So then the difference really is just in the GSO protection settings I guess.
The GSO protection is - to an extent - also patch protection. After all - who'd need satellite TV if everyone could watch the same TV via LEOs? But consider that:
- Interference to GEO sats from Starlink & Co. is out of band. I'm not sure what the out-of-band emissions profiles of dishys are, but I'd imagine we'd be looking at the usual few dozen dB below peak.
- Distance to GSO is around 64 times (2^6) larger than to Starlink's orbits - GEO sats see around 1/4000th ((2^6)^-2) of the power from a ground station that the Starlink satellite in front of it sees. That's 36 dB in extra separation.
- Dishy is comparatively small in cross-section, and that severely limits its gain. Most serious GEO uplink applications that I'm aware of use dishes more like 8 times dishy's cross section. That's another 9 dB or so in separation between a Starlink dishy signal and signal directed specifically at a GEO sat just from the overall size. Uplinks from gateways are likely to be much more of an issue (always on and probably higher power as well as ~6 dB higher antenna gain judging from the photos I've seen), but then again these point away from the GSO when serving birds further north.
- My understanding (correct me if you think I'm wrong here): It's also possible that Dishy's nature as a phased array helps here. Why? Out-of-band interference results from intermodulation and other unwanted emissions from the power amplifiers (PA) of transmitters. Where these are fed into a dish from a single PA, their amplitudes get amplified by the gain of the dish. Now say we're trying to replace that dish by an array with N antennas and N associated PAs that feed at the appropriate phase. Then each element (individual PA with associated antenna) needs to contribute P/N of the total transmit power P of the big PA & dish. Now remember that power is proportional to the square of the amplitude. As long as the wanted signal components from the PA are correlated - and they have to be for the phased array to work - their N amplitudes add up, meaning the total output power of the wanted signal across all elements is proportional to N^2. So each element only needs to contribute an amplitude proportional to 1/N in order to produce the wanted output at the correct power. However, now each PA produces its own dirt signal. But unlike the wanted signal, these unwanted signals aren't necessarily all correlated between the elements. As a result, the amplitudes of the unwanted signals from the PAs will partially cancel out as the signal combines into the beam that is being formed. So only the powers of these uncorrelated unwanted signals add up, but their amplitudes don't, meaning we now have an unwanted signal power that is proportional only to N rather than N^2, resulting in a higher ratio of wanted signal to unwanted signal. Again that'd help a lot with separation in an array with lots of elements and associated PAs. That said, published research into the unwanted emissions of phased arrays is still a bit in its infancy, and I can't profess to understand enough about Dishy's innards or the level of correlation between unwanted emissions in the array, but I'd consider it possible that this allows for some relaxation on the GSO protection parameters when it comes to Starlink.
On 26/07/2022 9:06 am, Mike Puchol via Starlink wrote:Your calculations are “Back of the Envelope Approved” :-)
My simulator can be found at https://starlink.sx and runs on your browser (desktop only). I have just released v1.10.6, which adds a slider for setting the GSO protection. This is the difference in Sweden from 18º (as used until recently) versus 10º (as found in recent ITU filings for STEAM-1B):
<18_vs_10_GSO.png>
Minimum elevation in both cases is 25º, so that variable is unchanged.
Best,
MikeOn Jul 25, 2022, 11:35 +0300, Ulrich Speidel via Starlink <starlink@lists.bufferbloat.net>, wrote:
I haven't got Mike's nice software but I got snail mail today, so let's try the back of one of the envelopes.
53 degrees north (where the existing constellation tops out) is roughly the latitude of Nottingham or Stoke-on-Trent. Note that this is also the latitude with the largest number of birds per km (or mile, if you so prefer) of parallel. They sit almost cheek to jowl there.
Now the northern tip of the UK's main islands is at around 59 degrees north - that's six degrees more. One degree of latitude is roughly equivalent to 10,000 km / 90 degrees = 111 km, so we're talking around a devilish 666 km to the north of Nottingham here as the crow flies.
Lets assume Dishy points itself due south at 59 degrees. The birds are at about 550 km. So doing a flat earth approximation for the moment, we're having to point at a satellite 550 km up from 660 km away - that gives an elevation of about 40 degrees (=arctan(550/660)). Ballpark. Now reduce that by the 6 degrees of difference to account for the curvature of the earth and we still have around 34 degrees, allowing for a bit of leeway either side if the closest bird isn't actually due south but a bit to the east or west. Again, ballpark, but we're nowhere near 25 degrees yet.
So I'd say that should cover it nicely? Where does my envelope err? GEO arc protection aside for the moment, of course. And of course just because Starlink offers you service doesn't mean that it's actually continuous.
On 25/07/2022 8:59 am, Dave Taht via Starlink wrote:For those of you that don't follow mike's twitter feed...
https://twitter.com/mikepuchol/status/1551288485713149952
--
FQ World Domination pending: https://blog.cerowrt.org/post/state_of_fq_codel/
Dave Täht CEO, TekLibre, LLC
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