d. Pulse Decay Time. The time required for the pulse to fall from 90 to 10 percent of the maximum voltage amplitude shall nominally be 2.5 microseconds. It shall not exceed 3.5 microseconds. The minimum decay time is governed by the spectrum requirements of paragraph 179.
174. PULSE POWER VARIATION. The peak power of the constituent pulses of any pair shall not differ by more than I dB.
Note: If the difference in level of the constituent pulses of interrogation pulse pairs is greater than 1.0 dB, ground components may not respond to interrogations.
175. PULSE CODING. Interrogation signals shall consist of paired pulses.
Spacings are measured at the 50 percent voltage amplitude points on the pulse rise time of each pulse. The pulse spacing shall be:
a. 12.0 ‡ 0.5 microseconds for X channels; or
b. 36.0 ‡ * 0.5 microseconds for Y channels.
176. INTERROGATION SIGNAL REPETITION RATE. The average interrogator pulse pair repetition rate shall not exceed 30 pairs of pulses per second. This assumes that interrogators are tracking at least 95 percent of the time. The repetition rate may be increased during search, but it shall not exceed 150 pulse pairs per second.
177. VARIATION OF REPETITION RATE. The variation in time between successive interrogations shall be sufficient to preclude mistaking distance reply pulses intended for another airborne component tuned to the same ground facility. It shall also preclude capture of the interrogations of one interrogator within the ground component dead time caused by the interrogations of other avionics.
178. SIGNAL STRENGTH IN SPACE.
a. When referenced to a point in space prior to the ground station anterna intercept, the airborne component of aircratt operating above 18,000 8w/m 5,486 m) AGL shall provide a minimum signal power density of -102.5 (5,486 m) Ape, cent timum sighil power density sari beating belim18,000
b. These specified power densities will assure a minimum reply efficiency of 70 percent when:
(1) the pulse frequency is within 1 100 kHz of the assigned center frequency;
(2) the pulse spacing is within ‡ 0.5 microseconds of nominal design center; and
(3) the beacon loading is 3375 pulse pairs per second with the echo suporession circuits disabled.
Chap 4 Page 30 Par 173
For the various operational service volumes currently employed in the ATC environment, the interrogator's EIRP shall be a minimum of +29.0 dBW for those aircraft operating above 18,000 feet (5,486 m) AGL and +19.0 dBW for those operating below 18,000 feet (5,486 m) AGL. These EIRP's will provide to the ground beacon the power densities necessary for satisfactory perfurmance.
Since EIRP levels greater than +33.0 dBW may impair system performance, the interrogator's EIRP shall not exceed this value. (See Appendix 1)
179. RADIO SPECTRUM. The spectrum of the RF interrogation signal shall be such that at least 90 percent of the energy, including FM components, in each pulse shall be within a 0.5 MHz band centered on the nominal channel frequency.
180. RADIATION. Radiation from airborne components shall not result in derogation of the operational use of this system by other users or in the derogation of other aeronautical services. Neither shall other users derogate the operational use of this system by valid DME or TACAN users. At all frequencies between 960 and 1215 MHz, the level of radiated CW signals, as referenced to an isotropic radiator, shall not exceed -60 dBW.
181. COMPONENT FUNCTIONAL CAPABILITIES AND PERFORMANCE. These subparagraphs identify functional and operational performance requirements applicable to the airborne component.
182. RECEIVER RADIO FREQUENCIES. For each chaniel in use, the receiver's center frequency shall be in accordance with Appendix 3.
183. RECEIVER SENSITIVITY. Based on the signal power density of paragraph 123, the airborne components shall provide the sensitivity necessary to acquire and display navigation information to the accuracy specified. Clear and distinct reproduction of identification signals shall be provided. (See Appendix 1)
184. REJECTION OF UNDESIRED SIGNALS. The airborne component shall provide undesired signal rejection characteristics adequate to assure the specified performance. For co-channel and adjacent channel DME and TACAN signals, this requirement shall be met when the respective signals provide desired to undesired (D/U) signal ratios up to the maximum values stated in paragraph
29.b. At these D/U ratios, a positive identification signal shall be provided to identify the ground component.
185. DISTANCE INFORMATION. The airborne component shall measure and display the slant range distance (in nautical miles) between the aircraft and the selected ground component.
Note: The airborne component should not be capable of displaying excessive distances. Distances should be considered excessive for a given receiver if that receiver cannot reasonably be expected to operate at that distance.
Chap 4 Par 178 Page 31
186. WARNING FUNCTION, DISTANCE. The airborne component shall provide a warning indication whenever the airborne component is neither tracking a distance reply signal nor operating from memory (see paragraph 188). This warning shall be clearly evident to the pilot.
187. ACCURACY OF DISTANCE INFORMATION. when the airborne component error 1S combined by root-sum-square with a ground component error of O.I nmi (185 m), the total error in slant range distance informatior, as displayed to the pilot, shall not (except during memory per paragraph l80) exceed O.s nmi (9zt
m) or 3 percent of the actual distance, whichever is greater (95 percent probability).
188. MEMORY FUNCTIONS. The airborne component shall provide a memory cause the display of distance information to continue for a period not io upon loss of a suitable reply signal while tracking, wi!!
exceed 15 seconds. The minimum distance memory shall be sufficient to cover the loss of distance reply signals during transmission of the ground component identification signal. The distance displayed during memory shall be within
1.0 nmi (1852 m) of the indication upon resumption of the tracking function.
189. TACAN BEARING AND COURSE DEVIATION INFORMATION. The airborne component shall provide devices for unambiquous determination of the aircraft magnetic bearing with respect to each selected ground component and for display of the aircraft deviation from the selected course.
190. COURSE DEVIATION DISPLAYS. The response, readability, and resolution of course deviation displays shall permit the pilot to determine the direction and extent of the aircraft deviation from the selected course.
191. WARNING FUNCTION, BEARING. The airborne component shall provide a warning indication whenever the azimuth signals are not present and when the component is not operating in memory. This warning shall be clearly evident to the pilot.
192. ACCURACY OF BEARING AND COURSE DEVIATION INFORMATION. At any bearing, the total airborne component error in bearing and course deviation information shall not exceed ‡ 3.0 degrees (95 percent probability) as displayed to the pilot.
193.-209. RESERVED.
Chap 4 Page 32 Par 186
Appendix 1 APPENDIX 1. COVERAGE.
RADIO PROPAGATION. Propagation losses vary as a function of time. For a given EIRP, the power density at a point in space will fluctuate. In addition, propagation loss will vary from site to site. This variation is a function of several factors including atmospheric refractivity, surface reflectivity, wave polarization, etc. Since the VOR/DME/TACAN system is a safety service, signals must be highly reliable. With this in mind, conservative assumptions have been made for the inputs to the propagation model. Figures 2 through 13 show standard VOR, DME, and TACAN signal strengths (95 percent time availability) at varicus points in space. A time availability of 95 percent means that, at a given point in space, the instantaneous signa! strength will be greater than or equal to the value showr 95 percent of the time. Although propagation losses will differ from site to site, the signal strength curves of this appendix are recommended for purposes of standardization. It is important to remember that for the distance function of DME and TACAN both the ground-to-air and air-to-ground transmissions must be considered.
GROUND STATION. The EIRP's and resultant signal strength in space curves given in Figures 2 through 13 are nominal. Allowances should be made for different EIRP's and for other variations in stalion characteristics.
Some ground stations may have higher cable losses (e.g., mountain top installations sometimes require longer cable runs). In addition, intervening terrain may increase ihe transmission path loss. Should this occur, it may be necessary to reduce the service volume accordingly.
VOR COVERAGE.
The power density specified in paragraph 53 (-120 dBW/m?) is equivalent to -123 dBW power available at the output of a lossless isotropic antenna (at 118 MHz). Allowance nust be made for the airborne antenna and for cable and insertion losses. Users will balance their airborne power budgets in different ways. Two examples are shown below.
Еx3mple 1 Example 2 Power Available - 123 dBW - 123 dBW Airborne Antenna Gain* +2.2 dBi -l dBi Airborne Losses -2.2 dB -4 d3 Receiver Sensitivity - 123 dB!! - 728 dBW Many other examples could be given. It is the user's responsibility to insure that the airborne installation will allow proper operation at the minimum signal strength of paragraph 53. (In considering airborne antenna gain, antenna patiern variation should not be neglected.)
*NOTE: Does not consider variations due to antenna locations or other factors which influence antenna gain due to shielding.
Par 1 Page I
Appendix 1
4. DME/TACAN (UPLINK) COVERAGE
a. The power densities specified in paragraph 123 are referenced to the powers available at a point in space. Allowance must be made for the airborne antenna and for cable and insertion losses. Users will balance their airborne power budgets in different ways. Two examples of how users might balance their airborne power budgets for the DME and TACAN are shown as follows:
Example 1 Example 2
DME DME (100W) TACAN (100W) TACAN Power Available - 109.0 - 114.5 dBW - 109.0 - 114.5 dBW Airborne Antenna Gain* +2.0 • +2.0 dBi +0.5 +0.5 dBi Airborne Losses -3.0 -3.0 dB - 1.5 - 1.5 dB Rx Sensitivity - 110.0 - 115.5 dBW - 110.0 - 115.5 dBW
b. Many other examples could be given. It is the user's responsibility to insure that the airborne installation will allow proper operation at the minimum signal strengths of paragraph 123. An airborne DME installation with 1 dB of loss (antenna pattern, cable and insertion losses, signal splicters, etc.) would require a 1°1100 55M. An stone TAch In. When the sensitivity of -110.0 dBW. An airborne TACAN installation with | dB of loss would require a sensitivity of -115.5 dBW. When the total loss is different than 1 dB, a corresponding adjustment must be made to determine the required sensitivity. In considering airborne antenna gain, antenna pattern variation should not be neglected.
5. DME/TACAN (DOWNLINK) COVERAGE.
a. The minimum power density specified in paragraph 178 for the input to the ground station antenna is -99.0 dBW/m? for DME and - 102.5 dBW/m?
for TACAN. These power density values are necessary to generate a reply efficiency of 70 percent from the transponder during periods when the transponder is under heavy load and when the interrogation signal is within + 100 kHz of the nominal channel frequency and its pulse pair spacing is within +0.5 microseconds of the nominal system value. The association of Interrogator signal strength and transponder sensitivity is as follows.
*Note: Does not consider variations due to antenna locations or other factors which influence antenna gain due to shielding.
Page 2 Par 4
Appendix 1
DME TACAN -94.0 -97.0 dBm Transponder Sensitivity per paragraph 115 + 6.7 + 3.0 dB Net coupling to receiver per
(1) +8.2 dBi DME Antenna Gain on horizon -1.5 dB Net Losses (cable and insertion)
(2) +4.5 dBi TACAN Antenna Gain on horizon -1.5 dB Net Losses (cable and insertion) -87.3 -94.0 d8m Receiver input for 70 percent Efficiency per paragraph 115 + 2.0 + 2.0 dB Sensitivity Reduction per paragraphs 116 and 117 -85.3 -92.0 dBm : Receiver Input for 70 percent efficiency
b. It is the user's responsibility to provide that signal strength to insure proper operation for those conditions stated in paragraph 178. For the requirement of 70 percent reply efficiency:
DME TACAN -85.0 -92.0 dBm Receiver Input per conditions of paragraph 178
- 0.7 - 3.0 dB Net coupling per Antenna -91.7 -95.0 dBm Signal Strength at the antenna input -99.2 - 102.5 dBW/m2 Power Density at the antenna input -99.0 -102.5 dBW/mC Value used in paragraph 178
6. TIME AVAILABILITY. At a point in space, signal strength will vary with time even when the ground transmitter's EIRP remains constant. A similar statement could be made for D/U signal ratios. This variation is largely due to propagation effects under different environmental conditions. Examples include atmospheric refractivity, seasonal changes in ground cover, and as are aiso inund cover Although it surface reflectivity. Numerous other factors are also involved. Although it is difficult to describe accurately all the variables involved, the situation statistical discussion of reliability, is time availability. It is that can be adequately covered by statistical treatment. A helpful concept, in a that a given sight strengen or bly natio unction fe ne percentage of time that a given signal strength or D/U ratio is met or exceeded. Figure I shows an example of signal strength as a function of time availability. For a time availability of 95 percent, the signal strength is
- 120 dBW/m?. This means that a signal strength of -120 dBW/m? or greater is present 95 percent of the time. For a time availability of 50 percent, the signal strength is -117 dBW/m' at the same point in space assuming the same constant ground station characteristics. In a similar manner, the signal strength present for any given percentage of the time can be read from the graph. This graph does not, however, take transmitter downtime into account.
Par 5 Page 3
Appendix 1 FIGURE 1. SIGNAL TIME AVAILABILITY SIGNAL STRENGTH IN SPACE (dBW/m2) 130 150 -01 5 10 50 90 95 99 • 99.9 99.99
PERCENT TIME Page 4
Appendix 1
TUDE IN THOUSANDS OF I FIGURE 2. VOR SIGNAL STRENGTH IN SPACE - LONG RANGE DISTANCE FROM THE DESIRED STATION IN Em 180 200 220 240 200 280 200 320 /MA 3OL- DISTANCE FROM THE DESIRED STATION IN MO 100 120 140 160 VOR SIGNAL STRENGTH IN SPACE 100 W (20 OBW) Tranamitter Power 3 d8 Cable Losses (Ground) 22 dBi Mainbeem Antenna Gain 19.2 dBW EIRP, 118 MHz
- •
ALTITUDE (AGLI IN THOUSANDS OF Page 5
9/20/82 AC 00-31A Appendix 1
ALTITUDE IN THOUSANDS OF METER FIGURE 3. TAGAN (RTB-2) SIGNAL STRENGTH IN SPACE - LONG BANGE мар 9 DISTANCE FROM THE DESIRED STATION IN km 160 100 200 220 240 260 140 DISTANCE FROM THE DESIRED STATION IN mmi 120 M8P 100 120 140 60 20 SIGNAL STRENGTH IN SPACE OBWI Transmiller Powe Losses (Ground) beem Antenna Gain RIP. 1213 MHZ
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LTITUDE (AGLI IN OUSANUS OT Page 6
Appendix 1
ALTITUDE IN -ş FIGURE. CARDION OME SIGNAL STRENGTH IN SPACE - LONG BANGE 100 200 220 200 200 2á0 200 220 20 360 3é0 ISP DISTANCE FROM THE DESIRED STATION IN km do 100 120 140 180 SIGNAL STRENGTH IN SPACE dEW) Tramamitter Power Losses (Ground) nbeem Antenne Gain 1213 MHz EIRP.
CARDION 1000 W
ALTITUDE (AGL) IN THOUSANDS OF FEE Page 7
Appendix 1
- 30 FIGURE 5, CARDION DME SIGNAL STRENGTH IN SPACE - LONG RANGE 100 DISTANCE FROM THE DESIRED STATION IN mmi RDION DME SIGNAL STRENGTH IN SPACE ẨвУ 100 W (20 dEW) Transmitter Power seei (Ground) um Antenne Gain 1213 MHz
1.5 da Callo Lo .4 dBI Mainbe • dBW EIRP
11.
- &
ALTITUDE (AGLI IN THOUSANDS OF FEET Page 8
Appendix 1
ALTITUDE IN THOUSANDS OF METER FIGURE 6. MONTEX DME SIGNAL STRENGTH IN SPACE - LONG RANGE DISTANCE FROM THE DESIRED STATION IN km 140 DISTANCE FROM THE DESIRED STATION IN Ai 140 160 100 200 220 260 60 60 100 120 MONTEK DME SIGNAL STRENGTH IN SPACE 1000 W (30 dBW) Tranemitter Power 1.6 de Cable Losses (Ground) 8.8 dBI Mainbeem Antenna Gain 37.3 dBW EIRP. 1213 MHz M8P 9'6L 8P !
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