GPM Core Spacecraft Solar Array: Designed To Extend Mission Life

Every spacecraft in Low Earth Orbit (LEO) experiences the effects of atmospheric drag, especially those at altitudes less than 450 km. As a satellite races through space in LEO, it collides with molecules in Earth’s atmosphere that impart a drag force on the spacecraft. The amount of drag force exerted depends on the density of the molecules and the projected surface area of the satellite as it passes through the molecules. The density of the atmospheric particles can be quite variable, depending on the altitude, degree of solar activity, season, longitude, latitude, and magnetic storm conditions. The greater the projected area of the spacecraft, the more molecules it will hit, increasing the drag.

This constant barrage of molecules gradually causes the spacecraft to slow down and drop from its intended orbit to a lower orbit, where it encounters even more molecules, causing it to slow down further, and drop still lower, etc. If this process is left unchecked, the spacecraft will eventually re-enter Earth’s atmosphere. Therefore, propellant is required to periodically boost the spacecraft back into the proper orbit. Satellites can only carry a limited amount of propellant onboard, and thus, a LEO mission’s lifetime is often limited by its propellant supply.

We cannot control the density of Earth’s atmosphere in the orbit selected for the GPM Core Spacecraft (~400 km) but we do have control over the spacecraft design. Often the biggest culprits increasing a satellite’s surface area are the solar array panels, which generally must have a very large surface area to capture the Sun’s energy and convert it to electrical power.

For example, the Tropical Rainfall Measuring Mission (TRMM) spacecraft is currently in an orbit very similar to the one that will be utilized by the GPM Core Spacecraft. To maintain its orbit, TRMM consumes a great deal of propellant to counteract the effects of drag on the spacecraft. The biggest source of drag on TRMM is its solar array panels. GPM engineers are seeking to reduce this type of drag on the GPM Core Spacecraft, therefore lengthening the mission’s lifetime.

Typical LEO missions have solar arrays that articulate to track the Sun as the spacecraft travels around its orbit. The problem with this design is that during certain portions of the orbit the arrays are positioned so that their full area is projected in the velocity direction of the spacecraft, generating a significant drag force. To minimize the amount of drag the Core Spacecraft will encounter, its solar panels will remain stationary, in a position that is edge-on to the velocity vector of the spacecraft. They will not move to track the Sun as the satellite proceeds around Earth.

This fixed array scenario, on the other hand, presents a different problem: If the solar arrays do not track the Sun, how can they collect enough energy to sufficiently power the spacecraft? A typical satellite’s solar arrays are in view of the Sun for 60-70 percent of its orbit. The GPM Core Spacecraft’s solar panels, however, will be in view of the Sun only about 30-40 percent of the time at most beta angles, since they will not be capable of tracking the Sun. (Note: The beta angle is the angle between the orbit plane and the direction of the Sun.) To gather enough solar energy in a lesser amount of time, the Core Spacecraft’s fixed solar arrays will be about 70 percent larger than articulating arrays would have needed to be to support the mission.

The solar array design is further affected by the fact that one side of the Core Spacecraft must always face away from the Sun, to keep the instrument thermal radiator and the GMI cold space calibration field of view pointing toward cold space. The solar array on this “cold” side of the spacecraft will be fixed perpendicular to the spacecraft bus to accommodate the physical location of these pieces of equipment. The array on the side of the spacecraft that faces the Sun (the “hot” side) will be canted (fixed at an angle) to maximize the panel surface area facing the Sun.

Line drawing of the GPM spacecraft showing the configuration of the solar array.

GPM engineers analyzed many potential design options to determine the optimal cant for the Core Spacecraft’s “hot” side panel. The final design includes a cant of 20 degrees, which will allow a sufficient projected area of the “hot” side solar panel to face the Sun, and will enable the arrays to gather enough energy to meet the spacecraft’s power requirements.

In addition, the Core Spacecraft’s orbit precesses around Earth over the course of several weeks. GPM engineers had to develop an operations scheme enabling the “cold” side of the spacecraft to face away from the Sun (and the “hot” side to face toward the Sun) as much as possible. The Core Spacecraft orbit takes 36 days to precess 180 degrees. Therefore, to keep the proper sides of the Core Spacecraft facing toward or away from the Sun, the spacecraft operators will rotate (or “yaw”) the satellite 180 degrees every 36 days. This maneuver is expected to take 20 minutes for each rotation, and will not significantly impact the amount of time the spacecraft’s instruments will be available for science observations.

This optimum scenario should enable the GPM Core Spacecraft to realize a savings in propellant equal to approximately one third to one half the propellant mass on the TRMM spacecraft, even though the two spacecraft have identical mission lifespans. Thus, by employing careful planning and lessons learned from prior missions, GPM will maximize the lifetime of its Core Spacecraft.

By Lena Braatz and John Durning

For further information on the GPM Core Spacecraft, contact John Durning (jdurning@pop400.gsfc.nasa.gov).

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GPM Microwave Imager Will Provide Crucial Measurements

A passive microwave radiometer is an instrument that measures microwave emission from Earth’s atmosphere and surface. Microwave radiometers are versatile instruments; when properly configured, they can be used to infer a wide variety of phenomena, such as atmospheric moisture and temperature profiles, soil moisture, and sea surface temperature. By employing a microwave radiometer at the appropriate frequencies and utilizing carefully developed retrieval algorithms, scientists can make inferences about various meteorological, oceanographic, and surface conditions. The versatility of the microwave radiometer has made it the instrument of choice for a variety of space-based missions, including environmental sensing and weather forecasting programs.

GPM’s goal is to detect the presence of rain and determine the rain rate by frequently sampling conditions over the entire globe. The microwave radiometer is particularly well suited to these purposes for a variety of reasons. Microwave radiometers are capable of measuring radiation at frequencies that permit rain and rain rate to be pinpointed. In addition, a microwave radiometer can be designed with a broad measurement scan, rendering it capable of measuring a large segment of Earth from an orbiting satellite. This capability is important to GPM, because of the mission’s need to make frequent, global measurements.

The GPM team is currently in the process of developing the specifications defining the capabilities and performance level required of the radiometer to be used by GPM. The GPM Microwave Imager, or GMI, will most likely look very similar to an instrument used on the Tropical Rainfall Measuring Mission (TRMM)—the TRMM Microwave Imager, or TMI. An illustration of the TMI is shown below.

The TRMM Microwave Imager

GMI will be a conical-scan, passive microwave radiometer. NASA will procure two nearly identical GMI instruments from industry: one instrument to be placed on the Core Spacecraft, and the other on a NASA-provided Constellation Spacecraft. Although the vendor for GMI has not been selected at this time, the instrument's design will most likely incorporate substantial heritage from previous instruments of similar design. This heritage will reduce the technical risk, time required for design and fabrication, and procurement cost.

GMI will be designed to incorporate several microwave frequencies (e.g., 10.7, 19.3, 21, 37, and 89 GHz), allowing the instrument to measure a variety of rainfall rates and related environmental parameters. Inclusion of additional, higher frequency channels (150-166 and 183 GHz) to provide increased sensitivity for the measurement of light rains is under consideration, if the cost of including these channels is not restrictive. GMI will have an offset parabolic reflector 1.0-to-1.2 meters in diameter, which will rotate about the instrument's vertical axis. The antenna will point at an off-nadir angle of approximately 49 degrees, providing a ground measurement swath extending roughly 800 km from side-to-side for the Core Spacecraft.

The instrument’s speed of rotation has not been firmly established, but all heritage systems have used a rotational rate of about 32 rpm. During each revolution, GMI will take measurements using a 130 degree scan sector centered along the spacecraft velocity vector; the remaining 230 degree sector will be used to perform a hot and a cold calibration and other housekeeping functions. GMI will thus be calibrated every scan. Momentum compensation will be incorporated into the instrument, most likely by integrating a separate momentum wheel assembly. GMI is expected to have a mass of about 80 kg, with an electronics enclosure on the order of 0.5x0.5x0.5 m to support the reflector.

Illustration of GMI Scan Geometry

NASA will acquire GMI using a performance-based specification (the GMI Requirements Document) that will define the performance capabilities of the instrument. Vendors will be allowed, however, to make detailed design decisions necessary to ensure optimized performance while striving to minimize cost. Delivery of the first GMI will take place in the spring of 2006, to provide adequate opportunity for the integration and test of the instrument on the GPM Core Spacecraft prior to its launch in late 2007.

Specific factors that will influence the design of the GMI include:

· The accuracy and precision with which the measurements must be made
· Spatial resolution requirements
· System operating lifetime requirement
· Spacecraft accommodation constraints such as mass, power, and data rate
· Cost

During the era when GPM is in operation, numerous space-based programs will use microwave radiometers to measure microwave emission from Earth’s atmosphere and surface. GPM will endeavor to obtain measurements from these instruments to assist the program in meeting its objectives for frequent, global measurements of rainfall.

By Mark Flaming

For further information on GMI, please contact the author (Gilbert.M.Flaming.1@gsfc.nasa.gov).

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