How do spacecraft measure onboard (micro)gravity at any given point in time (especially when subject to the gravitational fields of multiple bodies)? I'm guessing that rudimentary accelerometers won't be sufficient.
It's fundamentally impossible to measure gravity from an object that is in freefall. This is the first principle of general relativity.
What accelerometers will give you is
- accelerations (linear or rotational) induced by thrusters, atmosperic drag, reaction wheels, etc.
- vibrations from rotating solar panels, crew-induced forces, etc.
The only thing you can measure from a body in freefall is inhomogeneities in the gravity field, or in other words, the gravity gradient. You can arrange a collection of accelerometers such that it becomes a gravity gradiometer. A gradiometer does not measure the gravity itself, but the change in the gravitational field as the gradiometer moves along it.
The best way to measure gravity from a spacecraft is not by using an instrument on a spacecraft, but using a pair of instruments on two spacecraft. The best example of this is Grail spacecraft. What they did in essence was to orbit around the Moon such that they had a more or less constant distance between the two. As an area of higher gravity passed, then the spacecraft would actually drop a bit. The two spacecraft were able to very accurately detect the range between the two spacecraft, and if the spacecraft went in a particular direction. Using some complex math, they were able to figure out the gravity map of the Moon.
Alternatively, this can be done using an Earth based radar type system, where the frequency of the spacecraft is very carefully monitored for Doppler shift, searching for small changes in the orbit caused by differing gravitational fields. This is complex, but has been done. The prime example of this was Magellan, but I believe it has been done by other spacecraft, and this Doppler effect will also be used to determine Jupiter's gravity field by gravity perturbed orbit of Juno spacecraft as it starts orbiting it in 2016.
As far as I can tell, we don't actually have reasonably-sized sensors capable of precisely determining microgravity, and that our official gravitational values are instead calculated from other information. With enough information about the masses of and distances to nearby objects, and the acceleration of a given spacecraft, we can pretty accurately calculate the gravitational force on the spacecraft without need for sensors.
For example, the International Space Station has two systems of accelerometers (SAMS-II and MAMS) used to detect small vibrations on the frame of the ISS, but neither of them are actually sensitive enough to measure the net effective gravitational force on the ISS.
You can determine the microgravity by observing two or more free-floating objects and measuring their motion towards or away from each other. This is the bowling ball method. How can you tell if you are in free space, or falling down a long elevator shaft? Watch a pair of "floating" bowling balls. If they are moving towards each other faster than their own gravitation should produce, then you are falling into a gravity well. There is always a pseudo microgravity in orbits because parts of the craft closer to the planet are trying to follow a faster orbit than the parts that are further away. It is often called a tidal effect.
The late Dr. Robert Forward of Hughes Research designed "space-time flatteners" for shuttle experiments. These are arrangements of tungsten disks and spheres and toruses that counteract the tidal effects and gave very good approximations of being in free space far from any significant mass, albeit in a small areas in the middle of the apparatus. He once lamented that he could not patent them because he had used them extensively (on a much larger scale) in his science fiction.