A Shuttle Interface for Small Experiments

REMOTE SENSING FROM THE SPACE SHUTTLE AND SPACE STATION

Lunar and Planetary Laboratory West, University of Arizona, Tucson, AZ 85721, USA

ABSTRACT

In the past few years we have prepared and flown telescopes and spectrographs on five Shuttle flights with considerable success. We believe that the attributes of the Shuttle and Space Station should be re-examined by the scientific community that uses remote sensing instrumentation as research tools. These space platforms should play a far greater role in future remote sensing research than presently envisioned. We have developed an interface to the Shuttle system that allows us to fly new instruments cheaper, faster, and better than the classical rocket approach.

INTRODUCTION

The author and his science team have had the opportunity to prepare and fly experiments on the Shuttle for the USAF Phillips Laboratory. We have flown five times, allowing us to develop an understanding of the facility and the flight system. We have spoken to many other experimenters about the use of the Shuttle as a space platform for experimentation and have found many misconceptions that cause them to dismiss this facility out of hand. This is unfortunate because the scientific community has lost several years of prime experimental time in space.

The Space Shuttle is one of the most important facilities we have for the development of remote sensing global research instrumentation. In the past the development path was from the laboratory to a rocket to a satellite. Modern instrumentation is much more sophisticated and requires time in space to demonstrate the instrumental capability in a satellite environment. The Shuttle is the ideal platform to use in preparing for long-term space missions. Experiments that are being considered for satellite missions can be demonstrated and fine-tuned with a few days in space to gather real data under real conditions. The availability of real data would allow the experiment team to prepare to receive and analyze flight data well before the satellite flies. The hiatus of dealing with the first years of flight data could be pushed back into the pre-flight period with the possibility of improving the experiment and in effect extending the mission by a year.

When considering satellites for remote sensing research opportunities, the International Space Station (ISS) should be high on our list of facilities. The Space Station orbit inclination is 51ø, which is close enough to the auroral oval for good auroral viewing half of the day. This is a good orbit for global remote sensing research. The way to develop instrumentation for the Space Station is of course to use the Shuttle as the test platform. When proven to be operationally sound, the experiment would be attached to the Space Station by the Shuttle remote manipulating arm (RMS). Control would be transferred and the experiment operated again from the ground through the Space Station.

The most pervasive error in the approach to Shuttle experimentation has been in considering this opportunity as an upgrading of our rocket experiment approach, rather than understanding the attributes of the Shuttle and moving to provide those attributes to the experimenter. The list of attributes that we have achieved in our flight experiments should be the baseline for an experimenter to expect. These capabilities are as follows:

• 7 to 14 days in space.
• High data rate, 7 Gbytes to the ground per day.
• Excellent pointing accuracy and stability.
• Two-way communication with an experiment.
• Time to flight - one year.
• Cost - comparable to rocket experimentation.

The main problem in reaching the reasonable level of performance stated above is dealing with the Shuttle interface. The Shuttle interface to the experimenter is incomplete. I come to this conclusion based on the amount of common hardware that experimenters would have to develop in order to optimize their valuable time in space. Our Shuttle interface is provided through the Hitchhiker office at GSFC. This is an excellent service. It consists of a team of people who work closely with the experimenter and represent the experimenter's needs and requirements to the Space Shuttle Program Office at JSC. This Hitchhiker program office is certainly the heart of an experimenters access to the Shuttle.

The Missing Pieces of the Interface

An autonomous experiment control unit is required on the Orbiter. Although the communications to and from the Shuttle are excellent, the communication link is not always available. High rate data is transmitted down through the Tracking and Data Relay Satellite System (TDRSS) but the link is available only about 50% of the time. For the experimenter to optimize his experiment time in space, an on-board computer is required as well as an on-board recorder to store data during periods of Loss of Signal (LOS). Our system uses an Intel 386-level single-board computer and a read-and-write optical disk. Two communications ports are utilized, one for real-time data and one for recorder playback. These elements with appropriate software allow the experimenter to operate continuously throughout the 7-14 day mission.

The other constraint on an experiment is the Shuttle attitude and the attitude control limit cycle motion. The Shuttle attitude changes periodically for different tasks and to support some experiments. The attitude is controlled within a "deadband" by vernier thrusters causing continuous but slow motion. Our dependence on the Shuttle attitude has been removed by placing the experiment on a two-axis scan platform to increase the opportunities that allow the instrument to point in preferred directions. The instrument we have been flying, the Arizona Airglow Experiment (GLO) can acquire valuable data almost continuously during a Shuttle flight. The second task for the scan platform is in stabilizing the pointing by cancelling the Shuttle attitude control limit cycle. We employ imagers to actively track stars or the Earth's limb. The Shuttle attitude is known very accurately with respect to its gyro platform and the gyros are calibrated regularly against the star field. Our images of the star field provide a good calibration of the scan platform with respect to the Shuttle coordinates.

The electrical interface to the GLO experiment has the functional relationships shown in Figure 1. The scientifically unique parts of the GLO experiment are contained in the box identified as customer experiment and in subroutines in a fraction of the control software.

The mechanical interface is shown in Figure 2, the scan platform. The GLO sensor head, which contains spectrographs and imagers and weighs 22 kg, is contained within the dashed box. It is noteworthy that the GLO is a cluster of experiments with several scientific objectives involving Shuttle environment, Earth atmospheric science, and solar EUV flux monitoring. Many kinds of remote sensing experiments could replace the GLO sensor head on the scan platform economically and have their opportunity in space.

Fig. 2 Scan Platform with GLO Experiment

Considerable analyses, tests, reviews, and documentation efforts are needed to integrate an experiment into the Shuttle system for the first time. However, once this work has been done and the items are flown they move into a new category. For future flights they are classified as "reflown hardware." Minor changes are recorded but reflight is mostly a matter of updating the documentation. If, in fact, a piece of hardware is duplicated, such as the scan platform in Figure 2, the copy is classified as "series hardware." The documentation and analysis exists and only the qualifying vibration tests are repeated for flight acceptance.

In a case where the GLO sensor head was replaced by another experiment, the effort would be somewhat greater. The structural integrity of the new instrument would be analyzed. The math model for the scan platform would be modified to include the new experiment. A mechanical test series would be performed. As long as the experiment mass was less than the GLO sensor head mass of 22 kg, little difficulty should be encountered qualifying for flight.

Software and Experienced Personnel

A non-trivial part of a Shuttle interface is the software required. Every aspect of a flight program from conception through data distribution and analysis is software driven. Consequently, personnel who are familiar with the software are also a part of the interface. After dealing with the Shuttle system for some time, one realizes that the system is optimized to run the Shuttle, which is naturally NASA's main concern. It is an impressive facility but an interpreter is required if first-time visitors are to achieve optimum results.

An Experimenter Interface Facility

The experimenter interface to the Shuttle system should be comparable to the interface to a rocket. The rocket experimenter is not required to provide his own rocket subsystems that are common to repetitive flights. This principle needs to be the baseline for Shuttle experiments. It is my thesis that there is an interface team missing which is required to get modern instrumentation and experiments into space faster, better, cheaper. The components of that interface team are listed in Figure 3. Given this added interface team, an experimenter could expect days in space on the Shuttle as economically as minutes in space on a rocket.

Fig. 3

Although the Shuttles will be heavily involved in transporting ISS hardware into orbit, there will be many opportunities to include sidewall Hitchhiker experiments. When Shuttle payloads are being assembled, mass reserves are carried by several of the Shuttle systems. The decision of whether or not there is excess mass and space available could be decided as late as the Cargo Integration Review (CIR), 10 to 12 months before launch. If Hitchhiker experiments are available which do not impact Shuttle expendables and timeline, those experiments could be carried in a truly Hitchhiker approach.

EDUCATIONAL OPPORTUNITIES

One of the concerns in experimental research is giving the student the opportunity to develop his own experiment, gather the data, and analyze it. A Shuttle experiment provides unique experience for young experimenters. The research environment is current and indicative of future opportunities in global research. The student would acquire "hands on" experience with his experiment in the Payload Operations Command Center during the flight. He could receive and review his data. He could repeat his experiment if the data set was not complete. At the same time the student would acquire first-hand knowledge of the Shuttle, the international Space Station, and a real introduction to space engineering and research.

Last Updated: 15 October, 1996