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

Transcription

1 FCI Technology Investigations: L band Compatibility Criteria and Interference Scenarios Study Deliverables C7: Assessment on the potential use of the onboard suppression bus for L-DACS operation Edition Number 1.0 Edition Date 15/05/2009 Status Final COOPERATIVE NETWORK DESIGN

2 Document information Document title Analysis on the potential use of the onboard suppression bus for L- DACS operation Author Produced by Produced for Helios contact Produced under contract John Micallef, Richard Womersley Helios Helios 29 Hercules Way Aerospace Boulevard - AeroPark Farnborough Hampshire GU14 6UU UK Version 1.0 EUROCONTROL John Micallef Tel: Fax: john.micallef@askhelios.com C Date of release Document reference P1031 C7 P1031 C7 v10 HELIOS 1 of 32

3 Executive Summary This document is a report on the analysis of the use of the mutual suppression bus that is installed on most civil Air Transport aircraft. The suppression bus is used as a common means to protect the receiver of some L-band radios from powerful transmissions of other systems operating on the same platform (cosite interference) and in the same frequency band. So far, the development of the candidate terrestrial data link systems (L-DACS options), are considering the existing suppression bus to be readily available for use with L-DACS. This report analyses in detail the range of usage profiles of L- DACS for a basket of communications services, and determines on this basis whether or not the use of the suppression bus is necessary. The three main uses for the suppression bus that are identified in the present analysis are: Protection from strong signal damage Notification of potential interference Muting of other L-band transmitters to avoid spikes in RF field strength Considering these uses, two cases are investigated: Current protection offered to legacy systems Potential protection to be offered to L-DACS For protection to legacy systems, it is found that, given the information currently available on the L-DACS candidate waveforms, the L-DACS transmissions in the worst case represent a very small fraction of the overall channel time 1 and the typical communications usage profile is not expected to pose any significant constraints on the operation of the legacy systems. The advantages of not requiring L-DACS systems to activate the suppression bus when transmitting can be gained if it is demonstrated, through trials, that the level of L-DACS interference is insufficient to pose any danger to other avionics, and that no dangerous field strengths are produced when L-DACS transmit and other L-band systems receive simultaneously. From power budget considerations, the level of protection built into both the L- DACS systems as well as the legacy avionics appears sufficient to allow simultaneous operation without the need for integrating the L-DACS system on the suppression bus. The analysis presented in this report concludes that, considering the characteristics of the majority of avionics systems available on the commercial market, there are no compelling technical or operational cases for integrating L- DACS transmitters to the suppression bus chain neither for the protection of existing L-band systems nor for the protection of L-DACS. 1 Based on the communications loading profiles considered in this study (see Annex C). P1031 C7 v10 HELIOS 2 of 32

4 Contents 1 Introduction General About this document Background Structure of this document Existing use of the suppression bus Introduction Operation of mutual suppression pulses SSR transponder TCAS transponder DME UAT Output to suppression bus Input from suppression bus Analysis of L-DACS contribution to suppression bus activity Introduction Message Analysis ATS ATS ATS+AOC ATS+AOC L-DACS/ L-DACS/ L-DACS Impact Analysis Protection from strong signal damage Notification of Potential Interference Muting of other L-Band transmitters Conclusions Overall considerations Conclusions and observations Specific considerations Protection of legacy systems Protection of L-DACS A References P1031 C7 v10 HELIOS 3 of 32

5 B Abbreviations and acronyms C Description of scenarios C.1 Traffic C.2 Types of scenarios C.3 Message profile P1031 C7 v10 HELIOS 4 of 32

6 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. This study aims to define the interference scenarios to be investigated for the case in which the LDACS system is the victim system and the other systems in the L band are the interfering systems. The list of potentially interfering systems includes those considered in the Study deliverables C1-C72, with the addition of the military system JTDS/MIDS. 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 2 DME/TACAN, SSR, UAT, GSM/UMTS and GNSS. P1031 C7 v10 HELIOS 5 of 32

7 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 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 C7 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 is a report on the analysis of the use of the mutual suppression bus that is installed on most civil Air Transport aircraft. The suppression bus is used as a common means to protect the receiver of some L-band radios from powerful transmissions of other systems operating on the same platform (cosite interference) and in the same frequency band. The co-site interference case is largely considered to be the most critical interference case due to the small isolations available on board aircraft and the high powered nature of transmissions in the L-band. So far, the development of the candidate terrestrial data link systems (L-DACS options), have considered the potential usage of the existing suppression bus. However there has been no formal study to address the specific issues in light of the current constraints on the use of the bus, and indeed, whether the bus is required at all given the concept of operation of L-DACS. In particular, should it be necessary to use the suppression bus, any additional usage must not impact the acceptable operation of the existing systems already using the bus. 1.3 Background As there are a number of devices on-board an aircraft which operate in the L- band, there is a need to ensure that when one piece of equipment transmits, others are notified and can take appropriate action such as to mute or protect receivers, as required, and to prevent other equipment transmitting at the same time to avoid potentially damaging spikes in RF activity. P1031 C7 v10 HELIOS 6 of 32

8 Mutual suppression takes place as a result of a synchronous pulse that is relayed between transmitting equipment on board the aircraft. This pulse is used to suppress transmission of competing transmitters for the duration of their active RF transmissions 3. A given reception may be suppressed by an external source announcing activity on the suppression bus, and other equipment on board may choose to suppress their transmissions in response to suppression pulses generated by other equipment. This feature is designed to limit mutual interference between systems operating in the same frequency band. Depending on the characteristics of the systems and most notably the transmitter power, this mutual interference can have one of two effects: Damage to the receiver front end circuitry and PA if the signal is stronger than that which can be tolerated by the components; Undesired operation of the victim system if the signal is low enough to be processed by the receiver but follows a profile that allows it to be misinterpreted as a genuine signal. 1.4 Structure of this document Section 2: describes the current use of the suppression bus and the limitations imposed by the legacy L-band systems; Section 3: describes the outcome of an analysis of expected L-DACS communications loading and the implications on the use by L-DACS of the onboard suppression bus; Section 4: provides a summing up of observations and the conclusions of this report. 3 For instance, a DME interrogator transmits nominally 25 times per second (although this could be less). Each of these transmissions actually consists of a series of pulse pairs separated by random periods of time. P1031 C7 v10 HELIOS 7 of 32

9 2 Existing use of the suppression bus 2.1 Introduction The Mutual Suppression Bus (MSB) is used in aircraft for L-Band systems such as Secondary Surveillance Radar (SSR) transponders, TCAS/ACAS and DMEs. These L-Band avionics systems physically connect to the common bus. The L- Band systems that are connected to the bus drive the bus to announce to other systems that a transmission is taking place throughout the interval that the bus is activated. There are two forms of suppression defined and implemented in current avionics systems: bi-directional (two way) suppression: that allows the DME to suppress the transponder when the DME is transmitting, and also for the transponder to suppress the DME when the transponder is transmitting. one way suppression: used for the lower cost panel mount equipment found in general aviation market. It allows the DME to suppress the Transponder when the DME is transmitting. The systems that are connected to the bus can react in a variety of ways to the presence of a suppression pulse. In most cases, they desensitise their receivers for the duration of the pulse. The connected systems may also monitor the bus to react to other L-Band transmissions on the aircraft by delaying their own transmissions so as not to simultaneously transmit while another L-Band system is transmitting. Overall implementation guidance is provided for the operation of the Mutual Suppression Bus circuitry for each of the on board systems so that it is designed to prevent malfunctioning devices connected to the bus from impacting other systems connected to it. The MSB effectively limits the access of systems to the aircraft s RF resources in order to avoid mutual interference. Therefore, a major consideration of systems connecting to and driving the MSB is to minimize as much as possible the duration of the suppression period to minimize the impact on channel availability of the other connected systems. 2.2 Operation of mutual suppression pulses The suppression bus is normally implemented as a connection via a standard ARINC 429 communication link between TCAS transponders and SSR transponders and any other avionics on the host aircraft that transmit in the L- band. This intersystem connection is also used to prevent simultaneous transmissions which could interfere with the system's independent functions or cause equipment damage. Most modem transponders are capable of responding to suppression signals in accordance with published standards. When an appropriate suppression pulse is supplied to a transponder over the suppression line connected between the TCAS system and the SSR/Mode-S system, the receiver portion of the transponder is disabled so that the transponder does not generate unsolicited reply signals. This suppression feature prevents interference by other pulsed equipment, such as DME. P1031 C7 v10 HELIOS 8 of 32

10 Typically, the suppression input is AC coupled to the transponder, with a time constant typically in the order of about 5 ms. This time constant limits the effective suppression period to a maximum of about 2 ms. Suppression input may also be DC coupled to some other transponders SSR transponder During periods when the suppression bus is active, a Mode A/C SSR transponder may suppress its reply to interrogations received in the side lobes of the ground based antenna [1]. During this suppression period, the transponder does not accept any interrogations. Consequently, any interrogation arriving at that transponder will be ignored and no reply issued. The duration of suppression is defined in the SSR SARPs as being 35 µs (±10 µs). 2.4 TCAS transponder 2.5 DME 2.6 UAT The TCAS system typically requires the on-board transponder to have an interface to the suppression circuit, much like the SSR transponder. The newer TCAS II system, typically has several modes of operation selectable via a control panel coupled to the TCAS processor. The control panel is also coupled to transmit control signals to the TCAS processor. The DME interrogator is linked to the output interface of the suppression bus to prevent the SSR transponder from generating unsolicited replies in response to the DME interrogations generated on board the aircraft. The DME suppression output prevents cross-talk with the SSR onboard transponder, especially on closely coupled frequencies. The DME receiver is also linked to the input interface of the suppression bus to avoid cross talk with the SSR squitters generated by the transponder. The interface varies according to the aircraft type. In some aircraft, the DME suppression circuit suppresses the SSR transponder when the aircraft is on the ground. It was determined through trial during the UAT standards validation process that in the absence of any protection or coordination mechanism, SSR transponders generate some unsolicited replies during UAT transmission. This is caused by the transponder interpreting interrogation pulses from the UAT signal, and found to occur over a broad range and combinations of antenna separation, UAT transmit power and transponder sensitivity. The UAT standards therefore require avionics to output suppression pulses during UAT transmission 5 [2]. The principle of operation of UAT is rather different from the 4 Note that some transponders do not have provisions for suppression. 5 The UAT equipment must adhere to the electrical characteristics of the onboard mutual suppression bus and there exists a recommendation to provide protection circuitry to prevent against UAT equipment failure disabling the mutual suppression. P1031 C7 v10 HELIOS 9 of 32

11 other radionavigation systems implemented in the L-band, considerable study has been devoted to ensuring that UAT equipment uses the MSB in the appropriate way to ensure compatibility Output to suppression bus The UAT transponder provides a mutual suppression signal whenever the transmitter output power exceeds 20 dbm 6. Given the RF isolation available on typical aircraft, suppression therefore happens during the ramp up phase of any signal. This is explained further below. In addition, the suppression signal is required to not become active prior to 5 µs before the start of the ADS-B Message Transmission Interval, and shall not remain active later than 5 µs after the end of the transmission. This overhang on either side of the suppression interval exists to ensure that the suppression period covers that the entire duration of the transmission to prevent excessive receiver blanking of onboard equipment sharing the mutual suppression bus, while offering adequate protection to the SSR transponder from triggering of UAT transmissions. UAT averages one transmission per second and transmits either a Basic Message, which is 280 µs in duration, or a Long Message, which is 420 µs long. The suppression interval is required to be active during the transmission interval when the power is -20 dbm or higher. The origin of the -20 dbm requirement was the maximum (UAT) power allowable without SSR transponders generating unsolicited replies when signal levels from the UAT frequency skirts (i.e. the edges of the UAT transmit filter) within the transponder receiver pass band are above the transponder receiver threshold. The worst case maximum suppression interval is 430 µs due to a long ADS-B message (including the 5 µs overhangs). During the SARPs validation process, the impact of the mutual suppression interval as a result of on-board UAT transmissions on DME systems was assessed. With respect to the impact on the DME transmitting function, the short duration of the UAT suppression was found to be insignificant to the DME transmitter/interrogator. The DME system can operate safely while suffering interrogation delays of 430 µs that may result if worst case delay were imposed by a long UAT ADS-B transmission. With respect to the impact on the DME receiving function, the receiver blanking of DME that would result from UAT suppression activity the worst case was estimated at 0.043% duty cycle. This was found to be insignificant when considering that DME operation was found to be acceptable (through lab testing) at very low reply efficiencies 7. Similarly, it was also found that the short length of the UAT transmission also has negligible impact or no impact on any of the TCAS/ACAS functions. The worst case SSR transponder availability is calculated to be around 90%. 10% is lost due to system inefficiency and interference from other SSR signals. The addition of the UAT Mutual Suppression Bus blanking of the SSR transponder receiver reduces the availability at most from 90% to %. This reduction has been considered to have an insignificant impact to the operation of SSR transponders. 6 Per section 5.2 of the UAT Technical Manual [2]. 7 Well below 70%. P1031 C7 v10 HELIOS 10 of 32

12 2.6.2 Input from suppression bus The impact to the UAT receiver during other L-Band transmissions was considered when assessing UAT receiver performance in high density airspace. The UAT receiver is considered to be completely blanked during on-board L-Band transmissions from TCAS/ACAS, SSR transponders and DME due to their high power and the small isolation. However it was found that since the duration of the pulses (even if frequent) is very short compared to the UAT symbol period, the UAT decoder is able to recovered the lost data. Moreover, the protection built into the front end is capable of protecting the receiver front end during the high powered pulses. Therefore, the UAT avionics of any equipment class is not required to respond to suppression signals. This means that the UAT transponder does not monitor the Mutual Suppression Bus, nor does it attempt to inhibit or delay its own transmissions when other systems are actively transmitting. P1031 C7 v10 HELIOS 11 of 32

13 3 Analysis of L-DACS contribution to suppression bus activity 3.1 Introduction The use of the existing suppression bus by new technology is dependent on there being a suitable interface to the system i.e. the avionics units concerned need to provide a discrete output conforming to an ARINC specification that allows other systems to interpret a signal as an active period of transmission. In the case of TDD systems, this is rather trivial because the avionics can generate a signal on the bus every time a burst is generated. and sufficient probability of transmissions being allowed. There has been already recent experience on the integration of a new data link system on the suppression bus, with the introduction of UAT. The short transmission times for UAT allowed it to be introduced without noticable impact on DME or TCAS/ACAS. 3.2 Message Analysis In order to ascertain the level of activity of the suppression bus which will manifest from the imposition of L-DACS, it is first important to understand the level of traffic which will be generated by L-DACS usage. To assess this, we have analysed the traffic generated in three different scenarios, derived from [3], namely: A medium sized en route sector with ATS loading only (ATS 1); A large sized en route sector with ATS only loading (ATS 2); A medium sized en route sector with combined ATS and AOC loading (ATS+AOC 1); A large sized en route sector with combined ATS and AOC loading (ATS+AOC 2) ATS1 The medium scenario consists of 64 aircraft and the large scenario consists in 204 aircraft operating the same cell served by one local ground station. The ATS and AOC loading profiles are described in further detail in Annex C. It should be stressed that since the message loading is considered from the aircraft s viewpoint in the cosite case i.e. high powered transmissions perceived on the channel on the current platform only, the aggregate number of aircraft present on the channel should be of no consequence. Even though increased aircraft increase the number of ground transmissions overall, the victim transmissions of interest to the target aircraft remain unaffected. In order to provide a proof case for this, two ATS profiles are considered for two differently sized scenarios. The main difference in loading profile is between ATS only and combined ATS and AOC cases. In this scenario, the average message size is 1523 bytes. This is distributed between various message sizes as indicated in the chart below. P1031 C7 v10 HELIOS 12 of 32

14 45.0% 40.0% 35.0% Number of Messages Sent (%) 30.0% 25.0% 20.0% 15.0% 10.0% 5.0% 0.0% Size of Message It can be seen that messages of length 1380 and 2763 bytes dominate. The average number of messages sent per aircraft per hour is This is distributed as shown in the chart below. 25.0% 20.0% Number of Aircraft (%) 15.0% 10.0% 5.0% 0.0% Messages Sent Note that on both these charts, only the actual message sizes and the number of messages sent in the actual scenario have been shown. Other message sizes are not present in the scenario, and equally the number of messages sent per aircraft falls into the various discrete values shown. P1031 C7 v10 HELIOS 13 of 32

15 3.2.2 ATS2 In this scenario, the average message length is 1483 bytes. This is distributed as snow in the chart below. 45.0% 40.0% 35.0% Number of Messages sent (%) 30.0% 25.0% 20.0% 15.0% 10.0% 5.0% 0.0% Size of Message As with ATS1, messages of length 1380 and 2763 bytes dominate. The average number of messages per aircraft per hour is This is distributed as shown in the chart below. As expected, the figures are very close to those in ATS % 20.0% Number of Aircraft (%) 15.0% 10.0% 5.0% 0.0% Messages Sent P1031 C7 v10 HELIOS 14 of 32

16 3.2.3 ATS+AOC 1 In this combined ATS+AOC scenario, the average message size is 714 bytes. This is distributed as shown in the chart below. 25.0% 20.0% Number of Messages sent (%) 15.0% 10.0% 5.0% 0.0% Size of Message In this scenario, message of length 93, 127, 1380 and 2763 bytes dominate. The average number of messages sent per aircraft per hour is 87. distributed as shown in the chart below. This is 12.0% 10.0% 8.0% Number of Aircraft (%) 6.0% 4.0% 2.0% 0.0% Messages Sent P1031 C7 v10 HELIOS 15 of 32

17 3.2.4 ATS+AOC 2 In this combined ATS+AOC scenario, the average message size is 702 bytes. This is distributed as shown in the chart below. 30.0% 25.0% Number of Messages Sent (%) 20.0% 15.0% 10.0% 5.0% 0.0% Size of Message In this scenario, message of length 93, 127, 1380 and 2763 bytes dominate. The average number of messages sent per aircraft per hour is 88. distributed as shown in the chart below. This is 9.0% 8.0% 7.0% Number of Aircraft (%) 6.0% 5.0% 4.0% 3.0% 2.0% 1.0% 0.0% Messages Sent P1031 C7 v10 HELIOS 16 of 32

18 3.2.5 L-DACS/1 The proposed L-DACS/1 frame structure [4] is based on frame size 8 of 1.44 ms and within each frame there are two tiles in the frequency direction (each spanning 24 carriers). The use of one tile is required in order to request permission to transmit data. Both tiles in a frame can be used simultaneously for the transfer of data. Each tile can carry a payload 9 of 14 bytes. The table below therefore shows the number of tiles required for an average aircraft 10 in each of the scenarios together with the equivalent transmit time (transmit time assumes that where possible, two simultaneous tiles are used). Situation Number of single tile/frame requests Number of data frames required Time in Transmit per hour % of Total Time in Transmit ATS seconds % ATS seconds % ATS+AOC seconds % ATS+AOC seconds % L-DACS/2 The proposed L-DACS/2 frame structure [5] allows the mobile station (the aircraft) to transmit in one of three possible configurations, a full, half or quarter slot. The duration of these transmissions together with the associated data capacity (the actual payload capacity after synchronisation and FEC) is shown in the table below. Full Slot (CoS2) Half Slot (LoG2) Quarter Slot (CoS1) Transmission period ms 3.33 ms 1.67 ms Data capacity 1264 bits 576 bits 224 bits The half slot (LoG2) is only used for login messages and not for user data. The Quarter Slot (CoS1) is only used to request capacity for full slots (CoS2). Message data payloads are only carried in CoS2 slots, and each message will also require one CoS1 request. Based upon the spread of message sizes shown above, an average aircraft will send the following messages an average hour: 8 Unit frame as opposed to OFDM frame. 9 Stripped of all overhead. 10 At any given point in time, the channel load of different aircraft may vary substantially due to the stochastic access profile. However, the average transmission profile of aircraft considered over a suitably long period of time (such as a flight hour) is very similar. P1031 C7 v10 HELIOS 17 of 32

19 Situation Number of CoS1 messages per hour Number of CoS2 messages per hour Time in Transmit per hour % of Total Time in Transmit ATS seconds % ATS seconds % ATS+AOC seconds % ATS+AOC seconds % 3.3 L-DACS Impact Analysis For both L-DACS1 and L-DACS the ATS+AOC scenarios represent the worst cases. The results presented in sections and show that in these worst cases, L-DACS transmissions represent at most % and, on average, no more than around 0.08% of an hour. For illustrative purposes, in an extreme case where all of the necessary transmissions were compressed into just 1 minute within that hour, they would represent only 4.8% of that minute, noting that for the rest of the hour there would be no transmissions at all (corresponding to the aforementioned loading of 0.08% over the one hour period). Whilst such usage could never be deemed completely insignificant it represents, if other systems were interrupted for the duration of L-DACS transmissions, a smaller outage than those various L-Band systems would typically incur under everyday conditions (eg caused by propagation and interference from other legacy systems). It is perhaps also worth noting that the scenarios presented here represent those based on current expectations of traffic levels. If L-DACS proves to be a successful method of communicating between aircraft and ground, whilst AOC traffic levels are unlikely to change, there is a large potential upside for the use of L-DACS for ATS traffic. Nonetheless, even a tenfold increase in traffic would still yield relatively low levels of average transmission time (<0.8% over an hour). A similar loading profile analysis previously carried out as part of the UAT standards validation framework concluded that [7]: commercially available SSR transponders could output replies during a UAT transmission. This was because of UAT transmissions signal contribution around the SSR transponder receiver 1030 MHz frequency. The high powered UAT transmission can raise the baseline at the transponder receiver above its amplitude threshold which can be detected as interrogation pulses from the amplitude variations that randomly exceed the threshold. These detected pulses could align in time to appear as a valid interrogation which triggers the transponder to transmit a reply. Further compatibility investigations carried out during this process found that the contributions to channel unavailability as a result of UAT transmissions are negligible [2]. These contributions were estimated in the region of 0.03%. One issue which needs consideration is whether or not L-DACS transmissions should be indicated on the suppression bus. If L-DACS transmitters use the MSB this could be used for the following purposes: To protect other L-Band receivers from potential damage from strong signals produced by the L-DACS transmitters; P1031 C7 v10 HELIOS 18 of 32

20 To notify other L-Band equipment that any transmissions received during the period could be subject to large potential levels of interference and as such should be treated with caution; To mute other L-Band transmitters to ensure that problems from multiple simultaneous transmissions are avoided (e.g. excess power consumption or the production of potentially dangerous field strengths). It is worth considering each of these impacts in turn to understand whether or not L-DACS use of the suppression bus is necessary or beneficial Protection from strong signal damage The proposed airborne transmitter powers for L-DACS/1 and L-DACS/2 are 41dBm (12.5 Watts) and 47dBm (50 Watts) respectively. Given the potentially close proximity of L-DACS antennas and other L-Band antennas on the same aircraft, studies have shown that isolation between antennas of order 30dB is common 11. This means that any other L-Band receivers would potentially be subject to incoming levels of 10 dbm (10 milliwatts approximately 2 Volts peakto-peak into 50 Ohms) and 20 dbm (100 milliwatts approximately 6.2 Volts peak-to-peak into 50 Ohms) for L-DACS/1 and L-DACS/2 respectively (assuming unity antenna gain). Such received power levels should be able to be tolerated without damage by most receivers without any special action requiring notification from the suppression bus. The DME MOPS require the receiver front end to have the capability of sustaining input signal levels of 20 dbm without damage and whilst this applies to the input signal at the ground station, dialogue with avionics equipment developers has confirmed that the same level of resilience is commonly built into the avionics radios. Note also that for both L-DACS systems, transmitter power control is used such that the maximum transmitter power levels are only present during certain circumstances and at other times the levels produced will be much lower. In some cases, it is possible that the avionics are not capable of withstanding a momentary 20 dbm signal without prior warning from the MSB. In these instances, it may therefore be necessary to connect the L-DACS equipment to the MSB, however this would be an installation by installation necessity and other mitigation techniques, such as protection filters of various kinds on the input of the affected equipment, may provide alternative methods of achieving the same outcome i.e. not requiring L-DACS to activate the MSB. Evidence gathered on the avionics equipment capabilities has shown that most DME suppliers apply the protection requirement stipulated in ED-57. This requires the front end to be able to sustain an input signal of +20 dbm. Discussions with some suppliers indicated that although this is current best practise, state of the art implementations can achieve protection from stronger signals. Feedback made available to this study from a survey 12 has indicated that that protection level of +20 dbm is common, and that up to +30 dbm is possible by using surge protecting diodes at the input of the radio (the latter suggestion is from Collins). It was also 11 This is typically the isolation available for antennas on the same side of the fuselage. 12 Rockwell Collins, Fernau Avionics, KAC. P1031 C7 v10 HELIOS 19 of 32

21 found that some equipment available on the market place offers less protection and at least one example has been found where the maximum input is +10 dbm 13. The observations presented above would need further verification through tests and through further dialogue with manufacturers, not least because the practical extent of the LDACS waveform properties generated by real equipment needs to be demonstrated. However should this prove to be the case, there would be no need for an L-DACS transmitter to use the suppression bus to protect other L- Band receivers Notification of Potential Interference Some modern digital receivers can use the fact that incoming signals may be suffering from interference at certain times to change the way they react to any data received during such notified periods. However in most cases it is likely that indications on the suppression bus that other L-Band systems are transmitting is simply used to mute the receiver such that it produced no output during periods when it is know that interference levels may be high. It is possible that, at times when incoming signals are strong (e.g. when an aircraft is in close proximity to an SSR interrogator or DME transponder) that the incoming signal is large enough to overcome L-DACS interference. In these cases triggering the suppression bus when the L-DACS equipment was transmitting would have the effect of denying reception of other L-Band signals when such reception was possible. All L-Band systems will have some level of outage given propagation and other factors. Not triggering the suppression bus when L-DACS is transmitting would cause unsuppressed interference, however the incidences of such interference will be small (of order 0.2%) which are likely to be below any outages which occur from other sources. Given the potential for suppression to be forced when it is not necessary, it therefore seems feasible that there is no need for L-DACS transmitters to notify the other L-Band systems that it is transmitting Muting of other L-Band transmitters Given the high transmitter powers of other L-Band systems (DME, SSR), it makes sense for them to avoid transmitting simultaneously both to avoid excessive power drain on aircraft systems, and to prevent potentially damaging field strengths being produced. However, L-DACS systems are significantly lower in transmitter power than other L-Band systems such that allowing other systems to transmit whilst L- DACS is transmitting has the potential not to have these problems arise. There therefore does not appear to be a strong rational for causing other L-Band systems to cease (or delay) transmission whilst L-DACS transmissions take place. Not least, whilst there are some potential benefits to informing other L-Band receivers that there are active L-DACS transmissions, there seems no rational to stop other L-Band equipment from transmitting, which would in essence serve to widen the impact of L-DACS transmissions from just affecting reception to affecting transmission, therefore amplifying the apparent outages caused by L- DACS. 13 CA series Combined NDB/DME from Longwater systems. P1031 C7 v10 HELIOS 20 of 32

22 4 Conclusions 4.1 Overall considerations There is a strong technical and operational case for considering not connecting L- DACS transmitters to the suppression bus. Of the three main reasons why L-Band systems are currently interlinked in this way, namely: To protect other L-Band receivers from potential damage from L-DACS; To notify other L-Band equipment of L-DACS transmissions to avoid compatibility issues; To mute other L-Band transmitters to ensure that problems from multiple simultaneous transmissions are avoided; it appears that none of them are necessarily relevant for L-DACS. Further, forcing existing L-Band systems not to transmit whilst L-DACS is transmitting significantly and unnecessarily doubles the potential outages that such systems may experience. The advantages of not requiring L-DACS systems to activate the suppression bus when transmitting can be gained if it can be clearly demonstrated that: the level of incoming interference caused by an L-DACS system is not sufficient to damage other L-Band avionics, the power drain on an aircraft s systems is not too great when L-DACS and another L-Band system are transmitting simultaneously; no dangerous field strengths are produced when L-DACS and another L-Band system are transmitting simultaneously. since both L-DACS options have modest transmitting power ranging from 41 dbm to 47 dbm. 4.2 Conclusions and observations The key issues tackled in this investigation are: Whether the L-DACS specifications should support suppression output Whether the use of this capability should be mandatory. To reach a conclusion on these questions the following issues were investigated: 1. Is the assumed cosite power of L-DACS likely to interfere with other L- band avionics? From a power budget perspective, almost certainly yes. Other recent studies such as that led by DSNA reached the same conclusion in their recent study paper presented in WG-F [9]. From a waveform compatibility perspective, there is no evidence for incompatibility. The L-DACS bursts are considerably longer than the DME or SSR pulses and, unlike UAT, can not be mistaken for pulses i.e. they will not stimulate unsolicited replies. P1031 C7 v10 HELIOS 21 of 32

23 The conclusion on this issue is that while L-DACS is transmitting the DME receiver will be impaired, but will not trigger compatibility issues like unsolicited replies. 2. Is the assumed cosite power of L-DACS damaging to neighbouring avionics? The answer to this depends to a large extent on: the implementation platform (such as the size of the aircraft), the protection characteristics of the legacy avionics radios, and the output power of L-DACS. To estimate this we assumed the worst conditions (smallest isolation, maximum L- DACS power) and the outcome is "no interference" in the majority of the cases. Most mainstream DME avionics sustain +20 dbm, but for those units that are capable of less protection, the link budget deficit is marginal (in the range of 2 db). However, for smaller aircraft that offer less than 35 db of antenna port isolation, protection will be required. In this case, use of the suppression bus may be a viable option, but not the only option available. Other options include using surge protectors (see below) 14. Furthermore, the estimates provided in this report consider the worst case assuming the largest possible L-DACS power. Both L-DACS candidates offer a power control scheme whereby the closer one gets to a DME station (especially DMEs at airports which transmit at much lower power), the stronger the DME signal and therefore the weaker the L-DACS signal emitted on board the aircraft. This means that when not at the edge of coverage, the DME receiver may still be able to operate in the presence of L-DACS transmissions, without the use of suppression. If suppression were used universally, one would be reducing the availability of the legacy systems needlessly. The conclusion on this issue is that the activation of the suppression bus to protect legacy avionics is not necessary in the large majority of the cases. Legacy equipment that offer low protection e.g. 10dBm are marginal cases and the ned for further protection measures will need to be evaluated on a case by case basis. 3. Is the RF interference caused by L-DACS likely to impinge on the operation of legacy systems like DME and SSR? From the analysis of the traffic profiles (using a typical service profile at 2020 traffic levels), If the answer to (1) is yes and the answer to (2) is no, then L-DACS is not likely to cause any undue interference to legacy system operation. This is because the aggregate contribution to unavailability is so low that it is negligible. The conclusion on this issue is irrespective of the presence of a suppression bus, the duration of the interference is so short that it will have a negligible impact on the availability of the legacy systems. 14 Note further that there is the uncertainty of the practical maximum power of the L-DACS transmitter (in particular L-DACS/1) which needs to be confirmed through practical implementation and prototpying. P1031 C7 v10 HELIOS 22 of 32

24 4. Will L-DACS survive the pulsed interference environment in the L-band? Given the protection levels assumed for L-DACS (+20 dbm), the answer is yes. L- DACS/2 specification needs to be clear on the protection it requires, but best practice protection built into avionics today appears sufficient this applies irrespective which L-DACS option is considered. The conclusion on this issue is that L-DACS does not need to monitor the suppression bus. The suppression bus is currently used by DME/SSR to protect the avionics from high pulsed powers AND to avoid compatibility issues like unsolicited replies. In the case of UAT (lower power, but short bursts), it is used only to avoid the latter case. L-DACS is dissimilar in both power and burst profile. Therefore considering the role of the suppression bus today, this is consistent with the conclusions presented above. 4.3 Specific considerations Protection of legacy systems The two candidate L-DACS options have similar output power requirements for the avionics. Due to the properties of the OFDM waveform, the L-DACS/1 signal 15 is more prone to experiencing spikes in signal RF power leading to an RF signal that is several db larger than the nominal maximum power of 41 dbm. However the L_DACS/1 specification has built in contingencies that claim to limit this effect 16. In addition, both systems implement a power control scheme to limit RF pollution that can be nuisance to other equipment on board the aircraft. Notwithstanding this, given the minimum level of isolation available on board air transport aircraft, it is found that the L-DACS signal levels are unlikely to be damaging to the receivers of the legacy avionics systems operating in the L-band. Furthermore, application of a typical service demand profile on an L-DACS channel (applying both candidate options) revealed that the aggregate channel occupation time of the L-DACS transmissions 17 is under 0.2%. Considering the nominal SSR channel availability of about 90%, and the proven ability of the DME system to achieve the required operational performance at reply efficiencies significantly lower than the nominal 70% 18, the impact of onboard L-DACS signals 15 Which in practise is a composite of several signals transmitted on the various subchannels. 16 Although there effectiveness has yet to be confirmed by practical testing. 17 Per aircraft. 18 That is, the interrogator retains its capability of scanning and tracking a DME transponder on the ground even if less than 70% of the replies are successfully detected. Trials have shown that the DME interrogator can keep tracking DME ground stations with reply efficiencies as low as 30%. P1031 C7 v10 HELIOS 23 of 32

25 on DME and SSR is considered negligible, even if the latter are rendered incapable of their reception function during such periods of L-DACS activity Protection of L-DACS The L-DACS/1 specification provides a specific requirement that the receiver shall be capable to tolerate a pulsed interference signal of 25 dbm without sustaining damage 20. Considering the typical DME interrogator output power of 700 Watts (58.5 dbm), and the typical minimum isolation of 35 db for antennas on the same side of the fuselage [8], the residual power presented in the L-DACS receiver pass band is about 23.5 dbm. Although this is close to the protection limit mentioned above, dialogue with avionics developers on state of the art implementations revealed that greater protection levels of 30 dbm are achievable and do not add significantly to cost and complexity, provided the receiver is not required to decode on the reference channel during such periods of high activity. Smaller airframes that offer less than the above level of isolation may however require an interface to the suppression bus if operating at high power levels as those considered above. Finally, while the duty cycle of the pulsed systems (in the order of a few microseconds) is very small even when compared to the duration of a single L- DACS symbol, temporary desensitisation (and consequential recovery time) of the L-DACS receiver front end due to high powered pulses may cause the loss of some data, but not to the extent that it is not recoverable by the powerful error correction schemes defined in the L-DACS specifications. 19 For example through desensitisation, or application of pulse blanking. 20 The L-DACS/2 specification does not provide such a requirement. P1031 C7 v10 HELIOS 24 of 32

26 A References The following is a list of reports and other technical material referenced in this report. [1] ICAO Annex 10 Volume 4. [2] UAT Manual of Technical Specifications, ICAO. [3] FCI Technology Investigations: Communications operating concept and requirements, Eurocontrol/FAA, version 2.0, [4] L-DACS/1 System Definition Proposal: Deliverable D2, Eurocontrol, Edition 1, February [5] D1: Initial L-DACS/2 System Specifications, Eurocontrol, Edition 1.0, May [6] FCI Technology Investigations: Common evaluation scenarios, Eurocontrol, version 1.0, 2007 [7] Mutual Suppression Rationale, FAA Technical Centre, ICAO ACP, UAT SWG08 WP18, 31 January [8] Additional Technologies and Investigations for Provision of Future Aeronautical Communications, NASA/CR , WP13, February [9] Heuristic assessment of the compatibility issues between the aeronautical FRS (Future Radio System) and Radionavigation DME/TACAN systems in the Band MHz, 18 th Meeting of ACP/WG-F, May P1031 C7 v10 HELIOS 25 of 32

27 B Abbreviations and acronyms ACAS AC ATS AOC CoS DC DME FEC L-DACS LoG MSB RF SSR TCAS UAT Airborne Collision Avoidance System Alternating Current Air Traffic Services Airline Operational Communications Class of Service (slot) Direct Current Distance Measuring Equipment Forward Error Correcting Scheme L-Band Digital Aeronautical Communications System Log In (slot) Mutual Suppression Bus Radio Frequency Secondary Surveillance Radar Traffic Collision Avoidance System Universal Access Transceiver

28 C Description of scenarios C.1 Traffic C.1.1 All aircraft are assumed to be randomly distributed within the coverage of a single ground station. A typical illustration of the distribution of the aircraft in relation to the ground station is shown below. Figure 2 - Distribution of aircraft around ground station C.2 Types of scenarios C.2.1 A range of five different scenarios have been defined in support of the L-DACS development activities. The scenarios applicable to terrestrial systems are: Large TMA (TMA LRG) Small Enroute sector (ENR SML) Medium Enroute sector (ENR MED) Large Enroute Sector (ENR LRG) Super large Enroute Sector (ENR SLG) C.2.2 C.2.3 Communications loading using ATS traffic only as well as with a combination of ATS and AOC traffic were considered. The following table illustrates the complete set of evaluation scenarios: P1031 C7 v10 HELIOS 27 of 32