In real aircraft, how do instruments determine the distance to a beacon (VORTAC or ILS)?There are only two ways to find the distance to an object that I can think of: 1. triangulate - but this requires two sensors separated by some distance. 2. Time delay - Measure the time a signal originating at the beacon takes to reach the observer - but this requires synchronized clocks which aircraft and the beacons don't have. So, I'm at a loss.Does anyone know how these things work?

I have a great memory but it is short. So, here is what I remember :)The Plane's DME sends out a signal to the ground station (encoded unique to your set). The ground station recieves the signal - waits 50 microseconds - then transmits back your signal (again encoded unique to your set). Your set subtracts the 50 microsecond delay from the total time (send to recieve) and then computes the line-of-sight distance to the staion. Seeya,Kathy.Born to Fly.My Dad, Me, My Son.____________________http://ftp.avsim.com/dcforum/User_files/3d32decc3494b57c.jpg

It's #2 - time delay.The DME unit in the plane sends paired pulses to the ground station. These internogation signals are received by the ground station, which then responds with a paired pulse on a different frequency. Based on the latency between the interrogation and the response, the DME unit computes a slant range distance to the station.Since DME provides slant range distance, it is least accurate at high altitudes when close to the station. If you are at least 1000' AGL for each mile you are from the station, the slant range error is considered negligible.There's some good info in the Aeronautical Information Manual, section 1-1-7. You can read it on line at:John

Thanks for the confirmation that my memory is still intact John. :)See ya,Kathy.Born to Fly.My Dad, Me, My Son.____________________http://ftp.avsim.com/dcforum/User_files/3d32decc3494b57c.jpg

You're welcome, but actually I wrote my response before seeing you had already answered the question! You must have responded while I was typing. :-)BTW, what kind of plane is that in the photo next to your father?John

Hi John.That was a plane my dad and his partner owned. I recall him stating it was a Stearman model "C". They got rid of it many (like 15)years before I was born.See ya,Kathy.Born to Fly.My Dad, Me, My Son.____________________http://ftp.avsim.com/dcforum/User_files/3d32decc3494b57c.jpg

Gee, thought KP launch into the whole "Slant Range" discussion vs INS distance.....I wonder if MS modeled this......? That us a huge Stearman KP....in my next life I want a Staggerwing Beech.Timothy(too much time on my hands.....still waiting on my salad KP)