What kind of instruments does a rocket use to track its trajectory and velocity during ascent? Do they use pitot tubes or barometric sensors? What exactly are the instruments used?
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$\begingroup$ The PIGA is still the bomb. $\endgroup$– Phil SweetOct 15, 2020 at 1:34
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$\begingroup$ @PhilSweet in all seriousness, how does PIGA's accuracy, precision, and operational robustness compare with a MEMS implementation of accelerometer? $\endgroup$– Carl WitthoftOct 15, 2020 at 11:57
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2 Answers
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First things first: Many launch vehicles initially operate open loop, with guidance and control based solely on time since ignition until main engine cutoff. Precise guidance, navigation, and control becomes much easier when the launch vehicle has left the bulk of the atmosphere behind.
A wide variety of sensors exist for those vehicles that instead use closed loop guidance starting from launch. At a minimum, all modern launch vehicles have inertial measurement units (IMUs) that sense non-gravitational acceleration and sense change in orientation, and GPS receivers (or the equivalent for non-US rockets) that tell the rocket's navigation system where it is. Some rockets have angle of attack sensors, but this is not uniform. Some use pressure sensors so as to address issues related to maximum dynamic pressure, but once again, that is not uniform.
answered Oct 14, 2020 at 7:13
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1$\begingroup$ Pretty sure the equivalent to GPS for non-US vehicles is generally GPS. $\endgroup$– user21103Oct 14, 2020 at 11:36
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5$\begingroup$ @tfb That's perhaps not true. The country launching rockets with their own navigation satellites is likely to use their own satellites for launching rockets. $\endgroup$ Oct 14, 2020 at 12:03
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6$\begingroup$ @tfb It is highly illegal for US-based companies to sell GPS receivers that work above 30 km in altitude or that exceed 950 meters per second. The laws threaten to provide the CEO of any such company that does so with years and years of government funded room and board. The US government has convinced many of its allies adopt similar laws. $\endgroup$ Oct 14, 2020 at 12:22
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1$\begingroup$ @DavidHammen: yes, that's a good point and I should have known that. But what do people like ESA use if not GPS? They can probably use Galileo now, but they could not ten years ago. It would be ironic if the Russians were happy to sell them usable receivers for GLONASS. I kind of assumed that people like this came to special arrangements, but perhaps not. $\endgroup$– user21103Oct 14, 2020 at 13:02
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4$\begingroup$ @DavidHammen It isn't illegal to sell that stuff, only to sell it without a permit. $\endgroup$– hobbsOct 15, 2020 at 1:09
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Ground control of the rocket should know the speed. A proven method used since the V-2 of WWII uses the doppler frequency shift of radio signals. A signal with known and very stable constant frequency is send from ground to the rocket. The rocket sends a signal back derived from the received signal, for instance the doubled frequency ( or another constant factor).
The ground control compares the frequencies of the send and received signals to determine the speed of the rocket very precisely. A small change of the ground frequency is compensated by sending the signal in both directions.
If the rocket and ground control both use ultra stable atomic clocks, the rocket may determine its own speed using the frequency difference between the received and the built in clocks.
Rockets were launched decades before GPS, so a method without GPS was needed. When the first GPS satellites were launched, GPS could not be used by those rockets.
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$\begingroup$ Good point; do you have some references which verify that, e.g., NASA ground control in fact uses this methodology? $\endgroup$ Oct 15, 2020 at 11:59
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1$\begingroup$ @CarlWitthoft recent NASA launch vehicles (since 1980) do not use this Doppler method for ascent. $\endgroup$ Oct 15, 2020 at 12:45
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$\begingroup$ @OrganicMarble So the Doppler Method was used for all NASA launchers before the Shuttle? Mercury, Gemini and Apollo? $\endgroup$– UweOct 15, 2020 at 13:36
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$\begingroup$ @Uwe don't remember saying that. I'm not interested enough to look it up, but my impression is that is a really old method. Maybe some of the first US ICBMs used it. $\endgroup$ Oct 15, 2020 at 13:41