SECURE ATC SURVEILLANCE FOR MILITARY APPLICATIONS

978-1-4244-2677-5/08/$25.00 ©2008 IEEE
SECURE ATC SURVEILLANCE FOR MILITARY APPLICATIONS
This paper presents a system in concept development that
combines the integrity of the civil Automatic Dependant
Surveillance – Broadcast (ADS-B) system for air-to-air
conflict avoidance and air traffic management (ATM)
surveillance with the military secure Mark XIIA Mode 5
Identification Friend or Foe (IFF) system as the data link.
This new system, coined Military ADS-B (MADS-B),
expands the traditional IFF surveillance data of range,
azimuth, altitude, and identity (ID) to include each
platform’s dynamic state of motion and status. This selfreporting
system significantly improves positioning and
status data between Tri-Service platforms and with Allies –
who are also transitioning to the common Mode 5 system.
Beyond MADS-B’s securing of ADS-B data, it enables
overcoming other surveillance and navigation shortfalls
through future software based message additions. The
paper summarizes ADS-B operations, the conversion of
data to the Mode 5 Level 2 formats, and examples of
message extensions providing surveillance enhancements.
INTRODUCTION
The objective of the secure surveillance capability is to
bring all the benefits of the civil air traffic management
(ATM) Automatic Dependent Surveillance – Broadcast
(ADS-B) system to the military Identification Friend or
Foe (IFF) system. The proposed solution improves system
integrity, enables common flight procedures, and increases
link efficiency.
Civilian air traffic control (ATC) and military IFF share
dedicated communications channels, with an exclusive
international frequency allocation of 1030 MHz for the
interrogations (uplink) and 1090 MHz for the replies
(downlink). They have evolved from radar beacon based
question and answer systems. The initial civil component
is known as the Air Traffic Control Radar Beacon System
(ATCRBS). Legacy military IFF supplemented the
ATCRBS modes with an unsecured Selective
Identification Facility (SIF) for additional platform data in
addition to a secure ID only mode. As civil air traffic
density increased, an addressed interrogation mode, Mode
S, was developed to obtain platform data and reduce
system self-interference. It also introduced a self reporting
acquisition mode to supplement the question and answer
mode. Increasing traffic density continued to grow,
challenging the paradigm of question and answer ATM
Larry Kenney
Joe Dietrich
Jerry Woodall
Raytheon Network Centric Systems
Towson, MD
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systems. ADS-B is the civil solution to the Mode S overinterrogation
and Traffic Alert and Collision Avoidance
System (TCAS) generated false replies unsynchronized to
interrogator transmissions (fruit) that jeopardize ATC
integrity. ADS-B is the affordable solution to safely
increase traffic density, ensure conflict avoidance with
free-flight, and manage self-interference.
IFF over-interrogation also generates excess fruit that not
only jeopardizes IFF operational integrity, but also
increases fratricide risk and produces surveillance gaps. A
secure form of ADS-B that complements IFF and offers a
common set of training, tactics, and procedures with
seamless process for military aviators to transition from
the peace-time civil airspace to the hostile battle space.
With self-reporting reducing interrogation rates, the ADS-
B can efficiently use the signal bandwidth at 1090 MHz to
transmit an increased amount of information about the
vehicle position, velocity, and status. A secure ADS-B like
system of surveillance and air-to-air conflict avoidance can
be can be the system that provides superior IFF
performance into the next century, overcoming the
operational utility shortfalls in the question and answer
component of the IFF system.
ADS-B OVERVIEW
The ADS-B system is strongly positioned in universal
situational awareness. It is based on the concept of aircraft
self-reporting their geodetic location, both in the air and on
the ground. It has been developed and peer reviewed
internationally over the last 20 years. ADS-B builds on
Mode S with requirements established by national
authorities such as the FAA, RTCA, EUROCAE, and
ICAO. RTCA/DO-260A [1] establishes the minimum
operational standards (MOPS) for airborne equipment for
ADS-B utilizing 1090 MHz Mode S Extended Squitter
(1090ES).
Since the goal is to routinely use the ADS-B self reports as
a primary source of aircraft surveillance, the US and other
nations plan to replace the majority of ATC interrogators
with ADS-B ground receiving stations by ~2018. Some
nations plan to bypass upgrades with Mode S interrogators
and go directly from the ATCRBS to ADS-B as the
designated surveillance upgrade for the future.

ADS-B will be required internationally for civil
ATC/ATM. A clear case has been made for the installation
of ADS-B in military platforms [2, 3]. However, ADS-B
dictates that all surveillance be open, and therefore nonsecure.
This is a recognized vulnerability, but civil
authorities accept it in civil environments. Note that
1090ES ADS-B messages are equally important military
surveillance input.
The open nature of the ADS-B data transmission is
incompatible with IFF objectives in a hostile environment.
It would be vulnerable to the classic IFF threats of
spoofing and exploitation. Spoofers could attack the
system by generating false reports containing contrived
position data. Exploiters could listen to the ADS-B reports
to easily track and then advantageously attack the
vulnerable aircraft.
MARK XIIA MODE 5 OVERVIEW
The new Mark XIIA Mode 5 waveform for airborne
Identification Friend or Foe (IFF) offers the first
significant upgrade to airborne cooperative identification
systems since the Mark XII Mode 4 development in the
1960s. The US military is preparing to field this new
international standard which features secure data
transmission in addition to providing high assurance friend
determination. Mode 5 uses minimum shift keying (MSK)
modulation and spread spectrum techniques to realize a
processing gain waveform. The waveform defined by
STANAG 4193 [4] includes error control and methods for
reducing self-interference. Security is provided by a Type
1 algorithm embedded in a programmable crypto engine.
IFF Mode 5 capabilities are defined for four levels. Levels
1 and 2 are well defined and are supported by equipment
being delivered now. Levels 3 and 4 have less definition
and are intended for capability growth.
Data from
GNSS/GPS
& Aircraft
Military Aircraft Transponder
ADS-B Class B
Register Data
Generation
Added for ADS-B
M5 Encryption
& M5L2 MSK
Waveform
Mode S DF-17
PPM Modulated
Waveform
Added for MADS-B
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Mode 5 Level 1 (M5L1) is the synchronous question and
answer mode. It offers significant improvements in secure
friend determination over the legacy Mode 4, and includes
a secure data transmission capability. With M5L1, an IFF
interrogator can simultaneously determine friend status
and securely query altitude, mission code, platform
identification number (PIN), and national origin (NO).
M5L1 introduces the lethal interrogation mode, where a
reply is required from a transponder even when it is in
standby mode.
Mode 5 Level 2 (M5L2) is a new asynchronous mode for
secure self reporting. Report formats include ADS-B like
GPS data in 77 bit tactical data report messages. There are
two squittering (self-reporting) rates, a long term low rate
squitter, similar to the ADS-B squitters, and a short
duration high rate squitter, which can be triggered by a
scanning beam interrogator.
MILITARY SECURE ADS-B
For the military secure ABS-B (MADS-B), we recommend
combining the tracking accuracy and integrity of ADS-B
data with the security of the M5L2 waveform as the
available, secure-interoperable military communications
link. The ADS-B data would be reformatted into a subset
of the 8 unused M5L2 formats. The original formats would
be undisturbed. All of the M5L1 formats would also be
retained for interoperability and lethal use.
Figure 1 shows a simplified block diagram of a MADS-B
capable transponder. The core 1090ES, Mode S Enhanced
Surveillance, modes are current equipment capabilities
realized in the pulse position modulation (PPM) and
demodulation functions in the transponder transmitter and
interrogator receiver.
M5L2 MSK
Demodulation &
M5 Decryption
Tracking & Status
Data over Either Path
Military Aircraft Class A Transponder
or Ground/Surface Class C Receiver
Mode S DF-17
PPM Waveform
Decoding
Figure 1 MADS-B System Components
ADS-B
and
MADS-B
Target Data
to Display
ADS-B Class A
Track & Display
Generation

For ADS-B support, data is accepted from aircraft’s
navigation and global navigation satellite system (GNSS)
source, processed and loaded into the Comm-B Data
Select (BDS) registers. In ADS-B operation, data from
specified registers are sampled at random intervals
(~2/second rate) and used to create the 112 bit downlink
message. For military-secure MADS-B operation, the data
in the 112 bit downlink message would be remapped into
the 77 bit tactical data field payload of the M5L2 message.
That M5L2 message would then be encrypted and
modulated in the same manor as any other M5L2
messages.
On the receiving end, there is a MADS-B/ADS-B Class A
track and report generator. It accepts data from either the
demodulated 1090ES replies or the decrypted,
demodulated, and reformatted data from selected M5L2
messages. The track and report generator function is
essentially independent of the source of the ADS-B data.
With judicious re-formatting of the data from the normal
1090ES payload to the M5L2 payload, MADS-B provides
all of the advantages of the civil ADS-B operational use
and integrity, while providing critical military grade
security.
MADS-B MODE 5 LEVEL 2 FORMATS
Mode 5 provides up to 16 level 2 message formats, 11 of
which are assigned to standard Mode 5 interrogate-reply
functions and 5 are proposed for assignment to MADS-B
functions. The Mode 5 level 2 formats proposed for
MADS-B are shown in Table 1.
Table 1 Mode 5 Level 2 Message Formats 24 - 31
Squitter Transmission
Format Subformat
Used for Report Type
24 None Air Surveillance Airborne Position
25 None Surface/Ground
Surveillance
Surface Position
26 0 Air Surveillance Airborne
Velocity*
1 Platform Type Airborne
& Status Capability**
2 Surface
Capability**
3 Reserved for IFF & Military Status
4-7 Unassigned MADS-B Subformats
27-28 Reserved for MADS-B***
* Required with Format 24.
** Supplements Formats 24 and 25.
*** Formats 29-31 reserved for standard M5L2 future use
The airborne position and the airborne velocity report
types are explored in detail below to show the reformatting
of the ADS-B data into the M5L2 payload constraints.
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AIRBORNE POSITION
Figure 2 illustrates the mapping of the 1090ES, downlink
format (DF) 17 airborne position from the BDS register
(0,5) into the MADS-B M5L2 format 24. Recall that the
payload for the M5L2 is limited to 77 bits and that the long
1090ES formats are 112 bits long.
112 Bits
Mode S DF=17
Airborne Position (BDS 0,5)
5
3
24
5
2
1
12
1
1
17
17
24
DF Code
CA Status
AA Mode S
Address
Type Code
Surv Status
Single Ant
Altitude
Time (T)
CPR Format
CPR
Encoded
Latitude
CPR
Encoded
Longitude
PI
Parity
Identity Code
1
2
3
4
5
Identical
6
5
14 - MIL PIN
19 Address
5 - Nat Origin
4 Add’l Addr
4 Type Code
2 Surv Status
12
1 Time (T)
1 CPR Format
CPR
17 Encoded
Latitude
17
MADS-B
Format 24
Altitude
CPR
Encoded
Longitude
Figure 2 Airborne Position Report
Header
The most critical data which is the essence of the position
report is identically formatted in the MADS-B position
report. The 2 bit surveillance status field (Surv Status)
signals emergency and identity pulse (IP) in four states.
The 12 bit altitude is the GNSS height. The time flag field
signals the synchronization method. The compact position
report (CPR) format flag signals whether the odd or even
CPR data is included in this report. The latitude and
longitude data are CPR encoded.
The Mode S DF and civil aviation (CA) status codes (1)
are not mapped into the MADS-B report. The differences
begin with (2) the 24 bit aircraft address (AA) field being
dropped in favor of the 19 bits of the military platform
identification number (PIN) and national origin (NO),
along with (6) 4 additional bits reserved for military
identifier expansion. The type code (3) is compacted from
5 bits to 4 bits to report all accuracy and integrity levels of
ADS-B airborne position except for one type code. The
single antenna flag (4) is not mapped into the MADS-B
report because military aircraft typically have dual
antennas.
77 Bit Tactical Data Payload

The parity identity code (PI) is dropped because the M5L2
has a CRC integrity check on the data within the
cryptography, and the waveform has Reed-Solomon error
correction coding on the transmitted data.
The surface position report is mapped similarly, with the
movement, heading, and heading status fields being
included in the identically mapped from the DBS register
(0,6) to M5L2 format 25.
AIRBORNE VELOCITY
MADS-B Airborne Velocity Format 26 Subformat 0
information is contained in four different ADS-B velocity
subtype messages. Except for modifications to the Type
Code field, it is completely included in MADS-B Format
26. ADS-B Subtypes 1 and 2 give velocity over ground
for subsonic and supersonic velocities, respectively. Figure
3 illustrates the mapping of the 1090ES, DF 17 airborne
position from the BDS register (0,5) into the MADS-B
M5L2 format 26 for supersonic platforms.
112 Bits
Mode S DF=17
Airborne Velocity (BDS 0,9)
Subtype 1 shown, also 2,3,4
5
3
24
AA Mode S
Address
5 Type Code
3 Subtype
1 Int Ch Flag
1 IFR Flag
13 NACv
1 E/W Dir
10 E/W Velocity
1 N/S Dir
10 N/S Velocity
1 VR Source
1 VR Sign
9 Vert Rate
2 Reserved
1 Diff Baro Sign
7 Geo Baro Diff
24
1
DF Code
CA Status
PI
Parity
Identity Code
Identical
MADS-B
5 Format 26
3 Subformat
14 - MIL PIN
19 Address
5 - Nat Origin
4 Add’l Addr
3 Subtype
1 Int Ch Flag
1 IFR Flag
3 NACv
1 E/W Dir
10 E/W Velocity
1 N/S Dir
10 N/S Velocity
1 VR Source
1 VR Sign
9 Vert Rate
2 Reserved
1 Diff Baro Sign
7 Geo Baro Diff
Figure 3 Airborne Velocity Report
Header
Similar to the airborne position report mapping, the DF
code and CA status (1) are not included in the mapping.
The convention of using the PIN and NO in place of the
AA data are also consistent with the MADS-B position
report. The subformat field uses 3 bits of the tactical data
payload space. The critical 51 bits of ADS-B velocity data
are mapped one for one into the MADS-B payload.
77 Bit Tactical Data Payload
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The subtype defines whether the velocity is available and
whether it is in the normal or supersonic range. The intent
change flag is growth function to signal a planned flight
attitude change. The instrument flight rules (IFR) flag
signals the air navigation capability of the platform. The
navigation accuracy category for velocity (NACv) is a
look-up table for different velocity sources. The East/West
(E/W) and North/South (N/S) velocities and direction flags
provides the amplitude and sign of the velocity vector
components in those two dimensions. The vertical rate
source indicates whether a GNSS or barometric source is
being used. The vertical rate and its sign provide the
component of the velocity vector in a third direction. The
difference from barometric and its sign provide the index
into a look-up table to determine the difference between
GNSS and barometric altitude.
These three formats, airborne position and velocity, and
surface position constitute the core ADS-B position reports
that are supplemented by several status messages that
provide information on the type of aircraft, fixed obstacle,
or airport surface vehicle whose position is being reported.
Because these messages have certain redundant
information and information that only applies to civil
transport aircraft, not military tactical aircraft, the ADS-B
status message data can be compressed into a single
MADS-B message without the loss of essential safety and
tracking information.
These four message formats, three position-velocity and
one status message, may be thought of as level one of
MADS-B in that it provides a secure air-to-air conflict
avoidance and airborne surveillance system that directly
complements ADS-B and supports civil (ADS-B) and
secure military (MADS-B) operations.

Traditional and
M5L1 Q&A
Provides Track History
• Interrogator determines
track from past & current
position reports
M5L2 with GPS
Position Reports
1 2 3
Provides more Accurate
Track History
• Interrogator still tracks
from history data
ADVANTAGES BEYOND SECURITY
Having the ADS-B velocity component advances the
quality of situational awareness in the critical air-to-air
case. Figure 4 illustrates the progression of track quality
improvement as the IFF system evolves from traditional
question and answer operation to a self reporting MADS-B
paradigm.
In the traditional Mode 5 Level 1 (1), a track history is
accumulated from past position reports. Each position
report is estimated by interrogating a target across a
beamwidth and centroiding multiple replies. With the
current Mode 5 Level 2 squittered position reports (2), an
estimate of the target position is self reported and the
tracker has a more accurate track history to extrapolate
future positions. However, the MADS-B squittered reports
(3) include both position and velocity measured by the
reporting platform – minimizing the complexity and
uncertainty in the receiver tracker.
An additional level of total friend situational awareness is
also enabled by the rich data content of the MADS-B
status messages derived from the ADS-B catalog. The selfreporting
form of surveillance building on ADS-B and
Mode S information exchanges also provides a mechanism
for communicating military-only messages that provide
state and status information on the ADS-B/MADS-B
communications channel at 1090 MHz and the
interrogation channel used by civil Mode S and M5L1 at
1030 MHz.
Knowing and mapping the interference levels – power and
message rate information – at the aircraft source can
enable the surveillance network to identify sources of
over-interrogation and jamming, and where and when
Figure 4 Tracking Improvements with MADS-B
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MADS-B Provides
Position & Velocity
Reports + Status Data
Provides Accurate Predicted
Track from Aircraft NAV
System – Best data source!
• Receiver tracker minimal!
alternative methods of friend identification and ATM are
required.
In addition to creating a ‘network view’ of the ATM/IFF
communications channels, there are additional
opportunities to expand MADS-B capabilities to meet a
number of unmet military surveillance and identification
needs using the self-reporting method to minimize selfinterference
and exploit the interoperability advantage
offered within M5L2. These include providing greatly
expanded information on the state and status of each
friendly aircraft and its crew. This is a major leap from the
pre-Mode 5 IFF ID where the result of an interrogation
was simply whether you were a friend and your
approximate location, to MADS-B with extensive
information on aircraft and crew in real-time that would be
universally available to US and Allied militaries.
CONCLUSION
MADS-B is a robust, universal IFF upgrade that tracks the
most significant civil ATC surveillance update in the last
50 years. It provides a low-signal-density solution to the
IFF problem of self-interference losses due to over
interrogation. MADS-B provides for a transition to a
network enabled IFF system to solve the most severe IFF
operational problem of fratricide due to the IFF
transponder being ‘turned off’ because of excessive
interrogation. Performance and integrity are now
monitored largely within the system. Use of the existing
Mark XIIA Mode 5 Level 2 waveform and key ADS-B
tracking system elements assures that MADS-B
performance will remain current with any mandated civil
ADS-B tracking updates without need for additional
military interoperability coordination.

REFERENCES
[1.] RTCA/DO-260A, Minimum Operational Performance
Standards for 1090 MHz Extended Squitter Automatic
Dependent Surveillance – Broadcast (ADS-B) and Traffic
Information Services – Broadcast (TIS-B), April 10, 2003
[2.] Frain, S., Van Sickle, G., “CNS/ATM for Tactical
Military Aircraft”, Digital Avionics Systems Conference,
2003, Indianapolis, IN, pp. 4.D.1-1 – 4.D.1-9.
[3.] Van Sickle, G. The Military Utility of ADS-B,
presented at the Air Traffic Control Radar Beacon System
(ATCRBS), Identification Friend or Foe (IFF), Mark XIIA
System (AIMS) Conference 2008, San Antonio, TX
[4.] STANAG 4193, North Atlantic Treaty Organization
(NATO) NATO Standardization Agency (NSA)
Standardization Agreement (STANAG), Technical
Characteristics of IFF Mk XA and Mk XII, Interrogators
and Transponders - Part V: Technical Description of the
Mk XIIA System, 2007
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