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[aprssig] New APRS satellites!

Bob Bruninga bruninga at usna.edu
Sat Nov 20 14:24:35 UTC 2010

Several new AMSAT's appear to be successfully deployed last night.  The UT Austin team has suggested that after their primary mission is complete, they may allow APRS digipeating on their two birds.  They both operate on the same space APRS frequency as PCSAT and ISS, 145.825.  


If these satellites do eventually get turned on for APRS it has the potential to greatly extend global APRS and text messaging to mobiles and handhelds everywhere.  Although the operators have not discussed details, we can day dream about how these could be used for APRS.  

TEXT-MESSAGING:  Of course APRS status/position packets are an obvious application.  But the real goal of the APRS system is hand-held/mobile text messaging via HAM radio on a global scale.  And with FAST1, FAST2, ISS, PCSAT1 (sometimes) and our coming PSAT1 and PSAT2 next year, we could have a constellation of 5 birds giving passes as often as about every hour throughout the day.  Remember an APRS Text message can contain email and tweets making it fully compatible with ANY handheld wireless device in the hand of a HAM.  Please see www.aprs.org/aprs-messaging.html

The salient characteristcs of the two UT Austin satellites are:

Downlink(1200/9600) 437.345 MHz (beacon 1200) 
Uplink (1200 Baud)  145.980 MHz
Uplink (9600 Baud)  145.825 MHz

Downlink(1200/9600) 145.825 MHz (Beacon 1200)
Uplink (1200 Baud)  435.025 MHz 
Uplink (9600 Baud)  437.345 MHz

Their hardware is similar to PCSAT1 as they are using 1200/9600 baud KPC-9612's which can have one input at 1200 and one at 9600 and then the combine the outputs to one downlink.  They can also have a cross-baud conversion from 1200 to 9600 or 9600 to 1200. Notice they are a "couple" as each one crossbands to the other.

This makes one useable on 145.825 as an uplink and the other delivering packets to mobiles on a 145.825 downlink.   Here are some subtle considerations about mobile and handheld links:

VHF UPLINK: Since VHF has a 9 dB (omni-to-omni) advantage over UHF, then 5W handhelds would best use the VHF uplink.

BAUD: Since 1200 baud appears to have a 7 dB advantage over 9600 then handhelds would use 1200 baud.  (We measured this difference using D7 handhelds back on PCSAT1 (which may not be applicable here)) 

MOBILES:  Mobiles have 50 watts and can easily be heard omni-to-omni by any LEO satellite on an uncongested uplink.

DOPPLER:  VHF requires no doppler adjustments on either the uplink or the downlink.  But UHF requires 5 tuning steps on the downlink -10, -5, 0, +5 and +10 KHz.  The critical issue here is whether the UHF uplink receiver on FASTRAC2 was widebanded.  If so, then at most two steps might be needed +5 for half the pass and -5 for the second half.  Or just 0 and only use the middle.  But if the uplink UHF receiver was not brodbanded like it was on PCSAT1, then uplinks will have to be done in the blind using these 5 steps.

TXD:  The #1 problem with using 9600 baud is the vast majority of TM-D700 mobiles out there that can only do 9600 baud with a TXD of 500ms.  This is a huge waste of time since the APRS data is only about 20ms long.  SO these radios would best be used for receivng the downlink, not operating on the ALOHA uplink channel.  Or they should stick to 1200 baud.

We need to flesh out details about what the minimum operational TXD is for all the other APRS radios now on the market.  ANYONE?  We also do not know the TXD of the satellites, but it does not matter, because they are operating full duplex.

RANGE GAIN:  A huge advantage to mobile and handheld operation is the simple fact that overhead passes yield almost a 10 dB advantage compared to horizon passes.  And since there will be at least one good overhead pass in the morning, and one in the evening, everyone even with a omni HT has the potential to getting in at least twice a day.

SATGATES:  OSCAR-CLASS automatic ground stations can serve best for tracking the high doppler downlink of FAST1.  WIth their horizon-to-horizon link capability, only a handful of these SATGATES are needed per continent.  But for FAST2, with minimal doppler on 2m, ANYONE with short coax and a 19" vertical whip over a large ground plane can be a good SATGATE for FAST2.  They wont get packets on the horizon (but someone else will) and they will get good packets on their no-moving-parts-omni for the middle of the pass.  A dozen of these passive SATGATES can cover the whole USA.

Remember, once a packet is heard by ANY satgate only once, the packt is then distributed worldwide to its recepient via the gloabl APRS internet network.

More when I think of it...

AGAIN.  DO NOT UPLINK.  THESE SATELLITES ARE NOT OPEN to USERS.  They only have the potential for users later after they complete they primary mission and only if the operators chose to do so.


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