Data Transmission between Planes and Control Centers

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Summary The purpose of these notes is to give or to recall some major facts about the precarity of the exchange of data betwee~ plane and ground centers. There are no technical gaps for the implementation of automattc data links. Only procedural problems are still pending. It is mainly a political problem. I wish that at the end of this "case study" firm recommendations could be stated. TMA-2, CRT, ATZ ... ) will be (or already is) divided il controlled space classes -there will be 7 clases- with specific rules in each of these classes. Each geographical space wil be qualified by one letter A to G.

1. STATEMENT OF THE PROBLEM 1.1 . How to manage air traffic for all kinds of air vehicles with a high degree of safety and without too many constraints as to the knowledge imposed to the pilots.

1.2. Definitions

The classes are defined as follow:

VFR stands for Visual Right Rules IRF stands for Instrument Flying Rules

Class A controlled airspace Airspace in within IFR but not VFR flights are permitted. In this airspace the ATC units are responsible for IFR flight spacing.

This implies a distinction concerning the type of meteorological conditions. They are equally divided in 2 types: VMC Visual Meteorological Conditions -see table 1IMC Instrument Meteorological Conditions.

Class B controlled airspace Airspace within which both IFR and VFR flights are permitted. In this airspace the control units are responsible for IFR, IFR/VFR and VFR flight spacing.

Landings are classified in precision approach which implies that an accurate guidance is provided by ILS, MLS or PAR equipements (2) and non precision approach for all the other types of approaches (3).

Class C controlled airspace

Prior to departure, a "flight plan" (FPL) which describes the profile of the trajectory (plus additional information concerning the plane) is sent to the ATC (Air Traffic Control) for approval. FPL are not mandatory for VFR flights, but recommended.

Airspace within which both IFR and VFR flights are permitted. In this airspace the ATC units are responsible for IFR and IFR/YFR flight spacing as well as the provision to VFR flights or traffic data on other VFR flights.

1.3. Controlled airspace

Class D controlled airspace

The controlled airspace as it appears today , i.e. with a qualification attached to the geographic situation (Airways, Terminal Areas with a subset of spaces such as TMA-I

Airspace within which both IFR and VFR flights are permitted. In this airspace the ATC units are responsible for IFR flights spacing and the provision to IFR flights of VFR traffic data as well as the provision to VFR flights or IFR and other VFR traffic data.

(I) large parts of this text have been extracted from the

following articles to be published soon in Concise Encyclopedia published by Pergamon : - Trends in ATC - VFR-IFR flights - Data links in aeronautics This text also takes into account the conclusion of an fFAC/Working Group, on the same subject, which was implemented in 1988.

Class E controlled airspace Airspace of defined dimensions intended for the performance of specific defence activities and the control of military (OAT) fights requiring special operational and technical procedures.

(2) ILS Instrument Landing System MLS Microwave Landing System PAR Precision Approach Radar (a variety of GCA : Ground Control Approach)

In this airspace the general air traffic services provided to GAT flights are identical to those provided in Class A, B, C, D and E controlled airspaces as the case may be. Clearances are issued by the military A TC units in the light of specific constraints connected with defence activities.

(3) Non precision approach does not mean that the accuracy of the landing (touch down point on the runway) is degraded or could be degraded; very often non precision approaches may require high accuracy in the touch down point and alignement because, if the runway is not equipped with radioaids, there is high probability that it is also a shorter and narrow runway!

N.B. Except where otherwise stated, the term "controlled airspace" used in this context embraces military controlled airspace also. 41

Airspace within which both IFR and VFR flights are permined. In this airspace the ATC units provide advisory service.

- data transmission from A TC to planes (and back) and from planes to Companies Technical Centers (and back) and from Meteo to planes (and back, in order that planes following the same routes may benefit from the experience gained by planes which are ahead).

Class G uncontrolled airspace

- compatibility with modes A and C.

Airspace within which both IFR and VFR flights are permined. In this airspace the A TC units provide flight information service and alerting service only.

The solution is :

Class F uncontrolled airspace (advisory airspace)

2. THE PRESENT STATE (1989) AND THE NEXT FUTURE

2.1. VHF and Radar communications All data concerning the navigation of the plane in IFR conditions, from the time the plane is ready to have the engines started to the time when the plane has reached its parking space at the destination are transmitted throught the VHF. We will comment on that type of transmission in § 3. Radio navigation is provided by various means , summarized in Table 2. Each mean, let say a VOR beacon, is identified by its frequency and an identification code, in that case a 2 or 3 letters, regularly transmitted (every minute) by their Morse code. This is a kind of data transmission. Transponders are radar receivers on a prescribed frequency, the one of the "secondary radars ". These radars cover the upper levels (roughly above 10 or 15000 feet) in developed countries. When a plane receives the signals, it answers it by transmitting a signal which is rigourously delayed with regard to the incoming signal -in order to give a precise measurement of the distance to the ground site- and which contains data :

- a unique address, once for all, for each plane : a 24 bit messages allow 16 millions of addresses - data messages are 56 or 112 bits long, with a possibility of grouping messages up to 1280 bits (ground to air). - rate: 4 Mb/s (ground to air) I Mb/s (air to ground). - as to the threat avoidance collision system (TCAS) it will be possible to use the on-board transponder, Mode C or S (if available). Each plane equipped with such a transponder periodically broadcasts interrogation codes -the range is about 20 NM. It collects the responses distance (accurate data), bearing (non accurate) altitude (very accurate). An on-board computer detects and propose evasive menoeuvers to avoid collision -this is a vertical manoeuver which is radioed to the other plane and to ATC (ex. for a altitude difference of 750 feet a 20 s warning may be reached). Note : this sytem works only when potentially colliding planes are equipped with Mode S transponders and, at least one, with the interrogator possibility, plus the on-board computer. In 1989 the experience gained with these equipments leads to: I I I I

warning for each 3 hour flight period conflit resolution for 50 hours flight period "air-miss" for 100 000 hour flight period collision for IOexp6 hour flight period.

2.2. Naystar/Glonass. - in "Mode A " a code (4 digits 0 to 7) identifies the plane: this code is given through the VHF communication channels prior to take-off by the "Tower", or by the "En Route Control" or "Approach Centers", when it is necessary to use a code. Then at the radar site, the identity of the plane and the distance appear, the bearing and the ground speed are derived from these data by a computer on the radar site. - in "Mode C" the coded flight level (FL) of the plane is retransmitted. It is recalled that FL is the pseudo altitude, expressed in hundred of feet, read on an altimeter set at 1013.5 HPa whatever be the present barometric reference pressure (Ex. FL 290 means that an altimeter set at 1013.5 HPa, indicates 29.000 feet).

It is worthwhile noting that worlwide communications sytems do not provide voice or data transmission. Both systems are not widely developped and most commercial long range planes rely in INS for navigation. In some countries (US, Europe and Japan namely), communications satellites are studied to provide worldwide voice and data communications (see § 4).

3. DEFICIENCIES OF THE PRESENT SITUATION

3.1. Quality of the transmission channels Due to the fact that message transmitted by the on-board transponder comprises only 12 bits (4096 possibilities) it is impossible to transmit both the identification of the plane and its FL. Messages are interlaced, twice "identification " (Mode A) and once "Right level (Mode C) if the transponder has this Mode. Modes A and C are currently used in commercial and general aviation. Secondary Radars are capable of a 3rd mode, that is the Mode S. - in "Mode S" a longer message is available. Let's say for the moment that this mode is not yet in application but will soon be. In dense traffic zones such as in the vicinity (50 to lOO nm) of major airports, saturation has already appeared; the answering message of several aircraft may overlap and consequently are not usable. In "Mode S" system the possibility of selective calls is provided. Each plane may be called when it is necessary.

It is far from a HiFi receiver ! It is surprising that no improvement has been made for decades though the control of internal jammers is well dominated at the present time. Anyway the background acoustical noise in the cockpit is always high and most of the crew members do not wear a headset. They listen through a loudspeaker which normaly is fed by two frequencies (2 VHF receivers) and sometimes by other frequencies coming from NA V receivers (VOR, ADF, ... ).

3.2. Misinterpretation of the instructions According to international rules the language (English, or in some cases, local language) and the "phraseology" are precisely defined. In the real world, large variations from the basic vocabulary, are used and confusion may lead to a catastrophe.

The requirements are : - selective interrogation in addition to general calls as they exist in Mode A and C. A plane can be addressed individualy, thus the supersposition of transponders messages in dense areas is avoided. - to provide some degree of anticollision (see "threat avoidance collision system" below).

Recall: The Tenerife accident -more than 500 dead- was due to errors both from the Control Center and the Crew. The Crew asked "authorization to penetrate and take-off'. The tower answered "OK to penetrate". The crew interpretated as "OK for my request" (penetration and takeoff) ; the tower should have said "Negative, authorization for penetration only". Many examples could be mentioned ...

3.3. Saturation

4.2. Objectiye of CNS-ATM systems

The management of many air vehicles in a given space in which the traffic is concentrated on airways or approach patterns around an airport, is a difficult problem to handle. The human operator behaves better than computers except for very large machines, which are not yet implemented in Air Traffic Control Centers. Besides, the separation of vehicles is under the responsibility of the Controller.

In 1983, it has been recognized by the International Civil Aviation Organization (ICAO) that recommendations for Future Air Navigation Systems (FANS) must be drawn up.

All the constraints make air traffic saturation often appear in peak hour every day in many major airports and often extend to all day during the holiday period. Controllers often are assisted by computers, mainly for conflicts detection. However, an instruction which the controller intends to broadcast is not checked by the computer before its transmission. This proved to be the cause of accidents (Zagreb, 1976). 3.4. H.F. communication over oceans or impopulated areas When line of sight communications are not possible, the use of HF frequencies is mandatory . It is well known that the propagation of such long waves is subject to fading and other disturbances. 4. THE CNS-A TM CONCEPT 4.1. NA VSTAR and GLONASS systems Let us recall the characteristics of these two navigation systems. NAVSTAR : it will comprise 21 satellites (18 in line, 3 in standby state to replace a failed satellite). The trajectories are circular at an altitude of 17600 km. The orbits are coordinated so that when one satellite sets below the horizon another rises to replace it, and that, from an ay point of the earth. To do so two orbital planes are or will used: 63 0 for the 7 satellites which are present today in the sky and 55 for the I1 other which will be launched at the rate of 6 per year during 1989 and 1990 (3 spares satellites will be also launched). Each satellite keeps continuous track thanks to signal transmitted by 6 tracking ground stations. The computed accuracy, then the 21 satellites will be operational will be of the order of 30 min x and y coordinates (or latitude and longitude) and 40 m for z (altitude above the geoi"d) (4). As previously said only 7 satellites are in operation (January 1989) leading to large uncovered areas on a 24 hour cycle (some highly populated area such as Los Angeles are covered only 7 hour per day). When 4 satellites are on sight, the computed accuracy is reached and no drift are noticed (the atomic clock which are on board the satellites has an accuracy of lOexp-12 per year. Their expected life is 5 years, however some Navstar satellites are still operational after 9 years (First satellite has been launched on February 22, 1978).

The recomendations claims for a fully integrated Communication - Navigation - Surveillance (CNS) and Air Traffic Management (ATM) system. At the present time the European Community (EC) is studying the implementation of such a system; in North America a plan for the development of an Advanced Automation System (AAS) is going on. Technologies available today such as satellite communications, satellite navigation, digital communications and in the very next future, Artificial Intelligence, in particular Expert Systgems, are not yet integrated but there public and commercial use have been largely prooved. The future CNS-ATM system will include public correspondence communications both for passengers and airlines operators. The difficulty comes more from political decision than from technical gaps. The CNS-A TM fully integrated system can be devided in three interelated domains: The communications sub-systems between air mobiles and ground stations. At present time all communications are exchanged on VHF or HF channels from the air mobiles but they can be relayed by satellites channels from ground stations. Ground stations providing various kind of services, en-route traffic, TMA traffic approach, company administration, public telephone service, etc ... must be fully integrated, at least for protocole message exchange policies. The integrated CNS-A TM system will use communications satellites with direct links between air mobiles and satellites. However the secondary radars Mode will also allow communication between air mobiles equipped with Mode S transponders and radar sites, thence with any other ground stations. Messages compatibility is mandatory. It will not seem wish able to abandon Mode S communication link when communication satellites is fully operational because of the general trend to achieve a certain degree of redundancy in critical phases such the approaches in dense traffic areas. 5. CONCLUSION AND OPENING OF THE DISCUSSION According to the Radio Conference for the Mobile Services, Geneva, 1987 the following frequency bands are allocated for public communication with air vehicles: 1545 - 1559 MHz and 1646,5 - 1160,5 MHz and for terrestrial satellite communications: 1593 - 1594 MHz (ground to air), 1625,5 1626,5 MHz (air to ground). Note: to take into account the Doppler shift due to satellite speed each communication system must have a bandwidth of 6 kHz.

GLONAS. The main characteristics are : Number of satellites Number of orbits Type or orbit Period of rotation Inclination Frequency hand

24 (3 standby)

3 circular /19.100 km 11 hours 15 mn 64.8 0 (1602.5625/1615.5) ± 0,5 MHz

Type of measurements: Non interrogative The accuracy is : Coordinates (x, y) ± lOO m Altitude (z) ± 150 m Velocity components ± 0,15 rn/s These two systems do not provide voice or data communication.

The navigation sub-system. It must be a 4-D Nav (3 dimension plus time position). It may include a combination of on board equipment and ground or satellite equipment. For example the NA VSTAR can be used by itself or can be used to reset an inertial navigation system. It is important to evaluate the maximum localisation error both on-board and, on ground station and in some areas or airways, minimum accuracy equipment are required. This is the case at the present time namely on high density traffic routes on the North Atlantic Ocean. Obviously radar coverage on such routes is not achievable -even in the long future-. (4) these accuracies are those which can be reached all over the world. The US-Defence users may have a more accurate coding system which leads to errors roughly divided by 10 (at least for x and y coordinates).

Several systems exist at the present time but their accuracy is varying according to the region in which the plane navigates and according to random parameters such as radio transmission perturbations . Solar activity, which is impredictable, except on a few-minute basis, may degrade the precision of localisation suddenly. The navigation sub-system will normally receive more than one signal coming from navigation ground-based or satellitebased transmitter. It must derive from the data received the best estimate (Kalman filter) given by each system and then derive the zone in which the vehicle lies with an error smaller than 3 cr.

- will the crew be able to "type" a message on a keyboard, or on an equivalent device, in turbulence zones. - automatic data transmission ground to air may need safe addressing codes so that a plane may capture the information addressed to him and no other (except if there are several destinaries). A look at the Mathematics of Public Key Cryptography, Manin Hellman, Scientific American, Aug. 1979, p. 130 may help to solve this problem. - the choice of the transmitting device must be transparent for the crew, namely when there is duplication between Mode S Radar and satellites. How to solve the problem automatically?

The navigation sub-system must be able to transmit the position (and 3 cr error-zone) computed on board to ground stations, through satellite if necessary. The ground en-route center must corrolate the data received from the mobile to its own estimation and in case of disagreement must automatically dialog with the mobile.

Surveillance sub-system. The European Community (EC) is presently defining the requirements for the surveillance finition of the CNS-A TM fully integrated system. It includes the monitoring and controlling planes throughout the EC airspace in priority but must be capable of extension to oceanic and low density traffic other countries. The development of airborne collision avoidance systems must be included in the study both when used in an autonomous way by aircraft and when used in cooperation with Air Traffic Control Centers. It is obvious that the sub-system surveillance is highly interelated with the communication and navigation subsystems. The combination of these means must lead to the derivation of a positional information which can be assimilated to a pseudo-radar traffic data presentation. A simplify -i.e. local- representation for the air situation may be presented on board. It is clear that the next technical step is not entirely a matter of A TC : it concerns the plane as well. The aeronautical community, including the airline operators, is aware of the present deficiency (the lack of an automatic data link between the plane and the ATC's) and claims for urgent internationally agreeded decisions in order to control an air traffic which is increasing by 6 to 8 % each year. Peak periods correspond to a traffic which can be twice the arrival average traffic. Safety must be maintained or increased, no compromise is acceptable in this matter. Once an automatic data link is provided, the traffic will be controlled much more smoothly for many reasons:

Table 1

The visual meteorological conditions (VMC) are defined like so in France at the present time (1989). Let's first define a "Surface S" : This is the maximum of : 300 m above ground level (AGL) or FL 30 (flight level 30) which means the altitude read on an altimeter which is set at 1013 HPa.

In uncontrolled airspace: Above S the meteorological conditions must be such that : - the horizontal visibility - distance to clouds horizontal vertical

8km 1.5 km 0 .3 km

Under S the meteorological conditions must be such that : - horizontal visibility - distance to clouds

1.5 km outside clouds

In controlled airspace Within airpon(s) vicinity, like above 8 km; 1,5 km; 0,3 km Outside airpon(s) Terminal Areas

a) the "saturaton" of the frequency of the Approach Control Center will no longer appear because messages will be transmitted in a much shorter time both ways and the "lisibility" will reach "5 over 5". b) the stress of the crew on board will be kept reasonable because the radio will not be speaking all the time. At the present time, in dense terminal area, the radio is continually speaking : if ten to twelve planes are under control by the "Approach" only 8 to 10 % of the messages concern a given plane. c) it will be possible to reach the maximum landing rate of the airpon due to the developement of 4-0 Navigation. However there still are some problems: - broadcast means that all the planes which use the same prescribed frequency receive all the messages. If it is too much in general, the messages which concern planes which are flying in the vicinity of this given plane help the crew of this plane to reconstruct the traffic around him. One may think that so much information is unecessary for a given plane, yet will the data link transmit only the information needed by a plane or will it also transmit data concerning the surrounding planes, as said previously.

- horizontal visibility - cieling

8km 0,450 km

Table

Navigation

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Navigation System Comparisons

Method Used : Coordinates ProvIded

eov.:rage ProvIded

Status of the System

Navstar GPS

Sphertcal Ranging : 3 DPosition 3 D Velocity Precise TIme

Global (24 hrs/day)

6 Satellites In 12 hr Orbits. Available Worldwide 1 to 4 hrs/day

Transit

Doppler Shift Longitude Latitude

Global Except at the Poles (Pertodic fixes only, 'TYPically 1/2 to 2 hrs apari)

LoranC/D

Hyperbolic Ranging Longitude Latitude

Regional Coverage I)t 10% of Earth

8 Loran C Chains with 34 Transmitters Cover about 10 % of the Earth

Omega

Hyperbolic Ranging: Longitude Latitude

Essentially Global 88% Coverage by Day 98 0,6 by Night

8 Transmitting Stations In Operation Worldwide

ADF

Goniometry on Omnidirectional beacon

lOOn m around the beacon not accurate in case of storm Oightning)

worldwide

WOR/DME Tacan

Lighthouse Signal + Sphertcal Ranging Heading Slant Range

Line of Sight Along Present AIr Routes

More than 1000 Transmitters in Operation. At least 250,000 Users,

lLS/MlS

Beam Steering: Heading Elevation Range

LIne of Sight: 17t035nm1 Available only At Properly Equipped AIrports

Hundreds of Systems Operating Worldwide 120,000 + Domestic Users

Inertial Navigation

Integrating Accelerometers : 3 D Position 3 D Velocity

Global with Periodic Updates

Thousands of SelfContained Units in Use on Civilian and Military Planes a nd Ships

5 Satellites In Polar "Birdcage" Orbits > 10,000 Sets In Use 80 % Civilian Users