[OZAPRS] HF Mobile beacon times

Richard Hoskin vk3jfk at amsat.org
Wed Jul 7 13:00:07 EST 2004

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.
> > 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
> HF.
> > Assuming a channel efficiency of 30% for ax25 this results in a
> 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
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
and a 50% chance of being heard by VK4AH-4. So he has a 83% chance of
heard. The WA station has a 50% chance and the NT station has a 33%

2 hrs later conditions have changed and VK3BYD-14 has move location. He
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
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
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
in providing a reliable communications tracking system for remote

> 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
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
when people are used to seeing their friends position being updated every

> 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.


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