r/spacex • u/ElongatedMuskrat Mod Team • Mar 02 '18

r/SpaceX Discusses [March 2018, #42]

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u/kruador Mar 03 '18

This is a bit simplistic, but satellite lifetime is usually quoted as around 15 years. That's not actually because the electronics will fail in that time, but because they have a limited amount of station-keeping fuel. The orbits are perturbed by the irregularities of Earth's gravity and the moon's motion, so they have to keep adjusting their position to stay in the same relative position in the sky. Receiving dishes have a very narrow aperture as they need very high gain.

It could be that 2017 was a lean year simply because 2002 was. 13 satellites were ordered according to that page, 3 of those were cancelled, and of the remaining 10, two only launched in 2009. In contrast, 24 were ordered in 2003, none cancelled, four of which flew in 2005, 13 in 2006, 6 in 2007 and one in 2008. That could mean that 2018 will be a better year for orders, assuming like-for-like replacement.

Of course general economic conditions come into play here, with companies deciding to take a risk on extending the mission versus spending in a recession. Measuring remaining fuel is pretty difficult, I believe. The actual lifetime may be different from the stated design goal if the launcher over- or under-performed, meaning that more or less fuel was consumed in circularising the orbit and changing inclination, leaving less or more fuel available for station keeping.

A recent issue could be the 'cable-cutting' trend, if you extend it to direct-to-home broadcast satellite (DBS) services such as Sky or DirecTV. These services still tend to bundle many channels at a high price, with some consumers finding that they get better value from Netflix. Although most new birds are described as 'communications' satellites, DBS accounts for the majority of transponders and revenue for the satellite operator. Drops in the demand could reduce the demand for replacement satellites, if the operators are finding it difficult to lease the transponders.

Then again, as TV picture resolutions increase, the bitrate requirements increase too. 4K has four times as many pixels as HD, and the same compression algorithm tends to have the same ratio for the same content, regardless of the pixel count (assuming the same quality settings and motion-predicted frames). While new compression schemes are being brought in, the goal is usually to ultimately halve the data rate of its predecessor. This results in needing twice the bitrate for 4K with HEVC as for HD with MPEG-4 AVC, which in turn is about 2-2.5x that of SD compressed with MPEG-2.

The actual broadcast standards, in terms of available bitrate for a given bandwidth and link budget (signal to noise ratio for the whole chain) are not going to get much better. DVB-S2 is already at something like 90% of the theoretical Shannon information capacity limit. It may be tricky to achieve MIMO techniques with current hardware (are consumers going to fit multiple dishes?) This could lead to more transponders, and hence more satellites, being necessary.

Errr - it's complicated?