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Sapphire (and other LEO) SatGates

Permanent Sapphire (and other LEO) SatGates needed:

As we prepare to activate Sapphire for UI digipeating we need a lot more
permanent satellite IGates feding the downlink into the WEB.  Anyone with
a permanent Internet connection, and a spare UHF receiver and  a
coathanger antenna can contribute.

Compared to PCsat and ARISS satgates, we need 10 times the number of
Sapphire satgates for several reasons:

1) The 437.1MHz UHF downlink has a 9dB greater path loss to an OMNI ant
2) Doppler is 18 KHz so each receiver will only hear about 15% of a pass

The optimum antenna is a 19" whip in the middle of a metal groundplane
(like a car roof).  There are several reasons why this is optimum:

1) It is a 3/4 wavelength vertical which has almost 7 dB gain from about
   25 to 70 degrees.
2) The gain also occurs where the satellites is closest
3) The gain and closeness occurs near the center of the Doppler

THus, a fixed tuned receiver using a Fixed antenna CAN decode good packets
whenver the satelite is in the cone above this ground station.
Unfortunately for any given station, the satelltie is only in this cone
for less than 15% of the time.  Thus we need 6 times the number of
Satgates for Sapphire as we did for PCsat.

The GOOD news however is that the 19" whip over the ground plane not only
is ideal for 437 MHz downlinks but also perfect for PCsat, ARISS, SAREX,
PCSAT2, ANDE, ECHO, SO41, SO50 and any other 2m or 435 MHz downlink.

AND if you connect it to a radio that can TUNE to each satellite that
comes over, then one such SATGATE can serve all these satellites.  I use a
Kenwood TH-D7 HT because it has these advantages:

1) UHF and VHF receiver with excellent sensitifvity.

2) Builtin 1200/9600 baud TNC fully integrated for optimum performance

3) Small and can run on a wall-wart transforemer so you can mount it
   in the attic or at work just inside the roof line

4) Total radio inteface is 3 conductors for the serial port.  This sertial
   port communicates with both the TNC's and the radio control.  You can
   run this for hundreds of feet to your old PC or laptop that auto
   tunes the radio to each satellite in turn, auto-tracks doppler, and
   on the same serial line, captures all downlinks at either 1200 or
   9600 baud.

Years ago, I wrote APRStk in Quick Basic that controls the radio, captures
all downlinks and even can point a radio shack $64 rotator for tracking if
you want to see and hear more for 100% of all passes.  It also plots any
APRS positions and allows keyboard APRS messaging.  THE PROBLEM is that
it does not SATGATE, because Qbasic doenst link to the internet.  APRStk
has been around since 1998 and it is frustrating to me that no modern
author has replicated this in internet capable software!

This is a shame that we have these dual band, dual TNC, satellite
radios that can do everyting with a single serial port and no one has
risen to tackle these simple tasks that APRStk does over a single serial
port and 2 bits on the LPT1 printer port

1) COmmands radio to proper UHF or VHF freq of any satellite in view
2) Accounts for 5 KHz step Doppler tuning
3) Selects correct TNC (1200 or 9600) for each satellite
4) Commands TNC to downlink band and separately to uplink band
5) Sends LEFT-RIGHT rotation commands to bit 2 and 3 on LPT1 Port to
   drive any optional 24VAC standard cheap TV rotator.
6) Igates all packets heard into the global APRS-Internet system
7) Adds all normal APRS functionailiy for the operator if present
8) Mutes or unmutes the speaker depending on thwether it is a digital bird
   or a Voice bird.  (this lets you never miss any pass of any FM
   satellite no matter what its mode...)

The only current way to have this fully automated ground station AND to
IGate is to have APRStk running on an old PC and paralleling the radio's
serial port output to both the DOS PC running APRStk, and to a modern
Internet connected PC running any other version of APRS that will IGate
what it hears...

To this end, I think it is best to add all the above Satellite stuff to a
SATELLITE program and then let it just hook over to an existing APRS
program for all that functionality.

It is important that we get away from these $1000 AZ/EL systems that are
just absolutely not required for LEO satelites and get back to simple TV
rotators that any HAM can use... see my simple TV ROTATOR sytem:


To see all about the overall concept, look down near the middle and end of
the ASTARS web page:


Its too bad that we are down to only a few working LEO's these days.  Back
when I wrote APRStk, we had all of these working LEOS that were compatible
with the simple Kenwood integrated TNC/Radios:

UO22, KO23, KO25, ARISS, AO27, UO14, PCsat, OPAL, Sapphire (LO19 CW).
Running my HT  automatically in my office tuning to each as it came into
view gave over 50 to 60 passes a day.  There was almost something always
in view...  These days ther are fewer, but there will be more!

de Wb4APR, Bob

Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
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