Q&A: How Satellites Point Themselves

Question: How is it that satellites can point themselves in specific directions without using any fuel? I mean, a finite supply of fuel would limit their service life, and the exhaust would probably mess with their sensors. Is there a way to point satellites using only electricity from onboard solar panels? — JT, Oklahoma City, OK

Answer: That’s exactly how they do it, JT. And they key to making it work is the gyroscopic effect. The video shows the remarkable stability of a spinning gyroscope. Once the gyroscope is set spinning, it resists any attempts to change the direction of its axis of rotation. The effect is especially visible in the weightless conditions aboard the ISS.

Of course, you can cause the gyroscope to tilt its axis of rotation (as is shown in the video), but that requires a force be applied to the gyroscope. Because of Newton’s 3rd Law of Motion, the gyroscope then exerts an equal and opposite force back against your finger:

When the gyro is mounted in a gimbal, that reaction force can be measured by sensors and used to calculate changes in the direction of the gimbal relative to the unmoving rotation axis of the gyro. There are other ways to measure directions changes, such as optically or magnetically, but for my explanation I’ll stick with force measurement — usually done with strain gauges or piezoelectric transducers.

That’s the way it’s done on the Hubble Space Telescope (HST). I’ll be using that satellite for my explanation, since NASA provides some nice graphics showing how the whole pointing system works. I’ve annotated it a bit to provide more info. It’s functionally similar to the systems all pointing satellites use. Here’s a simple block diagram of the HST pointing system:

When HST needs to be re-pointed, a command from mission control initiates a series of events. That command includes the coordinates to which HST needs to point. Here’s what happens next:

  • The fine guidance sensors verify HST’s current orientation by sensing the coordinates of the Sun and a few key stars, and Earth’s magnetic field. Then it calculates how much rotation is needed to point at the new coordinates.
  • HST then activates one or more of the four reaction wheels to initiate the rotation. These are gyroscopes themselves, but fixed in orientation relative to the telescope. As they accelerate to the calculated speeds, the HST itself begins to rotate in response, much like the classic bicycle wheel demo you saw in physics class. The school-bus-sized HST doesn’t rotate very rapidly though. Typical speeds are only around 6°/min — about as fast as the minute hand on a clock.
  • When HST approaches the new coordinates the reaction wheels begin to spin down with a calculated deceleration that will leave it very close to the new coordinates.
  • Based on data from the gyros and fine guidance sensors, HST calculates any small tweaks needed to achieve perfect pointing.
  • Those small tweaks can be done using the reaction wheels again, but are often done using the four magnetic torquers mounted every 90° around the hull. These are basically 8-foot-long iron rods wrapped with coils of wire (electromagnets) that when activated interact with Earth’s magnetic field. This produces a gentle torque to fine-tune the pointing. When all is said and done, NASA claims HST is accurate enough to “hold a laser beam on a human hair one mile away.” That translates to 7/1000 arcsecond!

You’ve probably heard that HST is currently running on only 3 of its original 6 gyros, 3 of which were backups. Over all its servicing missions it’s burned through some 22 more. The failures are due to the gyros’ mechanical nature with moving parts and flexible wires to deliver power and extract data. One of the remaining 3 gyros is having intermittent problems, and though they figured out how two run on only 2 gyros (using the fine guidance sensor to compensate) HST cannot point properly with a single gyro. It’s days are numbered even if everything else keeps working fine.

That’s why newer space telescopes like JWST use a hemispherical resonator gyroscope (HRG) with no moving parts. They have an accuracy better than mechanical gyroscopes, and are also smaller and lighter and use less power. Unfortunately HRG technology didn’t exist at the time HST was being built.

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