Proposed L-Band Interference Scenarios

Transcription

1 Proposed L-Band Interference Scenarios DOCUMENT IDENTIFIER: D1 ISSUE: 1.0 ISSUE DATE: AUTHOR: DISSEMINATION STATUS: DOCUMENT REF: DEUTSCHES ZENTRUM FÜR LUFT- UND RAUMFAHRT E.V. (DLR) CO CIEA15_EN511.10

2 History Chart Issue Date Changed Page (s) Cause of Change Implemented by DRAFT A All sections New document DLR All sections Finalisation and incorporation of review comments received by ECTL DLR Authorisation No. Action Name Signature Date 1 Prepared S. Brandes, M. Schnell (DLR) Approved M. Schnell (DLR) Released C. Rihacek (FRQ) The information in this document is subject to change without notice. All rights reserved. No part of the document may be reproduced or transmitted in any form or by any means, electronic or mechanical, for any purpose, without the written permission of FREQUENTIS AG. Company or product names mentioned in this document may be trademarks or registered trademarks of their respective companies. Page: I

3 Contents 1. Executive Summary Introduction Modelling of DME Interference Implementation of DME interference Consideration of duty cycle DME Interference Scenarios Involving DME GSs Static victim receiver Example: B-AMC forward link scenario with static victim RX Moving Victim Receiver Example: B-AMC forward link scenario with moving victim RX DME Interference Scenarios Involving Airborne DME Interrogators NAVSIM Procedure Determination of Interference Power Distribution Determination of Duty Cycle Example: B-AMC Reverse Link Scenario Consideration of Other Interference Sources JTIDS/MIDS Interference SSR Mode S Interference UAT Interference Conclusions References Abbreviations Page: II

4 Illustrations Figure 3-1: Block diagram of interference simulator Figure 3-2: Example of parameter file for DME interference Figure 3-3: Generation of DME pulse pairs assuming 3600 ppps duty cycle Figure 4-1: DME stations at Paris CDG, observed from FL Figure 4-2: Position of DME/TACAN stations around CDG, 995 MHz Figure 4-3: Position of DME/TACAN stations around CDG, 997 MHz Figure 4-4: DME and TACAN ground antenna vertical pattern GTx (φ) [D4] Figure 4-5: Vertical patterns of airborne DME and MIDS antennas GRx (α) [D4] Figure 4-6: PDF for four DME/TACAN stations at 994 MHz Figure 4-7: PDF for three DME/TACAN stations at 995 MHz Figure 4-8: PDF for two DME/TACAN stations at 996 MHz Figure 4-9: PDF for DME/TACAN stations at 997 MHz Figure 5-1: Snapshot of air traffic possibly interrogating DME stations at channel 35X/Y and corresponding interference power at victim ground station Figure 5-2: Topology of DME stations round CDG, 35X/Y Figure 5-3: Interference power PDFs originating from interrogations of DME stations at 1059 MHz located around Paris, CDG Figure 5-4: Topology of DME/TACAN stations at Paris CDG, 1059 MHz Figure 5-5: Cross section of relevant fraction of operational range within overlapping area Figure 5-6: Interference power PDFs originating from interrogations of DME stations at 1058 MHz located around Paris CDG Figure 6-1: Measured JTIDS/MIDS TX Spectrum Figure 6-2: JTIDS/MIDS TX Spectral Mask Figure 6-3: Distribution of interference power from JTIDS Figure 6-4: Required spectrum limits for transponder transmitter [Annex10] Figure 6-5: Required spectrum limits for interrogator transmitter [Annex10] Tables Table 4-1: NAVSIM data extraction, victim receiver in feet (r H = 261 nm) at CDG Page: III

5 Table 4-2: Example: DME/TACAN stations for B-AMC forward link scenario at FL 450 assuming frequency planning, static B-AMC victim RX Table 4-3: B-AMC forward link scenario with moving victim RX Table 5-1: DME/TACAN stations for RL scenario with frequency planning Table 6-1: Pulse shapes of Mode S replies [Annex10, Tab.3-2] Table 6-2: Pulse shapes of Mode S and interrogations Page: IV

6 1. Executive Summary In this deliverable, procedures for generating interference scenarios are described that model interference from existing L-band systems towards new candidate L-band systems. The candidate L-band systems intend to use different frequency ranges and therefore they have to cope with significantly different interference conditions. Hence, it is difficult to define detailed, universally applicable interference scenarios, which can be used for investigating any arbitrary L-band system. However, the general procedures outlined in this deliverable can be applied to any frequency range and any L-band system for generating appropriate interference scenarios. As DME is the main source of interference in the aeronautical L-band, particular emphasis is put on modelling of DME interference and defining appropriate interference scenarios. In Chapter 3, an interference simulator is described that generates the DME signal in time domain. Different interference conditions can be set by varying the power, the duty cycle, the distance between the two pulses of a pulse pair, the centre frequency of the DME signal as well as the number of different independent interferers having the same or different centre frequencies. In Chapter 4, the methodology for generating interference scenarios taking into account DME ground stations is described. With the NAVSIM tool, the interference power received from any DME ground station operating in a certain area can be determined based on actual DME channel assignments. The interference power is determined at the input of the victim receiver, taking into account elevation angle dependent antenna patterns of typical L-band devices, cable losses and free space loss due to the spatial separation between the interference source and the victim receiver. Thereby, the victim receiver can be considered either static resulting in fixed interference power values or mobile (moving in a certain range) resulting in a varying interference power described by a power probability density function (PDF). From these data, interference scenarios for any L-band system can be derived by simply considering those channels relevant for the victim system under investigation. As an example, an interference scenario has been defined for the B-AMC forward link (FL). In Chapter 5, interference originating from aircraft interrogating DME ground stations is investigated. The victim receiver is on ground. With the NAVSIM tool, the DME ground stations, which are interrogated by aircraft in the considered area, are identified and the resulting interference power received at the victim receiver on ground is determined. For each ground station in the considered area, an interference power PDF is obtained that represents the interference caused by interrogations of different aircraft at different distances to the respective DME station and to the victim ground station. Moreover, a procedure for determining the corresponding aggregate duty cycle is described. Interference scenarios are determined by generating the required data, i.e. interference power PDF and duty cycle for all relevant DME stations in the considered channels and in the considered area. As an example, an interference scenario for the B-AMC reverse link (RL) is given. Finally, interference from other L-band systems is addressed. In the time domain, the pulses of JTIDS, UAT and SSR can be modelled in a simplified way avoiding the detailed implementation of the signal generation of the considered systems. For the duration of the pulses, noise with a power level corresponding to the received interference power is added to the desired signal. Pulse duration and duty cycles are given in the specifications; the received power level can be derived from the distance in space and in frequency between the interfering and the victim system. Page: 1-1

7 With the described procedures, all building blocks are available that are required to generate interference scenarios for any arbitrary L-band system. Depending on the position in the L-band, different interference sources may be relevant and thus are requiring a combined application of different procedures for generating an overall interference scenario reflecting the interference situation for the considered L-band system END OF SECTION Page: 1-2

8 2. Introduction In this report, interference scenarios for simulating interference from existing L-band systems towards any candidate L-band radio system are defined. The L-band systems to be considered are:! DME,! JTIDS/MIDS,! SSR Mode S, and! UAT. In the first stage of the B-AMC study, the impact of these systems onto the B-AMC system has already been investigated. Based on these results summarized in the B-AMC Deliverable D5 [D5], the procedure for generating interference scenarios is generalized such that it can be applied to any candidate L-band system independent of its frequency offset to DME channels. In addition, the influence of JTIDS/MIDS, SSR Mode S, and UAT is considered and included into the interference scenarios. The proposed scenarios aim at enabling comparative investigations of typical performance/capacity of different candidate L-band systems in a typical environment comprising multiple interferers. Therefore, location, power, duty-cycle and other parameters of interfering sources are generally based on statistics (distributions). The exact laboratory assessment of the worst-case impact of interference (e.g. at closest TX-RX spacing between two systems) and frequency planning may require different kind of deterministic scenarios (one desired signal source, one undesired signal source, one victim receiver). Note: As the candidate L-band systems are intended to be operated in different parts of the aeronautical L-band each exhibiting significantly different interference conditions, in this document only the rationale for taking into account different interfering L-band systems and determining interference scenarios is described. It is difficult to define a detailed common interference scenario that can be used for each candidate L-band system. This deliverable is organized as follows. As DME is considered to be the main source of interference, emphasis is put on its accurate modelling. In Chapter 3, the methodology for modelling DME interference is described. In Chapters 4 and 5, interference scenarios with DME ground and airborne stations as interference source are defined taking into account real DME channel assignment and real air traffic. The impact of other L-band systems such as JTIDS/MIDS, SSR Mode S and UAT is addressed in Chapter END OF SECTION Page: 2-1

9 3. Modelling of DME Interference A simulator for modelling DME interference has to be designed to simulate the impact of DME interference on the victim receiver as realistically as possible. Hence, a realistic representation of the interference signals and their processing at the victim receiver is considered by means of this simulator. Therefore, pulse pairs with a Gaussian shape characteristic for DME signals are generated in time domain. At the input of the victim receiver, this signal is superimposed with the desired signal. Afterwards, it is filtered by the RF filter and processed in the same way as the desired signal. In the first part of the B-AMC study, it has been shown that it is important to consider B-AMC receiver processing (right part of Figure 3-1) as it may have several effects on the spectral shape of the DME signal as experienced by the victim B-AMC receiver. The same applies to any victim receiver. However, the task of modelling a particular victim receiver is technology-specific and must be done individually for each candidate victim receiver, which is out of the scope of this work. Figure 3-1: Block diagram of interference simulator The interference simulator is designed flexibly. Therefore, it is be able to simulate an arbitrary number of DME interferers each operating in an arbitrary channel within the L-band. The parameters for each interferer can be adjusted in a parameter file serving as input to the interference simulator. In the parameter file, the following values can be set:! TYPE DME! FREQ absolute DME transmitting frequency in MHz. The victim receiver centre frequency is given in the parameter file for the complete simulation chain. Hence, the DME interferer frequency relative to the victim system can be adjusted in the interference simulator.! DELTAT interval between two pulses of a pulse pair in µs, chosen according to operational mode of interferer! POWER received total power of DME interferer in dbw, measured at the input of the victim RX, i.e. after the RX antenna. The procedure for determining the received interference power is addressed in the next chapter. Instead of a constant interference power, a probability density function (PDF) of the power (that produces the corresponding dbw value) can be used in order Page: 3-1

10 to model a movement of the victim RX or the interfering airborne DME stations.! DUTY duty cycle of DME interferer in ppps Note: The parameters TYPE, FREQ and POWER apply to any type of L-band interferers; the parameters DUTY and DELTAT are DME-specific and are used for generating a time domain representation of the interfering DME signal. In case of other L-band systems, similar or different parameters can be used for describing the time behaviour of these signals. In Figure 3-2, an extraction from such a DME interference parameter file is shown. If multiple DME interferers occur in the same channel, e.g. two interferers at 992 MHz and four interferers at 995 MHz, a statistically independent interference signal for each interferer is generated. Finally, all interference signals are summed up and processed in the victim receiver as depicted in Figure 3-1. This is repeated for each simulation run. If the parameter "POWER" is not set to a fixed value as shown in Figure 3-2, but a power PDF will be chosen instead, in each simulation run, a new power value is generated from the power PDF of the corresponding interferer. TYPE=DME FREQ=992 DELTAT=12 POWER= DUTY=3600 TYPE=DME FREQ=992 DELTAT=12 POWER= DUTY=3600 TYPE=DME FREQ=994 DELTAT=12 POWER= DUTY=3600 TYPE=DME FREQ=995 DELTAT=12 POWER= DUTY=2700 TYPE=DME FREQ=995 DELTAT=12 POWER= DUTY=2700 TYPE=DME FREQ=995 DELTAT=12 POWER= DUTY=3600 TYPE=DME FREQ=995 DELTAT=12 POWER= DUTY=2700 Figure 3-2: Example of parameter file for DME interference Implementation of DME interference During the simulation run, an interference DME signal is generated according to the input from the parameter file. A pair of two Gaussian-shaped pulses with a distance of t (given by DELTAT) can be represented in time domain as 2 2 α/2 t α/2( t t) 11 2 dt () = e + e with α = s The factor α is chosen such as to obtain DME pulses with the correct width and rise time as defined in [Annex 10, I]. The baseband Gaussian DME pulses are used to amplitudemodulate the DME carrier. The DME carrier frequency (defining the relative offset fn to the centre frequency of the victim receiver) is set as a simulation parameter (FREQ). Note: The above model does not include any impact of the DME TX radiated broadband noise or spurious signals and is therefore applicable only for larger TX-RX distances where these contributions can be neglected. This is assumed to be well applicable to "typical" scenarios, however close or co-located systems must be investigated by using other methods. Page: 3-2

11 Finally, N D DME signals with different powers P n, different carrier frequency offsets fn, different phase ϕ n, and different start time t n are summed up. Power and frequency are given in the interference parameter file by "POWER" and "FREQ", respectively. The phase ϕ n is determined randomly for each interferer assuming a uniform distribution for ϕ n between 0 and 2π. The selection of the start time t n is explained in the next subsection. The resulting interference signal is given by ND j( 2 fnt n) xt () = Pn dt ( tn) e π + ϕ n= Consideration of duty cycle When considering the duty cycle in the simulations, it has to be taken into account that the DME/TACAN stations do not generate pulse pairs in absolutely regular intervals. At the same time, the pulse pairs are not completely randomly distributed such that several pulse pairs never occur in a short period of time followed by a longer period of silence. In the simulations, the DME duty cycle is considered as follows: At first, the duration of one transmit frame of the interfered system is cut into segments of equal length according to the duty cycle such that one pulse pair occurs in each segment. Assuming e.g. a duty cycle of 3600 ppps and a frame duration of 6.48 ms as in B-AMC, 23 pulse pairs can occur within the duration of one transmit frame. Hence, the transmit frame is cut into 23 segments of µs. In each segment, one pulse pair is generated starting at a random time within this segment. The random start time is constrained to the length of the segment shortened by the total duration of the pulse pair. That way, it is guaranteed that always complete pulse pairs are generated in each segment. In doing so, certain regularity as well as certain randomness of the interference signals is simulated with this approach which is illustrated in Figure 3-3. Alternatively, the inter-arrival time between DME pulse pairs can be assumed to be exponentially distributed. Hence, the number N of pulse pairs started in a certain time interval can be modelled as a Poisson process. This has also been proposed in [DO-292]. The probability that k DME pulse pairs have started before time instance t is then k ( λt) λt PN ( = k) = e k! with parameter λ corresponding to the average number of pulses per time interval. In the considered example λ = 23/ 6.48ms. Page: 3-3

12 Figure 3-3: Generation of DME pulse pairs assuming 3600 ppps duty cycle END OF SECTION Page: 3-4

13 4. DME Interference Scenarios Involving DME GSs In this interference scenario, DME ground stations are considered as interference source for the airborne receiver of the L-band communications system. All DME ground stations within the radio horizon of the considered aircraft are taken into account. First, interference scenarios for three different environments characterized by different flight phases and flight levels (FL) have been defined:! Airport (APT) scenario (FL 0-50),! TMA scenario (FL ), and! ENR scenario (FL ). For determining the radio horizon of the victim receiver, in each interference scenario, the maximum flight level is assumed in order to obtain a worst case scenario with respect to the number of interfering DME GSs. The flight level ranges are chosen in accordance to [COCR]. For determining interference scenarios, a realistic topology of DME and TACAN stations operating in Europe is considered. The relevant data of each DME or TACAN ground station including e.g. geographical position, EIRP, and reply frequency are taken from the COM3 data base [COM3]. Interference conditions are investigated in that area with the highest density of DME stations in Europe in order to simulate worst case conditions. The NAVSIM tool, which is based on realistic DME channel assignment [COM3], has identified the area around Paris, Airport Charles de Gaulle (CDG), as that area with highest density of DME/TACAN stations seen within the radio horizon (261 nm) from an aircraft flying at high altitude (FL 450). In the screenshot from the NAVSIM tool shown in Figure 4-1, all 675 DME and TACAN stations deployed within 261 nm from CDG are plotted, regardless of the DME channel they use. In addition, mobile TACAN stations used by the military have been considered in the NAVSIM tool. For each "ALL country_x" entry (e.g. "ALL FRANCE") in the COM3 data base one mobile TACAN station has been placed at the closest possible distance to the victim RX. For example, in Belgium mobile TACAN stations may be operated at 995 and 997 MHz, respectively. They are positioned on Belgium territory as close as possible to the victim receiver located at Paris, CDG 1. Screenshots from the NAVSIM tool depicted in Figure 4-2 and Figure 4-3 show the worst case position of the Belgian mobile TACAN station and all other stations operating in those channels around CDG. For each interference scenario, interference arising from DME ground stations in DME channels adjacent to the centre frequency of the victim receiver located in the considered area (see sections 4.1 and 4.3, respectively) is determined by means of the NAVSIM tool. As data about all DME channels used in Europe is available [COM3], the NAVSIM tool allows the same procedure to be applied to any DME channel. In the lower part of Figure 4-1, the power density produced by these GSs within the entire L-band from MHz is illustrated, as seen by an airborne receiver at FL450 1 B-AMC system investigations used CDG as the reference point. But, any other reference area and the observation point within that area may be selected, as well. Page: 4-1

14 above Paris, CDG. For each DME channel the power density received at the victim receiver antenna is given in dbw/m 2, based on known EIRP values of DME/TACAN GS transmitters. In the following, the procedure for determining the interference power at the input of the victim receiver is described for two different cases: First, the victim receiver is assumed to be at a fixed position in the centre of the considered area. Therefore, fixed values for interference power in each considered DME channel are obtained (see section 4.1). Second, the victim receiver is assumed to move within the considered area, which results in a probability density function (PDF) for interference power in each considered DME channel (see section 4.3). Both possibilities can serve as input to the interference simulator as described in the previous chapter. Figure 4-1: DME stations at Paris CDG, observed from FL450 Page: 4-2

15 Figure 4-2: Position of DME/TACAN stations around CDG, 995 MHz Page: 4-3

16 Figure 4-3: Position of DME/TACAN stations around CDG, 997 MHz 4.1. Static victim receiver The victim airborne receiver is positioned in the centre of the European area with most dense DME population, i.e. at Paris Airport CDG. The actual position of the victim receiver is at a MLS/DME station at CDG (N / E ). For FL 245 and FL 450, the victim aircraft is positioned exactly above the interfering GS, i.e. the aircraft altitude is assumed as the minimum vertical distance between DME GS and victim RX in order to simulate the worst case situation when the aircraft with the victim B-AMC RX on board flies over the DME GS. For the ground scenario (FL 0 - FL 50), a minimum horizontal distance of 600 m to the MLS/DME station at Paris CDG has been assumed. The radius of the area of interest is set according to the radio horizon seen at a certain flight level, namely:! Airport (APT) scenario (FL50): r H =28 nm! TMA scenario (FL245): r H = 192 nm! ENR scenario (FL450): r H = 261 nm Page: 4-4

17 Assuming an airborne victim receiver at a fixed position in the centre of the most dense area around Paris CDG, the NAVSIM tool provides data about all currently operated and planned DME and TACAN GSs in the surrounding area, including:! DME station name and ID,! distance and slant range between victim RX and interfering DME/TACAN station,! DME/TACAN GS channel/operating mode and reply frequency,! type and duty-cycle of DME/TACAN GS,! GS EIRP (dbw),! ground/airborne elevation angles at DME/TACAN ground station and victim RX including corresponding antenna gain [D4], and! interference power (dbw) received at input of victim receiver. The interference power P interf received at the input of the victim receiver is calculated as [ ] Tx ( ) ( ) Rx ( ) Rx Pinterf dbw = EIRP + G ϕ L d + G α + G C. The known EIRP of the interfering GS is assumed to contain the maximum peak ground antenna gain. In [DO-292], the maximum antenna gain of L-band ground equipment is specified as 8 dbi. In addition, the antenna pattern from Figure 4-4 (label: DME_G Tx composed) [D4] is considered and an antenna gain G ( ϕ ) dependent on the elevation angle ϕ between the interfering GS and the victim receiver is added to the transmitter EIRP. The spatial distance d between the interfering GS and the victim receiver is taken into account by means of a simple propagation model. Within the radio horizon, the free space loss is defined as where L free (d): free L ( d) = log d + 20 log f free transmission loss (db) between transmitter and receiver as a function of distance, d: distance between transmitter and receiver (nm), f: frequency (MHz). At the antenna of the victim receiver, the maximum peak ground antenna gain Rx Rx G = 5.4 dbi and an elevation angle dependent antenna gain G ( α ) is added. The antenna of the victim receiver is assumed to have the same antenna pattern as conventional airborne L-band antennas. A representative antenna pattern, that is also used here, is depicted in Figure 4-5 [D4]. In addition, cable losses C = 3 db are taken into account. Page: 4-5

18 Figure 4-4: DME and TACAN ground antenna vertical pattern GTx (φ) [D4] Figure 4-5: Vertical patterns of airborne DME and MIDS antennas GRx (α) [D4] Page: 4-6

19 Note: Please refer to [D4, Chap. 5.1] for a detailed definition of the elevation angles α and ϕ. From the available data, the total interference power within each DME channel and the data/constellation of all visible ground DME/TACAN stations contributing to this power can be derived. An example of an available data set is given in Table 4-1 for Paris CDG at FL 450. For reasons of clarity, the data set is reduced to its relevant columns, e.g. ground and airborne antenna gains are omitted. ID DME Station Name Distance in nm Slant Range in nm DME channel Reply Frequency in MHz Type EIRP in dbw Received power at RX input in dbw TST METZ/FRESCATY X 984 TACAN LRE LURE X 984 TACAN WOL WOLVERHAMPTON X 984 DME MAS MAASTRICHT X 984 VOR/DME DIJON/LONGVIC X 985 MLS/DME MAM MARHAM X 985 TACAN COL COLA/KOLN X 986 TACAN/VOR BLM BALE/MULHOUSE X 986 VOR/DME BT PARIS/LE BOURGET X 986 VOR/DME IBR BRUXELLES/NATIONAL X 987 ILS/DME KSL KASSEL-CALDEN X 987 MLS/DME AS ANGERS X 987 ILS/DME AT ANNECY-MEYTHET X 987 ILS/DME GSI GUISCRIFF X 987 ILS/DME SDI SAINT-DIZIER X 987 MLS/DME Table 4-1: NAVSIM data extraction, victim receiver in feet (r H = 261 nm) at CDG For generating interference scenarios, the victim receiver is adjusted to a certain frequency and the interference in all relevant DME channels can be determined from the available NAVSIM data. In each relevant DME channel, all DME stations within the radio horizon are considered with interference power calculated by NAVSIM. The duty cycle for each considered GS is assumed to be maximum, i.e ppps for DME and 3600 ppps for TACAN stations. If more than one DME/TACAN station is active per channel, the contributions of all individual stations are considered with their respective received powers and duty-cycles. The individually generated interference signals are summed up, resulting in the total interference signal in the respective channel Example: B-AMC forward link scenario with static victim RX As an example, an interference scenario specific for the B-AMC system is derived in the following. The B-AMC channels are placed at 0.5 MHz offset from the DME channel grid (1 MHz). Investigations in the first stage of the B-AMC study have shown that for TX-RX distances of 600 m or more only two adjacent DME channels (at +/-0.5 MHz and +/-1.5 Page: 4-7

20 MHz offset from the selected B-AMC channel) at each side of the B-AMC channel effectively contribute to the interference situation. It is assumed that frequency planning was applied for selecting the B-AMC centre frequency, i.e. the "most appropriate" B-AMC channel is selected such as to minimize interference from DME systems operating in the two adjacent channels. Note: For other victim receivers with broader reception bandwidth/lower selectivity, additional DME channels may become relevant (this fact would have to be included in interference investigations). For the considered scenario at FL 450, MHz has been selected (evaluation the data from NAVSIM) as B-AMC centre frequency when assuming MHz as the frequency range available for the B-AMC GSs. According to the NAVSIM data shown in Table 4-2, in the channels next to MHz, i.e MHz, only relatively small interference powers occur. Therefore, MHz has been selected as B-AMC operating frequency. In the channel with -0.5 MHz offset to the B-AMC centre frequency one TACAN interferer occurs. In the channel with +0.5 MHz offset, two DME/TACAN stations occur, one with a peak interference power of dbm, and another one with a peak interference power of dbm. Interference in the channels at +/-1.5 MHz offset from the B-AMC centre frequency listed in Table 4-2 is composed of the contributions of several DME/TACAN stations that also can be derived from the NAVSIM data (see Table 4-1). Each interferer is modelled separately with its respective power shown in Table 4-2. For the duty cycle, the maximum pulse rate is taken into account for each station. In reality, the duty cycle of some stations may be lower when stations operate in variable mode with a duty cycle ranging from 700 to 2700 ppps for DME stations. Frequency Distance to Type of relative to B-AMC victim station B-AMC system RX [nm] Pulse rate [ppps] Mode TACAN 3600 X TACAN 3600 X TACAN 3600 X TACAN 3600 X B-AMC TACAN 3600 X TACAN 3600 X MLS/DME 2700 X DME/ILS 2700 X DME 2700 X DME 2700 X DME 2700 X TACAN 3600 X Interference power at input of victim RX considering antenna patterns [dbm] Table 4-2: Example: DME/TACAN stations for B-AMC forward link scenario at FL 450 assuming frequency planning, static B-AMC victim RX Page: 4-8

21 The procedure for deriving interference scenarios for DME GSs described above can be extended and applied to any arbitrary system operating in the L-band. After determining the interference power at a certain location for the victim RX received from all ground stations in the frequency range of interest (for that victim RX), interference scenarios can be derived with known! offset to DME channels,! centre frequency of victim L-band RF channel, and! number of relevant DME channels around that RF channel causing harmful interference. That way, all relevant parameters required as input to the DME interference simulator can be determined Moving Victim Receiver Within this study, interference scenarios are refined such as to model the interference situation occurring during a flight through an entire cell, i.e. an area rather than just a selected point in space has to be considered. Note: This scenario is offered as an option it should be agreed which kind of scenarios (static - or moving victim RX) would be finally used for testing new aeronautical systems. For each interference scenario, a circular area is defined, in which the GS of the victim system communicates with the victim airborne receiver. The centre of this area can e.g. be at CDG (as in the first version of the interference scenarios) or at any other arbitrary position. In contrast to the first version of the interference scenarios, the location of the victim receiver is no longer considered to be fixed but varies within a service volume (ENR, TMA, or APT) as the victim receiver moves. The position is assumed to be equally distributed within the highest flight level of the considered service volume (worst case regarding the radio horizon). Due to the random locations of the victim receiver, the received interference power coming from a particular DME station varies and can be modelled by means of a PDF. The power PDF is generated for each DME channel by positioning the victim receiver at many different locations within the cell and determining the interference power received from each DME station in all relevant DME channels. For the link budget calculations, elevation angle dependent antenna characteristics of both the victim receiver and the interfering ground station have to be considered as described in the previous section. In order to simplify the interference simulator, for each DME channel and all DME stations within each channel, all determined values of the interference power in a certain channel can be collected and the probability of their occurrence can be determined. For each DME channel, all relevant DME ground stations can be summarized and a compound received power PDF is then produced for each DME channel. The duty cycle in each channel can be determined by summing up the duty cycles of all contributing DME/TACAN stations. Again, the duty cycle of any particular DME and TACAN station is assumed to correspond to 2700 ppps and 3600 ppps, respectively. Page: 4-9

22 4.4. Example: B-AMC forward link scenario with moving victim RX The generation of interference scenarios is explained in the following by using the B-AMC forward link as an example. Again, the B-AMC centre frequency is assumed to be set at MHz. DME stations in the channels at 994, 995, 996, and 997 MHz are considered as potential interferers. The area of interest is located around Paris CDG. For the ENR scenario, the interference occurring at the victim receiver moving within a radius of 120 nm around Paris CDG, is observed. In the NAVSIM tool, the interference situation has been monitored at FL 450 at 72 discrete positions equally distributed within the observation area. DME/TACAN stations identified to be relevant are listed in Table 4-3. Comparing Table 4-3 with the version for a static victim receiver (Table 4-2), obviously more DME/TACAN stations per channel have to be taken into account when considering an entire observation area. The power PDFs corresponding to each involved DME/TACAN station and each DME channel are depicted in Figure 4-6 to Figure 4-9. According to simulation results from [D3], interferers with powers smaller than -130 dbw have no impact on the B-AMC system. Hence, if the interference power originating from a certain DME/TACAN station at a certain position is below a threshold of -130 dbw = -100 dbm, the power of the interferer is assumed to be -135 dbw. If the values below the threshold are just skipped, the power valued above the threshold become more probable hence falsely increasing the average interference power. This explains the peaks at -135 dbw in the PDFs in Figure 4-6 to Figure 4-9. The average power of the contributing DME/TACAN stations is slightly increased by a few db when considering the entire observation area as compared to the situation with a single observation point. Frequency relative to B-AMC system Location Type of station Duty cycle [ppps] Mode Interference power at input of victim RX considering antenna patterns [dbm] -1.5 Kempten DME 2700 X -1.5 Kleine Brogel TACAN 3600 X -1.5 Odiham TACAN 3600 X -1.5 Sint-Truide TACAN 3600 X -0.5 All Belgium TACAN 3600 X -0.5 All Italy TACAN 3600 X -0.5 Wunstorf DME 2700 X 0 B-AMC 0.5 Kleine Brogel TACAN 3600 X 0.5 Lyneham TACAN 3600 X 1.5 All Belgium TACAN 3600 X 1.5 Brest DME 2700 X 1.5 Brussels DME 2700 X 1.5 Exeter DME 2700 X 1.5 Friedrichshafen DME 2700 X Power PDFs from Figure 4-6 Power PDFs from Figure 4-7 Power PDFs from Figure 4-8 Power PDFs from Figure 4-9 Page: 4-10

23 Frequency relative to B-AMC system Location Type of station Duty cycle [ppps] Mode Interference power at input of victim RX considering antenna patterns [dbm] 1.5 Geneva DME 2700 X 1.5 Groningen DME 2700 X 1.5 Luxembourg DME 2700 X 1.5 Nantes DME 2700 X 1.5 Remaining stations DME 2700 X Table 4-3: B-AMC forward link scenario with moving victim RX Note: At 997 MHz, many different DME stations occur at only a few discrete locations within the observation area. For reasons of simplicity all these stations have been summarized to one station with a representative power PDF, shown in Figure 4-9 with the label "remaining stations". Since these stations occur only seldom and their powers are very small, the total duty cycle is assumed to correspond to 2700 ppps. In a similar way, all other stations occurring in the same channel can be summarized to one representative station with a composite power PDF. The duty cycles of all stations sum up Kempten Kleine Brogel Odiham Sint-Truiden normalized probability power (dbw ) Figure 4-6: PDF for four DME/TACAN stations at 994 MHz Page: 4-11

24 all Italy all Belgium Wunstorf normalized probability power (dbw) Figure 4-7: PDF for three DME/TACAN stations at 995 MHz Kleine Brogel Lyneham 0.25 normalized probability power (dbw) Figure 4-8: PDF for two DME/TACAN stations at 996 MHz Page: 4-12

25 normalized probability all Belgium Brest Bruxelles Exeter Friedrichshafen Geneve Groningen Luxembourg Nantes remaining stations power (dbw) Figure 4-9: PDF for DME/TACAN stations at 997 MHz END OF SECTION Page: 4-13

26 5. DME Interference Scenarios Involving Airborne DME Interrogators In these interference scenarios, the victim receiver is the ground station of the future L-band communications system. Thus, interference arising from airborne DME stations has to be considered. All aircraft within the radio horizon of the considered victim receiver on ground are taken into account. Interference from DME ground stations is neglected as only a few DME ground stations are within the limited radio horizon and their interference contribution is small compared to interference from aircraft flying over the victim ground station. The interference situation at the victim ground station only depends on the spatial distribution of all aircraft using relevant DME channels for their DME interrogations, regardless of their flight level. Therefore, no distinction between different scenarios for different flight levels - as for the scenarios involving DME ground stations - is required. DME interference scenarios involving airborne DME stations are only relevant for L-band systems operating in the ranges MHz and MHz as DME airborne stations are only active at these frequencies NAVSIM Procedure For NAVSIM simulations, the same area as that used for the investigation of the forward link is considered, namely Paris CDG, which is the area with the highest density of DME/TACAN stations. In the airspace above this area aircraft are flying and interrogating DME/TACAN ground stations. At the same time they cause interference at the victim ground station, which is assumed to be in the centre of the considered area at Paris CDG (N / E ). Air traffic is simulated according to Eurocontrol data of real aircraft movements monitored on July 7, The data are relatively old, but they still represent current worst case conditions as a day with high air traffic load has been considered. At first, the DME/TACAN ground stations an aircraft can communicate with are identified. This is done by checking if the signal from a certain ground station is received at least with a certain power density. If the received power level exceeds the sensitivity threshold of a DME/TACAN airborne receiver, i.e. the signal strength is above -89 dbw/m 2, as specified in [D1], an aircraft can theoretically interrogate that ground station. At a certain time instance, the aircraft interrogates only that ground station from which it has received the strongest signal. NAVSIM simulations are based on the assumption that at each time instance each aircraft is interrogating one DME/TACAN ground station, namely the one from which it has received the strongest signal. Depending on the distance of the aircraft to the victim ground station, the interference power received by the victim ground station is determined by means of the free space propagation model described in the previous chapter. Moreover, elevation angles of the aircraft with respect to the victim ground stations are determined and the corresponding antenna patterns are considered in the same way as in the scenarios with DME ground stations involved. That way, the interference power received by the B-AMC victim ground station from each aircraft in the surrounding airspace is obtained. Thereby, the individual contributing DME/TACAN stations as well as the DME channels including X and Y mode are distinguished. Moreover, a time-variant distribution of air traffic is taken into account by Page: 5-1

27 taking snapshots at different time instances. In order to simulate worst-case conditions, air traffic is simulated during "rush hour", i.e. between 4 and 5 p.m. A snapshot of the air traffic including the interference power caused at the victim ground station is shown in Figure 5-1 for the DME channel at 1059 MHz. In this channel, all aircraft that interrogate DME ground stations on channels 35X and 35Y, i.e. on 1059 MHz, and receive responses at 996 MHz and 1122 MHz, have to be considered. For reasons of clarity, the topology of interrogated DME/TACAN ground stations is shown in Figure 5-2. With the NAVSIM tool, all possible interrogations from aircraft in the airspace above the victim ground station are simulated. From the obtained data, conclusions about the distribution of interference power can be drawn. However, real interrogations taking into account different modes of DME devices (i.e. scan mode or track mode) and different duty cycles have not been considered. Figure 5-1: Snapshot of air traffic possibly interrogating DME stations at channel 35X/Y and corresponding interference power at victim ground station Note: In Figure 5-1 yellow dots indicate aircraft and green lines indicate the communication with a DME/TACAN ground station. The colour of the Page: 5-2

28 interconnecting lines indicates the interference power received by the victim B-AMC ground station. A red line corresponds to the interference power exceeding -75 dbw, an orange line stands for an interference power level between -105 dbw and -75 dbw, and a yellow line indicates a received interference power level between -135 and -105 dbw. Figure 5-2: Topology of DME stations round CDG, 35X/Y Note: In Figure 5-2, ground stations with green labelling are DME TMA stations, yellow labelling indicates DME en-route stations and TACAN stations are labelled in red Determination of Interference Power Distribution From NAVSIM data, the distribution of interference power in a certain DME channel can be derived. Similar as for the scenarios with DME ground stations the interference power P interf at the receiver input is determined according to [ ] Tx ( ) ( ) Rx Rx P ( ) interf dbw = EIRP + G α L d + G + G ϕ C. free Page: 5-3

29 According to [D4], the EIRP of an airborne DME unit is assumed to be 33 dbw= 63 dbm. The maximum peak antenna gain G Tx of L-band airborne equipment is specified as Tx G = 5.4 dbi in [DO-292] and is already included in EIRP. In addition, the antenna Tx pattern from [D4] (see Figure 4-5) is assumed and an antenna gain G ( α ) dependent on the elevation angle α between the interfering airborne unit and the victim ground receiver is added to the EIRP. The antenna of the victim ground receiver is assumed to have the same antenna pattern as conventional ground L-band antennas. The maximum peak antenna gain G Rx Rx of L-band airborne equipment is specified as G = 8 dbi. In Rx addition, an elevation angle dependent antenna pattern defining G ( ϕ ) is considered. A representative antenna pattern is given in [D4] and is shown in Figure 4-4. Moreover, cable losses C = 2 db are taken into account. The values of interference power received at the input of the victim receiver and measured at different points in time are collected. Interference power in different DME channels and originating from interrogations of different DME stations in that channel are distinguished. The interference power PDF for each DME channel and each DME station within that channel is generated by evaluating the probability of each power value. In the considered area around Paris CDG all aircraft up to FL450 that are in the radio horizon (261 nm) of the victim ground station cause interference at the victim ground receiver. These aircraft may interrogate DME ground stations within the considered area and even beyond. Taking into account an operational range of 200 nm for a DME enroute or a TACAN station, all DME/TACAN stations within an area of 461 nm have to be considered. However, not all interrogations of remote DME/TACAN stations cause significant interference power levels. Hence, the actual number of DME/TACAN stations to be considered reduces. As an example, the interference power PDF for the channel at 1059 MHz is derived in the following. According to the topology of DME/TACAN stations interrogated at 1059 MHz (Figure 5-2), seven DME stations and one TACAN station are within the area of interest and can be interrogated by aircraft within the radio horizon as seen from the victim receiver at Paris CDG. Due to very small interference power caused by aircraft interrogating the DME station at Milan in a distance of 309 nm and at Schleswig in a distance of 438 nm from the victim receiver, these DME stations are omitted. For all other stations, the resulting power PDF is shown in Figure 5-3. The shape of the PDFs is as expected: There are few aircraft close to the victim ground station causing high interference power levels. With increasing distance to the victim ground station interference power decreases, but the number of aircraft causing interference grows. Nevertheless, it can be distinguished between two different shapes of power PDFs. DME/TACAN stations "Koksijde" and "Kleine Brogel" cause interference power levels up to -62 dbm. Their peak is at -82 dbm. All other stations cause interference power levels up to only -78 dbm and their peak is also slightly lower at about -84 dbm. This can be explained by the spatial distance between the interrogated ground stations and the victim receiver. "Koksijde" and "Kleine Brogel" are relatively close to the victim receiver such that aircraft in the proximity of the victim station interrogate these two stations and hence cause high interference levels. All other stations are farer away, such that an aircraft interrogating these stations never is in the direct proximity and hence does not cause maximum interference. All considered DME/TACAN stations can either be simulated separately with their respective power PDFs or alternatively, one compound power PDF can be generated Page: 5-4

30 which represents interference power received from all aircraft independent of which ground station they interrogate. The compound power PDF is shown in Figure 5-3, as well. normalized probability Fairoaks Kleine Brogel Koksijde La Rochelle Lyneham Dortmund compound pdf power (dbm) Figure 5-3: Interference power PDFs originating from interrogations of DME stations at 1059 MHz located around Paris, CDG Note: Interference power values below -100 dbm have been neglected as they are considered to provide only very minor contributions to the overall interference situation. It can be seen from the power PDFs that even values up to -90 dbm occur only seldom and hence can be omitted as well Determination of Duty Cycle Since no real interrogations were simulated in NAVSIM, the duty cycle for the aircraft interrogating each DME or TACAN ground station had to be estimated. Therefore, it is assumed that each ground station receives exactly that number of interrogations it can reply to at maximum duty cycle, i.e. full load is assumed for each ground station. For a DME and TACAN station it is assumed that the interrogations from all aircraft in the operational range give 2700 ppps and 3600 ppps, respectively. Moreover, it is assumed that all aircraft are equally distributed within the operational range. Consequently, if parts of the operational range are beyond the radio horizon, the number of aircraft to be considered reduces by that fraction which is beyond the radio horizon. At the same time, this translates into a reduction of the duty cycle. Page: 5-5

31 For determining that part of the operational range of the considered ground stations in which aircraft have impact on the victim ground station, the distance between the victim ground station and the DME or TACAN ground stations interrogated on DME channels close to the B-AMC RL centre frequency has to be known. The topology of DME and TACAN stations that are interrogated at 1058 MHz is shown in Figure 5-2 and redrawn in Figure 5-4, respectively. For evaluating the number of contributing DME and TACAN stations in the considered area, it has been assumed that the service volume of a DME TMA station is 130 nm in radius at a height of 18,000 ft [ED54]. Hence, all interrogating aircraft within a radio horizon of 177 nm, corresponding to FL180, have to be taken into account as they are seen from the B-AMC victim ground station. These aircraft can communicate with DME TMA ground stations up to 130 nm beyond the radio horizon. Hence, all DME TMA stations within a range of 130 nm nm = 307 nm have to be considered. For DME ENR and TACAN stations an operational range of 200 nm has been considered [ED54]. Assuming a maximum flight level at 45,000 ft, all aircraft within the radio horizon of 261 nm have a line of sight connection to the victim ground station. Consequently, all DME ENR and TACAN stations within a range of 261 nm nm = 461 nm have to be considered. In Figure 5-4, the victim ground station is located in the centre of the considered area around Paris CDG. When considering all DME TMA stations in a radius of 307 nm as explained above, five DME TMA stations at 1059 MHz have to be considered (these are the same as the ones considered for determining the power PDF). The aircraft interrogating these ground stations are assumed to be equally distributed within the operational range of 130 nm. However, due to the limited flight level of 18,000 ft, only that fraction of aircraft in a maximum distance of 177 nm, i.e. within the radio horizon corresponding to FL180, causes interference in the victim system. The overlapping area between the operational range of the DME TMA station and the radio horizon of the B-AMC victim ground station is determined by simple geometric calculations. The ratio of the overlapping area to the total operational area indicates which fraction of the duty cycle has to be considered. For the DME TMA station at a distance of 247 nm to the B-AMC victim ground station (highlighted in Figure 5-4), 13.8% of the operational range are within the radio horizon of the victim ground station. Hence, a pulse rate of * 2700 ppps = 373 ppps applies. However, not all aircraft in the overlapping area are visible within the radio horizon of the B-AMC ground station. Considering a DME TMA station, all aircraft in a distance of 177 nm which are below FL 180 can not be "seen" by the victim ground station. Hence, in a second step the fraction of aircraft in the overlapping area, which is visible within radio horizon of the victim ground station, is determined. As illustrated in Figure 5-5 the area that is visible within the radio horizon of the victim ground station is approximated by a cone. The operational range of a DME/TACAN station is also represented by a cone since, in particular for a DME TMA station, it can not be assumed that aircraft on the ground or at low altitudes in the outer area of the operational range interrogate the DME ground station. Again, the aircraft are assumed to be equally distributed in the operational range. For the DME TMA station at a distance of 247 nm to the victim ground station considered above, 42.35% of the aircraft within the overlapping area are within the radio horizon of the victim ground station. Hence, the duty cycle further reduces to * * 2700 ppps = 158 ppps. Page: 5-6

32 This procedure is repeated for all relevant DME TMA stations in the considered channels. In addition, as shown in Figure 5-4, the relevant duty cycle of the TACAN station interrogated at 1059 MHz is determined as an example. The overlapping area is determined based on a radio horizon of 261 nm, a maximum flight level of 45,000 ft, an operational range of 200 nm and a maximum pulse rate of 3600 ppps. For these parameters, a pulse rate of 2368 ppps is obtained. Figure 5-4: Topology of DME/TACAN stations at Paris CDG, 1059 MHz Page: 5-7

33 Figure 5-5: Cross section of relevant fraction of operational range within overlapping area 5.4. Example: B-AMC Reverse Link Scenario As another example, the generation of an interference scenario for the B-AMC reverse link is considered. The B-AMC victim receiver is again located at Paris CDG. The B-AMC centre frequency is set between the two DME channels with the smallest total interference power and the smallest number of contributing DME/TACAN stations. Within the frequency range MHz, assumed for the B-AMC reverse link [D4], MHz seems to be the best choice for the B-AMC centre frequency. However, this approach is very simple and needs further refinement. Hence, the selection of the B-AMC centre frequency may not be optimal. Form the available NAVSIM data, interference power PDFs and corresponding duty cycles have to be derived for the channels 1058 and 1059 MHz. For 1059 MHz, interference power PDFs as well as corresponding duty cycles have already been derived in the example considered above. For all DME stations at 1059 MHz this procedure has to be repeated. As already seen in [D5], interferers in the channels at 1057 and 1060 MHz, i.e. at +/- 1.5 MHz offset from the B-AMC centre frequency, have no impact on the B-AMC system and can be need not to be considered. The parameters for the RL interference scenario with frequency planning derived from NAVSIM data are listed in Table 5-1. Frequency Distance to Type of station relative to B-AMC victim B-AMC RX [nm] system Duty cycle [ppps] Mode Interference power at input of victim RX considering antenna patterns [dbw] DME TMA 1962 Y Power PDF from Figure DME TMA 1077 Y Power PDF from Figure DME TMA 455 Y Power PDF from Figure DME TMA 308 Y Power PDF from Figure DME TMA 174 Y Power PDF from Figure DME TMA 139 Y Power PDF from Figure DME TMA 117 Y Power PDF from Figure DME TMA 46 Y Power PDF from Figure TACAN 3223 X Power PDF from Figure 5-6 Page: 5-8