FCI Technology Investigations: L band Compatibility Criteria and Interference Scenarios Study

Transcription

1 FCI Technology Investigations: L band Compatibility Criteria and Interference Scenarios Study Deliverable C1: Compatibility criteria and test specification for DME Edition Number 1.0 Edition Date 24/08/2009 Status Final COOPERATIVE NETWORK DESIGN

2 Contents 1 Introduction General About this document System Overview Mode of operation Compatibility Criteria Interference Environment Standards and requirements Criteria Test setup Goal of the tests Test equipment Reference DME signal level Source of interference ASOP and BSOP Measurements RE Measurements A Tests procedures B Equipment list C Antenna characteristics P1031C1v10 HELIOS ii of 19

3 Document information Document title Deliverable C1: Compatibility criteria and test specification for DME Author John Micallef, Helios Produced by Helios 29 Hercules Way Aerospace Boulevard - AeroPark Farnborough Hampshire GU14 6UU UK Produced for Eurocontrol Helios contact John Micallef Tel: Fax: john.micallef@askhelios.com Produced under contract C Version 1.0 Date of release 24/08/2009 Document reference P1031C1 P1031C1v10 HELIOS 1 of 19

4 1 Introduction 1.1 General Recognising that there is insufficient spectrum in the standard VHF band to support future aeronautical communications needs, two options for an L-Band Digital Aeronautical Communications System (L-DACS) have been identified by the European and US ICAO ACP members under the joint development activity known as the Future Communications Study (Action Plan 17). The first option for L-DACS is a frequency division duplex (FDD) configuration utilizing OFDM modulation techniques. The second L-DACS option is a time division duplex (TDD) configuration utilising a binary (GMSK) modulation scheme One of the key questions with respect to these candidate L-DACS technologies which needs to be addressed is that of its compatibility with other, existing L-Band systems. Not only must the candidate systems be able to operate effectively whilst in the presence of interference from other systems, but they must also cause the minimum possible interference to the legacy systems. These compatibility analyses are required in order to assess the feasibility of using the competing L-DACS systems both in a ground, and in particular in an airborne environment As the specific details of the L-DACS implementation options, including the Air Interface, are currently being specified and validated, this document considers only the compatibility of the legacy systems i.e. the interference path from L- DACS TO the legacy systems. The development of the criteria in the reverse case (to L-DACS) will be carried out at a later stage. The following diagram outlines the process currently foreseen by EUROCONTROL, showing the steps to be undertaken to complete the L-DACS selection. Development of L-DACS1/2 TX prototype Testing and L-DACS Specifications Development of Evaluation Selection RX prototype Interference Scenarios, Criteria and Testing Plan Figure 1 Overall evaluation process currently foreseen This document is a deliverable of the study covering the grey box in Figure 1 addressing the interference criteria, scenarios and testing plan. The overall study addresses two aspects of the current systems. The first one considers the current systems as victims and aims to define the appropriate spectrum compatibility criteria with a new system. The second one considers the current systems as P1031C1v10 HELIOS 2 of 19

5 interferers and aims to define the appropriate interference scenarios to be used when evaluating the impact of the current systems to a new system For the first part, there are 5 deliverables covering DME, UAT, SSR, GSM/UMTS and GNSS (C1, C2, C3, C4 and C5). For the second part, there is one deliverable consolidating the interference scenarios for all the previously considered systems and JTDS/MIDS in addition. There is also a combined deliverable (C6/S6) covering both the criteria and scenarios for the RSBN system. Finally there is one deliverable C7 providing an analysis of the potential usage of the suppression bus by a new system. 1.2 About this document This document is deliverable C1 of the Spectrum Compatibility criteria and Interference Scenarios for existing systems operating in the L band study produced by Helios for Eurocontrol under Contract C as contribution to the Future Communication Study (FCS) activities, and in support of the work to realise one of the recommendations of the FCS to develop an L-band data link The development of the L-band data link is identified in the development activities for the SESAR Implementation Package 3 (IP3) in the post 2020 timeframe. Therefore, the outcome of this deliverable will be used as input to the SESAR JU development activities under WP This document identifies the key operating characteristics and the relevant performance criteria for DME equipment that need to be measured to ensure acceptable operation in the presence of interference from an L-band communications system. The relevant specifications referenced from standards are as follows: [1] ICAO DME SARPs (Annex 10, Volume 1) [2] ED-57 Minimum Operational Performance Specification for DME (Ground) Equipment DME has been operating in the ARNS band for several decades. Although the techniques used to implement the hardware have improved steadily in pace with technology, and large gains have been made in operating efficiency, the principle of the system remains the same. The spectral landscape in the ARNS band at the time when DME was introduced was altogether different from that of today. The system was largely unconstrained in terms of spectrum compatibility, with the exception of the SSR frequencies. More recently the ARNS band has seen the introduction of satellite navigation services and a surveillance data link with the arrival of the GPS system in the top part of the band and the UAT system in the lower part A number of compatibility assessments have therefore already been carried out under the auspices of ICAO, to provide validation of the new standards and specifications, and to ensure continued operation at the required level of service for the legacy systems, and the consideration of the additional impact of L-DACS can therefore be based around these existing analyses. The criteria identified in this document are based on the current expected operating conditions of the proposed L-DACS systems, and on the published protection and susceptibility criteria for the technology alongside which L-DACS must operate. As such, issues such as frequency separation and emission masks for L-DACS are based on current expectations of the system parameters and final test validation will be necessary in order to confirm the implementation and test setup details herein P1031C1v10 HELIOS 3 of 19

6 documented. This paper is thus intended to provide the groundwork for the prototyping and compatibility trials bearing in mind that the compatibility criteria identified so far address interference in one direction where the legacy system is the victim. P1031C1v10 HELIOS 4 of 19

7 2 System Overview 2.1 Mode of operation The general principle followed during system operation is such that the airborne interrogator transmits a pair of pulses spaced at a preset interval. This interval is12µs for mode X channels and 30µs for mode Y channels. Figure 2 - DME pulse pair characteristics The ground transponder receives the pair of pulses, waits 50µs and then transmits another pair of pulses back to the aircraft. The airborne transceiver measures the time between transmission and reception, subtracts the 50µs, multiplies by the speed of light and divides by two to estimate the range to the ground interrogator site In order to serve a large umber of aircraft (typically in excess of 100), the aircraft generates a random set of pulses and correlates the replies to distinguish which ones are in response to its own interrogations (as distinct from those which are generated by other aircraft). This effectively provides a signature that distinguishes the interrogations of each aircraft and the replies destined to it The ground station transmits 2700 pulse pairs per second regardless of the number of aircraft interrogating. These extra pulse pairs are called squitter. If there are not enough interrogations to make up 2700 pulse pairs, the ground receiver increases its sensitivity until noise pulses trigger enough replies to make up the difference. If there are too many interrogations, the receiver decreases its sensitivity so that the weakest interrogations become ignored As a consequence, the ground based transmitter average output power is constant and the airborne receiver AGC circuitry has a constant average signal to work with Operational performance As with most systems there is a standby transmitter which takes over when the main one fails. Availability is maintained at levels exceeding 99.9%. P1031C1v10 HELIOS 5 of 19

8 3 Compatibility Criteria 3.1 Interference Environment DME is the largest user of the ARNS band and is likely to remain the cornerstone of navigation services for the foreseeable future. The ARNS band is also heavily used by other systems - some are recognised and protected in ICAO while others (which mainly include tactical military systems) are not The common element shared by these legacy systems is that they use pulsed and generally high powered systems. This generates a finite probability of RF interference that has two predominant effects: reduction of performance levels of the legacy systems, and damage to the legacy systems front end(s) if the interference power is large enough. To counter the latter, use of suppression circuits ensure that all systems are able to render service at the appropriate level without sustaining damage Standards and requirements The following requirements are derived from ED Sensitivity The sensitivity of the DME receiver shall remain greater or equal to X dbm for a range of reply efficiencies, as defined in the following table. Equipment type Signal level (dbm) Reply Efficiency (%) DME/N (En-route) DME/N (terminal) DME/P (IA mode) DME/P (FA mode) Table 1 - Sensitivity requirements These figures have been derived from ICAO Annex 10 (ref ) The sensitivity of the DME equipment under test should be verified to ensure at a first step that the equipment is operating correctly before carrying out the measurement tests against interference criteria Reply efficiency The reply efficiency of a DME system is the ratio of the number of sent pulses to the number of received interrogation pulses from aircraft. A reply efficiency of 100 % is very rarely achieved since, as described below, there are several reasons why no reply pulse is sent for an interrogation pulse. 1 This is treated in further detail in P1031 C7. P1031C1v10 HELIOS 6 of 19

9 Interrogation pulse occurs in the dead time of the receiver - The efficiency drops as the number of aircraft that are sending interrogation pulses to a ground station increases. Interrogation pulse occurs in the key down time of an ID sequence - The efficiency drops to 0 % during these times. Level of the interrogation pulse drops below the receiver sensitivity of the ground station - The efficiency drops dramatically when the maximum distance to the ground station is reached The reply efficiency is also often used as the limit for certain tests at the receiver When the receiver sensitivity is tested, the minimum input level for a reply efficiency of 70 %, for example, is checked Whenever an interrogation signal is within the dynamic range of the receiver, and is 10 db or more above the level of an interfering Continuous Wave signal, the reply efficiency shall remain greater than 70%. This requirement applies to DME/N and DME/P in IA mode Whenever an interrogation signal is within the dynamic range of the receiver, and is 25 db or more above the level of an interfering Continuous Wave signal, the reply efficiency shall remain greater than 80%. This requirement applies to for DME/P in FA mode The minimum pulse pair requirement shall be maintained in the presence of an interfering CW signal. This interfering signal must be 10 db below the maximum interrogation level of -10 dbm Note there is a requirement for the ground equipment to resist damage for input pulse signals at the cabinet connector of up to 20 dbm. 3.3 Criteria The objective of this testing is to determine impact of L-DACS signals on DME interrogator avionics as well as DME ground transponders operating in the L-band. L-DACS operates in the bands MHz (L-DACS/1) and MHz (L- DACS/2). In these frequency ranges, DME desired signals emanate both from ground transponders as well as airborne interrogators, thus making the DME interrogator avionics and the ground transponder the victim receivers Note that for UAT it was not necessary to test RE because the frequency used for UAT (978 MHz) is only used for DME uplinks to aircraft and thus no interference was expected to be caused to ground equipment. For L-DACS, however, frequencies used by DME ground receivers may be used and as such the test is necessary The main compatibility criteria for the DME receiving equipment in the presence of undesired interference are: Acquire Stable Operation Point (ASOP) Break Stable Operation Point (BSOP) Reply Efficiency (RE) Time To Acquire (TTA) P1031C1v10 HELIOS 7 of 19

10 3.3.4 Reply efficiency is relevant to the ground transponder receiver. Interference will lower the amount of interrogations which are detected and therefore reduce the RE TTA is an integral part of the test for ASOP. If the TTA is greater than 2 minutes then the DME is said to have failed to have acquired stable operation. As such, there is no specific need to conduct TTA tests to validate compatibility as the result is inherent in the ASOP test. Therefore the criteria proposed for testing are: ASOP and BSOP (for avionics interrogators) RE (for ground transponders) The application and measurement of these criteria are described in Section 4. P1031C1v10 HELIOS 8 of 19

11 4 Test setup 4.1 Goal of the tests The tests determine the interference threshold for ASOP and BSOP under various interference conditions. This established interference threshold can then be evaluated against operational expectations of the interference signals under different operational scenarios The tests additionally establish the minimum reply efficiency that would support normal DME operation (see Section 4.5 for further detail). 4.2 Test equipment Recommended test equipment for the execution of the tests described in this section are: DME interrogator; DME ground beacon; Interference source emulator (including UAT, JTIDS and L-DACS waveforms); Navigation display indicator; Variable attenuators; Combiner Shielded connecting cables Test bed configuration The above items of equipment are set up in the following configuration with the variable attenuators and combiner having been pre-calibrated. Calibrating data should be recorded accurately and accompany all results obtained with any given setup. 2 2 See Annex A for further detail on test steps to be undertaken. P1031C1v10 HELIOS 9 of 19

12 DME interrogator Under Test Nav. Display Indicator Interrogate Reply Interference Source emulator DME Test Set (Emulates DME Ground Beacon) Figure 3 - Test equipment setup for ASOP and BSOP tests DME interrogator Under Test Nav. Display Indicator Reply Interrogate Interference Source emulator DME Test Set (Emulates DME Ground Beacon) Figure 4 - Test equipment setup for RE tests Device under test See Annex B for a list of recommended DME equipment. P1031C1v10 HELIOS 10 of 19

13 4.3 Reference DME signal level The reference DME signal applied during the test shall be kept 10 db above the level of the interfering (or composite) signal. 4.4 Source of interference Test results will collect data to assess DME performance against each interference source individually and in combination. The sources of interference to the DME receive function will include self interference from DME operating in adjacent channels, UAT transmitters operating on 978 MHz, JTIDS/MIDS signals that periodically occupy different parts of the frequency band, and L-DACS ASOP and BSOP Measurements The first measurement establishes the desired signal level needed to achieve ASOP in the absence of any interference at 70% reply efficiency Conditions for determining the ASOP and BSOP thresholds are: DME Test Set Reply Efficiency set to 70% Desired beacon reply level at 77 dbm, 83 dbm, and -89 dbm at interrogator input Ground beacon reply rate of 2700 ppps Test Set Range set to 200 NM DME device tested Sensitivity at 70% RE (ASOP) Equipment #1 Equipment #2 Equipment #n x dbm y dbm z dbm Table 2 - Example of tabulated results The second measurement determines the interference threshold for ASOP when each desired DME reply is overlapped with one, or a composite of, several interference transmissions of fixed amplitude(s). 3 The channel separation for the L-DACS source is to be determined. Note however that the distance is likely to be 500 khz for L-DACS-1 and a combination of separations ranging from 200 khz to several MHz for L-DACS-2. P1031C1v10 HELIOS 11 of 19

14 4.5.4 This continual overlap condition is more severe than that which is currently experienced in the L-band. However given that the duty cycle of an individual L- DACS transmission is likely to be several orders greater than that of any of the legacy systems, it is necessary to determine the impact on the interference threshold level when several DME pulses are overlapped by a single L-DACS burst. For this reason, this test is convenient for isolating the SIR threshold for DME operation caused by the UAT as well as the L-DACS waveforms. The following table illustrates an example of how these results may be recorded. DME device tested Desired Signal Level Interference 70% Reply efficiency Equipment #1 u dbm x dbm Equipment #2 v dbm y dbm Equipment #n w dbm z dbm Table 3 - Interference thresholds with continual overlap based on ASOP/BSOP The desired signal level in the second column is derived from the MOPS required sensitivity level for that class of equipment. The value recorded in the last column indicates the level of interference at which each DME device tested is able to operate to MOPS required sensitivity The final measurement determines the minimum reply efficiency that would support normal DME operation in the absence of any interference. Any reduction in reply efficiency below the required 70% level represents the portion of time during which interference on the reply channel could exceed the interference thresholds determined above while still maintaining track. It should be noted that the larger this difference, the larger the reply efficiency margin available. In order to carry out this test, the DME test set needs to be capable of operating and measuring on variable reply efficiency Note In past trials carried out during the UAT validation phase, documented results have shown that the DME interrogators could acquire and track in the same level of UAT interference as long as at least 30% of its interrogations resulted in received replies. 4.6 RE Measurements This test requires that the DME test set being used is capable of measuring variable reply efficiency. The test setup is the same as that for ASOP/BSOP (see Figure 4) The first objective is to determine the minimum reply efficiency that would support acceptable DME operation in the presence of interference. The outcome of this test will indicate, for any of the DME units tested, the level of DME interrogations that actually result in received replies i.e. are acquired and tracked, in the presence of interference. P1031C1v10 HELIOS 12 of 19

15 4.6.3 The interference considered is CW interference. The level of interference must be kept at least 10 db lower than the interrogation signal. In the case of DME/P operating in FA mode, the level of interference must be kept at least 25 db lower (instead of 10 db) To pass the test, the reply efficiency must: Remain greater than 70% for DME/N and IA mode of DME/P Remain greater than 80% for DME/P in FA mode The second objective is to determine the minimum reply efficiency that would support acceptable DME operation in the absence of any interference. Any reduction in reply efficiency below the values indicated above, can be viewed as an indicator of the portion of time during which interference on the reply channel could be allowed to exceed the interference thresholds determined above while still maintaining track. The larger this difference, the larger the reply efficiency margin available. P1031C1v10 HELIOS 13 of 19

16 A Tests procedures A.1 ASOP-BSOP test A.1.1 With the equipment set up as per the procedure described above, the following steps are executed: 1. The trial is started by setting the interference to a low power value, and steps 2 and 3 are repeated while increasing the interference power by 1 db in each iteration: 2. At each power setting an observation time of 2 minutes is maintained; 3. When distance or velocity indication of DME is changed permanently, this value must be recorded as the BSOP; 4. From this point onwards, the power of the interferer is decreased by increasing the attenuator in 1 db steps; 5. Again at each power setting, an observation time of 2 minutes is maintained and the to distance and velocity indication presented from the DUT monitored; 6. When the DUT is again able to re-acquire the stipulated distance/velocity figures within the 2 minute observation period, the interference power is recorded as the ASOP; 7. These ASOP values are then used to calculate required distances between interferer and victim based on free space loss. A.2 RE test Figure 5 - Typical plotted results of ASOP test A.2.1 With the equipment set up as per the procedure described above, the following steps are executed: P1031C1v10 HELIOS 14 of 19

17 1. The trial is started by setting the DME interrogator signal power to the minimum sensitivity level, and the CW interference 10 db (or 25 db) below this level; 2. An observation time of 2 minutes is maintained; 3. In this period, the number of DME replies generated by the transponder is recorded, together with the number of interrogations in this period. 4. Steps 2 and 3 are repeated while increasing the interrogation signal power in steps of 5 db 4 while maintaining the interference 10dB (or 25 db) lower, starting from the sensitivity level up until the maximum dynamic range of the equipment. The measured Reply Efficiency is recorded at each stage and clearly indicating the signal levels used for the measurement. 4 This granularity is suggested in order to assure a reasonable amount of measurements across the dynamic range of the receiver. An alternative step size may be used provided the measurements consider both the lower and upper bounds of the signal power envelope. P1031C1v10 HELIOS 15 of 19

18 B Equipment list B.1 The objective is to conduct tests on DME units that are representative of the majority of the DMEs used in the different categories of aviation equipage. B.2 Due to the large variety of equipment available on the market, a reasonable and representative cross-section is to be tested. As had been done for the UAT validation programme, it is recommended to consider at least four DME units for trials purposes. The following table suggests a list of DME equipment representing the diverse equipment commercially available on the market for the various categories of avionics instrumentation. Manufacturer/Model Technical Standard Order Category Bendix King KD-7000 Yes Air carrier class Narco DME 195 Yes General Aviation Narco DME 890 No General Aviation Honeywell KDM 706A Yes Air carrier class Rockwell Collins DME 900 Yes Air Carrier class Table 4 - DME Units B.3 Although this is not an exhaustive list, it is to be noted that the latter two were recommended for testing by Eurocontrol in the UAT validation activities as representative of equipment used in Europe. P1031C1v10 HELIOS 16 of 19

19 C Antenna characteristics C.1 The MOPS requires the following specifications to be met by the antenna on the assigned DME channel. C.2 It is a desirable characteristic that a single antenna design covers e whole DME band between 960 and 1215 MHz. C.3 The sensitivity requirements identified in Section assume: An antenna gain of 8dB A pattern loss of 2 db An RF cable loss of 2 db. C.2 Horizontal pattern C.2.1 C.2.2 For en route and terminal DME the antenna pattern is required to be omnidirectional in the horizontal plane. The difference between maximum and minimum gain should not exceed 3 db. DME equipment associated with ILS or MLS may have an antenna pattern that is either omni-directional or directional, as long as the DME coverage is at least as large as that of the landing system. C.3 Vertical pattern C.3.1 The direction of the maximum radiation (of the main lobe) in the vertical plane must be 3 degrees from horizontal, with a tolerance of 1 degree on either side. C.4 Overall gain C.4.1 The gain measured at the maximum of the vertical pattern and at the mean of the horizontal pattern must not be less than 8 dbi. C.5 Gain below horizontal C.5.1 The gain between -10 degrees and -50 degrees referenced to the horizontal must be at least 12 db below the maximum gain. This applies to both en route and terminal installations. C.6 Gain above horizontal C.6.1 The vertical pattern between 6 and 40 degrees shall not pass under a straight line joining the points of coordinates (+6 degrees, -15 dbm under the maximum) and (+40 degrees, -25 dbm under the maximum). C.7 Impedance C.7.1 The antenna shall be matched to a nominal 50 ohms impedance setting with a VSWR not exceeding 2:1 at the input connector. C.8 Polarisation C.8.1 The antenna shall be vertically polarised. P1031C1v10 HELIOS 17 of 19

20 C.9 Antenna feeder loss C.9.1 The feeder cable loss must be kept to a minimum and will not normally be greater than 2 db. An N type coaxial connector is recommended. Figure 6 Typical antenna pattern (vertical) C.10 Reference antenna gain model C.10.1 The following antenna characteristics are derived from a WP presenting excerpts from draft GNSSP Spectrum subgroup report Methodology for assessing the impact of the radionavigation-satellite service (space-to-earth) on the aeronautical radionavigation service (DME/TACAN) in the band MHz (AMCP WG- F/8 WP/41 as presented by the ICAO secretariat. P1031C1v10 HELIOS 18 of 19

21 Elevation angle in Antenna gain including circular- tolinear polarisation mismatch Elevation angle in Antenna gain including circular- tolinear polarisation mismatch Elevation angle in Antenna gain including circular- tolinear polarisation mismatch Gr/Grmax in db Gr/Grmax in db Gr/Grmax in db P1031C1v10 HELIOS 19 of 19

22 Table 5 - Reference ARNS antenna pattern C.10.2 Note that the maximum antenna gain (Grmax) is 3.4 dbi taking into consideration the 2 db circular-to-linear polarization mismatch. Linear interpolation should be used between the value of the elevation angle. AR NS receiver radiated pattern to be used for epfd calculation Gr/Grmax in db elevation angle in Figure 7 Reference ARNS antenna pattern P1031C1v10 HELIOS 20 of 19