1. Lecture 8 SSR Mode S

3. Uplink-formats in Mark X Standard

5. The Reply Message The SSR down link format consists of a number of pulses, nominally 0.45 µs (±0.1 µs). F1 and F2 are always present and separated by 20.3 µs (±0.1 µs) – they are often referred to as a bracket or framing pair. Other pulse positions within this framing pair are spaced by 1.45 µs and are used to convey the required reply information in answer to the specific interrogation (e.g Mode A identity or Mode C flight level values).

6. The pulses are identified to give the bits of an octal code (ABCD)
The pulses are identified to give the bits of an octal code (ABCD). The X pulse at the centre of the reply is not used. The three blank positions may not be occupied by pulses, otherwise some decoders may reject the entire answer as interference. Note that the reply information itself does not contain any information to indicate which mode it is a reply to. The interrogator will assume that the replies received are in answer to it latest mode of interrogation.

7. SSR down link format

8. In the case of Mode A, the octal code (ABCD) is set by a control panel in the cockpit. In the case of mode C, the flight level is encoded in a special way (by a special form of Gray code known as Gillham code - this has the characteristic of only one bit changing for each change in flight level).

9. The SPI (Special Purpose Identification) pulse is used by air traffic controllers to confirm the identity of certain aircraft. The controller will ask the pilot to squawk ident – the pilot pressing a button on the control panel which adds the SPI pulse to SSR replies for a certain period (18±1 s).

10. The display system will then highlight aircraft with SPI
The display system will then highlight aircraft with SPI. (The SPI pulse may have been appropriate to distinguish aircraft on older display systems before fully plot extracted displays became available). The out of frame position of the SPI pulse is somewhat strange and, as will be seen later, the SPI pulse position chosen introduces rather unfortunate complications for automatic decoding purposes. According to ICAO the SPI-pulse will be added to Mode A reply only.

12. Each answer receives its meaning only in connection with the respective question. For example: in Mode 3/A: general air emergency in Mode C: 20,000 ft height

13. Garbling/Degarbling Garbling is a fundamental problem in the design of the classical SSR system and the situation is made worse by increased traffic. Aircraft are often closely spaced in range and azimuth but at different heights. Replies from two aircraft will overlap if their range separation is within the equivalent of the 20.3 µs reply length. This is approximately 1.7 Nm.

15. The most serious garbling situations occur when the azimuth separation is very small such that replies from both aircraft are received from all interrogations across the beam. With advanced reply processing techniques and algorithms, it may sometimes be possible to extract all some or all of the replies from the received signal.

16. At this, in principle, one distinguishes two manners of the overlapping: - Non-synchronous Garbling; - Synchronous Garbling Two replies overlaps in time such that its time grids are not congruent, so one speaks about Non-synchronous Garbling. Such answers can be separated and one by one be decoded correctly!

18. Synchronous garbling

19. But if two or more replies overlap in time such that its time grids are congruent, so one speaks about Synchronous Garbling. It cannot to state in the decoding any more, whether this single impulse belongs to one or the other ones response telegrams. Through this it would come to the decoding of completely new and wrong replies and difficult from the original replies. These replies must therefore be disabled!

20. Mode S
Super Beacon
Discrete Address Beacon System (DABS) project, later renamed Mode S
Enable two way
ground-air data transmission
S = Select: Uses discrete addressing to interrogate just one aircraft
Started a project called the Discrete Address Beacon System (DABS), later called Mode S
Would enable two way ground-air data transmission (hoping to reduce pilot communication w/ ground station and thus ease both workloads)
S = Select : Proposed to use discrete addressing to interrogate just one aircraft (clean up the channel overload problem)
(hoping to reduce pilot communication w/ ground station and thus ease both workloads)
allows for ground surveillance
(clean up the channel overload problem)

21. Overview
reply
interrogation

22. Mode S

23. Interoperability Issues
Transparency: Mode S must not break existing systems
Backwards-compatibility: Existing systems must still see Mode S equipped planes
other aircraft
existing signal
Mode S
ground
station
new signal
existing
ground
station
Mode S equipped

24. Frequency New frequency: difficult to allocate
Same frequency as old system (1030/1090 MHz): interoperable, but may cause interference
300 MHz
1030 MHz
1090 MHz
3000 MHz
Conclusion: use same frequency.
VHF
UHF
SHF

25. Mode S Uplink Formats A conventional SSR interrogator may have a typical sequence of Mode A interrogation, followed by Mode C interrogation or other modes. This would be repeated continually at a high rate to ensure that a position/identity plot can be produced for all targets in line-of-sight range of the interrogator during each antenna revolution.

26. The Mode S ground station produces a larger variety of interrogation types. These types can be roughly classified into two types: - All-call interrogations - Roll-call interrogations

27. All-call interrogations obtain replays from all aircraft in the beam dwell, although, under certain circumstances, Mode S aircraft can be „locked out” to all interrogations so that they do not reply. Roll-call interrogations are selectively addressed to acquired Mode S equipped aircraft using the unique 24-bit address assigned to each aircraft. Only the addressed aircraft produce replies.

28. The first problem for the Mode S system is to find the addresses of aircraft that are in radar cover so that selective addressed interactions can be made with them. This is achieved by Mode S all-call interrogator witch are made periodically from the radar.

41. Transponders Standard doesn’t specify what ATCRBS transponders
should not do:
549 transponders on the market
Each had unique behavior
Reference Edison by name.

42. Side Lobe Suppression In secondary surveillance radar technology the side lobes of the antennae affect particularly unfavorably. Transponders also can be interrogated over the side lobes and then answer about these, too and a response telegram can be received also over the side lobes. This circumstance results from the fundamentally better transmitting- and reception conditions for secondary radar units.

43. Such answers cannot be assigned obviously on the radar screen
Such answers cannot be assigned obviously on the radar screen. They rather appear as several targets in the same range but in different directions. In the extreme case an airplane can be interrogated permanently during a turn of the antenna. Such an reply then appears on the PPI-scope as a „ring around”.

44. There are two principles of Side Lobe Suppression (SLS) - Interrogation Path Side Lobe Suppression (ISLS), and - Reply Path Side Lobe Suppression (RSLS) The techniques for ISLS are very similar to those for RSLS. A supplementary so called - Improved Interrogation Path Side Lobe Suppression (IISLS) method uses the techniques of ISLS to reduce the influence of false replays caused by reflection.

45. The Hack Existing ATCRBS transponders used sidelobe suppression
aircraft 1
aircraft 3
aircraft 2
Existing ATCRBS transponders used sidelobe suppression
P1 main lobe
side lobe
INTERFERENCE!!!
ground
station

46. The Hack Existing ATCRBS transponders used sidelobe suppression
aircraft 1
aircraft 3
Existing ATCRBS transponders used sidelobe suppression
P1 main lobe P2 P2 P1 A1
Hack in the “clever” sense
ground
station
side lobe P1 P2 A2
aircraft 2

47. Hacking the Hack Purposely send a small P1 and large P2
“Disables” ATCRBS transponders
Use the time to cram in Mode S data blocks
Limited number of bits can be sent in this window P1 P2
Mode S data block
35 microseconds

48. What Changed Things Mid-air collision in 1986
Congress passes a law mandating that all commercial aircraft be equipped with a Traffic Collision and Avoidance System (TCAS) by 1993
TCAS uses Mode S
TCAS is now an international standard
Mode S technology is now commercially available

49. Mode S Today
108 of the U.S.’s busiest airports have Mode S ground stations
Majority of aircraft landing at these airports have Mode S transponders
Without Mode S, the 1030/1090 Mhz band would be completely overloaded
Mode S used in TCAS and many other applications

50. Transponder Basics

51. Overview Secondary Surveillance Radar Mode A Mode A/C
Mode S (selective)
Mode S/ES (extended squitter)
ADS-B (Automatic Dependent Surveillance Broadcast)
STCA (Short Term Conflict Alert)
TCAS (Traffic alert Collision Avoidance System)
Flarm

52. Acronyms ACAS – Airborne Collision Avoidance System
ADS-B – Automatic Dependent Surveillance Broadcast
CAT – Commercial Air Traffic
EHS – (Mode S) Enhanced Surveillance
ELS – (Mode S) Elementary Surveillance
ES – Extended Squitter
GA – General Aviation
LAST – Light Aviation SSR Transponder
LPST – Low Power SSR Mode-S Transponder
SSR – Secondary Surveillance Radar
STCA – Short Term Conflict Alert
TCAS – Traffic alert Collision Avoidance System
TMZ – Transponder Mandatory Zone
TRA(G) – Temporary Reserved Areas (Gliders)
UAV – Unmanned Aerial Vehicle

53. Secondary Surveillance Radar
Primary radar relies on passive reflection
Secondary radar co-located with primary
SSR beam interrogates transponders
Transponder reply depends on its mode
ATC computers use SSR to tag primary echoes with identifier and height

54. Mode A Developed from 2nd World War IFF (Identification Friend or Foe)
Limited number of squawk codes (4096)
Non-selective – all transponders respond to every interrogation broadcast
Typically high power requirements:
Frequent transmissions
Power hungry transmitter

55. Mode A/C Incorporates mode A … plus …
Altitude (i.e. Flight Level) as well as identifier
Extra power required to heat the altimeter to a constant temperature

56. Mode S (selective) Incorporates mode A/C ability … plus …
Less power required because:
Transmitter is typically more efficient
Altimeter does not need a heater
Can respond selectively:
If the SSR interrogates selectively
Can reduce power even more
Two levels of function (and price!):
ELS: Elementary Surveillance (gliders & light aircraft)
EHS: Enhanced Surveillance (for CAT)

57. Mode S/ES (extended squitter)
As for mode S plus …
Periodically broadcasts GPS position & velocity
Potentially provides one half of an airborne collision avoidance system
Provides some interoperability with ADS-B

58. ADS-B Broadly equivalent to Mode S/ES 150 mile range
Does not need a ground-based radar system
Already in use in Alaska & Australia
Arguably the system of the future

59. STCA (Short Term Conflict Alert)
Automated system which alerts ATC
Requires transponders on both aircraft
Requires at least one aircraft to be in contact with ATC

60. TCAS (Traffic alert Collision Avoidance System)
Is an Airborne Collision Avoidance System
Receives transponder signals
Works with mode A/C, S, or S/ES
Mandatory for CAT
Useless at detecting non-transponder aircraft

61. Flarm Good value collision avoidance 11,000 in use
High take up in the Alps and Germany
Growing take up in UK
Short range (a few kilometres)
Ideal for gliders and slow speed aircraft
May be unsuitable for ATC or CAT
Unacceptable as an alternative to mode S

62. ASAS Enabling technologies
Aircraft Sensors
ASAS Display
ADS-B
ADS-B receiver ground station
TIS-B

63. ASAS Functional description
On-board systems and data sources
(FMS, GPS, pilot interface)
Airborne data processing, Display
Applications processing
ACAS
Aircraft A
ADS-B out
ADS-B in
Navigation information - any source including GPS
Ground vehicle
ADS-B out
On-board systems and data sources
(FMS, GPS, pilot interface)
Airborne data processing, Display
Applications processing
ACAS
Aircraft N
ADS-B out
ADS-B in
Ground systems
Ground ADS-B receiver
ATC surveillance
Controller Working Position

64. ADS-B Definition
ADS-B: « Automatic Dependent Surveillance – Broadcast »
« A function on an aircraft or surface vehicle that broadcasts position, altitude, vector and other information for use by other aircraft, vehicles and by ground facilities » (Draft ICAO ASAS Circular)
The broadcast is independent of any external stimuli
The originating aircraft does not know who receives and uses its broadcast
Users: surrounding aircraft/vehicle and/or ATC
Three candidate data link technologies
Secondary surveillance radar (SSR) Mode S Extended Squitter (SSR Mode S ES)
VHF digital link Mode 4 (VDL Mode 4)
Universal Access Transceiver (UAT)

65. ADS-B Message content
Data transmit by ADS-B generically defined by RTCA ADS-B MASPS DO-242A
Horizontal position and related data
Lat/Long, Horizontal velocity, Ground speed, Heading on surface
Integrity (Navigation Integrity Category, Surveillance Integrity Level )
Optional: Airspeed, Heading while airborne
Altitude and related data
Barometric altitude, Geometric altitude, Vertical rate, NIC baro
Status
ICAO address, Call sign, Emitter category, Length and width
Emergency/priority, Capability class codes
Target State: Target altitude and HDG/TRK
Trajectory Change (Under review)
Arrangement within one or several transmissions according to the data link technology

66. ADS-B ICAO data link policy
Air Navigation Conference (2003) Recommendations
Strategy for the near-term
ICAO encourages the selection of the SSR Mode S Extended Squitter as the initial data link
Regional implementations
Europe: VDL Mode 4
US: UAT
In the longer term
The current SSR Mode S extended squitter technology may not be able to fully satisfy all of the requirements for ADS-B services in all airspaces
Continue to develop standards for ADS-B link technologies, including VDL Mode 4 and UAT

67. SSR Mode S ES Main characteristics (1)
On 1090 MHz channel
Secondary Surveillance radar (SSR) Mode A and C
Aircraft SSR Mode S responses to interrogations from ground-based radars
Aircraft answers to ACAS interrogations including transmission of non-solicited answers
ADS-B SSR Mode S Extended Squitters
Based on (except Modes A and C)
Multiple Access: pseudo random timing of the transmissions
Pulses modulated at 1 MHz able to convey 56 or 112-bit messages
Downlink and Uplink predefined formats

68. SSR Mode S ES Main characteristics (2)
Squitters
Aircraft broadcast non-solicited long squitters (112 bits)
Data to be transmit are prepared in « registers »
Different types of SSR Mode S Extended Squitters
Airborne/Surface Position Squitter Content of registers 05/06 (rate: 0.5 s)
Airborne Velocity Squitter: register 09 rate: 0.5 s Aircraft Identification Squitter: register 08
Event-Driven Squitter: register 0A
=> Several Squitters are necessary to transmit data

69. SSR Mode S ES Standardisation status
ICAO SARPS: Annex 10 Vol III Ch 5 and Vol 4
The current standard is Amdt 77
Plans to modify the content of some registers in order to improve the characterisation of the safety level
RTCA MOPS
DO-260: Minimum Operational Performance Standards for 1090 MHz ADS-B
DO-260A: with proposed improvements on safety level characterisation

70. SSR Mode S ES Transmitters
On-board current Transponders
Mode S transponders transmit on 1090 MHz aircraft SSR Mode S responses to interrogations from ground-based radars
They are installed on most aircraft (mandatory on all IFR aircraft in Europe as of 31/03/2005)
Evolution towards ADS-B Out Transmitters
Transmission of Extended Squitters is part of the standard (provided Registers are loaded by the avionics)

71. SSR Mode S ES On-board Receivers
Current TCAS
The TCAS box includes a 1090 MHz reception function of
answers of surrounding aircraft to ACAS interrogations
short squitters
Evolution towards ADS-B receiver
SSR Mode S Extended Squitters can be received and extracted by TCAS
Independency between TCAS and SSR Mode S Extended Squitters reception is required: no common failure

72. VDL Mode 4 Main characteristics
Frequency Band : MHz (i.e. Nav and Comm bands)
Bandwidth : 25 KHz per channel
Time Division Multiple Access, self organised by the mean of a common clock available to all users : UTC time (GPS) VDL4 user terminal checks available slots and make reservations in subsequent frames for transmission
The VDL Mode 4 protocol allows to manage several VHF channels
2 Global Signalling Channels (GSC)
Regional and Local Signalling Channels (LSC) for additional services as required
4500 slots/minute
Bit rate : 19,200 bits/s

73. VDL Mode 4 Standardisation Status
ICAO SARPS: Annex 10 Vol III Ch Manual on VHF Digital Link (VDL) Mode 4
Airborne MOPS: ED-108A in preparation
Ground: on-going ETSI standardisation

74. UAT Main characteristics
Designed specifically for ADS-B with no constraints from legacy systems
Single common wideband channel 2-3MHz 1 Mbps channel rate in MHz (ARNS band)
Aircraft access the channel autonomously at random
Transmission rate: 1/sec.

75. UAT Status Standardisation status Product status
Frequency allocation: need to be co-ordinated on a world-wide basis
SARPs not yet available
MOPS available (DO-282)
Product status
UAT part of SafeFlight-21 Link Evaluation study
UAT selected by FAA Alaska Region ( installations in Western Alaska)

76. TIS-B Definition TIS-B Data link technologies identical to ADS-B
« A service provided by ground stations, broadcasting information relating to aircraft based on surveillance carried out by ground systems, using ADS-B signals, formats and protocols, compatible with ADS-B receiving equipment » (Draft ICAO ASAS Circular)
Data link technologies identical to ADS-B
Depends on a ground surveillance infrastructure
e.g. SSR, PSR, ADS-B, multilateration, ASDE
Users: surrounding Aircraft/Vehicle

77. TIS-B Role & Status Role Status
Provides aircraft with a complete picture of the traffic
Exact role to be defined (ADS-B gap filler, ADS-B data validation, Bridge between different ADS-B data link technologies, Primary source for some ASAS applications?)
Status
Elements in ICAO SARPS for SSR Mode S Extended Squitters and VDL Mode 4 technologies
Complete SARPS to be developed
RTCA MASPS DO-280

78. On-board Sensors ADS-B requires on-board availability of
Aircraft position
May be based on GPS or FMS multisensor position
Available on all RNAV capable aircraft
Integrity data from GPS (more difficult with FMS)
Altitude and related data
Barometric altitude: always available
Geometric altitude: if GPS
Status
Ident: ICAO address and Call sign may require specific Control Panel

79. On-board Processing Data Link layer Surveillance layer
Applications layer
TA/RA
ACAS tracks
ACAS tracks
CAS Logic
ACAS Surveillance
Active reply
Active Interrog
ASAS
tracks
ASAS tracks,
Selected target, Guidance data
Display Management
UAT / VDL M4
Receivers
ADS-B
TIS-B
1090 MHz Receiver
Display
Surveillance
Data Processing
ASAS Applications
Own position
Other Systems (FMS, MCDU…)

80. On-board Display Cockpit Display of Traffic Information (CDTI)
Example of Navigation (FMS), TCAS and ASAS sharing the same display