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[aprssig] ISS Tracking & Pass Information

Lynn W. Deffenbaugh (Mr) ldeffenb at homeside.to
Fri Oct 14 01:49:49 UTC 2011

Greetings APRS users,

There's a new ISS object flying in APRS-IS, but this object is different 
than others that have flown.  It is flying with a MultiLine 
(http://www.aprs-is.net/WX/MultilineProtocol.aspx) description of the 
approximate reception footprint.  It moves every two minutes, so if 
you're using an APRS client that displays MultiLine information 
(APRSISCE/32 and xastir at a minimum), you'll be able to see the 
footprint coming and know to anticipate a pass (or realize that you just 
missed one).

The ISS object has a comment that says "Msg4Pass".  That means to send 
an APRS message to "ISS" (without the quotes) from any message-capable 
APRS platform.  This can be APRS radios (if you're in range of a 
message-gating IGate), any of the PC-based APRS messaging clients (like 
UI-View), or any of the new mobile APRS-IS clients, provided that they 
do messaging.

When "ISS" (not the real space station, but the APRS satellite server) 
will calculate the current or next pass of the ISS for an observer at 
your station's last beaconed position (it will say "Please Beacon 
Position" if it doesn't know where you are).  The response will be one 
of the following:

Az: aaa El: lll LOS:xx:yyz - If you are in a pass
AOS: xx:yyz+aa:bbc - If you are not in a pass
AOS: NONE - If there are no passes for your location in the next two days.

aaa and lll are the azimuth and elevation for an observer at your 
location at the time the message was processed.  LOS is Loss of Signal 
and xx:yyz is the time remaining in the pass and will vary to show 
sufficient time periods (hh:mmh, mm:ssm, sss).

AOS is Acquisition of Signal with xx:yyz being the delta time until the 
pass begins and +aa:bbc is the expected duration of the upcoming pass.  
The longer the pass, the higher you can expect the max elevation to be 
and the more chance of working it as it flies by.

But Wait!  There's MORE!   If the text you send to ISS is a known 
station identifier (say KJ4ERJ-12), the returned pass information will 
not be for your location, but for that station's last known location.  
To be clear, "@ call-SSID" is appended to the end of the response so you 
know which observer it is for.  Of course, if the satellite server 
doesn't know where your specified station is locate, you will receive 
"Please Beacon Position" as a response.

And that's not all!  This doesn't only work for the ISS, but for any of 
the other satellites found in the TLE set from 
http://www.celestrak.com/NORAD/elements/amateur.txt  You can send an 
APRS message to "AO-51" to find out when that satellite will pass within 
range.  If you have any trouble figuring out which satellite name a 
message can be sent to, send an APRS message to KJ4ERJ-12 (or an e-mail 
to here) and I'll see if I can map the TLE entry to the responder 
station ID.  And of course, you can also send any other known station ID 
to any of the satellites to get the current or next pass information for 
that station's location.

Give the service a try, and let me know how it works out.  There's 
nothing to remember except the satellite name (including the dash in 
most cases) and the fact that you send a message to the satellite.  
Nothing like ANSRVR or CQSRVR or SATSRV, just send the message to the 
satellite by name and it will return the pass information.

Lynn (D) - KJ4ERJ - Author of APRSISCE for Windows Mobile and Win32

PS.  Don't look for ISS at aprs.fi as it flies too fast and is currently 
rejected by the speed limit filter.  If you want to track the ISS 
object, add o/ISS* to your APRS-IS filter string.  If you don't want to 
see it when it goes by, add -o/ISS* to your filter.

Here's a pair of chat transcripts with ISS and AO-51 (times are Eastern US):

21:35:39  New Chat Between KJ4ERJ-AP and ISS on 2011-10-13
21:35:41> foo (*2-21:35:41)
21:35:41< Az:325 El:12 LOS:4:19m
21:42:57> bar (*2-21:42:57)
21:42:57< AOS:4:44h+3:21m
21:48:41> WB4APR-15 (*2-21:48:41)
21:48:41< AOS:1:19h+8:23m @ WB4APR-15

21:43:09  New Chat Between KJ4ERJ-AP and AO-51 on 2011-10-13
21:43:11> FOO (*2-21:43:11)
21:43:11< AOS:5:45h+3:10m
21:43:26> ISS (*2-21:43:26)
21:43:26< AOS:5:45h+3:09m
21:43:37> N0GPS-3 (*2-21:43:37)
21:43:38< AOS:8:59h+12:24m @ N0GPS-3
21:48:58> WB4APR-15 (*2-21:48:58)
21:48:58< AOS:7:12h+13:23m @ WB4APR-15

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