NavigationCommission: talkingfornothing

Sextants have guided sailors around the world for centuries and the basic idea of figuring out your location by finding the intersection of two known points is used from school-day maths, to military ‘landnav’, and in space as early as Apollo-when ground based navigation would have become a problem on the dark side of the moon. Apollo relied heavily on earth based guidance, but periodically they used a heavily modified sextant to both verify their position and as a critical backup in case of communication loss. The Apollo sextant used the horizons of the moon, earth and a set number of stars picked out for reference. What they used was a dual telescope system of a one power star finder and a 28 power precision cross-hair scope that determined angles for intersection. The main issues they had were determining stars from specks of ice and dealing with the refraction of earth’s atmosphere in finding her horizon. Using a gyroscope as a zero, several entries of numbers as a control, and the different angles the sightings came out to be as a standard-the outrageously under powered computers of the 1960’s were able to reliably find accurate position and speed.

Now given that our ship is in space we can skip over initial guidance (take off) and focus on mid course correction (steering) and inertial guidance (drifting)- leaving terminal guidance (landing) for another day. The gist of mid course correction are finding your position turning your boat and pushing yourself to where you want to be, hopefully with enough precision that you don’t have to keep making minor adjustments back to back and run out of fuel-that would suck. Keeping track of the stars by eye is one way to go about things and certainly a needed backup, but having an automated system that keeps making steady measurements not only give you a good idea of where you are-but a very good idea about where you will be in the very near future. Having this information at hand quickly can mean the difference between a cool fly by, a near miss, and going splat on something bigger than you. No for drifting in space “Basically, an inertial guidance system consists of three accelerometers mounted on a gyro-stabilized platform, and some form of computer” (www.hq.nasa.gov…) I really couldn’t think of a better way of putting it so there you go. Remember when I said a gyroscope is like a zero*? A gyroscope will only give you data on how much force is applied when it is being acted on. So if you were to have one right now it would show 1G towards the floor. But in space, most of the time, only the most minuscule forces are acting on your ship, like radiation or solar wind and every now and then a micrometeorite. A gyroscope navigation set with enough robust sophistication could go it alone as far as determining true position, but lets be honest-they are rather expensive & stuff breaks, so it’s better to double up.

I’ve tried dead reckoning on a landnav course, it never seems to work out well for me, what gets a lot of guys is they drift way off course from what point they were aiming for. Integration drift is kinda the same thing, small errors add up until the become really big ones that make bad things happen fast. Hence the need for regular adjustment, and a progress log to make sure nothing out of the norm goes unnoticed. At the very end of this process the data is more or less averaged together (much more Math involved obviously) and you get highly precise readings on your position and how fast you are moving, and from there you chart your course.

Sources, I paraphrase a lot

http://www.ion.org/museum/item_view.cfm?cid=6&scid=5&iid=293

http://www.hq.nasa.gov/pao/History/conghand/guidance.htm

http://en.wikipedia.org/wiki/Inertial_navigation_system

* my mentor was big on having zeros in experiment design, what it means is that you are testing for the effect of your experiment on your materials-his go to example was injections of saline, alcohol, and adrenaline: adrenaline is a stimulant & alcohol is a depressant so you could figure which between them would be the control based on what you were looking for; the difference between them would be your standard of measurement; but saline has no effect at all so it’s testing the effect of being injected on the subject, and when you subtract the effect of your actions on the experiment from the results-you gain a greater understanding of what’s going on inside the system.