[Starlink] a puzzling starlink uplink trace

[Alexandre Petrescu]

Le 31/08/2023 à 15:36, Dave Taht a écrit :
[...]
> VLEO (I am not sure if that is an established acronym - very low
> earth orbit operations)

I saw very often the term vLEO (very low Earth orbit).  Sometimes it was
said to range between 100k and 400km, at which LEO would start.

However, it is indeed a surprise to see starlink sats below 500km, right
now.

I think the things advance so fast that even the terminology is
fluctuating a lot.  Add to that that the definition of these orbit
altitudes are very approximate, Space is approximate, smaller and
smaller devices are easier and easier to control and describe
trajectories of arbitrary forms.

> -  feasible. Dropping the things even lower, making the shell more
> aerodynamic, and burning fuel to stay there might well be an
> interesting option. It seems to me the current altitude(s) are pretty
> conservative and given the fuel consumption reported on the 
> maneuvering side, they can last at 530km much longer than 5 years,
> and with the pace of technology and launch rates, moving them lower
> would be a huge win for bandwidth and latency.

Yes, I agree.

I would add to that also the new characteristic of being practically
'disposable' - launch, orbit, re-entry - all during maybe 2 to 5 years,
maybe 4 times per year.  Compare that with other sats with lifespans in
the order of 20, 30 years and even more.

With that kind of short lifetime, the quantity of energy needed to
maintain in an 'orbit' at, say 50km altitude, might not be impossible.

If one asks Starlink, I think they'd surely want to make it that way.
But there could be another way in which somebody else than Starlink puts
these sats at lower altitudes, and make constellation-to-constellation
communications.

Also, one inconvenient of these lower altitudes (say, below 300km) might
be (if I understand correctly) that they necessitate smaller sats, i.e.
smaller computers, i.e. lower bandwidths.  These small sats might offer
lower latencies, but lower bandwidths too.  There might be a need to mix
the lower latency of lower altitude sats with the higher bandwidths of
bigger sats at higher altitudes, onto the end user (not onto an
intermediary aggregator).

Alex

> 
> On Thu, Aug 31, 2023 at 2:13 AM Alexandre Petrescu via Starlink 
> <starlink at lists.bufferbloat.net> wrote:
>> 
>> to clarify: I some times look with satmap.space and with n2yo.com
>> at sat altitudes.
>> 
>> Some times I see some starlink sats at lower altitudes than LEO
>> (some times at 70km).
>> 
>> Right now I see sat STARLINK-6065 at 360 km altitude, which is way
>> below LEO and typical 550km altitude of starlink sats.
>> 
>> https://www.n2yo.com/satellite/?s=56812
>> 
>> If these altitude reports are correct, then I think it is hard to
>> say Starlink is anymore simply a LEO constellation.  It is much
>> lower than that.
>> 
>> Further, if the 20ms latency report is due to that 365 km altitude
>> then it is very easy to imagine what lower altitudes would give.
>> 
>> If one reports a great ms latency then it would be great to tell
>> which sat at which altitude was there above at that timestamp.
>> 
>> Alex
>> 
>> Le 31/08/2023 à 10:56, Alexandre Petrescu via Starlink a écrit :
>>> 
>>> Le 30/08/2023 à 20:07, Dave Taht via Starlink a écrit :
>>>> In the attached 5 minute plot from a few days ago (I can supply
>>>> the flent.gz files if anyone wants them), I see a puzzling
>>>> spike at T+155s to nearly 90ms of baseline latency, then down
>>>> to 20ms.
>>> 
>>> 20ms?
>>> 
>>> A latency of 20ms might come if these low altitude starlink sats
>>> (70km or so) pass by there?
>>> 
>>> Or maybe I dont see quite well these sat altitudes.
>>> 
>>> Alex
>>> 
>>> 
>>>> No degree of orbital mechanics can apply to this change, even
>>>> factoring in an over the horizon connection, routing packets on
>>>> the ground through LA to seattle, and back, or using a couple
>>>> ISLs, can make this add up for me. A combination of all that,
>>>> kind of does make sense.
>>>> 
>>>> The trace otherwise shows the sawtooth pattern of a single tcp
>>>> flow , a loss (sometimes catastrophic) at every downward
>>>> bandwidth change.
>>>> 
>>>> An assumption I have long been making is the latency staircase
>>>> effect (see T+170) forward is achieving the best encoding rate
>>>> at the distance then seen, the distance growing and the
>>>> encoding rate falling in distinct steps, with a fixed amount of
>>>> buffering, until finally that sat starts falling out of range,
>>>> and it choses another at T+240s.
>>>> 
>>>> But jeeze, a 70ms baseline latency swing? What gives?  I
>>>> imagine somehow correlating this with a mpls enabled traceroute
>>>> might begin to make some sense of it, correlated by orbital
>>>> positions....
>>>> 
>>>> 
>>>> 
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