Operational Notes on Distance Measuring Equipment
1 - INTRODUCTION
The Australian D. M. E. employs a double pulse system of aircraft to ground beacon interrogation. The system uses only two carrier frequencies in the V.H.F. Band for its operation - 206 mc/s for the aircraft to ground beacon interrogation and 224 mc/s for the ground beacon to aircraft reply. Typical of V.H.F. transmission, coverage is nominally "line of sight".
Beacon selection is achieved by presetting each beacon to respond only to interrogating signals having a predetermined time interval between the pulse pairs. The airborne unit channel selector provides for twelve different time interval settings.
Other versions of Distance Measuring Equipment which have been developed overseas are not covered in this booklet.
2 - GROUND EQUIPMENT
The ground equipment consists basically of a high powered transponder beacon, with accessory equipment in the form of an aerial array, end test equipment.
In order to afford greater simplicity of the airborne unit, all ground beacons in the complete system operate on the same two frequencies: one for the ground-air link and the second for the air-ground link. Channelling is provided by the system of double pulse interrogation from the aircraft, together with the pulse separator discriminator in the beacon. The mode of channelling enables the pilot to select a particular beacon to the exclusion of all others which may be within range. Provision is made for the beacon to be adjusted to any one of twelve interrogating channels.
A single aerial is used for both transmission and reception, and it is mounted on a tower adjacent to the building housing the beacon. The main electrical requirements of the aerial are that it have a broadband frequency characteristic since the transmitting and receiving frequencies are 18 mc/s apart, and that it have an omnidirectional field pattern.
3 - AIRBORNE EQUIPMENT
The component parts of the airborne installation consist of the main interrogator-responsor unit, control unit, indicator meter and aerial. A photograph of the airborne equipment appears at Figure 2.
All electrical connections are made to a multi-pin connector at the rear. There are a number of internal pre-set controls for initial adjustment. The weight of the complete unit including mounting rack is approximately forty-two pounds.
A view of the chassis of the main unit is shown in Figure 3. The dynamotor seen at the rear has an extension shaft for a cooling fan. The receiver unit and the transmitter unit are built as separate items and the remaining sections comprise the transmitter modulator, ranging and coding circuits, power supply, etc. A total of 44 valves, including neon lamps, are employed in the equipment. The current required from the aircraft battery to operate the equipment is approximately 9 amps.
Three models of interrogator-responsor providing maximum ranges of 100, 200 and 300 nautical miles have been produced.
The standard type selector permits the selection of the ranges 0-10 or 0-100 nautical miles and other versions permit the selection of the ranges 0-20 or 0-200 n.miles and 0-30 or 0-300 n.miles.
4 - PRINCIPLES OF OPERATION
A simplified block diagram of a D.M.E.-to-beacon link is shown in Figure 5.
The airborne equipment transmits on a frequency of 206 mc/s and receives on 224 mc/s, whilst the ground equipment receives on 206 mc/s and transmits on 224 mc/s.
The airborne transmitter emits pairs of pulses on 206 mc/s, the pulses (of 2 micro-seconds duration) in each pair being separated by a selected time interval. This time interval can be varied from 10 to 54 micro-seconds in accurate 14 micro second steps. The channel selector enables the selection of any of the twelve different time intervals corresponding to the particular time interval which has been pre-set on the ground beacon.
The pulse repetition rate from the airborne unit is set at a nominal 100 pulses per second, but is permitted to vary between 90 and 115 per second to prevent any possibility of synchronism existing between any two airborne transmitters within range or the same ground beacon. By deliberately arranging the random arrival at the ground beacon of interrogations from different aircraft, maximum response is ensured to individual aircraft. The ground beacon is designed to be "triggered" by a pair of pulses which are separated by a selected fixed time interval, e.g., one beacon may be triggered if the pulse separation is 10 micro-seconds and another if the pulse separation is 22 micro-seconds. The unit of the ground equipment which discriminates between pulse intervals is called the "pulse selector." The "pulse selector" receives the first pulse of the pair transmitted from the aircraft and uses it as a "gate" for the second pulse. If the second pulse arriving corresponds to the pre-set time interval, it is allowed to pass the "gate" and triggers the ground beacon transmitter. It is the second pulse which, on leaving the aircraft, starts the timing circuit in the airborne equipment. The second pulse is delayed by the ground equipment for 12.4 micro-seconds (equivalent to one radar or return nautical mile) before the ground transmitter emits a reply pulse. One reply pulse (5 micro-seconds duration) is transmitted for each pair of pulses received by the ground beacon. The train of reply pulses is accepted by the airborne receiver which then electronically measures the time interval between the second pulse of any pair emitted from the airborne transmitter and the receipt of the ground reply pulse. This measurement is them presented on the indicator meter as a distance in nautical miles, the airborne unit being adjusted to allow for the ground beacon delay of 12.4 micro-seconds.
Positive identification of a ground beacon is provided by means of pulses transmitted from the beacon 20 micro-seconds after each reply pulse. The transmission of these pulses, in the form of morse symbols, is controlled by a keying wheel, the periphery of which is cut with the identification of the particular beacon. The airborne receiver effects discrimination between these code pulses and the ranging pulses, and the former are used to control the voltage applied to the neon tube in the coding lamp. Simultaneously with the activation of the neon tube, an audio tone is generated and fed to the headphone circuit for aural identification purposes.
When the airborne equipment is switched on the transmitter emits the two pulses with the selected time-interval difference. The automatic search circuits which commence in operation simultaneously with the transmission of the second pulse, search, in time, out to the equivalent of 0-12 miles (on the 0-10 mile range) and 0-120 miles (on the 0-120 mile range). This search action continues from zero to maximum range, each sweep taking approximately four seconds on the short range (0-10 miles) and 25 seconds on the long range (0-100 miles), until a synchronised beacon reply is received. When this occurs the equipment "locks on" to the reply signal which it continues to track and at the same time the timing circuits measure the time delay between the emission of the second pulse from the airborne transmitter and the reply pulse from the beacon. Since this time delay is a measure of the distance of the aircraft from the selected beacon, the measured time delay can be applied to the indicator calibrated in nautical miles.
Should the "lock on" reply pulses fade for any reason such as terrain or aircraft shielding or due to a beacon failure, a "memory circuit" will maintain the equipment in a tracking condition for a period of approximately four seconds. During the interval when the signal is absent the indicator needle will continue to register the distance reading indicated prior to the reply pulse fading. Should the beacon signal restore to normal before the expiry of the four seconds, the equipment will continue normal operation. In the event of the signal not restoring to normal within the memory time period the distance needle will return to zero and searching action will recommence.
Interference pulses (called echoes) or pulses from other beacons in operation within a coverage area are rejected by the search and timing circuits.
5 - AIRBORNE EQUIPMENT HANDLING
The following sequences should be adopted in the operation of the standard airborne D.M.E. equipment:
1. Switch the power on; the code lamp will then light. (After a warm-up period of about 15 seconds, the equipment will commence to transmit the interrogation pulses and search for a reply.)
2. Consult current publications for the location of ground beacons and channel assignment. Adjust the channel selection switch on the D.M.E. control unit to the channel assigned to the desired ground beacon.
3. Unless the distance to the beacon is known to be less than 10 miles, set the range switch to X10 which selects the 0-100 n.miles range.
4. Select D.M.E. on the audio selector box.
5. The time taken for "lock on" after the warmup period will vary according to range and density of beacon users, but normally will not exceed four seconds on the short range or 25 seconds on the long range.
6. When the equipment locks on to the ground beacon, the indicator meter presents the distance to the beacon, the code lamp will be extinguished and will then flash the beacon code every 20 seconds. The beacon code signal will also be heard over the aircraft audio system. The selected beacon should be identified by watching the code lamp, and listening to the aural identification.
7. D.M.E. should be checked, as indicated in paragraph 6 during routine use of the equipment.
6 - D.M.E. EQUIPMENT CHECK
Similar to the necessity to check the performance of other radio aids to navigation, it is required that the pilot shall satisfy himself that the D.M.E. equipment is functioning normally prior to descending to the lower approach altitudes which will be available to D.M.E. equipped aircraft and also during routine use of the facility.
Suggested methods of checking the equipment are as follows:
(a) Ensure that the correct beacon is being interrogated by listening to and visually observing the identification signal.
(b) When over a known fix point such as an N.D.B., marker beacon, range station or visual fix, check that the D.M.E. distance indication is the same as the known distance from the beacon to that fixed point.
(c) When on a known track, such as a range leg, note the distance from a particular D.M.E. beacon and confirm the position fixed by noting the distance from another D.M.E. beacon in the vicinity. A simple example of this check is when the track being flown is that joining two D.M.E. beacons, then the sum of the D.M.E. distance reading from each beacon should be equal to the known total distance, e.g., 30 miles from Sydney D.M.E. - 49 miles from West Maitland D.M.E., and the sum equal to the known total distance of 79 miles.
7 - THE USES OF D.M.E.
The common factor in the use of D.M.E. for any purpose is time saving with no reduction of safety margins; and in this regard there is promise of time saving in the fields of lower safe approach altitudes, approaches to fields not equipped with precision approach aids, in orbiting, reduction of separation standards, and in improved en route navigation.
Actual and envisaged uses of D.M.E. are outlined in the following paragraphs. It is anticipated that other uses of D.M.E. will eventuate with experience.
and Descent Procedures
(a) The V.H.F. characteristics of D.M.E. partly offset the shortcomings of the MF aid (A.D.F.) due to the limited performance of the latter under certain conditions of propagation and atmospheric noise.
(b) The N.D.B./D.M.E. provides a system of area navigation.
NOTE: Homing on a beacon by the sole use of the D.M.E. equipment in the event of a failure of the A.D.F., has been established as a simple and practical procedure. This procedure, known as the "30 Degree and Rate of Closure Method", is in use and appears in detail in Appendix "A". Instrument descent procedures associated with D.M.E. homing, once the aircraft is located over the beacon, have been prepared and issued for inclusion in the DCA Aeronautical Information Publication. Detail of the descent procedure appears at Appendix "B".
At present, it is sometimes necessary to hold en route at facilities which may be some distance from the terminal. D.M.E. will, in certain circumstances, enable en route holding to be accomplished closer to the terminal and so reduce the overall flight time under conditions when holding procedures are necessary. D.M.E. permits holding patterns to be flown with greater precision than hitherto and, therefore, defined areas for holding and control zone areas may be reduced at certain locations.
of Positive Terrain Clearance
Arriving aircraft will benefit considerably since, on the basis of the D.M.E. distance reading, it will be possible for ATC to expedite arrivals by assigning lower levels earlier than is the present practice.
on I.L.S. Approaches
The continuous presentation of distance along the flight path on a landing approach should simplify, to a considerable extent, pilot procedures on I.L.S. approaches.
of Cloud Base Minima
of Compulsory Reporting Points
in Turbo-prop and Jet Operation
The economic and safety aspects of jet and turbo-prop high fuel consumptions dictate efficient control on departure and arrival. D.M.E. will contribute substantially to the provision of more accurate and reliable information which will minimise control delays.
APPENDIX "A" - HOMING BY D.M.E.
It has been established that, by using the distance information from a D.M.E. unit, homing to a D.M.E. beacon is quite a simple and practical procedure. There are several procedures which maybe used but the method described below (known as the "30 Degree and Rate of Closure Method") is considered to be most satisfactory, bearing in mind simplicity and accuracy. The adoption of this procedure as a "standard" is recommended since it will then result in the pilot and Air Traffic Control having a common appreciation of the circumstances should it be necessary for an aircraft to use D.M.E. procedural homing under instrument conditions.
It will be readily appreciated that, in still air conditions and provided the aircraft is flying a steady course in a direction which results in a decrease in distance from the D.M.E. beacon, then either:
(i) the aircraft will fly over the top of the D.M.E. beacon ( this is unlikely); or
(ii) it will reach a position where the distance information will cease to decrease and than commence to increase.
At this position a turn of 90 degrees in the correct direction would take the aeroplane over the D.M.E. beacon. Should the 90 degree turn be made in the wrong direction than it will be immediately apparent by an increase in the distance indication and a further 180 degree turn would head the aeroplane towards the D.M.E. beacon.
described in the previous paragraph is simplest, but in certain circumstances
could result in a considerable amount of flying before arriving over
the beacon. The procedure developed as the "standard" allows
for the determination of the course which will take the aircraft to
the D.M.E. station as near as practicable by the shortest route.
(i) When the aircraft is tracking directly towards the beacon the rate of decrease in distance (i.e., the rate of closure) will be CONSTANT also the rate of closure will be at a MAXIMUM (see Note (vi)).
(ii) If nil rate of change in distance is obtained on a particular course then the D.M.E. station is on a relative bearing of 90 degrees to the left or right of the aircraft - the reciprocal heading to the one required to fly towards the beacon will be shown by an increase in distance indication.
(iii) If on a particular heading the rate of closure is observed to be decreasing then the aircraft is not tracking directly towards the beacon.
(iv) If a certain rate of closure is obtained when flying a particular heading and a turn is made to a new heading; then:
an increase in the rate of closure is obtained then the new heading
will take the aircraft by a shorter route to the
(b) if a decrease in the rate of closure is obtained then the new heading will take the aircraft on a track further displaced from the direct track to the beacon;
(c) if the same rate of closure is obtained, then the heading required to take the aircraft by the direct track to the beacon is between the two headings on which the same rate of closure was obtained.
"30 Degree and Rate of Closure Method"
1 - Select and identify the D.M.E. beacon.
2 - Fly a constant course and note whether the distance indication remains CONSTANT, is INCREASING or is DECREASING.
3 - If the distance indication is CONSTANT then the relative bearing of the station is 90 degrees to the left or right of the aircraft.
4 - If the distance indication is INCREASING then turn 180 degrees to a new heading which will then give a DECREASE in distance indication.
5 - When a heading which gives a DECREASE in distance, note the RATE OF CLOSURE - (e.g., 5 miles in 2 mins. 10 secs.) - see Notes.
6 - Turn
30 degrees LEFT and again note the RATE OF CLOSURE.
7 - If the RATE OF CLOSURE has INCREASED then turn another 30 degrees LEFT and note the new RATE OF CLOSURE. Now bracket the course which gives the highest RATE OF CLOSURE in order to find the course on which MAXIMUM RATE OF CLOSURE CAN BE OBTAINED.
8 - If the RATE OF CLOSURE has decreased (after Step 6) then turn 60 degrees RIGHT and note the new RATE OF CLOSURE. Now bracket the new course in order to find the course on which MAXIMUM RATE OF CLOSURE can be obtained.
9 - Having determined the course on which to fly, make frequent checks of the RATE OF CLOSURE. If the RATE OF CLOSURE remains CONSTANT maintain the heading until over the beacon - if the RATE OF CLOSURE decreases then make small alterations, e.g., 10 to 15 degrees to the heading in an endeavour to bracket the heading on which a constant RATE OF CLOSURE can be obtained.
10 - If
the track made good is not in fact the direct track to the beacon then,
when abeam the beacon, i.e., when the distance indication ceases to
decrease and then increases, turn 90 degrees to proceed to the beacon
- if the turn is not in the correct sense then continue the turn another
180 degrees in the same direction as the original turn.
(i) If on any course the rate of closure is such to indicate a closing speed approximately the same as the T.A.S. and provided that as the aircraft proceeds further the rate of closure remains relatively constant, it is advisable to hold the course being flown until either over the top or abeam the D.M.E. beacon.
(ii) If on any course the rate of closure is obviously very slow then do not wait to measure a fixed change in distance but turn 30 degrees left and proceed in accordance with Step 6.
(iii) When the rate of closure is being determined it is desirable to measure a precise distance between two calibration marks on the D.M.E. indicator and measure the time in minutes and seconds, and then compare the times taken for similar distances. If, however, the distance indication appears constant then the course should be held for, say, one minute and note made of the distance covered. When using the X10 scale it should be remembered that if the indicator is watched only for a matter of seconds a rate of closure of one mile per minute will not result in an obvious pointer movement. Pilots should guard against assuming a constant distance indication without observing the indicator for at least one minute.
(iv) During homing procedures, when within 10 miles of the D.M.E. beacon, it is suggested that the course being flown should be continued until either over the top or abeam the D.M.E. beacon. This will assist in orientation prior to commencing descent.
(v) When on a course that originally gave a reasonable rate of closure it is observed that the rate of closure is decreasing, then "bracketing" may be employed to find a more satisfactory course. When bracketing, if a turn of 15 degrees is made and it is in the wrong direction then a turn of at least 30 degrees in the opposite direction should be made. Turns of less than 10 degrees will not result in an obvious change of rate of closure.
(vi) It has been proved theoretically that in drift conditions it is possible to obtain the maximum rate of closure when flying on a course other then the course which gives a direct track towards the D.M.E. beacon. However, this very slight increase in rate of closure is not pronounced and as the aircraft proceeds further towards the beacon the rate of closure will decrease, whereas if the aeroplane is tracking towards the beacon the rate of closure will remain constant.
(vii) It will be found in practice that after the initial 30 degree turn, the problem of orientation becomes simply a matter of turning the aeroplane to a heading which gives a reasonable rate of closure and then bracketing, initially with turns of 15 degrees, to obtain the course which gives maximum rate of closure.
(viii) It is desirable, to assist in orientation, that all turns should be made to the LEFT unless it is apparent that such a turn is in the wrong sense. This procedure will prevent confusion when trying to remember the last direction turned.
APPENDIX "B" - D.M.E. DESCENT PROCEDURE
Several procedures have been evaluated and in an endeavour to have a simple basic standard procedure which can be safely applied to all locations irrespective of terrain difficulties the procedure described in the following paragraphs has been developed. It is to be known as the "DR Descent Procedure" since the procedure is basically a DR descent. The "D.M.E. Descent Procedure" for any particular location which is prescribed in AIP/RAC-2 has been developed having regard to the considerations which apply to the basic standard procedure.
The procedure is basically DR in that when outbound from the D.M.E. beacon and during the procedure turn there is no indication of position relative to the prescribed track. However, when inbound the procedure does allow for corrections to be made to overcome to some extent the effects of drift.
It will be appreciated that, in still air conditions, if an aircraft is flown outbound on a particular heading from over the top of a D.M.E. beacon and at a certain distance an 80 degree procedure turn is executed, and the aircraft is then flown on the reciprocal of the outbound heading, then the aircraft would eventually fly over or nearly over the top of the D.M.E. beacon. It has been established that an aircraft can be located over the top of a D.M. E. beacon by employing the homing procedure, and consequently to successfully descend the aircraft to the minimum altitude and to a position from which a visual circuit and landing can he accomplished requires that some method to negate the effect of drift during descent should be employed.
Several important considerations have therefore been applied in developing the basic standard procedure:
(i) Except at certain locations (i.e., where a "break away procedure is employed) the procedure is confined within a definite radius of the beacon and thus the effect of any drift is limited by the shortness of "exposure" time.
(ii) Under average drift conditions it will be found that by simply following the prescribed procedure, without corrections for drift, the aircraft will return to a position from which it will be possible to see the aerodrome.
(iii) Where excessive drift conditions are experienced the magnitude of the drift error will immediately be apparent once the aircraft is inbound since the rate of closure will show a marked decrease as the aircraft proceeds.
(iv) When on a heading towards the beacon and using the X1 scale on the D.M.E. indicator, the pointer has a speed of movement which is quite obvious and allows for rapid determination of variation in RATE OF CLOSURE.
e.g., assume -
(a) the aircraft is on the inbound heading of a descent procedure subsequent to a procedure turn being completed within seven miles of the beacon; and
track of the aircraft is not directly towards the beacon since it has
been subject to an abeam component of say greater than 20 knots during
(c) If when the aircraft is on the inbound heading the RATE OF CLOSURE initially appears to be quite high and any dropping off in the RATE OF CLOSURE does not become apparent until ever the final portion of the approach, i.e., within say 3-4 miles, then the course being flown should be maintained as it will be found in practice that the aircraft will arrive within two miles when abeam of the beacon.
At those locations where, because of terrain considerations, it has been necessary to employ a "break away" procedure it will be noted that subsequent to locating the aircraft over the top of the D.M.E. beacon the aircraft flies a constant course descending to a minimum altitude which permits the aircraft to then return to the field under visual flight conditions. This will result in some instances in rather a lengthy drawn-out procedure but it should be remembered that the procedure will in fact only used when the ADF facility has failed and instrument conditions prevail.
Before commencing the D.M.E. Descent Procedures prescribed in AIP/RAC-2, pilots should observe two important factors:
(i) Do not fly below the minimum altitude specified for use in conjunction with D.M.E. procedural homing; and
(ii) Locate the aeroplane within one mile of over the top of the D.M.E. beacon prior to turning onto the outbound heading. In this regard it should be remembered that the indicated D.M.E. distance also includes the height of the aircraft, i.e., when over the top of the D.M.E. beacon at 3,000 feet the distance scale will show ½ mile, except in those instances where a "drop out" might occur.
The following procedure has been evolved for the purposes of description only but is very similar, except for the prescribed tracks, to many actual published procedures.
D.R. DESCENT PROCEDURE
1 - Locate the aircraft over the D.M.E. beacon at 3,000 feet.
2 - Commence descent at 500 feet per minute to 1,500 feet on a course of 180 degrees(M).
3 - At a distance of 5 miles execute an 80 degree procedure turn to the right continuing descent if necessary to 1,500 feet.
4 - When on a course of 360 degrees(M) continue descent to 500 feet DAY, 700 feet NIGHT.
5 - When at the minimum altitude if unable to proceed visually pull up immediately on a course of 360 degrees(M) to 3,000 feet or as advised by ATC.
The following diagrams depict the flight path of an aircraft carrying out, under various conditions, the procedure described above, with appropriate comments.
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