GB2165414A - Runway occupancy warning system - Google Patents

Abstract

An airport runway occupancy warning system comprises a number of short range (of the order of 100m) radars strategically grouped in cells (1, 2....8, Fig. 1) besides the runways, taxiways etc. Each radar determines the range and range rate of any targets in its field of view and the resulting data for each cell, giving position and velocity in formation, is transmitted to the airport control tower. The radars disclosed are FMCW devices, but the type is not important. <IMAGE>

Description

SPECIFICATION Runway Occupancy Warning System This invention relates to a system and meansforwarning of runway occupancy at an airport to the airport control.

With ever increasing usage of airports the maximising of runway usage is obviously necessary.

However concommitant with this is the increased danger of collision between aircraft moving on the runways, particularly in bad weather conditions with poor visibility. A large airport like Heathrow, or Madrid (which has seen a serious accident) is a complex combination of aircraft movements. To detect such movements and to relate them to each other so that safe and unsafe combinations can be separated from each other, is inevitably a complex operation.

It is, however, possible to divide the task into groups of smaller tasks with å more local interest and where the decision making is distributed about the airport.

A Runway Occupied Warning System (ROWS) is required to provide the Air Traffic Control (ATC) authorities with warning of the possibility of such an accident. The ROWS shall provide a warning to airport ATC whenever an active runway(s) has a vehicle or obstruction on the runway surface and within the limits of the runway strip. A runway strip is a defined area including the runway and stopway, if provided, intended: a) to reduce the risk of damage to aircraft running off a runway, and b) to protect aircraft flying over it during take-off or landing operations.

A vehicle is defined as any aircraft or equipment capable of manoeuvring on the airfield surface, or any departing or arriving aircraft whilst it is over the area defined as the runway strip. The warning to the Air Traffic Control Officer (ATCO) is preferably by both audio and visual indications at the Air Traffic Control Console.

In addition, the warning to the ATCO should be such that if a tactical or an active ATC clearance is issued over a Radio Telephony (RTF) frequency for a vehicle to enterthe Runway Strip and that clearance conflicts with a vehicle or obstruction already occupying the runway, then an alarm to the ATCO should be activated.

In weather conditions below those that permit visual observation of the Runway area from ATC, the alarm system should not be capable of being over-ridden by the ATC personnel.

The manner in which runway occupancy has been monitored to date has been by means of radar (airport surface movement indication-ASMI or airport surface detection equipment-ASDE). These systems have been very expensive due to the need to obtain high definition and resolution in adverse weather. More recent solutions have utilised modern data processing and display techniques to present the required information and prices are thus falling rapidly.

However, ROWS that are based on airfield surveillance radar system tend to be very costly if they are to be fully automatic. The system must be capable of indicating when the runway has been entered by other than the specified routing during periods of Instrument Meterological Conditions (IMC) (poor visibility). The indication should be: a) To the Duty Air Traffic Control Officer (DATCO) in the Visual Control Room (VCR), Ground Movement Control (GMC) or Aerodrome Control (ADC) (as appropriate) when an aircraft, vehicle or large object (person size, including large animals) has entered the runway via any access. This may be by visual or audio visual means.

b) When a) above has occurred warning to both Duty Air Traffic Control Officer (DATCO) and Pilot, when an aircraft which is to take off enters the runway at the designated take off end. This should be an audio alarm on the Ground Movement Control (GMC) orAerodrome Control (ADC) VHF frequency.

The "sensing" of the aircraft, vehicle or large object should be in the manner analogous to the "electronic gateway" used for passenger screening, that is having a vertical coverage to a height capable of covering all sizes of aircraft.

The period of the interruption of the "gateways" which are sited at a point on all metalled accesses (taxiways and safety services roads) which are joined to the runway, should give an indication of what has passed through, i.e. a long break by the fuselage of an aircraft or short break by a man on a bicycle.

According to the present invention there is provided a runway occupancy warning system comprising a plurality of fixed shortrange microwave sensors (as hereinbefore defined) distributed strategically about an airport, each sensor being placed adjacent to a runway, taxiway or the like, each sensor including data acquisition and processing means and data transmission means whereby information concerning occupancy of different portions of the airport by aircraft or other vehicles, either moving or stationary, is determined and transmitted to the airport control.

A short-range microwave sensor is defined as a radar transmitterlreceiver having a maximum range of 100 metres.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Fig. lisa simple map of Tenerife airport, Fig. 2 is a detail enlargement of one cell from Fig. 1, Fig. 3 illustrates in block form the cell combination unit of Fig. 2, Fig. 4 is a detail enlargement of another cell from Fig. 1, Fig. 5 illustrates in block form an arrangement in which a number of sensors join to define a cell, Fig. 6 illustrates in blockform the elements of a sensor, and Fig. 7 illustrates an alternative arrangement for a sensor arranged to detect an aircraft airborne either about to touchdown on or having just taken off from a runway.

For illustrative purposes take as an example Tenerife airport which has a simple layout illustrated in Fig.

1. At this airport a total of 8 areas or "cells" are delineated each with its own local interest. Cells 1 and 6 are the runway ends, cells 2-5 are other points where aircraft can enter or leave the runway and cells 7 and 8 are points on the extended centreline where it is known the aircraft will overfly.

In Fig. 2 we examine in more detail the cell numbered 1. In this cell two sensors marked A and B are used each of which is designed to detect the presence of and the range to an object-aircraft or vehicle--on the runway ortaxistrips in the area.

In the figure three targets marked T1 T2 and T3 are indicated. The two sensors will each provide a range and possibly range rate for the three targets.

A 3rd sensor located on the extended centre line points upwards to detect overflying aircraft. These three sensors are connected to a cell combination unit (CCU) where a microprocessor receives the individual ranges and range rates and from them computes the x-y coordinates of each target. Such a cell combination unit is illustrated in block diagram form in Fig. 3.

Positional precision is not necessarily required in itself; what is required is sufficient information to enable the ATC officer to know whether the runway is occupied or whether a conflict situation has arisen or is about to arise. To this end the runway and taxiway of cell No. 1 can be divided into 5 boxes numbered 1-5 as shown in Fig. 2. In terms of potential conflicts there are in all only about 16 different states which between them define the traffic situation in cell No. 1.These states are defined in Table TABLE I Messages from Computer Cell 1 BOX1 CLEAR 01 STATIC TARGET 02 TARGET MOVES DIRECTION 30 03 TARGET MOVES DIRECTION 12 04 BOX 2 CLEAR 05 STATIC TARGET 06 TARGET MOVES TOWARDS RWY 07 TARGET MOVES FROM RWY 08 BOX 3 CLEAR - 09 TARGET PRESENT 10 BOX 4 CLEAR 11 TARGET OVERFLYING 12 BOX 5 CLEAR 13 TARGET STATIC 14 TARGET MOVES DIRECTION 30 15 TARGET MOVES DIRECTION 12 16 Thus a communications link between the CCU and ATC need do no more than to transmit regularly a number in the range 1-16 which adequately provides a central control unit with all the information required.A simple microprocessor at the CCU is capable of working out-at any time--which particular state applies, as long as that microprocessor is given information about the xy coordinates of each sensor (shown as xa ya, Xb Yb and x, ys in Fig. 3 and also x, y, in Fig. 2).

In Fig. 4 details are shown of a similar arrangement for cell No. 3 where two sensors marked D and E are detecting three targets; T1 taxiing along the taxiway, T2 stopped on the entry to the runway and T3 rolling on the runway.

These two sensors are able to determine the position of each target provided the sensors are linked to a cell combination unit for that cell and where that unit is given the x, y coordinate for each sensor. Again a.

set of states may be defined and the unit will transmit the 'state' of the cell through one or more of a limited number of messages.

Returning now to Fig. 1, note that cell No. 1 is similar to cell No. 6, that cell Nos. 2,3,4 and 5 are similar to each other and that cell No. 7 is similarto No. 8.

Fig. 5 illustrates the overall concept in which a number of sensors join to define a cell and where a number of CCUs are linked to a central control unit with its display or warning system, together with some sort of input device whereby the ATC officer can cause the system to perform a variety of functions.

This unit is subsequently referred to as the CDA. Table 2 suggests some criteria whereby the messages received from the various cell combination units can be combined to define some conflict. Several levels of information can be envisaged, level 1 could for instance be concerned oniy with identifying that a runway is occupied and level 2 could be used to convey that a real emergency exists.

TABLE II Tentative Warning Criteria Level 1 (Information) a. A target has entered RWY via one of the cells but exit is not yet detected.

Level 2 (Emergency) a. Target in cell box 5 moves direction 30 before earlier target has cleared RWY.

b. Target in 1-5 moves direction 30 and target in cell box 2 moves towards RWY.

c. Target has overflown cell 7 inbound and not yet passed 1 while other target moving towards RWY in 1-2 or 2.

d. Targets entering 1-1 and 6--1 simultaneously (no wind).

A key module in this system is clearly going to be the sensor itself. Figure 6 indicates in block diagram form one sensor unit. A static antenna 10 is connected via a circulator 11 to transmitter 12 and receiver 13.

The transmitter is a CW source operating at x-band and modulated by means of a varactor diode 14. The bias on this diode comes from a microprocessor 15 which by means of control lines b0 and b, can generate any of the waveforms shown at the top of the figure. The received microwave signal beats with a leaked portion of the transmitted signal and the beat frequency is extracted. This frequency is a measure of range and range rate, depending upon the form of modulation, and therefore enables the microprocessor to derive these parameters.

This outline description shows how a complex task of assessing conflict situations on a busy airport may be solved by breaking the overall task down into sub-tasks, each of which is relatively simple, utilising distributed processing power throughout the system.

It should be understood that the system is highly modular, all sensors are envisaged as identical and mutually interchangeable. The cell combinations are similar and the central control unit will be unique to a particular airport. It is, however, envisaged that the software will itself be largely modular such that changes in sensor layout or in cell definition can be catered for relatively simply.

Suitable emission frequencies can be found in the range 8.5 G Hz--14.0 GHz. Allocation of particular frequency bands are given in the official Radio Regulations, along with relevant notes and definitions.

One factor which may at first sight appear to constitute a problem is mutual interference. To assess the scale of the problem consider the layout of Madrid airport, which can be regarded as of average complexity.

It has been estimated that in order to cover that airport reasonably well we would need about 60 sensors.

Suppose now we operate with the following parameters: Carrier frequency x-band Swept frequency 200 MHz Modulation frequency 40 Hz Modulation form linear symmetric then we achieve a beat frequency (modulus of transmitter frequency-receiver frequency) which has: 106.7 Hz per metre range 34 Hz per knot of target speed.

Assuming we require: maximum speed 200 knots maximum range 100 metres then we require a receiver bandwidth of: 106.7x100+34x200=17.47 kHz If we allowed 20 kHz so as to be able to adjust dynamically the modulation parameters, we could fit about 10 000 units into 200 MHz without any significant problem. Thus it is apparent that there is no need to control precisely the carrier frequency. Instead, if interference arises the software is programmed simply to move the frequency some small amount and try again. Instantaneous interference does of course arise as other transmitters sweep through, but these are transient phenomena which processing can deal with.

Interference may also arise in the ROWS caused by airfield radar and aircraft weather radar and this would have to be examined.

To extract range and velocity information from the beat signal we have several possibilities for the modulation: (a) calculate beat frequency with sense during up modulation and during down modulation. From the sum of the two beat frequencies we can extract velocity and from the difference we can extract range.

(b) moduiate linearly up and down for a while. Range can now be computed from the mean beat frequency. Then stop modulation and compute speed from the Doppler shift.

(c) obtain velocity from past history of position. Adaptive software control on the modulation is envisaged and by utilising a receiver filter whose gain rises with frequency we obtain a signal level substantially independent of range.

Software can reduce interference between neighbouring modules by ensuring modulation is stopped at a different frequency slot-or by altering the phase of the modulation.

Software can identify changes in target distribution by studying time variations in the beat frequency spectrum (e.g. when one aircraft taxies very close to a stationary aircraft).

While detecting the presence of aircraft on the runway will meet stated requirements it would clearly enhance the usefulness of the system if it could also detect that there is an aircraft on the approach just about to enter the runway or just having taken off from the runway. k is therefore possible to locate one of the sensors at either middle or the outer marker, to point it up about 30 relative to the vertical and to detect when aircraft pass over that point. Figure 7 illustrates this. Assuming the middle marker (MM) is 1 km out, an aircraft on a 2.5 glide path would then pass the sensor about 50 metres away.

To avoid false signals from vehicles on the ground a range window may be employed coupled with some range rate window.

It is envisaged that such a sensor would be linked to the rest of the ROWs either by way of a telephone line associated with the marker beacon itself, or by means of a VHF1UHF communications link.

The present invention results from the current availability of data processing facilities in small equipments. By building into the local modules local intelligence the complexity of the overall problem is greatly reduced. Thus each module has the task of exploring the spectrum of the beat signal, adjusting the modulation characteristics of the transmitter to eliminate interference and ambiguity, looking for coherence by integrating in softwa re--at the end of which process each module has a simple message like "I am unit No. 23, I have targets at 36 metres and 82 metres, neither moving".

An important aspect of this system is that it may be installed on any part of an airport and then grow with the addition of more cells as the need changes. Thus it would be feasible to protect first a main runway, and to add protection for other parts of the airport later.

It is also possible to add features in the software whereby at first the system only detects.'runway occupied' but where enhancements in sophistication later examines conflicts which will arise in exceptional conditions, e.g. when two aircraft simultaneously seek to occupy the same "block".

Eventually, the system can be arranged to grow in concept through experience, thus if 'near misses' arise of a nature which previously were excluded from the warning criteria, then software changes may be effected without necessarily requiring extensive alterations in hardware or in airfield installation work.

The layout shows in the figures relate to a single runway airport where the runway itself is well defined as are the various turn-on-turn-off points. Many airport, however, have aircraft movements which are less restricted in terms of predefined tracks and 'gates'.

The system envisaged here is sufficiently adaptable to cover any area shape provided it is possible to locate the sensors within radio range of the CCU and with such a relative geometry that position can be derived with sufficient accuracy and without ambiguity.

Claims (10)

1. A runway occupancy warning system comprising a plurality of fixed shortrange microwave sensors (as hereinbefore defined) distributed strategically about an airport, each sensor being placed adjacent to a runwáy, taxiway or the like, each sensor including data acquisition and processing means and data transmission means whereby information concerning occupancy of different portions of the airport by aircraft or other vehicles, either moving or stationary, is determined and transmitted to the airport control.

2. A system according to claim 1 wherein each sensor data acquisition and processing means includes means for determining the range and range rate for any targets in the sensor field of view.

3. A system according to claim 1 including at least one sensor directed upwards to detect overflying aircraft.

4. A system according to claim 1,2 or 3 wherein groups of sensors are linked together to form "cells" each cell covering a defined area of local interest of the airport surface.

5. A system according to claim 4 wherein the sensors in a cell are connected to a common microprocessor programmed to compute the position of targets within the area of the cell.

6. A system according to any preceding claim wherein each sensor comprises static antenna means connected to a transmitter and a receiver, the transmitter having a modulated CW source the modulation of which is controlled by a microprocessor, and means for beating the received signals, with a portion of the signals from the transmitter to generate a beat frequency which is fed to the microprocessor.

7. A system according to claim 6 wherein the beat frequencies fed to the microprocessor are categorised into one or more defined categories and the microprocessor transmits to the airport control information concerning the category into which the beat frequency is categorised.

8. A system according to claim 6 or 7 wherein each sensor includes microprocessor controlled frequency control responsive to interference received from another sensor transmission whereby the sensor frequency is altered to avoid mutual interference with said other sensor.

9. A runway occupancy system substantially as described with reference to the accompanying drawings.

10. A method of determining runway occupancy of an airport by placing in strategic positions on the airport surface one or more fixed shortrange microwave sensors, as hereinbefore defined, to monitor individual portions of the runway, taxiway or the like and transmitting occupancy data from the sensors to the airport control.