Facebook, just over the horizon

Communication satellites are predominantly equatorial, orbiting the planet in a sinusoidal fashion over the tropics. Being right at the bottom of the planet therefore poses a difficulty is ‘seeing’ them, as they hidden by the curve of the earth for much of their trajectory. We have access to three different satellite constellations, each with different windows and bandwidths. NATO IV B, which is part of the NATO communication network is the first we see each day, followed by the more antiquated GOES satellite. Lastly, SPTR which is accessing the NASA TDRS constellation that is used to communicate with the International Space Station gives us their left over bandwidth, when not doing live video links or other work. It is via SPTR (‘spitter’) that we can have any hope of up or downloading photos, documents or anything more bandwidth intensive. There is hope to get access to a new satellite in the coming months, which is gradually decaying in its orbit as it ages and runs out of fuel to reposition. It is falling out of its path over the tropics, instead straying further down over the Southern Hemisphere, and becoming visible to us as it peeks over the horizon. The highest angle we can see it above the horizon is a miserly one degree, but that is enough to get a lock and a couple of extra hours of internet, to stay connected to the ‘real’ world. In fact, even when it is over the horizon, the atmosphere allows bending of the signal (’atmospheric ducting’) so a signal can be locked even if line of sight is not formed.

A day on earth (solar day) is measured by the time it takes us to go from having the sun in the highest position in the sky, to it being back to that position. That takes 24 hours. The earth however only takes 23 hours and 56 minutes to do one complete turn on its axis in relation to the background stars (sidereal day), but during that time, it has also orbited the sun by 1/365 of an orbit. So as we are orbiting in a circle, earth needs to turn a bit further to get to be looking back at the sun otherwise we’d be looking in the same direction as yesterday, which is now off to the right of the sun. This four minute difference of the sidereal day to the solar day over the course of the year equates to a further full turn. What that means for us on a day to day basis, is that as the satellites are locked in to a set position in the stars in the sky, we turn back to see them at that same spot every 23 hours 56 minutes. So out internet window gets 4 minutes earlier a day, or around 2 hours earlier a month. Now instead of getting internet from around 9 o’clock at night through the night when we first arrived, we get it around 4pm.

 

 

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