VOR question

[Mcstealth]

How far does a VOR signal transmit, or how far away can you be before you loose signal? Where can I look that up? Far/Aim maybe?

David

[N201MKTurbo]

SSV Class Designator Dimensions T (Terminal) From 1,000 feet above ground level (AGL) up to and including 12,000 feet AGL at radial distances out to 25 NM. L (Low Altitude) From 1,000 feet AGL up to and including 18,000 feet AGL at radial distances out to 40 NM. H (High Altitude) From 1,000 feet AGL up to and including 14,500 feet AGL at radial distances out to 40 NM. From 14,500 AGL up to and including 60,000 feet at radial distances out to 100 NM. From 18,000 feet AGL up to and including 45,000 feet AGL at radial distances out to 130 NM.

[jetdriven]

At FL350 or so, you can recieve VOR signals at a maximum of 199 NM. Perhaps slightly more, but its line of sight. The formula is

Dmax= 1.23*SQRT(h1) + 1.23*SQRT(h2)

Dmax is the maximum distance in miles, and h1 is the height (AGL) of the aircraft, h2 is the height (AGL) of the VOR or the control tower. This is theoretical distance, and the real story is 10% or so less.

[Jerry 5TJ]

Byron is correct, if you ignore refraction. At VHF it is common to use a 4/3 earth radius assumption due to 'bending' of the radio waves because of refraction. Using that model you may hear a useful signal from a VOR out as far as 250 nmi or so, but that's for a plane well up in the flight levels.

Of course, a VOR is a radial system, and the farther you are from the station the greater the error. At 180 miles out a 1 degree error is over 3 miles, and it is hard to get a VOR system to maintain 1 degree accuracy. Not bad for a system developed well before WWII, but not so hot in the era of sub-10 meter GDOP expectations.

[RJBrown]

Line of sight makes a big difference in the mountains. On airport VORs can dissapear within 10 miles when blocked by the terrain. On Low IFR charts they show a vor change over point. On Victor 134 between FFU (Fairfield) and PUC (Carbon) VORs the distance is 68 miles between but only useable from FFU for  20 miles while the signal from PUC is good for 48. Then out of PUC the signal is usable for only 25 before you must dial in JNC (Grand Junction) for the remaining 72. These examples are on ELUS 9.