interannual anomalies in the context of the
global water cycle.

### *2.4.2 Functionality*

Two types of validation sites will be needed. The first is called a
local area validation supersite which is to consist of a research
quality multiparameter ground radar, surrounded by a dense network of
research quality raingages and disdrometers. In addition, situated away
from the radar\'s ground clutter zone will be an upward looking
radiometer-radar system at matched frequencies to the core spacecraft,
and a 95 GHz cloud radar capable of detecting the pre-precipitation
stage of a developing rain storm. Finally, a meteorological tower system
equipped to measure the surface radiation, energy, water, and carbon
budgets will be deployed near the radiometer-radar site. Such an
instrument complement would ensure high caliber calibration-validation
analysis of the satellite measurements at the local area scale of
approximately 200 km.

In order to qualify as a validation supersite, a local area site will
have to operate the above instrumentation in conjunction with an on-site
science data processing facility staffed with 4-6 scientists and
equipment technicians. A supersite must be capable of ingesting the
satellite data streams and report, on an up-to-date basis, a continuous
time series of accuracy and precision factors as well as error
covariance information suitable for use at experimental and operational
forecast centers employing rainfall data assimilation. A validation
supersite must also be situated in an area which can accommodate
specialized field programs to be operated under GPM auspices, focused on
particular GPM science problems.

The second type of validation site is called a regional raingage network
made up of a densely populated network of research quality raingages but
spread over a regional area (order 600 km) to address
calibration-validation situations whose rainfall characteristics cannot
be adequately sampled by a local area validation site. Whereas this type
of validation site is not as high caliber as the validation supersite in
terms of assessing accuracy and precision of the satellite measurements,
it resolves these factors at the regional scale which represents
important information in calculating error covariance statistics for
data assimilation users.

### *2.4.3 Implementation*

NASA will provide two validation supersites, the first a tropical open
ocean site, the second a mid-latitude continental site. It will also
provide one regional validation site based on the raingage network
deployed around and west of the Kennedy Space Center (KSC) in central
Florida. Because development and operation of all 8-10 sites is
expensive and should not be the sole responsibility of NASA, NASA will
negotiate partnerships with other agencies both nationally and
internationally to ensure the total number of validation sites
adequately samples the variety of environmentally distinct rain systems
distributed over the Earth.

## 2.5 Data Processing Center

### *2.5.1 Data System Architecture*

The GPM science data system shall be compliant with the NEWDISS concepts
and science data centers shall be structured using heterogeneous
architecture.

### *2.5.2 Data System Functionality*

The GPM science data system shall include functions for: (1) data
acquisition; (2) data preservation, (3) data dissemination; (4)
algorithm improvement/testing; & (5) data stewardship.

### *2.5.3 Input Data Streams*

The GPM science data system shall accommodate the minimum number of data
streams required to meet the temporal sampling resolution goal of 3
hours.

# 3.0 Schedule

## 3.1 Mission Start

The operating capability of the GPM mission is expected to begin in
2007.

## 3.2 Ongoing Mission Period

There are no hard lifetime requirements for the GPM mission as it
concerns NASA, other than dictated by the survivability of the core
spacecraft and the availability of science funding. Moreover, there are
no requirements on launch phasing by partners who introduce drone
satellite members into the constellation during the useful life of the
mission. This eliminates the need to plan satellite launches with
respect to a pre-ordained launch schedule.

It will be the responsibility of the Joint GPM Science Team to
coordinate and direct science-related activity for the assimilation of
new constellation spacecraft into the mission, and to provide advice and
counsel on orbital architecture issues to the partner agencies in
seeking the most optimal data gathering design vis-á-vis the
constellation satellite orbits.

# 4.0 Constellation Orbit Integrity

### *4.1 Strategy*

It is recognized that in defining the orbit configuration strategy for
the GPM mission that no single agency will be able to select the orbit
profiles of all satellites included in the complete constellation. These
decisions are the responsibilities of the individual agencies that
operate the individual satellites, in some cases according to
constraints outside of the domain of GPM. Therefore, it will not be
possible to design an orthodox constellation orbit architecture that
would create ideal sampling for GPM purposes. Moreover, because the
expected set of constellation members will fly at different altitudes
and inclinations and will consist of a mix of sun-synchronous and
non-sun-synchronous satellite orbits, the notion of an idealized
orthodox orbit architecture for \"perfect sampling\" capability is
precluded. By the same token, the expected orbit profiles of the shared
constellation members and the flexibility available in designing orbit
profiles for the dedicated drones in moving toward an optimal orbit
architecture, allows for a number of robust orbit architecture solutions
insofar as GPM\'s temporal sampling goals.

However, all of these architectures will require fairly rigid
specifications insofar as keeping the orbit profiles nearly invariant.
This is because any orbit architecture solution designed to optimize
sampling according to some type of GPM optimization metric will quickly
deteriorate if any constellation member becomes slightly perturbed out
of its pre-defined orbit profile, as characteristic of satellites flying
at low altitude undergoing residual atmospheric drag.

### *4.2 Autonomous Orbit Control*

It is possible to achieve near invariance in orbit profiles through a
relatively recent technology called \"autonomous orbit control\", which
consists of an onborard software package continuously making orbit
adjustment by commanding thrusters according to the pre-defined orbit
parameters for a given satellite, only requiring for hardware an onboard
GPS receiver to enable continuous navigation fixes. NASA will likely
require this capability on any GPM satellite which it will operate, and
will strongly recommend that this capability be included on any
constellation satellite(s) provided by a GPM partner.

# 5.0 Public Outreach & Education

## 5.1 General Theme

The GPM mission shall provide a means to disseminate information to the
general public in a manner that promotes outreach and provides a
significant educational benefit across a full range of education and age
levels. This is predicated on the ability of the GPM data information
system to provide continually updated rain imagery on a near-real-time
basis through internet interfaces designed to provide the touch and feel
of communicating directly with satellites within the GPM constellation.
For a number of outreach applications, a data latency for \"broadcast
quality\" products of no later than 45 minutes would be desirable (e.g.,
by the Japanese JMA); for most other outreach applications, a data
latency for \"broadcast quality\" products of no later than 3 hours is
needed.

## 5.2 Priorities for Outreach

Based on the TRMM experience, current high priority recipients and their
presumed information interfaces would consist of: (1) students,
teachers, and researchers within all types of educational institutions
by virtue of direct network access to GPM level 1, 2, and 3 data and
derived products; (2) commercial and public television enterprises by
virtue of direct network access to near-real time graphical rain imagery
suitable for weathercasting programming; (3) National Weather Service
and affiliated operational forecast centers by virtue of direct network
access to data assimilation level \"near-real-time-products\"; and (4)
any government, private sector, and academic data user agency as well as
private homes that would find global rain products of value for their
institutional and/or personal initiatives by virtue of direct network
access to all near-real-time and archival products (note USDA, USGS, and
the FAA are good examples of such U.S. government institutions).