OGN-R max range

[Deker]

I was puzzled by why the max range of the Flarm far exceeded that of the p3i transmissions.
Particularly curious as to why the range heat map was perfectly circular at turweston and crowland for example and also other stations show a circular max.
The differing local topography 'must' give a varying heat map, more like the variation you see on the Flarm plot.

More 'suspicious is that Crowland and Tur max out at exactly the same range of 60.2KM

So, 'I think' that the reporting software blocks or junks any received data that exceeds 60.2 - 60.4km range?
Would be good to see what the p3i is actually reaching.

Deker.

[exfirepro]

I was puzzled by why the max range of the Flarm far exceeded that of the p3i transmissions.

That bit’s easy Derek - the gain of the OGN antenna is significantly higher so will receive transmissions from further away, though the furthest away ones are usually only received at very low power - so not necessarily reliably. We are restricted in transmission power on the PAW side to a maximum of 500mW ERP (Effective Radiated Power). As the PAW Bridge is already designed to produce the max 500mW, into its standard antenna, we can’t simply up the antenna gain to improve the range. In any case, we only want to advise PAW users of OGN traffic within the local area (of approx 20-30 miles), so there isn’t really much point.

Particularly curious as to why the range heat map was perfectly circular at turweston and crowland for example and also other stations show a circular max.
The differing local topography 'must' give a varying heat map, more like the variation you see on the Flarm plot.

More 'suspicious is that Crowland and Tur max out at exactly the same range of 60.2KM

So, 'I think' that the reporting software blocks or junks any received data that exceeds 60.2 - 60.4km range?
Would be good to see what the p3i is actually reaching.

Deker.

This bit isn’t so easy - and something I must admit I wasn’t aware of. I could try asking over on the OGN Forum I suppose.

Regards

Peter

[Admin]

So, 'I think' that the reporting software blocks or junks any received data that exceeds 60.2 - 60.4km range?
Would be good to see what the p3i is actually reaching.

You got it :-)
I discard anything over 60km's there is also a height filter as well but cannot recall what this is.
There is a good reason for this, when on the edge of range the data can arrive ever so slightly corrupted, such as 1 or 2 bit errors, and in the very few cases those errors also appear in the CRC check
so what looks like a valid packet usually has something very weird, eg an aircraft zoomed 15km in 1 second, or jumped 20,000ft in 1 second. So this is a way of using heuristics on top of the CRC

Thx
Lee

[Deker]

You got it :-)
I discard anything over 60km's there is also a height filter as well but cannot recall what this is.
There is a good reason for this, when on the edge of range the data can arrive ever so slightly corrupted, such as 1 or 2 bit errors, and in the very few cases those errors also appear in the CRC check
so what looks like a valid packet usually has something very weird, eg an aircraft zoomed 15km in 1 second, or jumped 20,000ft in 1 second. So this is a way of using heuristics on top of the CRC

Thx
Lee

Thanks for the confirmation that my brain cell made the correct assumption

...thinking again ....due to the varying nature of the G/stns and aircraft installs:  antenna, location, height, topography, flight direction etc  there will still be some aircraft that will be well within the 60km limit but still are providing a scratchy signal.
I wonder if you could use the shape of the flarm plot to 'shape' the p3i range limits thereby reducing the chances of a corrupt position in directions of poor signal and visa-versa
An unlimited plot would be useful in determining G/STN performance or the effect of change in equipment or antenna etc. ....but I guess there are more important things on the go :-)
The bridges don't 'do' RSSI?? or I guess you could just junk anything that didn't have a S/N ratio above a certain figure?

[Admin]

Hi Deker

What we are trying to do here is to suppress bad data going back to the APRS network which makes A/C appear to 'jump around'. Dont forget this is not a 'real limit' - it is a limit applied to suppress unrealistic data.

Imagine a co-ordinate such as longitude, a single bit flip in the MSB part of the data can make it 'appear' to jump a wild distance, or altitude, again a 1 bit flip in the MSB makes it jump. Bear in mind that a single bit flip in isolation will not cause this behavior, it must be accompanied by a corresponding bit flip in the CRC. This is a very unusual and non-probabilistic event, but when we are recording thousands of transmissions, then the probability is more pronounced.
We saw this on very very few occasions, but enough to introduce the guard.

The downside is that this is destroying the competition I understand that is going on to get the most far reaching detection from the ground stations by their installers 

Thx
Lee

[Deker]

The downside is that this is destroying the competition I understand that is going on to get the most far reaching detection from the ground stations by their installers 
Thx
Lee

Drat, we've been found out :-)