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Re: [aprssig] Re: GPS data from PCsat



On Mon, 14 Jan 2002, Jeff King wrote:

> Do you think it is a good idea to mix modes that cannot detect each
> other's DCD? Understand you might be between a rock and a hard place
> but in general I think it is not such a good idea to do this. The 1200
> baud ground stations are going to be transmitting on top of the
> satellite.

Yes, that is what we planned.  See the design approach in the PCsat
articles in QEX and 73 magazines and on the WEB page (below).

THe normal APRS satellite user channel is 144.827MHz 1200 baud digipeater
(Up and down).  THis can be accessed with an HT (Kenwood THD7, or any
combo of HT and TNC) and all mobiles.  This channel is worldwide..

THe 144.39 downlink is for message delivery to any North American APRS
mobile or handheld operator anywhere in the country who may be
temporarily out of range of the terrestrial network.  It can also be  used for
national bullettins and special (infrequent) events.  To this end, all
data intended for terrestrial users of 144.39 is transmittted at 1200
baud as one would expect.

During the brief experimental times (this week for example)) when we need
GPS data, we downlink that on 144.39.  Since it is only 2 packets every
30 seconds,  we use 9600 baud (tiny packets) to not only save power on the
satellite,  but also to minimize any terrestrial interference.  THis data
is not intended for users and so we designed it so that all terrestrial
Users and  digipeaters will ignore our -122 dBm signal from space and
continue operating normally.

Thus, by design, the 1200 baud terrestrial users TNC's are supposed to
ignore our .3 second bursts and use the channel normally including
transmitting on top of us..

We have no problem receiving the weak data even on a busy 144.39 channel
due to space and time diversity of multiple receivers receiving the data.
We chose to use 144.39 for APRS data since it was already coordinated
throughout the North American continent and that way we would not be a hog
of the very precious and limited dedicated satellite channels limted to
145.8 to 145.99 MHz of which then we only needed one, instead of two.

Even in the busiest APRS area (middle east coast for example) the 144.39
channel at any one station's location still hears a 30% or less busy
channel..  With just 4 monitoring stations across the country we have a
99% chance of getting every packet without collision.

de WB4APR, BOb

PCsat WEB  page     http://www.ew.usna.edu/~bruninga/pcsat.html
ISS-APRS FAQ:       http://www.ew.usna.edu/~bruninga/iss-faq.html
CUBESAT Designs     http://www.ew.usna.edu/~bruninga/cubesat.html
APRS LIVE pages     http://www.ew.usna.edu/~bruninga/aprs.html
APRS SATELLITES     http://www.ew.usna.edu/~bruninga/astars.html
MIM/Mic-E/Mic-Lite  http://www.toad.net/~wclement/bruninga/mic-lite.html

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