[AusNOG] NBN Satellite Latency

[Ross Wheeler]

On Fri, 3 Feb 2017, Jonathan Brewer wrote:

> On 3 February 2017 at 18:39, Paul Wilkins <paulwilkins369 at gmail.com> wrote:
>
>> Which is 460ms, almost double what you pay Einstein (240ms). Wonder where
>> it all goes? They would have to be using TDM, because only a time slot
>> delay is going to make that sort of difference. 700ms is pretty awful.
>>
>
> 460 is a bit light for what you pay for your speed of light journey, not
> double.
>
> Your request goes up 35,786 km, and that takes... 119.3ms
> It comes down from the satellite to the earth station, 119.3ms
> If all goes perfectly (you land at your content) you spend 1ms on the
> ground.
> Your reply goes up to the satellite, 119.3ms
> Your reply comes down from the satellite, 119.3ms
>
> Einstein says the best you're going to do is 478 ms.

Yeah, but as Geoff hinted - it's not even that simple.
Geostationary orbit is basically 26,200 miles from the earths centre 
(22,230 odd from sea level) - so 22,230/186282 = 119.3ms

BUT - that's if the bird were directly overhead...

If the sat is at say, 45 degrees elevation (and at the same latitude, 
which it probably isn't), then that path just became 31,438 miles = 169ms 
each way. The closer to the horizon, the longer the path and the higher 
the travel time...  and the further from the equator you are, the greater 
the distance too.

R.