Satellite latency

Wed Feb 27 20:58:51 UTC 2002

Not to mention that -5 degree or so look angle (negative elevation) from the
poles being hard to acquire. ;) But with enough elevation and a big enough
dish...

Also about that satellite drift...The earth wobbles during rotation meaning
geo stats do small figure 8s overhead. IIRC there is some computer software
out there (from my days of TVRO in Europe) that given a latitude/longitude
and sat placement would return a time where the sat was in the middle of
that 8 so you could best aim the dish. Or maybe I just had too much Heineken
at the bar, can't remember which.

Just my 2¢. The delete key is your friend.

-Al

-----Original Message-----
From: Michael Painter [mailto:tvhawaii at shaka.com]
Sent: Wednesday, February 27, 2002 12:22 PM
To: nanog at merit.edu
Subject: Re: Satellite latency

----- Original Message -----
From: "Leo Bicknell" <bicknell at ufp.org>
To: "Steven M. Bellovin" <smb at research.att.com>
Cc: "Tim Devries" <zsolutions at cogeco.ca>; <nanog at merit.edu>
Sent: Tuesday, February 26, 2002 5:27 PM
Subject: Re: Satellite latency

>>Remember that a geosynchronous satellte must orbit the equator.
Let's say for the sake of argument it's over mexico, you're in New
York, and the downlink station is in San Diego.  The 36,000 is the
distance straight "down" to mexico, It's probably more like 50,000
to New York, and 45,000 to San Diego.  And if you're in New York,
and your mail server is in New York, but the downlink was to San
Diego, you've got another 4,000 across country.  Now you're up
closer to 100,000km.<<

Not wanting to get picky about ~20,000 km., but the maximum -usable- slant
path is ~41,000 km.

--Michael

>
> In a message written on Tue, Feb 26, 2002 at 09:07:14PM -0500, Steven M.
Bellovin wrote:
> > Geosynchronous orbit is about 36,000 km from the center of the earth.
> > Round-trip to the satellite is ~72,000 km; the speed of light is
> > 300,000 km/sec.  That works out to 240 milliseconds at the minimum for
> > one-way packet delivery.
>
> Remember that a geosynchronous satellte must orbit the equator.
> Let's say for the sake of argument it's over mexico, you're in New
> York, and the downlink station is in San Diego.  The 36,000 is the
> distance straight "down" to mexico, It's probably more like 50,000
> to New York, and 45,000 to San Diego.  And if you're in New York,
> and your mail server is in New York, but the downlink was to San
> Diego, you've got another 4,000 across country.  Now you're up
> closer to 100,000km.
>
> Add to this some inefficient encoding done on satellites, and most
> (consumer) systems using a broadcast medium that can buffer packets
> and you see why people report 1 second RTT's with services like
> StarBand.
>
> It's better than nothing, but it's a rough primary connection.
>
> --
>        Leo Bicknell - bicknell at ufp.org - CCIE 3440
>         PGP keys at http://www.ufp.org/~bicknell/
> Read TMBG List - tmbg-list-request at tmbg.org, www.tmbg.org
>