\n<\/p>\n
Let\u2019s use our car again, but this time we\u2019ll get real numbers from the accelerometer in our smartphone. Say we start at a red light and then accelerate at 2 m\/s2<\/sup> (meters per second squared) for five seconds. From the equation above, \u0394v1<\/sub><\/strong> would be 2 x 5 = 10 m\/s, so that\u2019s our velocity. Now, after cruising for a while, we accelerate again at 1 m\/s2<\/sup> for five more seconds. \u0394v2<\/sub><\/strong> is then 1 x 5 = 5 m\/s. Adding these two changes, our velocity is now 15 m\/s. And so on.<\/p>\n The only problem is that inertial measurement isn\u2019t as accurate as the Doppler method over long periods, because small errors will keep accumulating. That means you need to recalibrate your system periodically using some other method.<\/p>\n On Earth, people have long navigated by the stars. In the northern hemisphere, just find Polaris. It\u2019s called the North Star because Earth\u2019s axis of rotation points right at it. That\u2019s why it appears stationary, while the other stars seem to revolve around it. If you point a finger at Polaris you\u2019ll be pointing north, and you can use that orientation to go in whatever direction you want.<\/p>\n Now, if you can measure the angle of Polaris above the horizon, you\u2019ll also know your latitude. If the angle is 30 degrees, you\u2019re at latitude 30 degrees. See, it\u2019s easy. And once you can measure position, you just need to do it twice and record the time interval to find your velocity.<\/p>\n But celestial navigation works because we know how the Earth rotates, and that doesn\u2019t help in a spacecraft. Oh well, can we just use the stars like you would use the cows on the side of the road? Nope. The stars are so far away, astronauts would need to travel for many, many generations to detect any shift in their position. Like the airplane flying over the sea, you\u2019d seem to be stationary, even while traveling 25,000 mph.<\/p>\n But we can still use the basic idea. For optical navigation in space, a spacecraft can locate other objects in the solar system. By knowing the precise location of these objects (which change over time) and where they appear relative to the viewer, it’s possible to triangulate a position. And again, by taking multiple position measurements over time, you can calculate a velocity.<\/p>\n In the end, even though spaceships lack speedometers, it\u2019s possible to track their speed indirectly with a little physics. But it\u2019s just another example of how flying in space is really, totally different\u2014and way more complicated\u2014than driving or flying on Earth.<\/p>\n<\/div>\nOptical Navigation<\/h2>\n