Compatibility criteria and test specification for SSR

FCI Technology Investigations: L band Compatibility Criteria and

Interference Scenarios Study

Deliverable C3: Compatibility criteria and test specification for SSR

systems

Edition Number 1.0 Edition Date 24/08/2009 Status Final

COOPERATIVE NETWORK DESIGN

P1031C3v10 HELIOS ii of 18

Document information

Document title Compatibility criteria and interference scenarios for SSR systems

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: +44 1252 451 651

Fax: +44 1252 451 652

Email: [email protected]

Produced under contract 08-111428-C

Version 1.0

Date of release 24/08/2009

Document reference P1031 C3

P1031C3v10 HELIOS iii of 18

Contents

1 Introduction ............................................................................................................... 1

1.1 General ....................................................................................................................... 1

1.2 About this document.................................................................................................... 2

2 System Overview....................................................................................................... 3

2.1 Mode of operation........................................................................................................ 3

2.2 Equipment onboard architecture.................................................................................. 5

2.3 Front end characteristics ............................................................................................. 5

3 Compatibility Criteria .............................................................................................. 10

3.1 Interference Environment .......................................................................................... 10

3.2 Criteria....................................................................................................................... 10

4 Test setup ................................................................................................................ 11

4.1 Goal of the tests ........................................................................................................ 11

4.2 Test equipment.......................................................................................................... 11

4.3 Equipment setup........................................................................................................ 11

4.4 Source of interference ............................................................................................... 11

A Tests procedures..................................................................................................... 11

B Equipment................................................................................................................ 11

C Antenna characteristics .......................................................................................... 11

D References ............................................................................................................... 11

P1031C3v10 HELIOS 1 of 18

1 Introduction

1.1 General

1.1.1 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 Eurocontrol. The first option for L-DACS is a frequency division duplex (FDD) configuration utilizing OFDM modulation techniques. The second LDACS option is a time division duplex (TDD) configuration utilising a binary (GMSK) modulation scheme.

1.1.2 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.

1.1.3 The analyses presented in this section 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 results herein documented. These analyses are 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

Figure 1 – Overall evaluation process currently foreseen

1.1.4 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

LDACS1/2

Specifications

Interference Scenarios,

Criteria and Testing Plan

Development of

TX prototype

Development of

RX prototype

Testing and

Evaluation

LDACS

Selection


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.

1.1.5 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

1.2.1 This document is deliverable C3 of the Spectrum Compatibility criteria and Interference Scenarios for existing systems operating in the L band study produced by Helios for Eurocontrol under Contract 08-111428-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.

1.2.2 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 WP15.2.4.

1.2.3 This document identifies the key operating characteristics and the relevant performance criteria for Secondary Surveillance Radar (SSR) equipment 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 SSR SARPs (Annex 10)

� [2] ED-73B/C Minimum Operational Performance Specification for SSR Equipment

1.2.4 SSR equipment operates on two frequencies globally. These are 1030 and 1090 MHz. This system operates by the ground radar interrogating an airborne transceiver which then responds. A new digital communications system will have to be compatible with this system. This document identifies the key criteria to be used to assess spectral compatibility of the equipment.


2 System Overview

2.1 Mode of operation

2.1.1 Secondary Surveillance Radar (SSR)

2.1.1.1 The basic ATC RADAR system relies on pulsed RF as its means of communications. These pulses are 0.8 µs wide and vary from 8.0 µs to 21 µs spacing for mode A and C respectively. The SLS (P2) pulse is also transmitted omni-directionally and is used to suppress any replies to side lobe interrogations. The mode S SSR will interrogate using a 1030 MHz carrier with differential phase shift keying (DPSK) modulation. DPSK allows the interrogation frequency to have much more efficiency in sending information without interfering with mode A and C interrogations. DPSK also allows for up to 4 MBps of data. As the 1030 MHz, DPSK signal sends out the interrogation, the airframe will receive it, verify the request and integrity of the signal, and reply using a 1090 MHz carrier with pulse positioning modulation (PPM) transmission.

2.1.1.2 SSR is central to the mode S system. Mode S interrogations are generated at a rate of 50 times per second or 50 Hz pulse repetition frequency (PRF) and approximately 230 Hz for mode A/C interrogations. The reply will happen at the same PRF although mode S SSR has the ability to tell the mode S transponder not to reply to every mode S interrogation it receives. Once the SSR has received the reply it will decode the mode (A, C or S) and demodulate the information within each mode.

Figure 2 - Pulse shapes

2.1.1.3 There are three interrogation types in a mode S SSR system (refer to table above for pulse definitions):

� All call: This interrogation consists of P1, P3 and a 0.8 µs P4 pulse. P2 SLS is transmitted as normal. All ATC RADAR transponders reply with the 4096 identification code for mode A interrogations and altitude data for mode C. Mode S transponders do not reply on this interrogation.

� ATC RADAR/mode S all call: This interrogation is identical to the former except P4 is 1.6 µs long. ATC transponders reply with the 4096 code or altitude data as per the ATC RADAR all call. Mode S transponders reply with a special code, which contains the identity and the aircraft's discrete address.

� Mode S discrete interrogation: This interrogation is directed at a specific mode S transponder-equipped aircraft. The interrogation consists of P1, P2 and P6. P2 is transmitted via the directional antenna and hence is the same amplitude as P1 and P3. This effectively suppresses ATC RADAR


transponders from replying. P6 is actually a DPSK data block that contains either a 56-bit or 112-bit message. The DPSK modulation produces a spread-spectrum signal, which has immunity to interference.

2.1.1.4 When the transponder receives a valid mode S discrete interrogation, it will return a reply 128 µs after reception. The reply is transmitted on 1090 MHz and uses a 56-bit or 112-bit PPM transmission.

2.1.1.5 Each mode S interrogation will have a 24-bit address unique to the aircraft as well as a 24-bit parity check for validation. In basic mode S surveillance, the information is limited to altitude reporting (DF0), aircraft identification (DF4) and basic airframe information (DF11). The SSR is the main component of the interrogation and reply. The interrogation happens about 50 Hz PRF. The reply will happen at the same PRF. Once the SSR has received the reply it will decode the mode (A, C or S) and demodulate the information within each mode.

2.1.2 Monopulse

2.1.2.1 In case of close targets or crowded airspaces, the monopulse technique allows a reduction of the Interrogation Repetition Frequency (IRF), in turn reducing RF pollution.

2.1.3 1090ES and TCAS

2.1.3.1 A 1090ES aircraft transponder will periodically emit an unsolicited transmission. This transmission is commonly referred to as a ‘Squitter’. The functionality can be used to support the passive acquisition of a Mode S target by either ground or airborne users. The Squitter transmission is issued on the Mode S reply frequency and its functionality includes the following:

� Acquisition Squitter: containng the aircraft address and used primarily by TCAS and by ground-based multi-lateration;

� Extended Squitter: containing additional information to provide Automatic Dependant Surveillance Broadcast (ADS-B) capability, and which relies on unsolicited broadcasts from aircraft.

2.1.3.2 A definition of squitter is a reply format transmission without being interrogated. These squitters or “unsolicited replies” are used to provide TCAS 2-equipped airframes with the discrete address of the squittering airframe, to enable the TCAS 2 system to acquire and track the airframe using specific mode S formats.

2.1.3.3 Squitter has its origins in distance-measuring equipment (DME) transmissions (see ref P1031 C1). The DME ground station would broadcast unsolicited replies or squitters. When the airborne DME interrogator was in range, the squitter would be seen and the DME interrogator would then transmit a range interrogation and receive range replies from the DME ground station. This served to limit unnecessary transmissions over the air and optimised DME ground station-handling capability. TCAS 2 systems use mode S squitters in a similar fashion. The TCAS just listens for the DF11 squitters (see Figure 3), which contain the sending aircraft's discrete address, thereby reducing the need to interrogate over the air. Once the discrete address is obtained, it is placed on the TCAS 2 processor's roll call of addresses for ongoing tracking. Mode S technology has two types of squitter, a short (56 bit) DF11 acquisition squitter and the extended (112 bit) DF17 squitter.


Figure 3 – Mode S sweep timing pattern1

2.2 Equipment onboard architecture

2.2.1 The following diagram illustrates a typical airborne configuration for the SSR transponder and TCAS equipment.

Figure 4 – Airborne system architecture

2.3 Front end characteristics

2.3.1 SSR Interrogator

2.3.1.1 Annex 10 defined the signal-in-space characteristics that define the front end of the SSR equipment. For Mode S interrogations, the carrier frequency is pulse modulated. The data pulse has, in addition, internal phase modulation. Inter-mode (A/C/S) and Mode S interrogations consist of a sequence of pulses. Pulse shapes shall be as defined in Figure 5.

1 Courtesy of SELEX SI.


Figure 5 - Pulse shape

2.3.1.2 The short (16.25-microsecond) and long (30.25-microsecond) pulses that carry data have internal binary differential phase modulation consisting of 180-degree phase reversals of the carrier at a 4 megabit per second rate.

2.3.1.3 The power limits of the spectrum of a Mode S interrogation about the carrier frequency are illustrated in Figure 6.

Figure 6 – Spectrum mask for interrogator transmitter

2.3.1.4 The uplink signal power of the interrogator as well as the sensitivity of the transponder on the aircraft may vary. Similarly, the downlink signal power will depend on the transmitted power of the airborne transponder and the sensitivity


of the SSR at the antenna. The weaker of these two links determine the range of the system. The following table illustrates a typical SSR RF characteristic [3].

Parameter Value

Power 32 dBW

Sensitivity -90 dBm

Cable loss 3 dB

Maximum antenna gain 27 dBi

Table 1 – Typical SSR interrogator characteristics

2.3.2 SSR Transponder

2.3.2.1 The Mode S reply consists of a preamble and a data block. The preamble is a 4- pulse sequence and the data block is binary pulse-position modulated at a 1 megabit per second data rate.

2.3.2.2 The following figure shows the power spectrum centred on the carrier frequency and will therefore shift in its entirety plus or minus 1 MHz along with the carrier frequency. Further details on the antenna characteristics are furnished in the Annex.

Figure 7 - Spectrum mask for transponder transmitter

2.3.2.3 The SSR transponder MOPS (ED-73B) provide specifications of the RF peak output power. This is summarised in the following table.

Equipment class Peak output power dBW (Watts)


Min Max Min Max

Class 1 21 27 (125) (500)

Class 2 18.5 27 (70) (500)

Table 2 - ED-73B peak output power specification for SSR transponder

2.3.3 TCAS equipment

2.3.3.1 TCAS equipment is functionally similar to that of SSR but slightly different from the RF and signal profile standpoint because it can have a number of beams with potentially a range of gains. The antenna is fixed and the beams are therefore not rotated.

2.3.3.2 A typical arrangement for the TCAS antennas is a directional antenna on top of the fuselage and an omnidirectional antenna on the bottom of the fuselage. Given that the most likely incidence of the TCAS signal occurs in the forward facing beam which has the highest gain. The following table illustrates a typical signal power profile for TCAS.

Parameter Value

Power 21.5 dBW

Sensitivity -71.5 dBm

Cable loss 0 dB

Maximum antenna gain 7.5 dBi

Table 3 – Typical TCAS signal properties

2.3.4 Standards

2.3.4.1.1 Detailed definitions of all the parameters required for Elementary and Enhanced Surveillance are given in the ICAO Manual on Mode S Specific Services. Transponder certification is further guided through the Minimum Operational Performance Specifications as laid down in RTCA-181C and EUROCAE ED-73B. These MOPS documents are intended to ensure that aircraft Mode S SSR transponders certified to it will be compatible with ICAO Annex 10 standards. They include performance characteristics and tests under normal operating conditions and simulated airborne conditions. ED-73B provides requirements and tests for several areas including Level 5 transponders, Extended Squitter Functions and ACAS.

2.3.4.1.2 Performance standards for the operation of SSR based equipment are geared towards assessing the performance of the transponder in the highly pulsed L-band environment. This includes a large component of self interference originating from other SSR installations operating on the 1030 and 1090 MHz channels. A considerable number of tests have therefore been developed to test for correct operation of the system in the self interference environment. Examples of such tests are:

� Reply delay and jitter,

� Side lobe suppression


� Pulse level tolerances

� Transponder recovery and desensitisation

� Response to interference

2.3.4.1.3 The tests which are most relevant to assessing compatibility between SSR systems and LDACS in the L-band is the response to interference tests. The criteria used for the tests are described in the next section.

2.3.4.1.4 The ICAO SARPs2 includes a requirements on:

� Random trigger rate

� Reply ratio in the presence of interference3

2.3.4.1.5 “Random triggering rate. In the absence of valid interrogation signals, Mode A/C transponders shall not generate more than 30 unwanted Mode A or Mode C replies per second as integrated over an interval equivalent to at least 300 random triggers, or 30 seconds, whichever is less. This random triggering rate shall not be exceeded when all possible interfering equipments installed in the same aircraft are operating at maximum interference levels.”

2.3.4.1.6 “Reply ratio in the presence of low level asynchronous interference. For all received signals between – 65 dBm and –21 dBm and given a Mode S interrogation that requires a reply according to 3.1.2.4 and if no lockout condition is in effect, the transponder shall reply correctly with at least 95 per cent reply ratio in the presence of asynchronous interference. Asynchronous interference shall be taken to be a single Mode A/C interrogation pulse occurring at all repetition rates up to 10 000 Hz at a level 12 dB or more below the level of the Mode S signal.”

2 Annex 10 Volume IV Chapter 3.

3 Note that this requirement in the SARPs is intended to present measures of the performance of the

Mode S transponder in the presence of interfering Mode A/C interrogation pulses.


3 Compatibility Criteria

3.1 Interference Environment

3.1.1 SSR equipment operates on two frequencies globally. These are 1030 and 1090 MHz. This system operates by the ground radar interrogating an airborne transceiver which then responds. SSR operates in a highly pulsed environment, so the main performance criteria are intended to test operation of the transponder in the presence of random interfering pulses, and as such, the test specification only makes provisions for pulsed type interference, notably DME/TACAN. This is the transponder reply efficiency test.

3.1.2 However there are other tests designed to evaluate the performance of the transponder’s response in the presence of CW interference.

3.2 Criteria

3.2.1 The main compatibility criteria for the SSR/Mode S receiving equipment in the presence of undesired interference is:

� Signal-to-Interference ratio (S/I)

� Undesired reply rate

3.2.2 These criteria are described in below, and their application is described in Section 4.

3.2.3 The interference criterion of the aircraft transponder is dependent on the ability of its receiver to demodulate and decode requests that were transmitted from the ground based interrogator. The minimum triggering level (MTL) is defined as the minimum input power level referred to the sensor RF port that results in a 90 percent reply ratio if the interrogation signal has all nominal pulse spacings and widths and if the replies are the correct replies assigned to the interrogation format.

3.2.4 The key criterion is that the transponder receiver is able to demodulate and decode 90 percent of the interrogations that are transmitted in its direction with a signal to interference (S/I) ratio of 12 dB.

3.2.2 Signal-to-Interference ratio

3.2.1 Based on the SARPs requirement for performance in the presence of low level asynchronous interference, the interference protection criteria for SSR based transponders and interrogators are [5]:

� S/I ≥ 12 dB for interrogator;

� S/I ≥ 12 dB for transponder.

3.2.3 Undesired reply rate

3.2.3.1 In the presence of non-coherent Continuous Wave (CW) interference at a frequency of 1030 +/- 0.2 MHz, at signal levels of 20 dB or more below the


desired Mode A/C or Mode S interrogation signal level, the transponder shall reply correctly to at least 90 percent of the interrogations [6].

3.2.3.2 Mode A/C

a) The random trigger rate squitter on all ATCRBS modes shall not be greater than 5 reply pulse groups or suppressions per second, averaged over a period of at least 30 seconds.

b) The random trigger rate on all ATCRBS modes shall not be greater than 10 reply pulse groups or suppressions per second, averaged over a period of 30 seconds, when operated in the presence of non-coherent CW interference at a frequency of 1030 +/- 0.2 MHz. and a signal levels of -60 dBm or less.

3.2.3.3 Mode S

c) In the absence of valid interrogation signals, Mode S transponders shall not generate unwanted Mode S replies more often than once per 10 seconds.

d) In the presence of non-coherent CW interference at a frequency of 1030 +/- 0.2 MHz. and at signal levels of -60 dBm or less, and in the absence of valid interrogation signals, Mode S transponders shall not generate unwanted Mode S replies more often than once per 10 seconds.


4 Test setup

4.1 Goal of the tests

4.1.1 The transponder reply efficiency and undesired reply rate are measured as a function of desired signal level for various combinations of interference. The steps to be undertaken to determine these quantities are described in the Annex.

4.2 Test equipment

4.2.1 Recommended test equipment for the execution of the tests described in this section are:

� Transponder Test Set (T-50-3A/4A or equivalent)

� Wide Band Dual Channel Oscilloscope (HP1710B or equivalent)

� Frequency counter (HP 5381A or equivalent)

� 3 Port Power Dividers.

4.3 Equipment setup

4.3.1 ED-73 prescribes conditions with which to set up the tests. The conditions are as follows:

� Unless otherwise specified, all tests shall be conducted with the power input voltage adjusted to design voltage ±2%.

� For ground equipment, the input frequency shall be adjusted to design frequency ±2%.

� For avionics equipment, tests shall be conducted with the input frequency adjusted to within 5% of a declared frequency within the range for which the equipment is designed.

� All tests shall be performed with the equipment connected to loads having the impedance values for which it is designed.

� When test results are being recorded for incorporation in the type test report, it is not sufficient to note merely that the specified performance was achieved. The actual numerical values obtained for each of the parameters tested must be recorded to enable verification of the statement of compliance with the relevant performance specifications of ED73.


Figure 8 - Test equipment setup

4.4 Source of interference

4.4.1 Test results will collect data to assess SSR/Mode S/TCAS receiver performance against each interference source individually and in combination. The sources of interference to the SSR receiver will include on-channel self- interference from other SSR/Mode S transmitters, DME adjacent channel, JTIDS/MIDS signals and digital communication signals that periodically occupy the frequency band at or near the near band.


A Tests procedures

A.1 Spurious pulse interference tests

With the equipment connected as shown in Figure 8, interrogate the transponder with the Mode S-Only All-Call interrogation at a signal level of -50 dBm and follow Steps 1 through 4 below:

A.2 Undesired reply rate tests

With the equipment connected as per Figure 8, connect the RF terminal of the Transponder Test set to the transponder antenna terminal, and carry out the following two steps:

Step 1: Non-Interference environment

Set the ATC test set to zero interrogation rate. Set the oscilloscope to a total sweep time of 100 microseconds and the internal trigger to allow one sweep for each reply group transmitted by the transponder.

Count the number of replies for a minimum of one minute. The squitter generation function may be disabled for this test.

Step 2: Random Trigger Rate

Repeat the procedure provided in step 1 while injecting non-coherent CW interference at a frequency of 1030 +/- 0.2 MHz. and starting at a signal level of MTL – 3 dB.

Count the number of replies for a minimum of one minute and verify that the reply rate does not exceed the limits specified in 3.2.3.1 above.

Repeat the procedure as needed while increasing the interference signal level MTL-3 dB to -24 dBm in 5 dB increments.

Note 1: In both steps, do not interrogate the Transponder.

Note 2: Squitter and Self Test transmissions, if not inhibited, must be disabled in this test.


B Equipment

The following two classes of equipment are considered in the MOPS tests.

B.1 CLASS 1 equipment

This equipment is intended for use in aircraft that operate at altitudes above 15000 ft, or have a maximum cruising true airspeed in excess of 175 knots (324 km/h), and shall meet the following requirements:

� RF peak output power > 21 dBW

� Reply transmission frequency tolerance + 1 MHz C.

� Mode A and C reply rate capability > 1200 replies/second

B.2 CLASS 2 equipment

This equipment may be used in aircraft that operate at altitudes not exceeding 15 000 ft, and have a maximum cruising true airspeed not exceeding 175 knots (324 km/h), and shall meet the following requirements:

� RF peak output power > 18.5 dBW

� Reply transmission frequency tolerance + 1 MHz

� Mode A and C reply rate capability > 1000 replies/second


C Antenna characteristics

C.1 SSR antenna

The MOPS requires the following specifications to be met by the antenna on the assigned DME channel.

The MOPS assume antennas having the characteristics defined below.

C.1.1 Tuned frequencies

The antenna shall receive and transmit signals on the nominal operating frequencies of 1 030 and 1 090 MHz.

C.1.2 Impedance

The antenna must have a nominal impedance of 50 ohms. When terminated in a transmission line representative of the aircraft installation, the VSWR at the transponder end must not exceed 1.5:1 over the above operating frequency range.

C.1.3 Radiation Pattern

When mounted at the centre of a 1.2 m diameter (or larger) flat, circular ground plane, the antenna gain shall not be less than that of a matched quarter-wave stub, minus 3 dB, over 90% of a coverage volume of:

� azimuth: 0° to 360°

� elevation: 5° to 30° above the ground plane.

Figure 9 – Typical SSR antenna gain pattern (dBi) [3]


C.1.4 Polarisation

The antenna radiation pattern shall be predominantly vertically polarised.

C.2 TCAS antenna

The following diagram illustrates a typical antenna gain pattern for a single TCAS beam [3].

Figure 10 - Typical antenna gain pattern for TCAS


D References

1. Fact Sheet - SSR Mode S , Directorate of Airspace Policy, UK CAA

2. ED-73B and ED-73C MINIMUM OPERATIONAL PERFORMANCE SPECIFICATION FOR SECONDARY SURVEILLANCE RADAR MODE S TRANSPONDERS, January 2003 and December 2008.

3. Study of Low Power Transponder link Margins, QinetiQ/07/03229/1.0, January 2008.

4. ICAO Annex 10 Volume IV.

5. Interference protection criteria, Phase 1 – Compilation from existing sources, NTIA Technical Report 05-432,

6. RTCA DO-181C Minimum Operational Performance Standards for Air Traffic Control Radar Beacon System/Mode Select (ATCRB/Mode S) Airborne Equipment.

Documents

Compatibility criteria and test specification for SSR