[OZAPRS] HF Mobile beacon times

[Richard Hoskin]

Hi Ron,


> -----Original Message-----
> From: Ron Perry [mailto:ronk at sunlinux.com.au]
> On Wed, 2004-07-07 at 08:38, Richard Hoskin wrote:
> >
> > Hi Tony,
> >
> > The general concessus is to set HF posit transmissions to 10 minutes.
I
> > believe that it should be 15 minutes especially if the number of HF
> mobiles
> > increases. You just need to do the sums.
> >
> > An average length APRS Posit packet takes 3 to 4 seconds to transmit
on
> HF.
> > Assuming a channel efficiency of 30% for ax25 this results in a
maximum
> of 7
> > stations being able to transmit per minute. At a transmission rate of
> one
> > posit per 10 minutes the maximum number of stations that can be in a
> single
> > APRS Gate's coverage area is 70 stations. This will be less as the Net
> > stations transmit frequently and messages  and/or tuning may be under
> way on
> > the channel. HF Propagation is also an important factor in limiting
> station
> > numbers.
> >
> 
> I've tried to explain the HF propagation factor below.
> The HF propagation means that we could have (upto) 70 HF mobiles in VK4
> being gated by VK3MY and 70 HF mobiles in VK2 being gated by VK5AH-4,
> and none of them interferring with each other.  The lack of propagation
> becomes an advantage.

This can be an advantage or disadvantage. 

If say VK3BYD-14 txed and the conditions where right he would access both
VK4DMI-4 and VK5AH-4. (may be even VK3MY-4 and others as well.) This means
that a station in WA that can only access VK5AH-4 at the time will have
less
chance of being rxed. There is also an HF station near Darwin who can only
get to VK4DMI-4 and one is ZL that VK4DMI-4 can here but cant decode
(thought VK3MY-4 can).

So what do we end up with?

In the example above  VK3BYD-14 has a 33% chance of being heard by
VK4DMI-4
and a 50% chance of being heard by VK4AH-4. So he has a 83% chance of
being
heard. The WA station has a 50% chance and the NT station has a 33%
chance.

2 hrs later conditions have changed and VK3BYD-14 has move location. He
now
can access VK3MY-4. The WA station can access VK4DMI-4, the NT station can
access VK5AH-4 and interferes with VK4DMI-4 and the ZL station can access
VK4DMI-4, VK3MY-4 and interferes with VK5AH-4.

3 hrs later we have the scenario you suggested where the WA station has
access to VK5AH-4, The NT station as access to VK4DMI-4, the ZL station is
communicating with VK3MY-4 and VK3BYD-1 can not access any HF Base
station.
But VK4BBS-4 is on his way home and is wiping out 50% of the NT stations
access time.

This is complex system of chance and probability. What we know is that if
we
limit the traffic on the channel then the probability of a single
transmission being decoded increases as the competition is decreased. This
is one of the few items in the system we have some control over to assist
us
in providing a reliable communications tracking system for remote
travellers.

> 
> So, IMHO, at this stage with the limited number of HF mobiles, I don't
> think a beacon at 5 minutes would hurt the network, but being mindful
> that as more HF mobiles become active, I agree, then the outcome of
> Richard's sums would be implemented.

>From experience it is very hard to get all operators to change their
tracker
setting once a standard is in place.

5 min may be ok now but try and get it change to 10 or 15 min in the
future
when people are used to seeing their friends position being updated every
5
min.

> 
> And just musing, would the HF network handle 200 HF mobiles?  (just
> dreaming)
> 

A few extra HF RX Base Stations around the country would assist in this.

Cheers
Richard.
VK3JFK

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