Responsive Environments Group--Projects
2013 - present
Circuit stickers are peel-and-stick electronics for crafting circuits.
Crafttechnology for artistic expression
2012 - present
A GSM payments enabled powerstrip to enable sharing of electricity via informal connections.
Energy Management
2011 - present
A handheld digital milling device for craft and fabrication
Ubiquitous ComputingHuman-Computer InteractionDigital FabricationCraft
2010 - present
Using a game engine to browse and interact with data from densely-deployed sensor/actuator networks
Sensor NetworksDynamic VisualizationCross-Reality
2010 - present
Adaptive Lighting
have constructed a variety of lighting networks to optimize the
electrical and photometric properties of solid-state lighting
while meeting the illumination levels set by the
Sensor Networks
Energy Management
Control & Optimization
Passive Ring-like gestural finger controller for a wearable computer system..
Passive Sensing
Wearable Computing
2009 - 2010
Interactive Story Capture Camera
A robotic camera programmed with
an interactive scenario designed to pull stories from people, captured as structured media streams.
Human-Robot Interaction
Social Computing
Interactive Media
2008 - 2009
Personalized Building Comfort Control
A novel air-conditioning control system, focused around the individual, with body worn sensors determining comfort in real-time.
Multi-modal actuation and sensing allows the building to respond to the user wherever he or she is located.
Issues of arbitration between users and comfort/cost tradeoffs are addressed.
Sensor Networks
Building Energy Systems
Low-Power Circuit Design
Reinforcement Learning
2008 - 2010
Ubiquitous Sensor
multimodal sensor/actuator network installed throughout the
Media Lab to support a variety of projects.
Sensor Networks
Ubiquitous Computing
Pervasive Dynamic Media
2008 - 2010
NONO Badge
are constructing a configurable infrastructure to protect
users' dynamic levels of privacy in a pervasive sensor network.
The work is based around a configurable privacy badge that can
alert the user to the presence of participating sensor
networks, plus emit an RF beacon with which the network can
gauge the level of privacy desired.
Sensor Networks
Ubiquitous Computing
2007 - 2008
The Ubicorder is a mobile, location and orientation
aware device that enables users to browse and interact with
real-time data from sensor networks.
Sensor Networks
Ubiquitous Computing
Mobile Devices
ultradense multimodal sensor network as an electronic sensate
Sensor Networks
Electronic Skin
Tactile sing
2006 - present
sportSemble
high speed wearable wireless sensor network that enables direct
measurement and analysis of the extreme forces that an
athlete's body experiences during activity. These measurements
allow sports medicine doctors to understand, treat and prevent
sports injuries.
Sensor Networks
Biomechanics
Sports Medicine
2006 - 2010
Application of a lab-wide sensor network platform
designed to detect and capture fragmented events of human
behavior that can be collected and sequenced into a cohesive
narrative that conveys a larger overall meaning. This project
also looks at the development of parametric models of narrative
that can be mapped on to sensor-detectable elements of human
Human-Centric Sensing
Video Networks
Phenomenology
Experiential Design
Narratology
2007 - 2008
Interactive music running on a Nokia 800 (phone-like
PDA) exploiting a Pure Data Compiler (PuDAC) and wireless
wearable sensors for mobile interactive music and
exercise-generated media - a glimpse at the future of the
portable music player.
Sensor Networks
Interactive Music
Mobile Devices
2006 - 2008
Micropower sensate tags for supply-chain management
and security applications. These wireless sensor nodes are
low-cost devices that monitor many modalities relevant to
shipping and supply-chain needs. They keep extremely low
standby current, asychronously waking up to exceptional
phenomena with dynamically adjustable thresholds.
Sensor Networks
Power Management
2006 - 2010
Trek style Tricorder that lets you 'See through the walls' by
using multi hop sensor networks. An Augmented Reality platform
for sensor networks.
Sensor Networks
Augmented Reality
2006 - 2010
Dual Reality
Virtual and real worlds that reflect, influence, and
merge into each other by means of deeply embedded
sensor/actuator networks.
Sensor Networks
2005 - 2008
power strip imbued with sensing, computational, and
communication capabilities in order to form the backbone of a
sensor network in domestic and occupational environments.
Sensor Networks
Ubiquitous Computing
2005 - 2007
Development of a low-cost system to track the precise
(cm-scale) 3-D position of large numbers of objects tagged with
passive SAW microwave RF transponders at short-range (3-100m)
and in real time.
Ubiquitous Computing
Mobile Sensor Networks
Localization
2005 - 2008
automated framework for generating a population of classifiers
for power-efficient detection in wearable sensors.
Power Management
Adaptive Systems
Wearable Computing
2003 - 2004
Parasitic Mobility
Research into devices that can use the actuation and
navigational intelligence of hosts present in the
environment.
Power Harvesting
Mobile Sensor Networks
FlexiGesture
sensor-rich adaptive gesture and affordance learning platform
for electronic music control.
Adaptive interfaces
Multi-DOF sensing
Electronic music control
The UberBadge
versatile wearable computer platform in the form of a badge,
featuring three processors, RF and IR communication, built in
microphone, audio output, large LED display and LCD capability,
pager vibrator, large flash memory capacity, and extensive
expansion possibilities. Designed to enable a wide variety of
applications where mobile and social computing intersect.
Wireless Sensor Clusters
Wearable Systems
RF Random Access Integrated
compact RF transceiver card, originally developed for the
UberBadge project, but useful for many different
applications.
Wireless Sensor Clusters
Wearable Systems
Sensate Cooktops
In association with TTT sponsor Schott Glass, this
project explores various techniques for transduction in glass
ceramic cooktops. We have developed a capacitive slider for
sensing continuous fingertip position across the glass, several
techniques of sensing the presence and size of pots above the
glass, haptic response to capacitive button activation,
micropower wakeup of cooktop electronics with quasipassive
sensing, and remote measurement of glass temperature above a
burner via IR and acoustic monitoring.
Sensor Architectures
Interactive Surfaces
The "Trible" (Tactile Reactive Interface Based on
Linked Elements) is an excursive in dense distributed sensing,
computation, and actuation. It has the form factor of a ball,
tiled with a multimodal sensate "skin" consisting of 32
networked elements. Each tile measures pressure at 3 locations,
temperature, sound via a microphone, illumination, and dynamic
tactile stimulation with up to 12 channels of protruding,
touch-sensitive "whiskers". Each tile can also respond with a
small audio speaker, a pager vibrator, and a bright RGB LED.
There is no central processor - each tile talks to its
neighbors through conductors in the frame. The Trible is a
research platform for the application of decentralized control
and distributed estimation to human-computer
interaction.
Sensate Media
Perceptive Objects
Small bottlecap-sized computers, each with two
"thumbtack-like" rear-protruding pins, draw their power from a
layered composite into which they are pushed and communicate
with neighbors via infrared or capacitive coupling. Provisions
are made for simple sensors to be easily added to each pushpin,
enabling the system to serve as a testbed for developing
algorithms for extremely high-density, smart sensor
Sensate Media
Stack Sensor Architecture
compact, configurable wireless sensing system, for which
several sensing boards (e.g., tactile, inertial, sonar, etc.)
have been designed. These boards can be stacked in any order
and configuration atop a main processor/RF board, allowing the
sensor suite to be easily customized. A TDMA polling scheme
enables multiple stacks to be used simultaneously. Although it
has been primarily designed for wearable applications, this
device serves as a general platform for compact multimodal
Sensor Architectures
Wireless Sensor Clusters
Wearable Systems
The Gait Shoe
By measuring many (e.g., 20) different parameters at
each foot using our Sensor Stack, these shoes become a wearable
gait laboratory. Data is analyzed for features indicative of
particular gait problems. This system, in collaboration with
the MGH Biomotion Laboratory, is intended to diagnose gait
conditions and provide real-time corrective feedback to the
Embedded Healthcare
Wireless Sensor Cluster
Phenomenological Model
for Distributed Systems
An sensorcentric object model for distributed
Wireless Sensor Clusters
Sensate Media
Gesture-Sensing Radar
This project explores microwave sensing for detecting
noncontact gesture. Several systems have been developed and
deployed, including very-low powered Doppler motion-sensing
radars with onboard digital feature extraction and modified
swept-Doppler ranging radars.
Sensor Architectures
Perceptive Spaces
collaboration with the University of Limerick a sensate floor
is made from networked sensor tiles, each of which has over a
dozen small pressure sensors connected to an embedded computer.
All tiles are networked when they are snapped together. Local
communication between tiles establishes an ad-hoc network.
Sensor signals are compared across tiles, enable stimuli from
dynamic sources, such as footsteps, to be clustered and
abstracted into a basic set of parameters, which are routed
peer-peer across connected tiles to an external
Sensate Media
Expressive Interfaces
Perceptive Spaces
Precision capacitive sensing is used to determine the
dimension of an actively-controlled nanometer-scale gap. This
project, a collaboration with Alex Slocum's group in MIT's
Mechanical Engineering Department, has many applications in
biochemistry and nanoscale chemistry.
Sensor Architectures
The FindIT
Flashlight
flashlight with an optically coded beam quasi-passively wakes
up active tags sleeping at extremely low power drain. If the
tag's ID matches the interrogating beam's request, a
tag-mounted LED flashes. The compact tags can be mounted on the
edge of removable media, for example, allowing a particular
item on a crowded shelf to be found by merely scanning the
flashlight across. The metaphor of seeking real-world objects
with a flashlight is thus extended into the realm of data.
Micro/Self Power
Perceptive Spaces
Wireless Sensor Clusters
Window Tap
Technology
Knocks and taps across a large sheet of glass are
detected by a set of pickup transducers, then characterized by
their frequency content and located via differential timing.
Essentially any single-paned window can be converted into the
equivalent of a touch screen by this inexpensive technique,
with no hardware mounted outside the glass.
Interactive Surfaces
Perceptive Spaces
Expressive Interfaces
An extremely low cost and
compact wireless motion sensor sends out a brief RF pulse when
jerked. As this device is very inexpensive, it can be given
away as a "ticket" to an event, allowing large audiences to
kinetically interact with responsive media. Feature extraction
algorithms and content mapping strategies to exploit the human
"schooling" and self-organizing reflexes with this system.
Although our research looks at interactive dance as an
application, other uses (e.g., interactive gesture and games at
sports events, conventions, and large concerts, very low-cost,
wireless shock monitoring of packages, etc.) abound.
Wireless Sensor Clusters
Expressive Interfaces
Micro/Self Power
Navigatrics
Swept-RF magnetically coupled resonant tag reader has been
developed that exhibits enhanced stability, high resolution,
and the ability to address up to three reader coils. This
enables an ensemble of tagged objects to be wirelessly tracked
in 3 dimensions. This device has been used in a musical
environment that explores high-level gestural control of
musical structure (e.g., the definition, manipulation, and
overdubbing of musical sequences and arpeggiation) in live
performance.
Expressive Interfaces
Magnetic Tag Tracking
Magic Carpet
An interactive space has been designed that consists
of a carpet atop a mesh of piezoelectric wire (tracking foot
position and dynamic pressure) and a pair of Doppler microwave
motion sensors (to respond to movement of the arms and upper
body). This system has been used for several immersive musical
installations, and is now a permanent installation at the MIT
Perceptive Spaces
Interactive Surfaces
Expressive Interfaces
Self-Powered
Pushbutton
This project has resulted in a batteryless pushbutton
that wirelessly transmits a 12-bit digital code to the vicinity
(e.g., 25-100 feet, depending on the RF environment) when
pressed. This enables digital controls, from light switches to
garage door openers, to be embedded essentially anywhere,
without worrying about battery life or wiring.
Micro/Self Power
Wireless Sensor Clusters
very low cost active magnetic tag tracker has been developed to
track the position and absolute orientation of a PDA around a
set of reader coils. Three such tracking stations have been
built and installed for the "Atmospheres" installation at the
MOMA "Workspheres" design exhibition.
Magnetic Tag Tracking
Expressive Interfaces
Free-gesture musical controllers suffer from the
drawback of having no tactile (and often no visual) reference
for their player, hence they are difficult to competently
master. This project has developed a hybrid musical controller,
consisting of a Theremin-like, free-gesture capacitive hand
sensor in the horizontal axis, punctuated by an array of
vertically-directed rangefinding lasers on mechanical carriages
that can be moved to preassigned positions. The lasers, in
light fog, are seen as references (or frets) - the position of
the hand is also detected along the laser beams, affording
another channel of expressive control.
Expressive Interfaces
project draws on ideas from distributed, embedded, and parallel
computing in order to address the creation and management of
databases composed of large collections of physical objects
(e.g. mini DVs in a rack or books in a library). An IR
interrogation beamed at one object is transferred between
neighboring objects peer-peer, diffusing to all objects in the
environment (e.g., in the bookshelf). The sought-after item
then illuminates an indicator.
Sensate Media
Perceptive Spaces
Micropower Wireless Embedded
project is a design study for a TDMA basestation to be used
with very low power CMOS digital transmitters developed at
MIT's MTL for wireless sensor systems.
Wireless Sensor Clusters
Swept RF Tagging
Several reader systems have been developed to
wirelessly track the position of magnetically-coupled resonant
tags. By making the tag's resonance frequency a function of a
local parameter (e.g., pressure or controller displacement),
the tag can also become a sensor. These systems have been used
to develop several demonstrations of "tangible bits" (where
simple objects become controllers), the most engaging of which
is a multimodal music environment called "Musical
Trinkets."
Magnetic Tag Tracking
Expressive Interfaces
inexpensive scanning laser rangefinder has been developed to
track bare hands against a large interactive display. This
system provides a simple retrofit to any large surface - by
putting the rangefinder in one corner, a large virtual
"touchscreen" is created, with hand tracking insensitive to
background light. This system has been used in public with
several interactive applications ranging from information
browsers to musical interfaces.
Interactive Surfaces
Expressive Interfaces
inexpensive, compact package has been developed to monitor
traffic (magnetically sensing passing cars) and detect local
road conditions (e.g., temperature and moisture). A summary is
wirelessly broadcast to a receiver mounted above the road
within circa 300 meters of the sensor package. As the specified
battery is projected to last for over a decade (depending on
conditions), a network of these low-cost packages can be
embedded into the roadbed, allowing the traffic report to be
generated right in the street.
Wireless Sensor Clusters
Perceptive Spaces
IMU for User Interfaces & Atomic Gesture
simple scripting framework has been created to recognize
combinations of simple "atomic" gesture components in data from
body-mounted sensors. Much as the way in which phonemes combine
to form words, the user is able to specify a script of
sequential microgestures that form a desired macroscopic
gesture. When sensor data makes a good match with a specified
script, the gesture is detected. This algorithm is extremely
efficient and is able to run in real time on a low-end PDA, for
example. A pair of small, wireless, handheld, 6-axis inertial
measurement units (as shown here) were constructed to capture
gesture and test this framework.
Wireless Sensor Clusters
Perceptive Objects
Low-Cost Portable
Telediagnostics
compact suite of medical instruments has been integrated into a
kit with a connected palmtop computer. Software has been
written to guide a medical assistant through the process of
running the needed tests on a patient, collecting the required
data, and transmitting it to a remote medical facility for
diagnosis. This system has been used with the LINCOS
installations to provide medical care in rural Costa
Embedded Healthcare
Wireless Sensor Clusters
Power Harvesting in
Electricity is generated while walking by flexing and
pressing compact piezoelectric elements unobtrusively embedded
into a shoe sole. As a demonstration, a self-powered wireless
digital ID code is transmit directly from the shoe after every
few steps.
Micro/Self Power
Wearable Systems
Expressive Footware
suite of 16 diverse sensor channels is embedded into each of a
pair of shoes. All parameters are wirelessly broadcast directly
from the shoes to a nearby basestation with 50 Hz full-state
updates. A very early example of compact multimodal wireless
sensing, this system has been designed and used for interactive
dance. With so many degrees of freedom measured, even a simple
musical rulebase enables a dancer to produce an engaging sonic
accompaniment.
Wireless Sensor Clusters
Expressive Interfaces
Wearable Systems
wideband, dispersionless monopulse acoustic receiver can
theoretically be constructed with continuously-tapered,
co-located subapertures. Such a device would have the
capability of locating the direction of arrival of sonic
transients by calculating a simple sum and ratio. This project
has built and characterized such a device.
Sensor Architectures
Interactive
large mylar balloon has been bonded to a piezoelectric foil
sheet so it can act as both a microphone and a loudspeaker. A
set of electronics has been designed for the balloon to detect
significant audio activity, and play a selection from a set of
prerecorded audio samples when the activity ceases,essentially
enabling one to have a "dialog" with the balloon. Over 50 such
balloon systems were produced and installed all over the
Weisner Building for the Media Lab's 10'th anniversary
celebration.
Perceptive Objects
Sensor Architectures
The ScanFish
16-element array of 4 x 4 programmable electrodes was built in
order to study electric field imaging in air. Each electrode
could dynamically be assigned to either transmit or receive.
Sensor Architectures
Brain Opera Technology
different musical controllers (ranging from digital batons to
sensor chairs) were designed and constructed for the "Brain
Opera", a very large interactive musical installation that was
produced at the MIT Media Lab and toured throughout the
Expressive InterfacesFrom Wikipedia, the free encyclopedia
This article needs additional citations for . Please help
by . Unsourced material may be challenged and removed. (December 2007)
Secondary surveillance radar (SSR) is a
system used in
(ATC), that not only detects and measures the position of aircraft i.e. range and bearing, but also requests additional information from the aircraft itself such as its identity and altitude. Unlike primary radar systems that measure only the range and bearing of targets by detecting reflected radio signals, SSR relies on targets equipped with a radar , that replies to each interrogation signal by transmitting a response containing encoded data. SSR is based on the military
(IFF) technology originally developed during , therefore the two systems are still compatible. Monopulse secondary surveillance radar (MSSR), ,
are similar modern methods of secondary surveillance.
The rapid wartime development of radar had obvious applications for
(ATC) as a means of providing continuous surveillance of air traffic disposition. Precise knowledge of the positions of aircraft would permit a reduction in the normal procedural separation standards, which in turn promised considerable increases in the efficiency of the airways system. This type of radar (now called a primary radar) can detect and report the position of anything that reflects its transmitted radio signals including, depending on its design, aircraft, birds, weather and land features. For air traffic control purposes this is both an advantage and a disadvantage. Its targets do not have to co-operate, they only have to be within its coverage and be able to reflect radio waves, but it only indicates the position of the targets, it does not identify them. When primary radar was the only type of radar available, the correlation of individual radar returns with specific aircraft typically was achieved by the controller observing a directed turn by the aircraft. Primary radar is still used by ATC today as a backup/complementary system to secondary radar, although its coverage and information is more limited.
Installation of mode S antenna on top of existing primary antenna
The need to be able to identify aircraft more easily and reliably led to another wartime radar development, the
(IFF) system, which had been created as a means of positively identifying friendly aircraft from enemy. This system, which became known in civil use as secondary surveillance radar (SSR), or in the USA as the
(ATCRBS), relies on a piece of equipment aboard the aircraft known as a "." The transponder is a radio receiver and transmitter pair which receives on ;MHz and transmits on ;MHz. The target aircraft transponder replies to signals from an interrogator (usually, but not necessarily, a ground station co-located with a primary radar) by transmitting a coded reply signal containing the requested information.
Independent secondary surveillance radar (ISSR), designation YMT, north of Chibougamau, Quebec, Canada
Both the civilian SSR and the military IFF have become much more complex than their war-time ancestors, but remain compatible with each other, not least to allow military aircraft to operate in civil airspace. Today's SSR can provide much more detailed information, for example, the aircraft altitude, as well as enabling the direct exchange of data between aircraft for collision avoidance. Most SSR systems rely on
transponders, which report the aircraft . On the ground, the pressure altitude is adjusted, based on local air pressure readings, to calculate the true altitude of the aircraft. Inside the aircraft, pilots use a similar procedure, by adjusting their altimeter settings with respect to the local air pressure. Pilots may obtain the local air pressure information from air traffic control or from the
(ATIS). If the transponder is faulty, it may report the wrong pressure altitude for the aircraft. This has led to accidents, such as the case of .
Given its primary military role of reliably identifying friends, IFF has much more secure (encrypted) messages to prevent "spoofing" by the enemy, and is used on many types of military platforms including air, sea and land vehicles.[]
(ICAO) is a branch of the United Nations and its headquarters are in , Canada. It publishes annexes to the Convention and Annex 10 addresses Standards and Recommended Practices for Aeronautical Telecommunications. The objective is to ensure that aircraft crossing international boundaries are compatible with the Air Traffic Control systems in all countries that may be visited. Volume III, Part 1 is concerned with digital data communication systems including the data link functions of Mode S while volume IV defines its operation and signals in space.
The American
(RTCA) and the European Organization for Civil Aviation Equipment (Eurocae) produce Minimum Operational Performance Standards for both ground and airborne equipment in accordance with the standards specified in ICAO Annex 10. Both organisations frequently work together and produce common documents.[]
(Aeronautical Radio, Incorporated) is an airline run organisation concerned with the form, fit and function of equipment carried in aircraft. Its main purpose is to ensure competition between manufacturers by specifying the size, power requirements, interfaces and performance of equipment to be located in the equipment bay of the aircraft.[]
The purpose of SSR is to improve the ability to detect and identify aircraft while automatically providing the
(pressure altitude) of an aircraft. An SSR ground station transmits interrogation pulses on ;MHz (continuously in Modes A, C and selectively, in Mode S) as its antenna rotates, or is electronically scanned, in space. An aircraft
within line-of-sight range 'listens' for the SSR interrogation signal and transmits a reply on ;MHz that provides aircraft information. The reply sent depends on the interrogation mode. The aircraft is displayed as a tagged
on the controller's radar screen at the measured bearing and range. An aircraft without an operating transponder still may be observed by primary radar, but would be displayed to the controller without the benefit of SSR derived data. It is typically a requirement to have a working transponder in order to fly in controlled air space and many aircraft have a back-up transponder to ensure that condition is met.
There are several modes of interrogation, each indicated by the difference in spacing between two transmitter pulses, known as P1 and P3. Each mode produces a different response from the aircraft. A third pulse, P2, is for side lobe suppression and is described later. Not included are additional military (or IFF) modes, which are described in .
Mode A and C interrogation format
P1–P3 Pulse spacing
17 us
21 us
25 us
3.5 us
multipurpose
Sum and Control antenna beams
A mode-A interrogation elicits a 12-pulse reply, indicating an identity number associated with that aircraft. The 12 data pulses are bracketed by two framing pulses, F1 and F2. The X pulse is not used. A mode-C interrogation produces an 11-pulse response (pulse D1 is not used), indicating aircraft altitude as indicated by its altimeter in 100-foot increments. Mode B gave a similar response to mode A and was at one time used in Australia. Mode D has never been used operationally.[]
The new mode, Mode S, has different interrogation characteristics. It comprises pulses P1 and P2 from the antenna main beam to ensure that Mode-A and Mode-C transponders do not reply, followed by a long phase-modulated pulse.
The ground antenna is highly directional but cannot be designed without sidelobes. Aircraft could also detect interrogations coming from these sidelobes and reply appropriately. However these replies can not be differentiated from the intended replies from the main beam and can give rise to a false aircraft indication at an erroneous bearing. To overcome this problem the ground antenna is provided with a second, mainly omni-directional, beam with a gain which exceeds that of the sidelobes but not that of the main beam. A third pulse, P2, is transmitted from this second beam 2 us after P1. An aircraft detecting P2 stronger than P1 (therefore in the sidelobe and at the incorrect main lobe bearing), does not reply.
A number of problems are described in an ICAO publication of 1983 entitled Secondary Surveillance Radar Mode S Advisory Circular.
Mode A and C reply format
Although 4,096 different identity codes available in a mode A reply may seem enough, once particular codes have been reserved for emergency and other purposes, the number is significantly reduced. Ideally an aircraft would keep the same code from take-off until landing even when crossing international boundaries, as it is used at the air traffic control centre to display the aircraft's callsign using a process known as code/callsign conversion. Clearly the same mode A code should not be given to two aircraft at the same time as the controller on the ground could be given the wrong callsign with which to communicate with the aircraft.
The mode C reply provides height increments of 100 feet, which was initially adequate for monitoring aircraft separated by at least 1000 feet. However, as airspace became increasingly congested, it became important to monitor whether aircraft were not moving out of their assigned flight level. A slight change of a few feet could cross a threshold and be indicated as the next increment up and a change of 100 feet. Smaller increments were desirable.[]
Since all aircraft reply on the same frequency of ;MHz, a ground station will also receive aircraft replies originating from responses to other ground stations. These unwanted replies are known as FRUIT (False Replies Unsynchronized with Interrogator Transmissions or alternatively False Replies Unsynchronized In Time). Several successive fruit replies could combine and appear to indicate an aircraft which does not exist. As air transport expands and more aircraft occupy the airspace, the amount of fruit generated will also increase.
Fruit replies can overlap with wanted replies at a ground receiver, thus causing errors in extracting the included data. A solution is to increase the interrogation rate so as to receive more replies, in the hope that some would be clear of interference. The process is self-defeating as increasing the reply rate only increases the interference to other users and vice versa.
If two aircraft paths cross within about two miles slant range from the ground interrogator, their replies will overlap and the interference caused will make their detection difficult. Typically the controller will lose the longer range, and later to reply, aircraft just when the former may be most interested in monitoring them closely.
While an aircraft is replying to one ground interrogation it is unable to respond to another interrogation, reducing detection efficiency. For a Mode A or C interrogation the transponder reply may take up to 120 us before it can reply to a further interrogation.
Original SSR antenna providing a narrow horizontal beam and a wide vertical beam
Regions of weak signal due to ground reflection
The ground antenna has a typical horizontal 3 dB beamwidth of 2.5° which limits the accuracy in determining the bearing of the aircraft. Accuracy can be improved by making many interrogations as the antenna beam scans an aircraft and a better estimate can be obtained by noting where the replies started and where stopped and taking the centre of the replies as the direction of the aircraft. This is known as a sliding window process.
The early system used an antenna known as a hogtrough. This has a large horizontal dimension to produce a narrow horizontal beam and a small vertical dimension to provide cover from close to the horizon to nearly overhead. There were two problems with this antenna. First, nearly half the energy is directed at the ground where it is reflected back up, and interferes with, the upward energy causing deep nulls at certain elevation angles and loss of contact with aircraft. Second, if the surrounding ground is sloping, then the reflected energy is partly offset horizontally, distorting the beam shape and the indicated bearing of the aircraft. This was particularly important in a monopulse system with its much improved bearing measurement accuracy.
The deficiencies in modes A and C were recognised quite early in the use of SSR and in 1967 Ullyatt published a paper and in 1969 an expanded paper, which proposed improvements to SSR to address the problems. The essence of the proposals was new interrogation and reply formats. Aircraft identity and altitude were to be included in the one reply so collation of the two data items would not be needed. To protect against errors a simple parity system was proposed – see Secondary Surveillance Radar – Today and Tomorrow. Monopulse would be used to determine the bearing of the aircraft thereby reducing to one the number of interrogations/replies per aircraft on each scan of the antenna. Further each interrogation would be preceded by main beam pulses P1 and P2 separated by 2 us so that transponders operating on modes A and C would take it as coming from the antenna sidelobe and not reply and not cause unnecessary fruit.
The FAA were also considering similar problems but were assuming that a new pair of frequencies would be required. Ullyatt showed that the existing ;MHz and ;MHz frequencies could be retained and the existing ground interrogators and airbornes transponders, with suitable modifications, could be used. The result was a Memorandum of Understanding between the US and the UK to develop a common system. In the US the programme was called DABS (Discrete Address Beacon System), and in the UK Adsel (Address selective).
Monopulse, which means single pulse, had been used in military track-and-follow systems whereby the antenna was steered to follow a particular target by keeping the target in the centre of the beam. Ullyatt proposed the use of a continuously rotating beam with bearing measurement made wherever the pulse may arrive in the beam.
The FAA engaged Lincoln Laboratory of MIT to further design the system and it produced a series of ATC Reports defining all aspects of the new joint development. Notable additions to the concept proposed by Ullyatt was the use of a more powerful 24-bit parity system using a , which not only ensured the accuracy of the received data without the need for repetition but also allowed errors caused by an overlapping fruit reply to be corrected. Further the proposed aircraft identity code also comprised 24 bits with 16 million permutations. This allowed each aircraft to be wired with its own unique address. Blocks of addresses are allocated to different countries and further allocated to particular airlines so that knowledge of the address could identify a particular aircraft. The Lincoln Laboratory report ATC 42 entitled Mode S Beacon System: Functional Description gave details on the proposed new system.
The two countries reported the results of their development in a joint paper, ADSEL/DABS – A Selective Address Secondary Surveillance Radar. This was followed at a conference at ICAO Headquarters in Montreal, at which a low-power interrogation constructed by Lincoln Laboratory successfully communicated with an upgraded commercial SSR transponder of UK manufacture.[]
Comparison of the vertical beam shapes of the old and new antennas
The only thing needed was an international name. Much had been made of the proposed new features but the existing ground SSR interrogators would still be used, albeit with modification, and the existing airbound transponders, again with modification. The best way of showing that this was an evolution not a revolution was to still call it SSR but with a new mode letter. Mode S was the obvious choice, with the S standing for select. In 1983 ICAO issued an advisory circular, which described the new system.
The problem with the existing standard "hogtrough" antenna was caused by the energy radiated toward the ground, which was reflected up and interfered with the upwards directed energy. The answer was to shape the vertical beam. This necessitated a vertical array of dipoles suitably fed to produce the desired shape. A five-foot vertical dimension was found to be optimum and this has become the international standard.
Antenna main beam with difference beam
The new Mode S system was intended to operate with just a single reply from an aircraft, a system known as monopulse. The accompanying diagram shows a conventional main or "sum" beam of an SSR antenna to which has been added a "difference" beam. To produce the sum beam the signal is distributed horizontally across the antenna aperture. This feed system is divided into two equal halves and the two parts summed again to produce the original sum beam. However the two halves are also subtracted to produce a difference output. A signal arriving exactly normal, or boresight, to the antenna will produce a maximum output in the sum beam but a zero signal in the difference beam. Away from boresight the signal in the sum beam will be less but there will be a non-zero signal in the difference beam. The angle of arrival of the signal can be determined by measuring the ratio of the signals between the sum and difference beams. The ambiguity about boresight can be resolved as there is a 180° phase change in the difference signal either side of boresight. Bearing measurements can be made on a single pulse, hence monopulse, but accuracy can be improved by averaging measurements made on several or all of the pulses received in a reply from an aircraft. A monopulse receiver was developed early in the UK Adsel programme and this design is still used widely today. Mode S reply pulses are deliberately designed to be similar to mode A and C replies so the same receiver can be used to provide improved bearing measurement for the SSR mode A and C system with the advantage that the interrogation rate can be substantially reduced thereby reducing the interference caused to other users of the system.
Lincoln Laboratory exploited the availability of a separate bearing measurement on each reply pulse to overcome some of the problems of garble whereby two replies overlap making associating the pulses with the two replies. Since each pulse is separately labelled with direction this information can be used to unscramble two overlapping mode A or C replies. The process is presented in ATC-65 "The ATCRBS Mode of DABS". The approach can be taken further by also measuring the strength of each reply pulse and using that as a discriminate as well. The following table compares the performance of conventional SSR, monopulse SSR (MSSR) and Mode S.
Standard SSR
Monopulse SSR
Replies per scan
Range accuracy
230 m rms
13 m rms
7 m rms
Bearing accuracy
0.08° rms
0.04° rms
0.04° rms
Height resolution
100 ft (30 m)
100 ft
25 ft (7.6 m)
Garble resistance
Data capacity (uplink)
56–1,280 bits
Data capacity (downlink)
56–1,280 bits
Identity permutations
16 million
The MSSR replaced most of the existing SSRs by the 1990s and its accuracy provided for a reduction of separation minima in en-route
from 10 nautical miles (19  12 mi) to 5 nautical miles (9.3  5.8 mi)
MSSR resolved many of the system problems of SSR, as changes to the ground system only, were required. The existing transponders installed in aircraft were unaffected. It undoubtedly resulted in the delay of Mode S.
Mode S interrogation, short and long
Mode S reply, short and long
A more detailed description of Mode S is given in the Eurocontrol publication Principles of Mode S and Interrogator Codes and the ICAO circular 174-AN/110 Secondary Surveillance Radar Mode S Advisory Circular. The 16 million permutations of the 24 bit aircraft address codes have been allocated in blocks to individual states and the assignment is given in ICAO Annex 10, Volume III, Chapter 9.
A mode S interrogation comprises two 0.8 us wide pulses, which are interpreted by a mode A & C transponder as coming from an antenna sidelobe and therefore a reply is not required. The following long P6 pulse is phase modulated with the first phase reversal, after 1.25 us, synchronising the transponder's phase detector. Subsequent phase reversals indicate a data bit of 1, with no phase reversal indicating a bit of value 0. This form of modulation provides some resistance to corruption by a chance overlapping pulse from another ground interrogator. The interrogation may be short with P6 = 16.125 us, mainly used to obtain a position update, or long, P6 = 30.25 us, if an additional 56 data bits are included. The final 24 bits contain both the parity and address of the aircraft. On receiving an interrogation, an aircraft will decode the data and calculate the parity. If the remainder is not the address of the aircraft then either the interrogation was not intended for it or it was corrupted. In either case it will not reply. If the ground station was expecting a reply and did not receive one then it will re-interrogate.
The aircraft reply consists of a preamble of four pulses spaced so that they cannot be erroneously formed from overlapping mode A or C replies. The remaining pulses contain data using . Each 1 us interval is divided into two parts. If a 0.5 us pulse occupies the first half and there is no pulse in the second half then a binary 1 is indicated. If it is the other way round then it represents a binary 0. In effect the data is transmitted twice, the second time in inverted form. This format is very resistant to error due to a garbling reply from another aircraft. To cause a hard error one pulse has to be cancelled and a second pulse inserted in the other half of the bit period. Much more likely is that both halves are confused and the decoded bit is flagged as "low confidence".
The reply also has parity and address in the final 24 bits. The ground station tracks the aircraft and uses the predicted position to indicate the range and bearing of the aircraft so it can interrogate again and get an update of its position. If it is expecting a reply and if it receives one then it checks the remainder from the parity check against the address of the expected aircraft. If it is not the same then either it is the wrong aircraft and a re-interrogation is necessary, or the reply has been corrupted by interference by being garbled by another reply. The parity system has the power to correct errors as long as they do not exceed 24 us, which embraces the duration of a mode A or C reply, the most expected source of interference in the early days of Mode S. The pulses in the reply have individual monopulse angle measurements available, and in some implementations also signal strength measurements, which can indicate bits that are inconsistent with the majority of the other bits, thereby indicating possible corruption. A test is made by inverting the state of some or all of these bits (a 0 changed to a 1 or vice versa) and if the parity check now succeeds the changes are made permanent and the reply accepted. If it fails then a re-interrogation is required.
Mode S operates on the principle that interrogations are directed to a specific aircraft using that aircraft's unique address. This results in a single reply with aircraft range determined by the time taken to receive the reply and monopulse providing an accurate bearing measurement. In order to interrogate an aircraft its address must be known. To meet this requirement the ground interrogator also broadcasts All-Call interrogations, which are in two forms.
Mode A/C/S All-Call interrogation
In one form, the Mode A/C/S All-Call looks like a conventional Mode A or C interrogation at first and a transponder will start the reply process on receipt of pulse P3. However a Mode S transponder will abort this procedure upon the detection of pulse P4, and instead respond with a short Mode S reply containing its 24 bit address. This form of All-Call interrogation is now not much used as it will continue to obtain replies from aircraft already known and give rise to unnecessary interference. The alternative form of All-Call uses short Mode S interrogation with a 16.125 us data block. This can include an indication of the interrogator transmitting the All-Call with the request that if the aircraft has already replied to this interrogator then do not reply again as aircraft is already known and a reply unnecessary.
The Mode S interrogation can take three forms:
Surveillance
position update
contains 56 data bits
up to 16 long interrogations strung together to transmit up to 1280 bits
The first five bits, known as the uplink field (UF) in the data block indicate the type of interrogation. The final 24 bits in each case is combined aircraft address and parity. Not all permutations have yet been allocated but those that have are shown:
application
short air-air surveillance (TCAS)
surveillance, altitude request
surveillance, Mode A identity request}
Mode S only All-Call
long air-air surveillance (TCAS)
Comm-A including altitude request
Comm-A including Mode A identity request
Comm-C (extended length message)
Similarly the Mode S reply can take three forms:
Surveillance
position update
contains 56 data bits
up to 16 long interrogations strung together to transmit up to 1280 bits
The first five bits, known as the downlink field (DF) in the data block indicate the type of reply. The final 24 bits in each case is combined aircraft address and parity. Eleven permutations have been allocated.
DF decimal
application
short air-air surveillance (TCAS)
surveillance, altitude reply
surveillance, Mode A identity reply
All-Call reply containing aircraft address
long air-air surveillance (TCAS)
extended squitter
military extended squitter
Comm-B including altitude reply
Comm-B reply including Mode A identity
military use
up to 16 long replies strung together to transmit up to 1280 bits
A transponder equipped to transmit Comm-B replies is fitted with 256 data registers each of 56 bits. The contents of these registers are filled and maintained from on-board data sources. If the ground system requires this data then it requests it by a Surveillance or Comm-A interrogation.
ICAO Annex 10 Volume III, Chapter 5 lists the contents of all those currently allocated. A reduced number are required for current operational use. Other registers are intended for use with TCAS and ADS-B. The Comm-B Data Selector (BDS) numbers are in hexadecimal notation.
magnetic heading
indicated airspeed
Mach number
vertical rate
roll angle
track angle rate
true track angle
ground speed
selected vertical intent
Starting in 2009, the ICAO defined an "extended " it supplements the requirements contained in ICAO Annex 10, Volumes III and IV. The first edition specified earlier versions of extended squitter messages:
Extends Mode S to deal with basic ADS-B exchanges, to add
(TIS-B) format information, as well as uplink and downlink broadcast
Better describes surveillance accuracy and integrity information (navigation accuracy category, navigation integrity category, surveillance integrity level), and additional parameters for TIS-B and
rebroadcast (ADS-R).
The second edition introduced yet a new version of extended squitter formats and protocols to:
enhance integrity and accuracy reporting
add a number of additional parameters to support identified operational needs for the use of
not covered by Version 1 (including capabilities to support airport surface applications)
modify several parameters, and remove a number of parameters, which are no longer required to support ADS-B applications
, all encompassing description
, free flight enhancement
Secondary Surveillance Radar, Stevens M.C. Artech House,
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Illman, Paul E. (1998). The pilot's radio communications handbook (Fifth Edition, Paperback). McGraw-Hill. p. 111.  .
() hosted at
The original Spanish report, available via mail, is by the Accident Investigation Board, Directorate General of Air Transport, .
ICAO Annex 10, Volume IV
ICAO Circular 174-AN/110 Secondary Surveillance Radar Mode S Advisory Circular
Stevens, M.C. "Multipath and interference effects in secondary surveillance radar systems", Proc. Inst.Electr. Eng., Part F, 128(1), 43–53, 1981
Ullyatt, C. Secondary radar in the era of auto-tracking, IEE Comf. Pub., 28, 140, 1967
Ullyatt, C. Sensors for the ATC environment with special reference to SSR, Electron. Civil Aviat., 3, C1–C3, 1969
Stevens, M. C., Secondary Surveillance Radar – Today and Tomorrow, SERT Avionics Symposium, Swansea, July 1974.
Bowes R.C., Drouilhet P.R., Weiss H.G. and Stevens M.C., ADSEL/DABS – A Selective Address Secondary Surveillance Radar,AGARD Conference Proceedings No. 188. 20th Symposium of the Guidance and Control Panel held in Cambridge, Massachusetts, USA, 20–23 May 1975
Stevens, M.C. Precision secondary radar, Proc. Inst. Electr. Eng., 118(12), , 1971
: Mode S Today, Chang E., Hu R., Lai D., Li R., Scott Q., Tyan T., December 2000
ICAO Annex 10 (PDF). Volume III, chap. 9. ICAO . Archived from
(PDF) on .
Orlando V.A., Drouilhet P.R. (August 1986).
(PDF). Lincoln Laboratory 2014.
Stevens, M.C. Surveillance in the Mode S Era, CAA/IEE Symposium on ATC, London. March 1990
Gertz J. L. (January 1977).
(PDF). Lincoln Laboratory (MIT) 2014.
FAA (2004). . DIANE Publishing Company.  .
Manual on Mode S Specific Services, Panel Working Group B Surveillance and Conflict Resolution Systems, September 2001
Carriage of SSR Mode S Transponders for IFR Flights Operating as General Traffic, www.caa.co.uk/docs/810/
ICAO (2008).
(1 ed.). International Civil Aviation Organization.  .
ICAO (2012).
(2 ed.). International Civil Aviation Organization.  .
Industry specifications
; 4th E ICAO; 280 2007.
; Rev E; RTCA; 2011.
a 1961 Flight article on SSR
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