Deep space probe communication problems:

Question

Back in 1989 (I think!) I watched the "live from Uranus" {though it could have been "live from Neptune"; it's been so long ago I can't remember}.

I am assuming that the trajectory has already been programmed for most of the trip (planning for the 'sling shot' effect of the gravitational pull of Jupiter and Saturn).

Course corrections were made, and I think that the probe missed it's target by < 90 miles (after making a voyage of about 12 years). How is this done?

Answer 1

How is it done?  Very carefully!

I&#039;m only half kidding.  The individual procedures are fairly simple, but must be carried out with tremendous precision.

A trajectory per se isn&#039;t programmed.  A trajectory just occurs as the natural consequence of momentum and gravity.  What is programmed are the engine maneuvers, and they don&#039;t have to be programmed necessarily ahead of time.

To fly a spacecraft to a destination, you first have to know where it is.  That&#039;s the notion of a &quot;state vector,&quot; the spacecraft&#039;s position and velocity in three dimensions.  Those are the product of the cosmic gravitational ballet, and engine burns.  Engine maneuvers register on the spacecraft&#039;s accelerometers, but speed changes due to gravity are not.

Rather than try too hard to deduce the spacecraft&#039;s location that way, it&#039;s often better simply to measure it.  That&#039;s done by listening carefully to its radio signal.  A combination of active and passive radio techniques allows the spacecraft&#039;s velocity along a line between it and the Earth to be very precisely measured (i.e., fractions of meters per second).  A sequence of several such observations fit only one orbit (trajectory).

Armed with the spacecraft&#039;s position and velocity, flight dynamics officers compare it with the values that would be true along the spacecraft&#039;s desired course.  That is, they find the difference (&quot;error&quot;) between the precomputed path and the actual path.

When that error becomes too great, the spacecraft must fire its motors to correct the trajectory.  That may mean speeding up, slowing down, or nudging left, right, up, or down.  This works because nudges early in the trajectory can have large consequences at the end.

So to accomplish a midcourse correct, the spacecraft has to change its velocity in a certain specific direction, computed by flight dynamicists to be the one that corrects its specific error.  Since most spacecraft motors are fixed to the frame and can&#039;t be steered themselves, you have to point the entire spacecraft in the right direction.  Small steering jets or electric gyrodynes turn the spacecraft.

But that means you have to be able to measure the spacecraft&#039;s orientation and changes in it.  That requires another component of the guidance system, the guidance platform stable member.  This is usually a gyroscopically stabilized chunk of metal that is left free o rotate within gimbals.  Or more accurately, the spacecraft rotates around it without it changing attitude.  It becomes a fixed reference against which rotation can be measured.

The platform is calibrated to star sightings, employing special purpose star-sensors aligned to the platform, so that when the spacecraft is &quot;homed&quot; to its prime orientation, the stars should appear in the star sensors.  Any drift can be measured and applied to a corrective computation in the alignment program.

The engine burns are usually accomplished in closed-loop fashion.  This means that a certain required acceleration is precomputed.  Then the engine is fired up, and operates until the desired acceleration is reached, then it shuts down.  Onboard accelerometers in the guidance platform (sometimes on the stable member itself) register the engine burn in three dimensions.

A high-precision guidance platform can measure alignments to a hundredth or thousandth of a degree, and linear accelerations to hundredths of a meter per second.  This technology has evolved since the late 1950s when it was developed for ICBMs.  It is quite mature, and even being replaced in many cases by solid-state, non-moving equivalents!