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Rockets contain accelerometers and gyroscopes to measure changes in velocity and angular rates.

Gyroscopes detect changes of the orientation in space (=attitude). You can integrate these changes to get the cumulative change in attitude and given one knows the initial attitude, the attitude can be estimated. But, gyros do not measure attitude. Bias and noise will accumulate and thus after some time the attitude estimate won't be accurate at all.

If you are not accelerating (i.e. standing on the ground), accelerometers can be used as inclinometers to determine the vector of the gravitational force, or less complicated, the downward direction. Quite often gyros and accelerometers are thus used in combination (via sensor fusion, e.g. complementary filter) to determine attitude.

But: In free fall (e.g. in a rocket after booster burnout), the accelerometer cannot be used to determine the downward direction. The accelerometer would measure an accerleration opposing the drag force. Thus, the accelerometer can't be used to determine absolute attitude in free fall. (More general it can't determine the vector of the gravitational force if the acceleration is not known).

Once in orbit, sun sensors and star trackers are used but I don't think they would work during ascent or descent.

For rockets at launch and landing (think Falcon 9) I suppose it is quite important to estimate attitude. To counter error buildup of gyros some absolute attitude sensor would be needed.

So, my question is: How do NASA or SpaceX or even missiles measure absolute attitude during launch or landing? Which sensors are being used?

I'd be glad about any responses.

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    $\begingroup$ The drift rate of high quality inertial navigation gyroscopes is very small, typically less than 0.01° per hour. Thus it doesn't seem like much of a problem during launch, though the high accelerations probably increase disturbances. $\endgroup$
    – jpa
    Sep 8, 2022 at 7:29
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    $\begingroup$ This is a good question but will get few responses for modern vehicles. The problem is that the technology used for launch and landing can also be used by other countries to make their missiles more accurate. In addition, SpaceX is very protective of their intellectual property. The combination of possibly going to jail and possibly losing ones life savings are enough to prevent people who might know the answer from providing an answer. $\endgroup$
    – David Hammen
    Sep 8, 2022 at 9:11
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    $\begingroup$ GPS is a thing. Other options are radar, lidar, and optical (camera) tracking systems - either onboard or at a tracking station. $\endgroup$
    – Wyck
    Sep 8, 2022 at 14:51
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    $\begingroup$ Non-MEMS gyroscopes are really quite accurate nowadays. For example aerospace.honeywell.com/us/en/products-and-services/product/… has 0.004 deg/hour and 22 G acceleration limit. The gyroscope on Gravity Probe B measured 10^-7 degrees per year in 0 G environment. $\endgroup$
    – jpa
    Sep 8, 2022 at 17:23
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    $\begingroup$ Lidar can scan for features on the ground and match to known features of the topography -- same with camera systems. (I thought you asked about absolute altitude -- oops, my bad, I misread) Gyros and accelerometers measure relative orientation. Magnetometers can give an absolute orientation, provided you have mapped good baseline field data but it's fussy. Model rocketry uses a secret sauce hybrid of absolute and relative sensors, mag, accel, grav, and gyros. It's usually acceptable use a gyro provided you don't do acrobatics. $\endgroup$
    – Wyck
    Sep 8, 2022 at 17:28

3 Answers 3

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If you want a historical answer, the space shuttle used its Inertial Measurement Units (IMUs) and Rate Gyro Assemblies (RGAs) to determine attitude during ascent.

The function of the attitude processor is to derive attitude related data for several user principal functions. ...

Attitude Processor is structured to derive the vehicle attitude quaternion, using a selected IMU, at a low rate, 1.04 Hz, and to propagate the attitude by integrating a quaternion differential equation, driven by selected, prefiltered RGA outputs at a high rate, 12.5 Hz. There are some users, namely flight control, that require the attitude quaternion at 12.5 Hz, so that is why the RGAs (SRB RGA's) are used to propagate the attitude quaternion.

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Source: Shuttle/JSC GNC ASC 2102 Ascent Guidance and Navigation and Flight Control Workbook Chapter 3.3 ASCENT FLIGHT CONTROL

The IMUs were "stable platforms" described as follows:

There are three IMUs on the orbiter. Each contains three accelerometers and two two-axis gyros mounted on an inertially stabilized four-gimbal platform. The IMUs provide inertial attitude and velocity data to the GNC software functions.

The Rate Gyro Assemblies are described as follows:

The orbiter has four RGAs. Each RGA contains three identical single-degree-of-freedom rate gyros so that each gyro senses rotation about one of the vehicle axes. Thus, each RGA includes one gyro sensing roll rate (about the X axis), one gyro sensing pitch rate (about the Y axis), and one gyro sensing yaw rate (about the Z axis). These rates are the primary feedback to the FCS during ascent, entry, insertion, and deorbit.

Additional RGAs were mounted in the Solid Rocket Booster forward skirts.

These last two quotes are from the Shuttle Crew Operations Manual

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    $\begingroup$ Thanks for your answer. Do you have any idea about more recent rockets, like SpaceX Falcon 9? I think, landing would require quite accurate atttitude. $\endgroup$
    – user50224
    Sep 7, 2022 at 17:52
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    $\begingroup$ Most (all?) recent vehicles do not use the Apollo era / Shuttle era concept of an inertially stabilized platform. In addition, there are rate gyros that much, much smaller, much much cheaper, and much less noisy than the Shuttle-era rate gyros. $\endgroup$
    – David Hammen
    Sep 8, 2022 at 8:59
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Another good @OrganicMarble answer to this question is What sensors or combination of sensors do rockets use during takeoff for their orientation?

Adding my own experience with other systems, star trackers are a lot more useful than you give them credit. They were already quite good 30 years ago, and they keep getting better all the time. One reason that even old ones were very helpful is in many applications, you don't actually need to know attitude to better than the gyros can do during the first stage of launch. Once you're in space and stop vibrating so much, you can look around, and figure out where you actually are, and are pointing. When you compare that to your Inertial Navigation System (INS)'s filter estimate, you can solve for the biases in the Inertial Measurement Unit (IMU), and use that to improve INS performance for the rest of the flight, as well as retroactively re-estimate where exactly you were before then. The job of the first stage is to get you close to your desired orbit, not into it exactly --- that's what stage two is for, to make a small course correction from wherever you ended up to wherever you wanted to be. The amount of wiggle room depends on your mission requirements, which drives how good your IMU and other sensors need to be, and thus how much they cost (and weigh!).

On landing, since you're slowing down and getting closer to the surface, you can use traditional airplane sensors like radar altimeters, radio navigation beacons, airspeed and pressure indicators, etc. These generally work better for landing trajectories than they do for launch trajectories.

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  • $\begingroup$ Thanks a lot for your answer. So, for launch, the IMU is sufficient until you get into orbit. Do you have any idea about landing? (like Falcon 9) $\endgroup$
    – user50224
    Sep 7, 2022 at 17:50
  • $\begingroup$ Thanks! I knew it sounded familiar! $\endgroup$
    – Organic Marble
    Sep 7, 2022 at 18:05
  • $\begingroup$ I couldn't find the meaning of INS by Googling. (I did find that IMU is inertial measurement unit.) $\endgroup$
    – WaterMolecule
    Sep 8, 2022 at 16:52
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    $\begingroup$ @WaterMolecule oh, sorry! Inertial Navigation System -- the combination of the IMU sensors with the software that interprets the data. $\endgroup$
    – Ryan C
    Sep 8, 2022 at 17:32
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But: In free fall (e.g. in a rocket after booster burnout), the accelerometer cannot be used to determine the downward direction. The accelerometer would measure an accerleration opposing the drag force. Thus, the accelerometer can't be used to determine absolute attitude in free fall. (More general it can't determine the vector of the gravitational force if the acceleration is not known).

Even before burnout, you can't simply integrate acceleration to determine the trajectory. What an accelerometer tells you is the rate of change of your trajectory relative to a free fall trajectory. So, the machinery keeps track of what the free fall trajectory is and adjusts according to the accelerometer readings. In vacuum after burnout, the rocket will continue to follow a free fall trajectory. So, you may determine the altitude.

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    $\begingroup$ Sorry for not being clear. I meant attitude = orientation in space, not altitude. $\endgroup$
    – user50224
    Sep 8, 2022 at 20:07

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