ACARS distance?

[Jay911]

Hi folks,

When I'm receiving ACARS data from aircraft, is the signal being relayed to me from the a/c itself or from a ground station?

I'm hoping this doesn't sound too noobish, but I'm getting some signals that, if the positions are correct, are coming in from absolutely ridiculous distances.

My setup: BC72XLT with an RH77CA antenna, set to 136.85 (one of two active freqs in my area), running into a PC running ACARSD. I am at least 30 miles line-of-sight to the airport (and the lat/long indicated on the government license for the above frequency).

I am at (roughly) N50°56'34'' W114°37'00'', in the foothills of the Rocky Mountains.

The license for that freq is N51°05'49'' W114°02'21'', on the grounds of the Calgary International Airport (CYYC).

I just received an ACARS packet which reported a position of N50°06'21'' W093°27'36'' which is over 900 miles almost due east of me.

Did I legitimately receive a signal from an aircraft over Ontario while I'm in Alberta? Or is it being retransmitted to me from a ground station?

If it matters: I never see signals originating from ground stations. I see aircraft sending messages TO ground stations all the time, including queries, but never see responses back.

Your help in educating this relative newbie to aeronautical monitoring is appreciated!

 

[SkipSanders]

You are receiving from the aircraft itself (except the ones sent from the ground station TO the aircraft, of course).

900 miles would indicate a very, very good path (tropospheric ducting) was occuring at the time, it doesn't represent 'expected' distance. The 'usual' max range from an aircraft at 10,000 feet would be 100-200 miles. Anything past 300 is probably an unusual weather related path.

Of course, your software allows you to link in via the internet with other receiving stations, and if that's turned on, well, you might be getting info from anywhere on the planet.

 

[w0fg]

900 miles is unusual but not impossible, particularly if the aircraft was at FL400 instead of FL100. I occasionaly hear aircraft calling Cheyenne or even San Franciso from here in Iowa. Altitude, aircraft attitude, and transmitter power are all variable factors.

 

[Yokoshibu]

My average farthest is from 380-500 consistent... tropospheric ducting sounds like a good possibility on that one! however I have seen aircraft relay multiple points in one packet and acarsd plot the wrong position so it appears to be farther away than it is... usually results in a track that looks like a Z going past it's first tracked point. ... that is usually obvious though!

 

[SkipSanders]

Acarsd has a 'bug' where it assumes aircraft position reports are in degrees, minutes, where they're actually being reported as decimal degrees (or vice versa, not sure), resulting in major position errors. Sadly, some aircraft report in one format, some the other, with no obvious way except 'reality check' by brain to decide which they're using. It's probably by airline, and THEY know which their aircraft use...

This can mean an A/C reports 32 degrees, 59 minutes, and is 'seen' at 32.6 degrees, etc.

 

[Jay911]

Aha.
I for example am getting a message that contains, in part, N49328W119575 which ACARS reports as North: 49.33 West: 119.58. What I think I'm getting from what you're saying is I should be reading it instead as North 49°32.8' by West 119°57.5'.

The message format I show above is the one I usually see on acarsd - there are many more numeric values after the 6 digits after the W, but I figure they're part of the rest of the message - that they're not computing longitude out to some insane value of precision..

 

[SkipSanders]

That's the idea, yep. I suggested the acarsd folks add a 'switch' to let you choose which format it assumes lat/long is in, but no response. If you use the acarsd ability to 'link' to another program such as PosFix, THAT program seems to read the data correctly, even if acarsd doesn't.

 

[morfis]

That's the idea, yep. I suggested the acarsd folks add a 'switch' to let you choose which format it assumes lat/long is in, but no response. If you use the acarsd ability to 'link' to another program such as PosFix, THAT program seems to read the data correctly, even if acarsd doesn't.

As you point out, airlines will use the format that their software is set to work with. In several cases the airline will use more than one format depending on the aircraft TYPE. Posfix was written to try and sort out this jumble after WACARS made the first real progress at mapping aircraft positions based on the ACARS data. ACARSD is a decoder primarily and perhaps a descision has been made not to re-invent the wheel given that posfix is easy to integrate with it?

 

[crayon]

Altitude, aircraft attitude, and transmitter power are all variable factors.

Agreed. However, it is always nice to have some math on hand to compare the real world against.



The rule-of-thumb to figure the maximum distance to the horizon between a ground station and an aircraft, in nautical miles, for VHF transmissions, is equal to the square root of the height of the aircraft in feet. This is assuming, of course, the underlying terrain is flat.

Actually, it is 1.06 times the square root of the height but, conversationally, that can be ignored. Also remember that other standard propagation disclaimers apply.

 

[SkipSanders]

The quoted 'rule of thumb' is Motorola's rule for 'reliable communications'. Most likely, you can get out to 1.3 or 1.4 times that, most cases. Up to 2 times that, when the gods smile upon you. It's not intended as anything more than a simple, easy to remember rule for 'what you can expect'.