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[aprssig] New APRS Satellite Support!

Bob Bruninga bruninga at usna.edu
Sun Sep 16 15:57:06 UTC 2007


The GO-32 Satellite TECHSAT-1B team has asked me to announce experimental 9600 baud APRS access for mobile and tactical APRS on a not-to-interfere basis with the existing PACSAT BBS.   

The uplink is possible with 5 Watts on 2 meters, but the downlink is 9 dB worse to an omni antenna on UHF and requires tuning 10 to 20 KHz for Doppler.  SO far, we are starting with zero permanent IGates to bring the downlink into the global APRS system, but we hope volunteers will surface with permanent satgates.  So initally, do NOT expect to see yourself on FINDU.COM.  Enjoy ham radio,  Make a contact instead!

Experimental testing shows that an APRS HT with a 19.5" whip can get a lucky shot into this satellite (but not when there is congestion on the uplink).  With a handheld beam, it should be no problem for an HT, and the beam is needed on reception anyway.  Fifty Watt mobiles should have no problem using a stock mobile whip.  Due to the absence of APRS signals to date, we do not have good statistics on reception in a mobile yet...

Operating on a 9600 baud FULL DUPLEX APRS satellite is MUCH different than ARISS, or any of the other 1200 baud APRS PCSATS, because the turn-around is so fast, that you cannot see your own digipeated packets on the same radio (usually)...  Please read and understand this entire email before attempting to operate.

The easiest way to get on GO-32 with 9600 baud APRS is to use a Kenwood D7, D700 or D710 radio.  These radios since 1998 have been satellite 9600 baud ready, just waiting on a satellite!.

Read these notes on how to operate GO-32 with a D7 or D700:

Set APRS Baudrate to 9600 baud.  
Set A band to uplink on 145.93
Set B band to receive 435.225 +/- 10 KHz
Set Path to be via 4XTECH
Set MYCALL to a unique SSID
Set TX method to AUTO
Set TX RATE 1 min for HT. 2 min for D700
Put something useful in your STATUS text maybe describing your setup: "50W mobile, 1/4 wave, 2m rate" or "5W HT, long whip, 1m rate"

Save in a PM for use anytime you are outside of the terrestrial APRS network.

SATELLITE OPERATING NOTES:

The GO32 uplinks and downlinks are for the PACSAT store and forward system and users.  APRS is on a secondary basis and should not be operated unattended.  If you see that the BBS PBLIST is full of other users, do not enable your APRS since the uplink will be busy.  RIght now, the PBLIST is not in APRS format so you cannot see it unless you are running normal packet mode.  But we will work on that...

On the D700 you can press PMON on the front panel and see these packets... but they FLY by...

ACCESS TIMES:  GO-32 is sun synchronous and so it comes over everywhere three times between about 8 AM to Noon and again between 8 PM to midnight local sun time.  During these two windows at least one pass each will be an overhead pass which might also work for an HT.  The other passes will be lower to the East or West and will work fine for a 50W mobile.

WHAT YOU HEAR:  9600 baud sounds almost exactly like open squelch, though the tuned ear can soon distinguish the difference.  Before the pass, set your squelch normally to quiet the speaker.  When you hear the satellite, the squelch will open and you may see up to 3 bars on your S meter.  Tune to the "best sounding" noise.

DOPPLER:  Depending on how low to the horizon you can see, the satelite approaches 10 KHz high at 435.235 MHz... But it is maybe 3000 km away.  As it gets higher, and 6 dB closer, it will be on 435.230 MHz, passing through 435.225 published center frequency at the middle point, and then drop down through 435.220 and ending at 435.215.

But since it is 6 to 10 dB closer (and stronger) towards the center of the pass (800 km overhead), the mobile antenna is probably only going to hear the middle 435.230, .225, .220 portion easily.  So I would start my receiption at 435.230...

UPLINK CHANNELS:  GO-32 allows two APRS uplinks.  One is exclusive to D7/D700 tactical position reporting and the other exclusive to messaging.  This is in hardware, not policy...

1) All APRS messaging (or fixed station non-Mic-E positions) must use the 145.85 uplink where GO32 only digipeats APRS packets with TOCALLs that begin with the usual "APxxxx".  (Even the D7 and D700 use "APKxxx" for messages.)
 
2) All APRS Mic-E position uplinks (D7, D700 and D710s) must be on 145.93 MHz and they must have the position comment set to "Committed, Special or PRIORITY"...  With those comment settings then the TOCALL first LATITUDE digit will be 1,2,3,4,5,6,7 and only these will be accepted by GO32 for 
digipeating from 145.93.

DATA CARRIER DETECT: The D700 and non(g) model D7's will NOT TX if they are hearing the downlink at the same time due to CARRIER DETECT.  The D7(g) model has DCD IGNORE that *will* let it TX anyway.  So use separate rigs for TX and for RX if you want to see yourself.

Otherwise stick to the receommended TX rates and know that you are getting in if you stick to the protocol.  Sticking to the recommended rates also keeps channel loading low, so that everyone gets in with less congestion.

PERMANENT SATGATES:  Here is a great place to use your D7 HT when you are not using it otherwise.  Simply connect it to a 19" whip over a ground plane and to  your APRS IGate system.  The antenna does not even need to be high, since it cannot hear, nor will it be on frequency for low packets near the horizon 3000 km away.  Set it to 9600 baud RX and tune to 435.230 (which is 5 KHz high).  This Doppler setting will match the stronger signals.

This 19.5" whip (3/4 wave on 435) does not need to see below 25 degrees, since its max gain (almost 7 dB) is between 30 to 70 degrees anwyay.  This also protects your HT from lightning, since it can be low, below all of your other antnenas.  Yes, your station will only see about 30% of all possible packets and only on the best two passes per day, but combined with dozens of other such unattended SATgates, all packets should be heard somewhere by someone and injected into the APRS Internet system.

I hope to have an APRS-via-GO32 web page up in a few weeks, but in the mean time, use this message for deails.

Here is a link to AMSAT's description of GO-32:

http://www.amsat.org/amsat-new/satellites/satInfo.php?satID=14&retURL=/satellites/status.php

Good luck!

Bob, Wb4APR




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