DISTANCE MEASURING EQUIPMENT (DME)

DME is a ranging guidance equipment which gives the slant range from the component/ground station installed

Principle: Secondary Surveillance Radar (SSR) Principle

i.e. When a transmitted coded radar pulse is reflected by an object and received back at the source, the time taken is used to find the distance using the simple formula,

Distance= Speed x Time

Any information can be exchanged using the SSR but not via a Primary Radar because the transmission is in Analogue Signal and works on Echo Principle.

Requires,

• Vertical Polarization

• Line of Sight (LOS)

Antenna type: Omnidirectional Antenna

Freq : Ultra High Frequency (UHF)

Freq Range: 962-1213 MHz

(Previously it was 978-1213 MHz)

Channel spacing: 1 MHz

Therefore, total number of channels – 252

The 252 channels are subdivided into X and Y channels,

X – 126 channels

Y – 126 channels

The X and Y channels are further subdivided into 2-2 sub group of channels respectively.

X – 126 channels ( 63 channels(A1) – 63 channels(B1))

Y- 126 channels ( 63 channels(A2 – 63 channels(B2)).

Note: The designations A1,A2,B1,B2 are assigned solely for the purpose of understanding

The range of

A1: 962 MHz – 1024 MHz

(1x, 2x,…. 63x channel)

B1: 1025 MHz – 1087 MHz

(64y, 65y, ….. 126y channel)

A2: 1088 MHz – 1150 MHz

(1y, 2y, …. 63y channel)

B2: 1151 MHz to 1213 MHz

(64x, 65x, …. 126x channel)

The purpose of this is to clarify the fact that

Aircraft can only interrogate from 1Y channel to 126Y channel, But GRD station can reply in all channels. WHY?

To ensure that when an interrogative signal is sent from the aircraft, the DME station needs to have an option to reply either +63 MHz/ – 63 MHz,

Just to ensure that the working of both the components doesn’t exceed the range of 962-1213 MHz.

Example: If the aircraft, transmits at 1000 MHz, the DME GRD station can reply either in 937 MHz or 1063 MHz,Though 1063 MHz in range but 937 MHz isn’t.( 962 MHz to 1213 MHz)So, aircraft cannot transmit in 1000 MHz

1Y to 126 Y Channel frequency range is 1025 MHz to 1150 MHzSo, if aircraft transmits in 1025 MHz,Then the reply can be either in1025 – 63 = 962 MHz1025 +63 = 1088 MHz

CHARACTERISTICS OF X AND Y CHANNEL

Pulse Repetition Frequency

X channel- 12 microseconds

Y channel- 36 microseconds

Pulse width Frequency

Both X and Y channel – 3.5 microseconds

In DME,

Aircraft is interrogator

Interrogates by Jitter Pulse

GRD station is Transponder

Replies by Squitter Pulse

JITTER PULSE

Deliberate random variation of the time interval between successive interrogations. Each interrogator produces a Jittering pulse (pair)repetition frequency (p.r.f.) which, over a period of several interrogations, describes a unique pattern since the variations are random.With an interrogation rate of, say, 100, the average interval between interrogations will be l0 ms, with any particular interval being between say 9 and 11ms. The unique interrogation pattern enables the DME to recognize replies to its own interrogation by stroboscopic techniques.

SQUITTER PULSE

The auto standby circuit will not allow interrogations to commence until it detects signals from the GRD STN. When a sufficient number of interrogating aircraft are within range of the station, there is no problem, since another aircraft coming within range will receive all the replies and thus begin to interrogate. If, however, we consider the GRD STN having just come on line or the first flight, after a quiet period, approaching the STN, we have a-chicken-and-egg situation: the beacon will not reply unless interrogated; the interrogator will not interrogate unless it receives signals. Thus far there are infact signals available, namely ident, but this means an aircraft may have to wait 30 s, perhaps more in weak signal areas, before coming out of auto standby.This is unacceptable; consequently the beacon is made to transmit pulse pairs even in the absence of interrogations. Such transmissions from the ground beacon/STN are known collectively as Squitter‘ to distinguish them from replies.

When the random squitter pulse pairs are received the airborne equipment starts to interrogate.

The DME in the aircraft asks continuously asks question/interrogates the nearby GRD station by transmitting coded pulses.

When the transponder in the GRD receives the coded pulses, It replies with a delay of 50 microseconds.

To find the accurate distance,

The aircraft itself compensates for the delay of 50 microseconds and therefore calculates the slant range to the GRD station.

This data can also be used to calculate the Ground Speed (GS) and Time to Station (TTS).

Once every 30 seconds, the Ground Station transmits its identity which is detected by the pilot as a Morse code, with a burst of three letters at an audio tone of 1350 Hz.

It should be noted that the R.F. radiated from the beacon/GRD STATION during identification is of the same form as when transmitting replies, i.e. pulse pairs.

The difference is that when replying there are random intervals between transmissions whereas during identification the intervals are constant at 1/1350th of a second.

MODES

SEARCH MODE: In this mode, the aircraft tries to reach out a GRD station within its range with a high interrogating pulse rate.

Pulse Rate: 40-150 Interrogating Pulses/Sec

When it finds a GRD station, the a/c goes into TRACK MODE

TRACK MODE: In this mode, the aircraft constantly tracks the GRD station and therefore the pulse rate of interrogation reduces.

Pulse Rate: 10-30 Interrogating Pulses/sec

When the aircraft goes into TRACK MODE,

After every 4- 12 seconds aircraft waits for DME pulses,

If it finds the pulses, it stays in TRACK MODE,

else it will again revert back to SEARCH MODE

MEMORY MODE

MEMORY MODE has two kinds of memory displays, in case if the aircrafts reverts back to SEARCH MODE

• Static Memory Display – Display will be freezed up for about 100 NM.

• Dynamic Memory Display- Display will continuosly change because at the time of interrogation the True Airspeed (TAS) in one second is already noted by the component.

ADVANTAGES

1. Ranging accuracy is very high, i.e. 0.1 -0.3 NM

2. Accurate Distance to the touchdown is available.

DISADVANTAGES

DME can only reply to a maximum number of 100 aircrafts at a time whose interrogation signal are stronger.