Le 12/11/2023 à 00:47, Ulrich Speidel via Starlink a écrit :
On 11/11/2023 6:09 pm, Alexandre Petrescu via Starlink wrote:
I want to say that I think this hexagon is an imaginative idea of the
GUI designer. I think it does not correspond to reality. I am not sure
about even the most basic fact such as the dimension of the hexagon, or
of a more circular 'spot' radius.
Well it's the basis on which SpaceX will sell you a fixed subscription
or not, so it's a bit more than just a GUI designer's fancy.
Thanks, it is good to know.
The satellitemap.space site displays relatively large hexagons, but says
they are not affiliated with starlink.
The availabaility map published by starlink.com shows hexagons much
smaller in size.
It was the case like that with earlier maps of cellular network
deployments. The name 'cell' itself that comes from it - it's a
hexagon, like in a honey pot. In practice, no cellular network
deployment I am aware of has cells of that kind of precise shape. The
base stations themselves are not following such precise patterns. The
precise forms of coverage shapes can not be given by operators because
it is unique, difficult to calculate, and depends on many landscape
factors and other propagation conditions. What can be given is the type
of antennas, their precise placement and orientation. That is public
info of cellular systems in some countries.
Yes, but I wouldn't think of the Starlink hexagons as "cells" in the
cellphone sense. In the cellphone sense, you'd have a base station per
cell - and some seem to think that Starlink has something like a "spot
beam per cell". But that's clearly not what it is.
YEs, I fully agree.
Starlink cells are quite obviously predominantly a tool to control
user density on the ground.
We know well that Dishy orients itself to the area of the sky where it
can expect to see the largest number of satellites without falling
foul of GSO protection rules where applicable. Dishy then associates
with one of them at a time for period that are multiples of 15 second
intervals (we know that from the obstruction maps available from Dishy
via grpc). We also know that the capacity we get via these
associations fluctuates along with the 15 second intervals.
These are all hallmarks of a burst slot based system where each
satellite handles a set of time and frequency slots (TDM+FDM), such
that a combination of a periodic time slot, a frequency channel and
perhaps a number of other parameters (spreading code, polarisation,
spatial multiplexing through beamforming at the satellite) defines a
channel through which Dishy talks to the satellite or receives from
the satellite. This sort of technology has been around for decades
(see GSM mobile comms). During your slots, the satellite your Dishy is
associated with will project a beam towards you.
Fluctuations in the capacity are a result of a Dishy having more or
fewer of these slots assigned during subsequent 15 second intervals.
If a satellite picks up more users for the next interval, the number
of slots it can make available to your Dishy goes down. If it sheds
users as it moves along but you hang on, then you get a few more
slots, until either the satellite picks up more user or you change
satellite.
For a scheme like this to work, you need to ensure that the number of
slots that the visible satellites can offer to their users on the
ground works out to an acceptable average minimum number of slots per
user at all times. As the number of slots per satellite is likely
fixed (at least for the same generation satellite), that puts a limit
on the number of users within view on the ground. This is where the
hexagons come in - they help ensure that the user density doesn't grow
to a level anywhere where a Dishy would struggle to get enough slots.
Sounds as 'beamforming'. Could be.
I'd presume that the size of the hexagons was chosen to reflect the
ability of the beamformers on the satellites to resolve a locality on
the ground (no need to go for higher resolution then).
It can make sense. Whether it is true or not is another matter.
For my part, I think there is a need for a description of that, which
comes from starlink. It should tell the sat antenna numbers, positions,
aperture, orientation.
I'd also presume that the number of fixed users allowed per hexagon
would depend a bit on geographical latitude, visible satellite density
and load contributions from a location's surroundings (Colorado
farmers would likely see more of those from their neighbours down the
road than folks on Rapa Nui). Roaming subscribers aren't guaranteed
the same service levels (read: their number of slots is allowed to dip
further than those of fixed subscribers), but as they can't be told
where to be, SpaceX uses pricing to control user density indirectly.
With starlink antennas there is no authoritative public info (from
starlink) about the precise orientation and types of antennas of the
sats. The reported positions of sats are rather irregular - much more
irregular than that precise shape of hexagons shown in the photo.
Yes - see above.
The placement of teleports is also unknown, but speculated by end users.
In some cases, their locations are precisely known from their spectrum
licenses.
That is a question.
For where I live, the spectrum licenses could come from several sources:
2 country authorities, 1 Europe authority and ITU.
I am trying to look at them, but it is not easy because they differ in
some respects.
The info about satellite tracking, their precise situation: I still need
to find out where that info comes from more precisely and how is it
obtain (is it reported by sats or is it other cameras/radars that
'range' each one of them, or is it simply speculated from an initial
plan of trajectory; and what is the delay between the actual fact and
what is seen on GUI: seconds, minutes or hours delays).
https://celestrak.org/NORAD/elements/index.php
It has a Donate button, which seems to make it a site built by an
independent end user. It is a great tool already! With respect to the
starlink sats positions: celestrak might simply reflect an original data
which is made by space-track.org which is a US authority. Or maybe not,
they dont say it. I dont know.
My question is how is the original data on space-track.org (or other
original source of sat position data) created: do they range the sats
(i.e. point lasers at them and wait for replies, radar, or similar) or
do the sats transmit their positions on a voluntary and cooperative basis?
Another question is about which starlink sats are 'in-service' and
deliver service, and which not? It is not only a matter of altitude.
The current websites telling 'in-service' or similar attributes, do not
seem to be related to starlink, and do not seem to take that data from
DISHYs. They seem to be simply telling that if it is at a 550km
altitude then they're in service.
There is also a question of latency between what is shown on a computer
screen about the position of a sat, and where it actually is. For
example, if I see it now on my screen to be above my head, will I see it
if I look up?
Alex
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