Global Precipitation Measurement mission - architecture and mission concept

The Global Precipitation Measurement (GPM) mission is a collaboration between the National Aeronautics and Space Administration (NASA) and the Japanese Aerospace Exploration Agency (JAXA), and other US and international partners, with the goal of monitoring the diurnal and seasonal variations in precipitation over the surface of the Earth. These measurements will be used to improve current climate models and weather forecasting, and enable improved storm and flood warnings. This paper gives an overview of the mission architecture and addresses some of the key trades that have been completed, including the selection of the Core Observatory's orbit, orbit maintenance trades, and design issues related to meeting orbital debris requirements


INTRODUCTION
The GPM Mission plays a key role in the National Aeronautics and Space Administration's (NASA) Earth measurements crucial to answering key questions related to the Earth's water and energy cycle (http:l'kwec.gsfc.nasa.gov). The GPM mission will provide this data by extending and improving the measurements made by the successful Tropical Rainfall Measurement Mission (TRMM).
The key elements of the GPM mission are its constellation of spacecraft containing microwave imagers, a Core

Mimion ArchitecturdOperations Concept
The GPM Mission Architecture (see Figure 1) consists of a constellation of satellites provides continuous monitoring of precipitation over the globe. The GPM project provides two of these spacecraft, which are designated the Core Spacecraft and the Constellation Spacecraft. The remainder of the satellites in the constellation consists of other existing and planned US government resources (e.g., NPOESS), as well as contributed resources from international partners (e.g., Megha-Tropiques). Each of these missions contributes key passive microwave radiometer data to enable the GPM mission to achieve its global coverage requirement.
Information System (GDIS) collects science data from the GPM constellation of satellites, processes it and provides products to the user community. A separate Ground Validation System (GVS) is being developed to provide information for the improved accuracy of modeling based on precipitation measuremen the world.

2.2.
Ground System Architecture The GPM Ground System consists of three major items: the ~i~~i~~ Operations center (~0~1 , the precipitation processing system (pps), and the system (GVS).
The MOC, developed and control and monitoring of the spacecraft. It is responsible for maintaining the health and safety of the satellite, as well as managing the flow of science data from the satellite to the PPS. Once an orbit, a command load is relayed through white Sands complex to the spacecraft via a TDRSS SSA link. Continuous monitoring of spacecraft health and safety is provided the TDRSS Demand A~~~~~ system (DAS). Science data from the GMI is provided via DAS every 5 minutes to satisfy data product laten quiremen&. The Instrumentation flying on the Core spacecraft consists of the DPR and GMI instruments. The electronically scanned DPR provides a 3-dimensional mapping of precipitation over both sea and land, and is used to calibrate the passive microwave radiometers that contribute precipitation data to the mission* The conically-scmed GMI provides precipitation rate information over a much wider swath and estimates of global precipitation rates.
Science data from the GMI is continuously relayed over TDRSS to the Mission Operations Center (Mot) at GSFC, while science data from the DPR is stored and downlinked through scheduled TDRSS S-band Single Access @SA) forward link contacts Once Per Orbit* The GPM Data a!% Figure 1. GPM Mission Architecture outreach products. These products include surface rain Assembly rates, calibrated brightness temperatures, and rain images. The GVS," by comparing ground-based precipitation measurements with those obtained from space, will enable improvements in the algorithms used to Process future precipitation data products.

2.3.
Core Spacecraft The Core spacecraft, managed and developed by GSFC, will provide the platform for the instrument payload, including accommodating the thermal, mechanical, electrical, and data requirements of the instruments. Figure 2 shows the current layout of the spacecraft. Communications with the instruments is provided over a MIL-STD-1553 data bus.
The spacecraft will be maintained in a nadir-pointing configuration, with infrequent yaw maneuvers to adjust for beta angle changes.
Instrument operations require calibration maneuvers roughly once a month that involve leaving the nadir-pointing mode for periods of a few orbits. Spacecraft power, sized for a load of 1900 Watts, is provided by dual single-axis-articulated solar arrays, a Lithium-Ion Battery, and a direct energy transfer power system. The cant of the solar arrays ensures a minimum power production given any beta angle. Communications is provided via a steerable high gain antenna and two hemispherical omni antennas. GPS receivers on-board provide accurate orbit and time information to support the instrument geo-location requirements. A hydrazine propulsion system provides for orbit maintenance to meet the tight orbit control requirements of the DPR.   Debris. Future trades include optimizing the orbit of the at Ku-band at 13.6 GHz, and the other spacecraft* at 35.5 GHz, are designed to provide ofiles of precipitation with greater sensitivity (down to 1 dB). They will also aid in the calibration of the passive microwave radiometers that make up the rest of the GPM constellation of satellites. A 'key driver on the design of the spacecraft is the requirement to. keep the footprints of the two radars co-aligned so that they view the same column of atmosphere. By a technique that compares the signals returned from both radars, Drop Size Distribution (DSD) of precipitation in the overlapped beams can be deduced, a key scientific objective of the mission. The dual radars are quite large with the KaPR 1.4 m x 1.1 m x 0.7m, and the KuPR nearly twice the size. Other DPR instrument parameters are listed in Table 1. J A m and the National Institute 0 on and maintenance, and a design trade study to meet the ,.-j;,x

Orbit Selection 3.1.
One of the driving requirements of the mission is to optimize the coverage of precipitation rate information over the globe. Since the majority of other spacecraft in the proposed GPM constellation are polar orbiting spacecraft, it was decided to optimize the orbit of the Core Observatory to coverage of the tropics as well as the temperate zones, yielding coverage over a plus-and-minus 65-degree latitude range. This coverage requirement leads to trades in selecting orbits for the Core Observatory as well as the Constellation Observatory. A trade study [4] performed during mission development led to the selection of an orbit for the Core Observatory with a semimajor axis (sma) of 407 km and an inclination of 65 degrees. To compare the ground coverage results between different orbits, figures-of-merit to be used for each instrument were devised. They included looking at the time for complete earth coverage (GMI), and the percent coverage for a given time period (DPR). In optimizing the orbit height above about 405 km res'ults in marginal improvements to the time for complete earth coverage. Varying the inclination once above about 405 km to at the higher inclinations. For the radars, which have much smaller footprints, the orbit was again optimized over a range of altitudes. Here, the orbit selection shows sensitivity to orbits with repeating ground tracks. For the GPM mission, non-repeating ground tracks are desired so that gaps in can be minimized.

SYSTEMS TRADES
Over the Of GPM system GMJ coverage, due to the large swath width, increasing the trade studies have been conducted, including the following abbreviated list (The outcome of the trade is listed in parentheses): figuration (Sing1e axis optimize performance shows a slightly improved coverage canted arrays) . This proposed constellation covers 100% of the globe with average revisit times of 3 hours or less, fully satisfying the mission requirement for average revisit times of less than 3 hours over 80% of the globe.

3.2.
Orbit Maintenance Due to the tight orbital constraints imposed by the operation of the DPR, a calculation of estimated times between orbital maintenance maneuvers was performed [6]. This study showed that depending on the solar activity level, an orbit correction maneuver ad&d be required every few days to maintain the orbit within its control box. In order to keep operations costs low, a trade was performed that resulted in a requirement being placed on the observatory to provide for autonomous orbit maintenance. Recently, the analysis has been refined and the orbit control requirement has been slightly adjusted to allow for an orbit maneuver once per week in the worst case. It was felt that this level of ground intervention for orbit maintenance was acceptable, and therefore, it was decided that ground-based orbit maintenance procedures would be used for GPM.
Future trade studies will include looking at the fuel cost versus operations of the solar arrays. A study will examine the effect on mission lifetime and fuel usage versus the ability to keep the array's cross-sectional area parallel to the velocity direction while allowing for adequate solar array current input to the electrical power system. Typical space-based power systems have large margins at launch to account for various aging effects as well as capacity to recover from certain failures. This margin could be used save fuel by feathering the arrays.

3.3.
Orbital Debris NASA requirements for limiting orbital debris dictate that spacecraft developers show by analysis that most of the spacecraft burns up when it re-enters the Earth's atmosphere. If this is not the case, and considerable debris is estimated to re-enter, then a controlled re-entry is required. For GPM, a study looked at the survivability of the observatory design to re-entry. Typically, high strength metals like titanium and stainless steel are shown to largely survive the trip through the atmosphere. This is particularly true if the part is surrounded by other materials that must burn away before the atmosphere can begin ablating the surviving metal part. For GPM, a strategy was selected to, scrub the design and remove these survivable material&-, where possible. Several areas of study were undertaken to look at increasing the likelihood that the design would "demise" before reaching the ground.
Flexures. One design area studied was the use of titanium flexures to support the large radars. Titanium has the beneficial properties of being high strength as well as thermally isolating. Analyses were performed to examine changing this interface material to aluminum, and the results of this study were that the thermal isolation requirement was relaxed in order to bring down the total survivable debris.

CONCLUSIONS
The GPM mission architecture and operations concept have been reviewed, and some of the system trade studies that have been conducted have been described. The in-house development team at GSFC has been assembled to meet the requirements of the increased in-house portion of the core observatory development. Planning is underway for the GPM Mission Preliminary Design Review in Summer 2006.