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Mobile Satellite Comms



At the AMSAT conference, the new Kenwood Mobile Radio (not yet officially
announced) was demonstrated to receive UI packets from the 9600 Baud
pacsats direct to the front panel of the radio (No TNC,MODEM or Laptop
needed).  Here is a paper discussing the potential applications to the
mobile satellite environment:

                     APRSpace CONCEPT DISCUSSION          10 Oct 99
                        Bob Bruninga, WB4APR

APRSpace is a network concept I presented at the AMSAT-NA symposium to 
use amateur satellites to deliver messages/position and status between
mobiles and between the worldwide APRServe system.  With the imminent
introduction of the Kenwood APRS Mobile radio with built in 9600 baud TNC
and front-panel APRS message and position displays, any mobile traveling
can send and receive packets via a 9600 baud satellite.  The purpose of
this "white paper" is to begin the dialog to suggest how to use this
radio for mobile satellite communications.

    In the conference parking lot, using a handheld 3 element (1 foot)
UHF beam, the Kenwood radio received and displayed UI packets from both
the UO22 and KO25 downlinks on the front panel of the radio.  With
several new Amateur Satellites in design and on the launch pad, this
demonstrates a great satellite application for these new 9600 baud
transponders.  Remember, no PC, laptop, TNC or any other hardware other
than the mobile radio itself is required!

   Further, No on-orbit software, nor message storage nor processing
is required other than simply enabling digipeat for UI frames on the
satellite.  Mobile-to-mobile communications would be enabled instantly,
and mobile to-from any other APRS station in the world is possible via
only a few APRServe fixed stations serving as impromptu APRServe IGates.
Finally, no special ground segment software is required either, since
every copy of Mac/Win and +SA APRS software already includes automatic
background Gateway capability to APRServe whenever such a user is
connected to his internet ISP..

   Thus, in HAM radio we have the ability in only a few months from
now to activate a worldwide mobile Satellite comunications system
that rivals (in our own small way) the global presence of Iridium,
Globalstar and others...  What makes this possible is the availability
of the off-the-shelf Kenwood mobile radio (maybe by December 99) with
built in 1200/9600 baud packet radio and APRS front panel displays
and controls.

    NOTE, this does not replace, but only augments the existing efforts
using the 1200 baud AO16/LO19/IO26 satellites to accomplish the same
APRS mobile satellite obectives.  Similarly it does not compete with
the existing 9600 Baud PACSAT Protocols nor satellites.  These three
amateur satellite segments can be thought of as a means to satisfy a
variety of mobile satellite communications needs.  The following table
attempts to summarize these three areas and capabilities:

                  AO16/LO19/IO26   
PACKET FUNCTION   1200 BAUD SATS 9600 bd SATS  ALTERNATIVE GROUND STN
----------------  -------------- ------------  -----------------------

Posit reporting   $2 mod TNC-2   KWD Mobile    Mobile PACSAT station
                  and GPS        and GPS       with TNC, laptop, etc

Status Reporting  $2 mod TNC-2   KWD Mobile    Mobile PACSAT station
                  and laptop                   with TNC, laptop, etc

Message Xmission  $2 mod TNC-2   KWD Mobile    Mobile PACSAT station
                  and laptop                   with TNC, laptop, etc

Message reception PSK-SSB        KWD Mobile    Mobile PACSAT station
                  and laptop                   with TNC, laptop, etc

File Transfer     PACSAT TNC     KWD Mobile    Mobile PACSAT station
                  and laptop     and laptop    with TNC, laptop, etc
                                 UO22/KO23/25

Notice how versatile the Kenwood will be as a mobile satellite terminal
even without a laptop in most cases...

ANTENNAS:  Existing mobile whips for 2m will work fine for the mobile
uplinks.  But existing 9600 baud downlinks are below threshold for any
mobile omni antenna.  Thus mobiles need to begin working on 7 to 10 dB
gain antenna designs.  At least a 3 element (or equivalent) beam or
phased array is required.  And the vertical polarity of the signal and
the nadir pointing null from the satellite preclude the normal center-
of-pass advantages.   Fortunately, the future satellites may have more
power available on the downlink, due to the low duty cycle of these
APRS style UI messaging.

APRS GROUND STATION (IGATE) SOFTWARE MODS:  Although no mods are needed
for mobile-to-mobile and mobile-to-internet messsaging, some minor
software mods will be needed to existing APRS swoftware IGate handling
from APRServe back to the mobile via the satellite.  THis is due to the
short duration and infrequent satellite passes:

1)  APRSpace IGates will need to retain APRSpace mobiles in their "local"
lists for possibly "days" instead of hours.  (APRS Igate software uses
these "local" lists to determine whether or not they will forward the
traffic from the internet to the mobile via RF).

2)  APRSpace IGates will need to "buffer" APRServe traffic for APRSpace
mobiles until they next come into view.  The existing "send-on-heard"
algorithm will work once the packets are buffered.

3)  Because these Message packets will no longer be LIVE, they may need
to be TIME stamped.  All other APRS messages are delivered LIVE.  It
is important that there be a distinction to avoid user confusion.

4)  Because of this many-hour potential message latency, there will be
a problem in "old" messages, and conflicting messages via the terrestrial
and APRSpace link.  One possible solution is to assure that the APRSpace
mobile always uses a unique SSID while working the satellites.  This is
trivial to do since the Kenwood Mobile has 5 user "profiles", one of
which can be used for APRSpace unique settings.

FUTURE:  This paper only capatilizes on the impending introduction of
the new KENWOOD Mobile radio and its capability to use satellites for
mobile long distance communications by suggesting a starting point using
only existing hardware, software, APRServe, and on-orbit software with
no changes.  But this is only a starting point.  There are many possible
enhancements that can be added to future Satellite software:

1)  9600 Baud digipeating satellites could save downlink power by
keying up the transmitters only when there are packets for relay.  They
should buffer packets, say for 3 seconds, or so to avoid wasting TXD
key-up delays.  Such savings could allow a 6 dB power increase for the
same average DC power.  Each TXD Key up can be equivalent to as many as
EIGHT APRS packets!

2)  Satellites could store-and-forward packets from off-shore and other
countries when the downlink is idle.  This can provide worldwide APRS
capabilty for stations thousands of miles from any APRServie IGate.

3)  Satellites could transmit their own positions to aid users in antenna
pointing. (THis can actually be digipeated from a tracking program on the
ground in the interim).

4)  APRSpace digipeaters might not permit point-to-point link layer
connections.  It is well known that these are grossly inneffecient on
a shared ALOHA channel with 100% "hidden" transmitters.

5)  "Country Servers" can forward appropriate "Country wide" bulletins
announcements and possibly track data for special interest items

6)  APRSpace satellites can "ping" APRSpace mobiles so that the mobiles
only transmit when the satellites are in view.  This promotes spectrum
effeciency.

I invite others to consider how we can take advantage of the
impending new Kenwood APRS Mobile Radio for mobile operations via our
AMSAT Satellites.  I hope to see applications that encourage more
excitement in Amateur Radio and Amateur satellites to promote LIVE
human-to-human communications.  The digital equivalent of the good
aspects of AO27...

de WB4APR

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