DME operates on Frequency band of 962 to 1213 MHz.
Principle used is Pulse Technique on Secondary radar.
Ground Transponder operates on frequency band of 962 to 1024 & 1151 to 1213 MHz.
A/C system has an Interrogator which works on the Frequency of 1025-1150 MHz.
A beam of pulses is transmitted by the A/c Interrogator/Transmitter in short burst.
The beam of pulses are received at the ground station, amplified and transmitted back on a different frequency. Since the system works on the Secondary Radar principle it requires only energy to reach the transponder located on the ground, which has it,s own Transmitter to reply back on a different frequency. The A/c Interrogator measures the speed of Pulses with reference to time between the Transmission and reception and determines the range of the a/c.
The range obtained through this method is called Slant Range, which can be converted to the distance by the formulae-
Ground Range²= Slant Range² – Height²
Where all the Distance and height is converted into N.M
The accuracy of the equipment is ±3% of total distance.
The DME also provides Ground Speed of the A/c which is reliable only when the a/c is flying directly towards or away from the facility.
Although there is a very small cone of confusion exactly over head the beacon, like the VOR beacon but is very small and can be ignored, the range indications continuously shows the range due to Memory recall system built into the system
The DME never shows 0 N.M, if the a/c passes over the DME facility it will show the Altitude of the A/c in N.M.
DME Locking means A/c is getting DME indications.
When the DME is co-located with VOR, or are within 7 N.M of each other they are treated as co-located and a letter Z is included in their identification sign.
Errors-
- Beacon Saturation- When the maximum number of A/c (Usually taken as 100 A/c) are using the facility, the beacon gets saturated and replies to the a/c that are closer in range.
- Line of Sight Range- The Range depends upon the Height of the Transmitter and Receiver.
As a Thumb rule, when the Slant range is more than double the Altitude, the difference or the error between the Slant range and the Ground range is Zero.
In DME slant ranges are given. When the slant range is double the altitude, the error between the slant range and the ground distance is O.