Selected Articles from the
February 2001 Odyssey

Editor: Terry Hancock

• NEAR Shoemaker Touches Down, Terry Hancock
• Annual Activities Report for 2000, Steve Bartlett
• A Matter of Attitude, Part 2, Robert Gounley
• The Surf Report, Diane Rhodes

A Matter of Attitude
Part 2

By Robert Gounley

"You've got to be very careful ``You've got to be very careful if you don't know where you're going, because you might not get there." - Lawrence Peter "Yogi" Berra

In our previous installment, we were aimlessly adrift in an Earth-orbiting spacecraft. Our solar panels aren't getting enough sunlight, and our batteries are nearly drained. Electronics are running too hot because other parts of the ship are getting too much sun. With our dish antenna pointing at Alpha Centauri rather than a communications satellite, the reception has been quite poor. Lacking any sense of the right direction to aim it, our rocket engine is worthless for getting us home.

It's time to take action! We must correct our spaceship's orientation - an ``attitude adjustment'' some might say. But first we need to know which way we're pointing right now.

As for Earthbound mariners, the Sun is our first point of reference. Looking through a window, we can measure the angle between the Sun and the direction our ship is currently pointed. Track the Sun's apparent movement and we can tell how fast we're spinning. Unfiltered sunlight being a bit harsh for human eyes, most spacecraft use a set of sun sensors, anchored to a surface, to do the job instead. These sensors are simply solar cells connected to a sensitive current meter. By knowing how much electricity they produce when pointed directly at the Sun, we can get a good estimate of the angle they make relative to the Sun. Measure the how their electrical signal fluctuates, and we can infer spin-rate.

This is a good beginning, but not enough to direct us everywhere. In space, we may know that we're looking at the Sun, but that alone is not enough to tell if an observer on the Earth would see us right-side-up, upside-down, sideways, or something in between. Therefore, spacecraft in Earth orbit frequently orient themselves with both the Earth and the Sun.

If our orbit is extremely high, Earth is a compact disc like the Sun, making it easy to get our bearings. Aiming for the center of Earth's disc, we find the nadir, the direction folks on Earth think of as ``down''. Ironically, a low orbit makes things a bit harder. Here the Earth so fills our view that the naked eye finds it difficult to judge where nadir actually is. To find it, we must look to the horizons where the sunlit atmosphere marks a hazy edge against black space. Pick a point on the horizon and measure the angle to the opposite side of Earth's disc. Halfway between is nadir.

This works well when Earth's disc is in daylight, but what if the view below is of night? With difficulty, our eyes might sense Earth by absence of the stars, but most spacecraft use a different approach. To sensors designed to measure infrared radiation, the Earth's atmosphere glows in daytime or night against the relative darkness of space. By detecting where the boundaries of this infrared glow, horizon sensors allow spacecraft to locate nadir anywhere in Earth orbit.

For our relatively unsophisticated spaceship, knowing where Sun and Earth are satisfies most needs. However many applications, such as an orbiting telescope observing distant galaxies, demand greater precision. Stars provide an answer. Like sailors sighting off of the constellations, many spacecraft use digital cameras to image a part of the sky and computers to chart direction from the star field. Unlike horizon sensors, star trackers work anywhere, even outside of Earth's orbit. However, cosmic rays or floating debris sometimes create false signals that can fool software designed to identify stars. For this reason, most spacecraft with star trackers also have sun or horizon sensors to quickly regain bearing after a glitch.

Everything said so far assumes our spaceship is relatively steady. Consider what happens when we fire our rocket engine. The violent rumbling makes tracking the Sun or any other object difficult and prone to error. Even our human sense of balance is overwhelmed. While we shake, a slight tilt in the rocket nozzle could well be spinning our spaceship about. Gimbals on the engine nozzle could be used to tilt it back to the correct direction, if only we could sense the change quickly enough.

For situations like this, we use the electromechanical equivalents of the human sense of balance. Gyroscopes, by their spinning, keep their orientation while everything else turns about them. Measure movements relative to these ``inertial sensors'' and we can guide even the most ill tempered rocket.

Now we have the basic tool to know where we're pointed. All we need do is to turn our spaceship about until we're in the correct direction. Tilting our rocket nozzle and firing the engine could bring us about, but not very efficiently. Worse, we're also giving our spaceship a push that changes our orbit. That new orbit may not be to our liking.

There are alternatives. We'll discuss them in our next installment.

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