[Starlink] Dishy GRPC obstruction maps
Ulrich Speidel
u.speidel at auckland.ac.nz
Wed Jan 24 20:27:18 EST 2024
I've been playing a little bit with these:
https://github.com/sparky8512/starlink-grpc-tools
More specifically, I've been playing with the python script that's
getting the obstruction map here. This grabs an array of 123 row tuples
with 123 floating point number column entries each, which represent SNR
(signal to noise ratio) data, with the row and column indices of the
data structure being 2D coordinates of the direction in which this data
was observed. A value of -1 meaning that Dishy hasn't ever seen a
satellite in this direction, actual signal strengths are coded as values
between 0.0 and 1.0. You can output these as PNGs as well, like here
(with a slightly changed colour scheme):
What you are looking at is a screenshot (so not exactly 123x123 but very
close), with white pixels corresponding to good signal and anything
blueish to impaired signal, and anything black to no signal. North is
top, west is left and east is right. You're looking at the corner of my
house as seen by Dishy on my deck, the flattish bit on the left is a
wooden trellis getting in the way, and the light blue line crossing the
white top part is the aluminium front bar of my awning (the awning
fabric appears to be transparent to the RF signal).
My ultimate goal here is to be able to identify which satellite Dishy is
currently talking to, something the grpc interface doesn't seem to
reveal directly (anymore). This is of some interest in order to see
where signals enter the Starlink network, which ground stations the
satellite may relay to in bent pipe mode, and perhaps for an educated
guess as to which ISL hops it's taking.
I'm trying to do this essentially by comparing two successive retrievals
of this map and detecting which entry has changed. This is the easy part.
The hard part is trying to figure out which satellite this corresponds
to. Essentially, the idea is to translate this pixel data into a unit
vector pointing at the satellite, and then compare that with the unit
vectors from Dishy's location pointing at the thousands of Starlink sats
up there, and picking the one pair with the smallest angle. All this
takes conceptually are a few coordinate transforms to get everything
into the same coordinate system, with sat positions computed from NORAD
two line elements.
My initial thought was that:
1. Index coordinate (62,62) in the SNR data matrix corresponds to a
satellite that sits on the Dishy surface normal.
2. Indices minus 62 correspond to some sort of Cartesian x-y coordinate
that should let me derive a unit vector for the direction to the
satellite in a polar coordinate system based on Dishy's surface and
the surface normal.
3. That then needs transforming into a coordinate system based on
Dishy's location, removing Dishy orientation in the step. Dishy
location and orientation are kindly available from Dishy itself via
grpc.
4. Coming the other way, two line elements need to be turned into
global coordinates for satellites at the current time, and these
need to be turned into local coordinates in the system we're
transforming into under 3 above.
The crux is at step 1 and 2. If the assumption under 1 is correct and we
assume that the scales in row and column direction are the same, getting
at the azimuth is easy. But what does the distance of an entry (pixel)
to the centre of the map represent?
* A linear function of the elevation angle?
* A cosine projection of the elevation angle?
* Would a map position in the middle of each map edge represent an
elevation of 0 or, given the much rumoured phased array "cone" of
100 degrees, an angle of 40 degrees over Dishy surface?
* Something else?
But I'm not even sure that the assumption under 1 is correct. Note how
the area with valid SNR values in the map above is slightly elliptic and
offset a bit towards the bottom? This can't be due to Dishy's geometry
as the long dimension of it is top to bottom (north to south) rather
than east to west. Could this already be a projection into
the coordinate system based on Dishy's location, such that (62,62) is
straight up from the ground?
Anyone got any insights on this?
Thanks muchly in advance.
Ulrich
--
****************************************************************
Dr. Ulrich Speidel
School of Computer Science
Room 303S.594 (City Campus)
The University of Auckland
u.speidel at auckland.ac.nz
http://www.cs.auckland.ac.nz/~ulrich/
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