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Re: tracking through APRS



On Thu, 19 Oct 2000, Alex wrote:

> I'm writing some APRS software for a Linux platform which transmits
> satellite objects [onto the nationwide APRS frequency]. I think Bob
> Bruninga wrote something similar for DOS.. [APRSdata.exe]
> 
> The purpose of this is to give people a live feed of where the
> satellites are and where to point their antennas to if they are within
> a 30 miles radius of the transmitting station....
> 
> Now to my question. If I would broadcast the elevation and azimuth data
> for the position of the transmitting station, would this be accurate...?

No need to do this.  Since APRSdata transmits the actual position of
the satellite in real-time, and All APRS stations including the D700
mobile or D7 HT know where they are, they all compute and display the
actual AZIMUTH and DISTANCE to the satellite unique to them.

Thus, each radio already has precise and unique azimuth data, including
an arrow on the front panel pointing in that direction!  There is also
no need for elevation data Since most LEO satellits are about the same
height.  THus there is a one-to-one correlation between RANGE and
ELEVATION ANGLE.  If the satellite gets to within 800 mi, then it is about
45 degrees up.  If it is further than 1200, then it is around 30 deg or
less.  And so forth.

This way, only one APRSdata needs to be running in each major region and
it provides good updates to every HT and mobile within several hundred
miles.

> I tried to get an answer using tracking software....  

No need to do that.  Just look at the geometry.  If your position says 
the satelite is at 60 deg up to your west, then someone that is about
300 miles west of you will see it overhead (90 deg) etc..

> Any feedback is appreciated.

So, I chose to let the users HT and Mobile compute the AZ and DISTANCE
instead of burdening the channel with the same data that is only good for
your station.  Instead, APRSdata.exe uses the available 20 free characters
to transmit the satellite UPLINK and DOWNLINK frequencies and
instantaneous Doppler.  

Thus the HT or mobile user SEES ON HIS RADIO FRONT PANEL the following
data:

   AZimuth and distance
   Frequency of the UPLINK and current doppler
   Frequency of the DOWNLINK and current doppler

And that is ALL he needs.  He just tunes to those freqs, works the
satellite and then goes back to monitoring APRS on 144.39 until his HT
ALSERTS him to the next pass.

Also, APRSdata sends out a single packet in the DX Cluster Spot format
that the HT and MOBILE capture once every 10 minutes.  This single packet
contains the SCHEDULE of all ssatellites that will be in view in the next
80 minutes.  THus an HT or MOBILE user never needs a computer, laptop or
KEPS or list of freqs or anything.  His radio has everything he needs to
know about all satellties both IN VIEW and ANTICIPATED in the next 80
minutes.

I call it TINY WEB PAGES that are "pushed" to every radio on 144.39 in the
north american continent.  If there is not someone running APRSdata in
yoru area, then just download it and run it on an old DOS machine in the
corner to serve your fellow hams...


P.S. APRSdata also can transmit a single ONE-LINE-ELEMENT once an hour
24 hours a day so that after  monitroing the APRS channel for only about 
36 hours, your computer will be updated with the latest elements...
continuously if you want...

de WB4APR, Bob


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