Trade Study Determines Data Bus for Core Spacecraft

As Global Precipitation Measurement (GPM) crystallizes, a number of decisions must be made regarding the design and architecture of the project’s components. To aid in the decision process, GPM engineers often perform trade studies to compare potential design options. One recent study involved the GPM Core Spacecraft’s data bus system—the system that will control onboard communications among all of the spacecraft’s instruments. Two data bus systems are under consideration—a MIL-STD 1553 option and an Ethernet data bus system. Engineers analyzed the advantages and disadvantages of both systems.

The GPM Core Spacecraft

For example, the space community has utilized the MIL-STD 1553 data bus on spacecraft for over ten years. There is an abundance of 1553 hardware in existence, and this method of data transfer has proven itself to be a solid performer in the space environment. The main drawback of 1553 technology is the data rate—the nominal rate for 1553 data transfer is only 1 MB/second, which roughly translates to a mere 500 Kb/second actual transfer rate.

Ethernet technology, on the other hand, has a much faster actual data transfer rate of over 10 MB/second. In addition, Ethernet data bus systems employ standardized data transfer protocols such as Internet Protocol (IP), Transmission Control Protocol (TCP), and User Datagram Protocol (UDP). The commercial sector makes extensive use of Ethernet technology in ground-based systems, but has not yet extended Ethernet use to space-based applications. It will require significant effort and cost to develop an Ethernet data bus system that can withstand the rigors of space, particularly the radiation levels experienced by Low Earth Orbit (LEO) missions.

After much analysis and discussion, the GPM management team decided to utilize a MIL-STD 1553 data bus in the baseline design for the GPM Core Spacecraft. They determined that since the data rate requirement for the Core Spacecraft is 240 kbps (which can be accommodated with 1553), at this stage in GPM’s lifecycle it is wiser to make use of a proven data transfer system, rather than factor in an estimate for the cost and effort required to develop a “spaceworthy” Ethernet data bus. In parallel, however, GPM will continue to investigate Ethernet technology, with the intention of re-evaluating the baseline as the space flight Ethernet technology continues to mature. The final opportunity to switch to an Ethernet-based design is prior to the Preliminary Design Review (PDR), currently scheduled for the fall of 2003.

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The Design for Demise: A Less Painful End

The Core Spacecraft is a major element of Global Precipitation Measurement (GPM). The data gathered from GPM has the potential to positively affect the living conditions of humans on our planet, by improving our understanding of the global water cycle. In all the excitement surrounding the development of such a spacecraft, it would be easy to focus primarily on the launch and functional life of the satellite, ignoring its inevitable end. But an end-to-end design approach would be remiss if it did not consider end-of-life design options in the early part of the design effort. Decisions made now, as the spacecraft is designed, will affect the way in which we terminate its life.

Unless one takes action to counter the effects of gravity, all objects in Low Earth Orbit (LEO) will eventually fall to Earth. During a satellite’s functional life, controllers use onboard propulsion systems to maintain the desired orbit. But what happens when a satellite has ended its useful life? Engineers must incorporate special features into the design of a spacecraft to ensure that as the vehicle descends to Earth, it does not endanger human life or cause undue property damage.

There are two methods by which to accomplish this de-orbiting process. The spacecraft can descend to Earth in a controlled fashion or an uncontrolled fashion. In controlled re-entry, onboard propulsion systems are used to position a satellite so that its major components do not land in populated areas as it falls to Earth. Alternatively, a satellite can be carefully designed so that none of its components survive the re-entry process in significant shape to cause harm to life or property—the spacecraft can be allowed to simply fall to Earth under the influence of gravity alone. To ensure that one of these options is carried out properly for every mission, the National Aeronautics and Space Administration (NASA) has established stringent requirements for the satellite de-orbiting process.

In 2001, this titanium fuel tank
(from a successful Delta launch vehicle Mission)
survived the re-entry process, landing in Saudi Arabia.

To this end, GPM has come up with a “Design For Demise” philosophy for the Core Spacecraft. The plan calls for the satellite to be designed in such a way that the need for a controlled re-entry is eliminated. There are several advantages to this decision. First, it is inherently more reliable to design for an uncontrolled re-entry; no spacecraft system has to perform an action to ensure safe disposal of the spacecraft. Also, it will enable maximum use of spacecraft resources for scientific objectives, since no unique hardware will be needed to accomplish re-entry. Uncontrolled re-entry will also enable NASA to operate the spacecraft and its instruments until the bitter end, allowing its components to degrade gracefully, instead of having to end the mission prematurely so that the satellite can be maneuvered into proper position for safe re-entry. In addition, GPM will avoid the costs (which can easily exceed one million dollars) associated with the controlled re-entry process.

Of course, the “Design For Demise” philosophy does not come without some compromises. To ensure adequate destruction of spacecraft components upon re-entry, vehicle and instrument assembly materials must be carefully selected. Standard designs for spacecraft propellant tanks and reaction wheels utilize titanium and stainless steel, respectively. Titanium is also frequently used for satellite structural components. Although these materials are known for their strength and durability, they also tend to survive the re-entry process. So minimizing—if not completely eliminating—the use of these materials is critical to the success of the “Design for Demise.” In addition to these design complexities, uncontrolled re-entry will require extensive de-orbit analyses to ensure compliance with NASA regulations.

To implement the “Design For Demise,” GPM must accomplish a number of activities. Project engineers will need to qualify a new kind of propulsion tank, redesign and qualify reaction wheels with non-stainless flywheels, and develop non-stainless steel structural fittings and mounts for the Core Spacecraft. GPM personnel also must become proficient with the software (ORSAT) used for de-orbit analysis. GPM managers have developed a schedule and budget for these activities, and briefed NASA Headquarters in April 2002 regarding their plans.

With proper advanced planning and the “Design For Demise,” GPM can potentially extend the usefulness of its Core Spacecraft, and we on Earth will be assured of a safe end-of-life disposal of the spacecraft.

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