CHAPTER 4. DME AND TACAN SYSTEMS SECTION. OPERATIONAL CHARACTERISTICS FOR DME AND TACAN
GROUND COMPONENTS
110. INTRODUCTION. This section identifies standard signal and performance characteristics for DME and TACAN ground components. These characteristics represent the performance which shall be provided throughout the operational service volumes defined in paragraph 23.c. Requirements apply to both DME and TACAN components, unless noted otherwise.
111. POLARIZATION. The ground component antenna shall radiate and receive a vertically polarized signal. Any horizontally polarized signal components shall be at least 26 dB weaker than the vertically polarized component.
112. TRANSPONDER RESPONSE TO INTERROGATION SIGNALS. The response of the transponder to interrogation signals shall conform to paragraphs 113 through NOTE: At the ground component antenna, the presence of Ci:
signals within I 3.0 MHz of the nominal value of the interrogation frequency at a signal power density of -l13 dBW/m or more, will normally derogate the performance of the system. Steps should be taken to avoid this situation.
113. INTERROGATION RADIO FREQUENCY. For each channel in use, the center frequency of the transponder's interrogation and reply frequencies shall be in accordance with Appendix 3.
114. SENSITIVITY TO INTERROGATION SIGNALS. Transponder sensitivity is specified as that minimum value of peak pulse power density (prior to ntercept by the ground component antenna) which will result in a transpondel reply efficiency of 70 percent. (See Appendix l) NOTE: Ground components may not respond to interrogations as specified if the difference in level of the constituent pulses of interrogation pulse pairs is greater than 1 dB.
115. ON-CHANNEL SENSITIVITY. For interrogation signals within ‡ 100 kHz of the assigned frequency, having a repetition rate no greater than 200 pulse pairs per second and pulse pair spacing of exactly 12.0 microseconds (36.0 microseconds for Y channel), DME sensitivity shall be not less than -101.5 than -104.5 dBu/m same conditions, the TACAN sensitivity shall be not less NOTE: At 1150 MHz, the value -101.5 dBW/m2 corresponds to -124.0 dBW at the output of a lossless isotropic receiving antenna. Similarly, the value -104.5 dBW/m? corresponds to - 127.0 dBW. (See Appendix 1) Chap Par 110 Page 19
116. SENSITIVITY AT OTHER PULSE SPACINGS. When the spacing of the constituent pulses of interrogation pulse pairs differs from the design center value (12.0 microseconds for x channel; 36.0 microseconds for Y channel) by + 0.5 microseconds or less, the sensitivity of paragraph 115 shall not be reduced by more than 1 dB.
more from nominal, the reduction in sensitivity shall be at least 70 dB with respect to the level of paragraph l15.
117. VARIATION OF SENSITIVITY WITH INTERROGATION LOADING. When the beacon is loaded with 3175 additional pulse pairs per second at a level of -65 dBm (referenced at the input to the transponder receiver with the transponder's echo suppression circuits disabled), the sensitivity of the receiver shall not be reduced by more than 1 dB from the value measured in paragraph 115.
118. SENSITIVITY TO ADJACENT CHANNEL INTERROGATIONS. Interrogation signals removed 900 kHz or more from the assigned channel interrogation frequency and laving an amplitude up to 80 di above the on-channel sensitivity or the component shail not elicit a reply from that component.
IlS. TRANSPONDER DEAD TIME. Dead time is the time immediately following an accepted interrogation decode for reply processing, during which succeeding interrogations receive no reply. The transponder dead time shall normally be the treconds f or 60 microseconds for X channels (72 microseconds for Y channels).
120. ECHO SUPPRESSION DEAD TIME. The retriggerable blanking gate (RTBG) (see paragraph 145.b) is typically set such that the effective dead time is a nominal 150 microseconds. When required for a particular site, the effective dead time may be increased up to a nominal 250 microseconds.
121. TRANSPONDER OUTPUT SIGNAL CHARACTERISTICS. The transponder shall conform to paragraphs 122 through 150.
122. FREQUENCY STABILITY. For each channel in use, the center frequencies of the ground station's transmitter and receiver shall be in accordance with qui ment uresed arter liy ee tabi trent er reer stat ions shall be ‡ 0.002 percent or better.
NOTE: Stabilities of older equipment need not be increased to + 0.001 percent.
123. SIGNAL STRENGTH IN SPACE. Within that part of the operational service ve t part or che operatonat sor vice votume that a betor 18, 00 ret 15,.496 above 18,000 feet (5,486 m) AGL, a minimum signal power density
m) AGL, a minimum signal power density of -86.0 dBW/m< shall be provided.
Signal power shall be determined by the average over one second ot the equivalent peak pulse voltage waveform. At the nearest aircraft position expected during operations, the maxımum signal power density expected durıng liaht will be on the order of -17.0 dBW/m< .
Chap 4 Page 20 Par 116
NOTE: At 1213 MHz, the value -91.5 dBW/m2 corresponds to
- 114.5 dik at the output of a lossless isotropic receiving Similarly change in antenna. Similarly, -86.0 dBW/m- corresponds to -109.0 dBW. The apparent change in numbers from the old standard does not represent a change in beacon radiated power (for example, see Appendix 1). Therefore, increases in rece!ver sensitivities of existing airborne equipments are not. required to achieve the same service ranges that users have experienced in the past. Future designs should not be restricted by this fact.
124. RADIO SPECTRUM. The spectrum of the pulse modulated signal shall be as follows.
a. The equivalent isotropic radiated power (EIRP) contained in a 0.5 MHz band centered on a frequency either 0.8 MHz above or 0.8 MHz below the nominal channel frequency shall (in both instances) not exceed 200 milliwatts (except that the power relative to center frequency shall not exceed -50 dB).
b. The EIRP contained in a 0.5 MHz band centered on a frequency either
2.0 MHz above or 2.0 MHz below the nominal channel frequency shall (in both instances) not exceed 2 milliwatts (except that the power relative to center frequency shall not exceed -70 dB).
c. Each lobe of the spectrum will be generally of lesser amplitude than the adjacent lobe nearer the nominal channel frequency.
125. SPURIOUS RADIATION. The RF output level, during the interval between occurrence of the desired pulse pairs, shall not exceed a level which is 80 dB below the maximum power level during a pulse. In addition, between the pulses of each pair there shall be an interval for 1.0 microsecond or greater in length during which the RF output level does not exceed a level which is 50 dB below the maximum power level of the weaker pulse of the pair.
126. PULSE SHAPE. The following, as limited by the requirements of paragraphs 124 and 125, shall apply to all radiated pulses.
a. Pulse Rise Time. The time required for the pulse to rise from 10 to 90 percent of its maximum voltage amplitude shall not be less than 0.1 microsecond nor more than 3.0 microseconds.
b. Pulse Top. Between the points on the leading and trailing edges which are 95 percent of the maximum voltage amplitude, the instantaneous amplitude of the pulse shall not fall below a value which is 95 percent of the maximum voltage amplitude of the pulse.
c. Pulse Duration. The pulse duration, as measured at the 50 percent maximum voltage amplitude points, shall be 3.5 1 0.5 microseconds.
d. Pulse Decay Time. The pulse decay time, trom the 90 percent point to the 10 percent point of the maximum voltage amplitude, shall be such that the remaining requirements of this standard shall be satisfied.
Chap 4 Par 123 Page 21
12/. PULSE CODING. Transponder output signals shall consist of paired pulses. The spacing of the pulses is measured between the 50 percent maximum voltage amplitude points on the leading edge of each RF pulse. The pulse spacing shall be:
a. 12.0 ‡ 0.25 microseconds for x channels; or
b. 30.0 ‡ 0.25 microseconds for Y channels.
128. PULSE POWER VARIATION. The peak power of the constituent pulses of any pair shall not differ by more than 1 dB.
129. DISTANCE REPLY SIGNALS. Distance reply signals, consisting of pulse pairs, are transmitted in response to interrogations.
130. REPLY EFFICIENCY. Reply efficiency is defined as the percentage of interrogations from a specific interrogator to which the transponder replies within a given time interval when the transponder is under specified load conditions. The reply efficiency for interrogation signals at and above the minimum sensitivity levels of paragraph i14 shall be at least 70 percent for all interrogation loadings up to the maximum for which the transponder is designed (3375 interrogations per second).
NOTE: To provide service under adverse echo conditions or to handle increased traffic, it may be necessary to reduce the reply efficiency. To be able to receive the same service during traffic overload conditions, airborne receivers should be capable of operating properly with reply efficiencies at least as low as 50 percent.
131. REPLY DELAY TIME. Reply delay time is defined as the time of all delay introduced by the ground component in replying to interrogations. When airborne components are to indicate distance with respect to the transponder site, the zero-distance reply delay time shall be 50.0 microseconds for maels his resents the time eten the percent voltage point on the leading edge of the first pulse of the interrogation pulse pair and the corresponding point on the first pulse of the reply pulse pair.
NOTE: In older facilities, the reply delay time is referenced to the second pulse of interrogation and reply pulse pairs. The nominal value of reply delay time in these instances is 50 microseconds for both X and Y channels.
132. RANDOM PULSE PAIR SIGNALS. In addition to distance reply pairs, the ground component shall radiate random pulse pairs, defined as squitter, in order to maintain a total pulse pair rate in accordance with paragraphs 133 and 134.
133. DME COMPONENTS. For DME ground components, the total pulse rate, exclusive of identification pulses, shall be within the range of 700 up to 2850 pulse pairs per second, in the absence of high traffic density.
Chap 4 Page 22 Par 127
134. TACAN COMPONENTS. For TACAN ground components, the total pulse pair ate, exclusive of code identification signal and reference burst pulses, shall be 2700 - 90 pulse pairs per second, in the absence of high traffic density. For a transponder dead time of 60 microseconds (for X channels), the distribution of random pulse pairs shall conform to Figure 4-1.
PULSE PAIR RATE. To provide greater traffic handling capacity (than the total output pulse pair rates of paragraph 133 and 134), the equipment shall have the capability to increase to as high as 5000 ‡ 150 pulse pairs per second as a function of actual traffic loading. Under this condition it will be impossible to maintain the output pulse pair spacing distribution of Figure
4-1. Accordingly, new TACAN airborne equipment design should avoid the use of circuits in which azimuth indication is sensitive to transponder output pulse count and spacing distribution. (See paragraph 130.)
136. GROUND COMPONENT RANGE ACCURACY. The ground component shail not contribute more than O.T nmi (185 m) to overall system error.
paragraph 30.a. The spacing between the first and second pulse pairs constituting each pulse group, as measured between the Su pecent voltage amplitude points on the leading edge of the first pulse of each pair, shall be 100 ‡ 10 microseconds. The repetition rate shall conform to the following.
a. DME Components. For ground components providing DME service only, the identification signal may consist of either one or two pulse pairs in a group. The group repetition rate shall be 1350 1 10 groups per second.
. JACAN Components. For TACAN ground components, the repetition rate shall be 1350 groups per second I 0.23 percent) which is phase-lockec within 150.0 microseconds of the tenth harmonic of the 135 Hz bearing seen euront so mitrepuls after the infuse on an a dese group sector reference signals.
138. TACAN AZIMUTH SIGNAL CHARACTERISTICS. TACAN azimuth signals consist of North (main or coarse) and 40 degree sector (auxiliary or fine) bearing reference signals and 15 Hz (coarse) and 135 Hz (fine) amplitude modulation variable bearing signals. Ine azimuth signals radiated by the antenna shall conform to the following.
Chap 4 Par 134 Page 23
RANDOM PULSE DISTRIBUTION LIMITS Based on a minimum of 1000 reedings of the spacing of the 2700 random pulses with data recorded to the neares! 25 microsecond Internal. The ratative occurrence is aquai to: number of readings in each intervel total number of reading At leest 87% of the points must fall between the solid alk live: may te beren the over a end and lower dotted lines; and 3% may fall in the region outside of the NOTE: This distribution curve is representatire for x 71 channels with a transponder dead time of 60 microseconds. 1000 800 PULSE SPACING IN MICROSECONDS (TO NEAREST 25 MICROSECOND INTERVAL) FIGURE 41. RANDOM PULSE DISTRIBUTION LIMITS dotted lines 700 600 500 400
RELATIVE OCCURRENCE Page 24
139. BEARING REFERENCE SIGNALS. Transmission of the North and 40 degree sector bearing reference signals shall occur synchronously with antenna pattern rotation. For each rotation of the antenna pattern, one North reterence signal shall De transmitted. Following the North reterence signal at each of eight consecutive angular increments of 40 degrees, the 40 degree sector reference signal shall be transmitted. A ninth 40 degree sector reference signal, which otherwise would coincide in time with the North eterence signal, shall not be transmitted. The characteristics of reference signals shall be as follows.
a. North Reference Signal.
(1) For X channels, the North reference signal shall consist of a group of z pulse pairs. Spacing between the two pulses of a pair shall be l2 + 0.25 microseconds. Spacing between the first pulse of each pulse pair shal be 30.0 ‡ 0.3 microseconds. Spacings are measured at the 50 percent voltage amplitude points on the leading edges of the pulses.
(2) For Y channels, the North reference signal shall consist of a group of 13 single pulses. Spacing between the pulses shall be 30.0 + 0.3 microseconds. Spacings are measured at the 50 percent voltage amplitude points on the leading edges of the pulses.
b. 40 Degree Sector Reference Signals.
(1) For X channels, the 40 degree sector reference signal shall consist of a group of 6 pulse pairs. Spacing between the two pulses of a pair shall be 12 + 0.25 microseconds. Spacing between the first pulse of each pulse pair shall be 24.0 ‡ 0.25 microseconds. Spacings are measured at the 50 percent voltage amplitude points on the leading edges of the pulses.
(2) For Y channels, the 40 degree sector reference signal shali consist of a group of 13 single pulses having a spacing of 15.0 " 0.25 microseconds. Spacings are measured at the 50 percent voltage amplitude points on the leading edges of the pulses.
140. VARIABLE BEARING SIGNALS. The variable bearing signals shall be produced by a rotating directional antenna pattern which results in a composite amplitude modulation of the transponder radio frequency pulse signals at 15 and 135 Hz. The characteristics of the variable bearing signals shali be as follows.
a. Amplitude Modulation Frequencies and Accuracy. The amplitude modulation frequencies shall nominally be 15.0 and 135.0 Hz. Each frequency shall vary from the nominal values in exact synchronism with an antenna pattern rotation rate of 900 (1 0.23 percent) rotations per minute.
Chap 4 Par 139 Page 25
elative to the horizon. ive depth of Modulation individual elation comfrents shall degrees 9 percent. At all elevation angles from -2 to 50 degrees, the sum of the 15 and 135 Hz modulation components including the harmonics shall not exceed 55 percent. Further, within the angles from -2 to +2 degrees relative to the horizon, the maximum variation in depth of modulation for the 15 and 135 Hz components shall not, for each frequency, vary more than I 4 percent from the respective median values within the angles specified.
141. RELATIONSHIPS OF REFERENCE AND VARIABLE BEARING SIGNALS. On the magnetic North radial from the antenna, the relationships of the reference and variable signals shall conform to the following.
the 15 Hz amplitude modulation component shall coincide within Coarse Bearing Signal. The inflection point on the negative slope of 2.0 azimuth degrees of the 50 percent amplitude points on the leading edge of:
(1) the tenth pulse of the North reference signals for X channels; or
(2) the sixth pulse of the North reference signal for Y channe?s.
b. Fine Bearing Signals. The inflection point on the negative slope of the 135 Hz amplitude modulation component shall coincide within + 0.33 degrees azimuth of the average position of the 50 percent amplitude point on the leading edge of:
(1) the tweifth pulse of the 40 degree sector reference signal for X channels; or
(2) the eleventh pulse of the 40 degree sector reference signal for y channels.
142. PRECEDENCE OF PULSE TRANSMISSIONS. The order of precedence for transmission of transponder pulse signals shall be in accordance with the following.
a. DME Components. For ground components providing DME service only, the precedence shall be:
(1) International Morse Code Identification Signals
(2) Distance Reply Signals
(3) Random Pulse Pair Signals Neither distance reply nor random pulse pair signals shall be transmitted luring the "key-down" interval of International Morse code identificatior ional transmission.
Chap 4 Page 26 Par 140
TACAN Components. For TACAN components, the precedence shall be:
Bearing Reference Signals
(2) International Morse Code Identification Signals
(3) Distance Reply Signals
(4) Random Pulse Signals Neither code identification, distance reply, nor random pulse pair signals shall be transmitted during the interval required for transmission of all pulses in each bearing reference signal. Distance reply and random pulse pair signals shall not be transmitted during the "key-down" interval of code identification signal transmissions.
143. REJECTION OF UNDESIRED SIGNALS. The ground component shall provide undesired signal rejection characteristics adequate to assure the specified performance. This includes co-channel and adjacent-channel OME/TACAN signals.
144. RECEIVER DECODER. The decoder shall decode and produce an output pulse from interrogation signal pulse pairs occurring at spacings within the range of :
a. 12 1 0.5 microseconds for x channels, or
b. 36 ‡ 0.5 microseconds for Y channels.
145. ECHO SUPPRESSION. Echo suppressions shall be provided in accordance with the following subparagraphs.
a. Short Distance Echoes. Synchronous pulse signals occurring between the constituent pulses of a direct path interrogation pulse pair and which are also superimposed on the leading or trailing edge of the second pulse of the direct path pulse pair, shall not affect the time of decoding of the direct pulse pair by an amount in excess of 0.15 microseconds. Neither shall the reply efficiency be reduced by more than 10 percentage points from that measured in the absence of the echo pulse. These requirements shall be met when the RF input signal level of the direct path pulse pair has any level from 10 dB above thresholn triggering level to an absolute level of -10 dBm and the echo pulse has any level up to the level of the direct pulse pair and for all direct pulse pair spacings of paragraph 175.
Chap 4 Par 142 page 27
b. Long Distance Echoes. A separate echo suppression (Retriggerable Blanking Gate (RIBU)) circuit shall be provided in order to prevent the generation of multiple replies to aircraft interrogations having echoes which are delayed with respect to the direct path signal in excess of receiver dead time setting. The echo suppression circuit shall be triggered by the decoding of the direct signal pulse pair only when the level of the pulses exceeds a pre-established level. Such triggering shall result in the generation of a receiver desensitizing pulse starting at the time of pulse decoding nominally adjusted to 150 † 10.0 microseconds with an upper limit of 300 microseconds.
The degree of receiver desensitization shall be to a level 3.0 dB above the level of the direct path signal and shall hold over the entire duration of the echo suppression pulse, unless retriggered by a signal stronger by o to 6 dB chan the direct path signal, and over a range of input signals from 10 de above threshold triggering level to -15 dBm. The echo suppression circuit pre-established level will nominally.be -70 dBm.
146. DECODER DISCRIMINATION. The decoder shall provide a minimum of 70 dB rejection to:
a. paired pulses whose spacing differs by 3.0 microseconds or more from the nominal value given in paragraph 144,
b. paired pulses with spacings within the range of paragraph 144 where either pulse has a width of 0.8 microsecond or less, and
c. single pulses of any width including widths within the range of pulse spacings of paragraph 144.
147. RECEIVER RECOVERY TIME. The recovery time of the receiver and its associated video circuitry shall be such that the sensitivity to desired interrogations is not reduced by more than 1 dB when desired interrogations occur 8.0 microseconds and more after the reception of undesired pulses of any width having levels up to 60 dB above the sensitivity of the receiver in the absence of such undesired pulses. The desired interrogations shall be RF pulse pairs conforming to the characteristics specified in paragraphs 173 through
179. Undesired pulses shall conform to the same requirements except that the pulse spacing shall be outside the limits of paragraph 175.
148. DESENSITIZATION BY CW. The presence of CW interference signals on the assigned channel frequency or elsewhere within the receiver pass-band shall not reduce the on-channel sensitivity by more than 2 dB from the value measured in the absence of CW interference provided that the level of the interference is no higher than 10 dB below the level of the desired on-channel signal. Additionally, the reply efficiency to a single aircraft interrogation shall not be reduced by more than 10 percent when the level of the interrogation signal is 6 dB and more above the level of the interfering CW signal. The Cw sensing circuit shall not reduce the receiver gain by more chan 1 dB when pulses 8.0 microseconds wide or wider are received at a rate of 37,000 per second at a level 10 dB below receiver sensitivity. (This is to preclude premature receiver desensitization, if the receiver is exposed to a high Pulse Repetition Frequency (PRF) pulse multiplexed signal.)
Chap 4 Page 28 Par 145
149. MONITORING. The radiated signals shall be monitored and removed from service upon recognition of unsafe operations.
150.-169. RESERVED.
SECTION 2. OPERATIONAL CHARACTERISTICS FOR DME AND TACAN
AIRBORNE COMPONENTS
170. INTRODUCTION. This section specifies functional capability and erformance characteristics required of DME and TACAN airborne components.
he term "component" as used in this order, includes the complete aircraft installation. This includes such items as the antenna and its transmission line, the interrogator, the electrical power source(s), identification signal re also includes select and display instrumentation devices for bearing and and the distance indicator; when applicable, course indication. Airborne components used in the performance of aircraft operations under IF must meet all requirements. For other aircraft operations the requirements are limited to paragraphs 170, 175, 176, 179 and
180. Requirements apply to both DME and TACAN components unless otherwise noted. Components should be capable of performing as specified throughout the operational service volume of ground stations. The applicable performance requirements should be met when the ground stations are operating in accordance with this standard.
171. INTERROGATOR SIGNAL CHARACTERISTICS. Paragraphs 172 through 180 identify interrogation signal characteristics and tolerances applicable to the radiated signal.
172. INTERROGATION RADIO FREQUENCIES AND ACCURACY. The interrogatori shall transmit on the appropriate frequency in accordance with Appendix 3. The center frequency of the interrogation shall be within 100 kHz of the channel frequency.
173. PULSE CHARACTERISTICS. The radio frequency pulse envelope shall have characteristics as follows.
a. Pulse Rise Time. The time required for the pulse to rise from 10 to 90 percent of its maximum voltage amplitude shall nominally be 2.5 microseconds. It shall not exceed 3.0 microseconds. The minimum rise time is governed by the spectrum requirements of paragraph 179.
b. Pulse Top. Between the points on the leading and trailing edges which are 95 percent of the maximum voltage amplitude, the instantaneous amplitude of the pulse shall not fall below a value which is 95 percent of the maximum voltage amplitude of the pulse.
c. Pulse Duration. The pulse duration, as measured at the 50 percent maximum voltage amplitude points, shall be 3.5 = 0.5 microseconds.
Chap 4 Par 149 Page 29
149. MONITORING. The radiated signals shall be monitored and removed from service upon recognition of unsafe operations.
150.-169. RESERVED.
SECTION 2. OPERATIONAL CHARACTERISTICS FOR DME AND TACAN
AIRBORNE COMPONENTS
170. INTRODUCTION. This section specifies functional capability and performance characteristics required of DME and TACAN airborne components.
The term "component" as used in this order, includes the complete aircraft installation. This includes such items as the antenna and its transmission line, the interrogator, the electrical power source(s), identification signal reproduction or display devices, and the distance indicator; when applicable, it also includes select and display instrumentation devices for bearing and course indication. Airborne components used in the performance of aircraft open on the neut men.
operations under IF must meet all requirements. For other aircraft operations the requirements are limited to paragraphs 170, 175, 176, 179 and
180. Requirements apply to both DME and TACAN components unless otherwise noted. Components should be capable of performing as specified throughout the operational service volume of ground stations. The applicable performance requirements should be met when the ground stations are operating in accordance with this standard.
171. INTERROGATOR SIGNAL CHARACTERISTICS. Paragraphs 172 through 180 identify interrogation signal characteristics and tolerances applicable to the radiated signal.
172. INTERROGATION RADIO FREQUENCIES AND ACCURACY. The interrogator shall transmit on the appropriate frequency in accordance with Appendix 3. The center frequency of the interrogation shall be within I 100 kHz of the channel frequency.
173. PULSE CHARACTERISTICS. The radio frequency pulse envelope shall have characteristics as follows.
a. Pulse Rise Time. The time required for the pulse to rise from 10 to 90 percent of its maximum voltage amplitude shall nominally be 2.5 microseconds. It shall not exceed 3.0 microseconds. The minimum rise time is governed by the spectrum requirements of paragraph 179.
b. Pulse Top. Between the points on the leading and trailing edges which are 95 percent of the maximum voltage amplitude, the instantaneous amplitude of the pulse shall not fall below a value which is 95 percent of the maximum voltage amplitude of the pulse.
c. Pulse Duration. The pulse duration, as measured at the 50 percent maximum voltage amplitude points, shall be 3.5 I 0.5 micruseconds.
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