This week’s foray into the Technician pool is about satellites, specifically….
Which of the following are inputs to a satellite tracking program?
A. The weight of the satellite
B. The Keplerian elements
C. The last observed time of zero Doppler shift
D. All of these answers are correct
Well, we should probably eliminate the ones that don’t make any sense, and why they don’t.
A. The weight of the satellite, is incorrect, because is zero gravity, weight is irrelevant. Mass, on the other hand, counts, but really doesn’t concern us here.
By default now, D. All of these answers are correct. is also tossed out.
C. The last observed time of zero Doppler shift really is a meaningless statement. “Zero Doppler shift” only occurs when the satellite is neither moving away from or towards an observer. Its the exact moment when the shift changes. This is different for every observer, obviously.
The correct answer, therefore, is B. The Keplerian Elements.
Oh, yeah, what the heck are “The Keplerian Elements?” you are probably wondering about now….
Well, the short answer is that they’re just a set of numbers that have already been calculated for a given satellite, whether that’s the moon, the ISS, your TV satellite, or whatever. Somebody has crunched these values, and when you feed them into a satellite tracking program, it will calculate the orbit of the satellite.
The long answer? Well, back in the day, Johannes Kepler came up with the math for tracking the moon and the planets. He discovered that the orbits of these satellites could be accurately predicted by seven or eight variables.
I’m not even going to dive into explaining these variables here. It would require a lot of quite technical reading. If you’re really interested, check out AmSat’s explanation here: http://www.amsat.org/amsat/keps/kepmodel.html