System and method for avoidance of collision between vehicles

[0002] This application claims priority to a PCT Application filed on Dec. 8, 2000 and assigned Application No. PCT/SE00/02467 which was published on Jun. 28, 2001 and assigned International Publication No. WO 01/46933 A1; the PCT Application claims priority to an application filed in Sweden on Dec. 22, 1999 and assigned Application No. 9904717-7, the contents of both applications are incorporated herein by reference.

[0003] 1. Field of the Invention

[0004] The invention relates to a system and a method for avoidance of collision between vehicles.

[0005] 2. Description of the Related Art

[0006] Collisions between fighting airplanes during air combat training is frequently occurring and are normally caused by a pilot lacking of attention, for instance due to attention being directed to radar reconnaissance or weapon adjustments. These collisions cause costly losses in the form of crashed fighting airplanes and, in the worst case, result in the loss of life of an involved pilot. Consequently, there is a great need of a reliable system for collision avoidance.

[0007] Conventional collision avoidance systems are based on the idea to calculate the trajectories of the involved airplanes and predict a risk of collision with the aid thereof. Such a collision avoidance system, which primarily has been developed and used for civil air traffic, is known under the denomination TCAS (“Traffic alert and Collision Avoidance System”). Each airplane included by this system is provided with its own TCAS-device comprising inter alia a radio transponder. The radio transponder continuously transmits a radio signal and a radio transponder in another airplane, when receiving this radio signal, transmits a reply signal comprising data concerning the height of flight of the own airplane. The distance to the other airplane is obtained by measuring the time between transmitted and received signal and the bearing is obtained by means of a radio direction finder. By means of these values the TCAS-device continuously calculates and updates the flight trajectories for the TCAS-equipped airplanes located in the surroundings. If a flight trajectory calculated for another airplane is estimated to cross the planned flight trajectory of the own airplane, a warning is generated to the pilot at a moment with a predetermined time interval up to a so called “Closest Point of Approach”.

[0008] TCAS II is a further developed system which besides giving the pilot a warning also suggests an avoidance manoeuvre direction. The avoidance manoeuvre directions are co-ordinated between the involved airplanes via radio link. TCAS gives a bad accuracy in the bearing values and many warnings are generated even when the airplanes can pass each other without any risk of collision.

[0009] A collision avoidance system that gives many superfluous warnings implies an additional stress factor for the pilot, which is particularly inconvenient in air combat training when the pilot has a very high work load with many instruments and a large amount of data to handle under great pressure. During air combat training with flight manoeuvres that are difficult to predict and a close contact between the involved airplanes, the risk of superfluous warnings is as well larger than in commercial aviation.

[0010] An object of the present invention is to provide a system and a method for collision avoidance, particularly for use in fighting airplanes, by means of which collisions between vehicles can be averted with a high reliability at the same time as collision warnings and/or automatic avoidance manoeuvres are avoided to the utmost possible extent in the situations when the vehicles located in the vicinity of each other have a possibility to pass each other without any risk of collision.

[0011] The system and the method according to the invention are based on the strategy to seek for a possible avoidance manoeuvre for the respective vehicle and present a warning to a person manoeuvring the vehicle and/or automatically execute the suitable avoidance manoeuvre just before the possibility to perform this avoidance manoeuvre disappears, i.e. just before the involved vehicles are in such a position in relation to each other that a collision cannot be avoided with a satisfactory security. The collision avoidance system according to the invention is consequently restricted and a warning is generated and/or an automatic avoidance manoeuvre is performed only in case of an immediate risk of collision.

[0012] According to a preferred embodiment of the invention, a calculated avoidance manoeuvre trajectory is represented by a number of three dimensional objects, such as spheres, each of which objects being assigned a certain time value, the dimension of the separate objects being dependent on the stipulated minimum distance between the avoidance manoeuvre trajectories. In this connection, it is controlled whether any of the geometrical object representing the avoidance manoeuvre trajectory of the vehicle is overlapping a geometrical object with corresponding time value included in the representation of the avoidance manoeuvre trajectories of any of the other vehicles. In this way it becomes possible to control in an easy way whether the calculated avoidance manoeuvre trajectories are located to close to each other.

[0013] By making the dimension of the geometrical objects depend also on estimated errors of the calculated avoidance manoeuvre trajectories, an improved security against collisions caused by miscalculations is obtained.

[0014] According to a preferred embodiment of the invention, the avoidance manoeuvre direction for the respective vehicle is determined based on the criteria that the completion points of the avoidance manoeuvre trajectories of the vehicles are to be at as large a distance from each other as possible. In this way, the appropriate avoidance manoeuvre directions can be calculated by means of a relatively simple model of calculation.

[0015] With reference to the drawings, the invention will hereinbelow be more closely described by means of embodiment examples. It is shown in:

[0016] FIG. 1 schematically illustrates two airplanes equipped with a system according to the invention and an avoidance manoeuvre trajectory calculated for the respective airplane;

[0017] FIG. 2 is a block diagram illustrating a system according to the invention;

[0018] FIG. 3 schematically illustrates an avoidance manoeuvre trajectory calculated for an airplane and represented by geometrical objects in the form of spheres;

[0019] FIG. 4 illustrates two overlapping spheres belonging to the avoidance manoeuvre trajectories of two different vehicles;

[0020] FIG. 5 is a schematic illustration of certain parameters included in a co-ordinated calculation of avoidance manoeuvre directions;

[0021] FIG. 6 is a schematic illustration of a collision situation; and

[0022] FIG. 7 is a schematic illustration of an auxiliary parameter.

[0023] FIG. 1 schematically shows two airplanes F₁and F₂flying in the direction towards each other, which airplanes are equipped with a collision avoidance system according to the invention. The airplanes F₁and F₂are provided with means 2 (see FIG. 2) for continuous calculation of their respective future position P₁, P₂and means 4 for calculating, co-ordinated between the airplanes F₁, F₂, a respective avoidance manoeuvre direction D_i. Based on this avoidance manoeuvre direction D_iand the calculated future position P₁, P₂, an avoidance manoeuvre trajectory Tr₁, Tr₂for each airplane F₁, F₂is calculated by means of a means 6.

[0024] In the calculation of the avoidance manoeuvre trajectories Tr₁, Tr₂a formula is utilized which can be made more or less complex depending on, inter alia, the accuracy desired in the system and the computer power that can be used. The means 4 utilizes this formula for the calculation of values representing a number of possible avoidance manoeuvre trajectories, with different directions, for the respective airplane F₁, F₂in the calculated future position P₁, P₂. By means of a suitable method, an avoidance manoeuvre direction D_icorresponding to one of the possible avoidance manoeuvre trajectories of the airplane is then determined by a selection among said values. One possible method for this will be described hereinbelow. Based on the obtained avoidance manoeuvre direction D_i, the calculating means 6 thereafter calculates the avoidance manoeuvre trajectories Tr₁, Tr₂for the airplane by means of said formula.

[0025] The respective airplane F₁, F₂is also provided with means 8, such as a software module executed by at least one processor, for controlling whether the avoidance manoeuvre trajectory Tr₁, Tr₂calculated for the airplane F₁, F₂in each moment during its calculated lapse is located at a stipulated or predetermined minimum length distance d from the avoidance manoeuvre trajectories calculated for the other airplanes. If the control shows that this is the case, the data concerning the calculated avoidance manoeuvre trajectory Tr₁, Tr₂is stored by means of a suitable storage means 10, whereupon a new future position P₁, P₂for the airplane is calculated by the means 2 based on the present position of the airplane and the previous procedure is repeated. If, on the other hand, the control shows that the avoidance manoeuvre trajectory Tr₁, Tr₂of the airplane in any moment during its calculated lapse is located at a distance from the avoidance manoeuvre trajectories of any of the other airplanes that is smaller than the stipulated or predetermined minimum distance d, a means 12 is activated, which means 12 warns or communicates to the pilot an existing risk of collision.

[0026] The stipulated or predetermined minimum distance d corresponds to a predetermined safety distance d_safe, possibly supplemented with an error d_errorcorresponding to an estimated maximum error of the calculated avoidance manoeuvre trajectory Tr₁, Tr₂.

[0027] The warning means 12 can be arranged to present the warning in the form of an acoustic signal and/or a light signal. Preferably, the warning means is also arranged to present an appropriate avoidance manoeuvre to the pilot, which avoidance manoeuvre is based on the last stored safe avoidance manoeuvre trajectory Tr₁, Tr₂. The initial warning is presented a short time before the moment t_avoidwhen an avoidance manoeuvre based on the last stored safe avoidance manoeuvre trajectory Tr₁, Tr₂has to be initiated in order to be able to avoid a collision with certainty.

[0028] After the initial warning, the system continues to search after a safe avoidance manoeuvre trajectory Tr₁, Tr₂, in which connection a new future position P₁, P₂for the airplane is calculated by the means 2 based on the present position of the airplane and the previous process is repeated. If the control means 8 shows that a new safe avoidance manoeuvre trajectory Tr₁, Tr₂has been found, for instance due to the fact that any of the airplanes since the previous cycle of calculation has performed such a manoeuvre that an immediate risk of collision is no longer existing, the warning means 12 is inactivated. In this case, the new safe avoidance manoeuvre trajectory Tr₁, Tr₂is stored by the storage means 10 and a new cycle of calculation follows.

[0029] If no new safe avoidance manoeuvre trajectory Tr₁, Tr₂is found during the warning period, i.e., before the moment t_avoid, the system executes, via a calculation means 14 an automatic avoidance manoeuvre based on the last stored safe avoidance manoeuvre trajectory Tr₁, Tr₂. The means 14 is arranged to calculate control parameters based on the stored avoidance manoeuvre trajectory Tr₁, Tr₂, by means of which control parameters the airplane can be made to perform an avoidance manoeuvre corresponding to the stored avoidance manoeuvre trajectory Tr₁, Tr₂. The calculation means 14 supplies the calculated control parameters as inputs to the electronic flight manoeuvring system of the airplane, whereby the airplane can be made to perform the desired avoidance manoeuvre. In this connection, the system is adapted in such a way that the automatic avoidance manoeuvre is initiated at the moment t_avoid, which corresponds to the moment when the airplane is supposed to be in the calculated future position P₁, P₂based on which the stored avoidance manoeuvre trajectory Tr₁, Tr₂has been calculated.

[0030] When the automatic avoidance manoeuvre has been performed, the control of the manoeuvring system of the airplane is returned to the pilot as soon as possible after the situation is safe again.

[0031] The means 6 for calculation of the avoidance manoeuvre trajectories Tr₁, Tr₂is suitably arranged to make a calculated avoidance manoeuvre trajectory be represented by a number of three dimensional geometrical objects S₁-S₄, which objects are arranged close to each other along the avoidance manoeuvre trajectory and each of which is assigned a certain time value T₁-T₄. In this connection, the object S₁represents a volume of space in which the airplane is calculated to be located at the moment T₁, and so on. The dimension of the separate objects S₁-S₄is adapted taking the safety distance d_safebetween the avoidance manoeuvre trajectories Tr₁, Tr₂into account. In this connection, the control means 8 is arranged to control whether any of the geometrical objects S₁-S₄representing the avoidance manoeuvre trajectory Tr₁, Tr₂of a vehicle overlaps a geometrical object with corresponding time value included in the representation of the avoidance manoeuvre trajectories of any of the other vehicles.

[0032] The geometrical objects are preferably constituted by spheres, as shown in FIGS. 3 and 4, which gives a simple presentation of the avoidance manoeuvre trajectory since only the centre point, the radius r_nand the time value T_nof the respective sphere S_nhas to be determined. The number of spheres S_nin the representation of the respective avoidance manoeuvre trajectory Tr₁, Tr₂depends on the radius r_nof the spheres S_nand the length of the avoidance manoeuvre trajectory Tr₁, Tr₂. In this connection, the radius r_nof the respective sphere S_ncorresponds to d/2.

[0033] Since there are many sources of error in the calculation of the avoidance manoeuvre directions D_iand the avoidance manoeuvre trajectories Tr₁, Tr₂, inter alia, due to the unpredictable manoeuvres of the pilot, the avoidance manoeuvre trajectories Tr₁, Tr₂should be presented in such a way that the errors in the calculated avoidance manoeuvre direction D_iand avoidance manoeuvre trajectories Tr₁, Tr₂do not affect the security of the system. This is suitably achieved in that the calculation means is arranged to adapt the dimension of said geometrical objects S₁-S₄taking estimated errors of the respective calculated avoidance manoeuvre trajectory Tr₁, Tr₂into account.

[0034] In case the objects consist of spheres, the radius r_nof the respective sphere S_nis in this connection determined by two components, the possible error d_errorand half the safety distance d_safe/2, i.e. r_n=d_safe/2+d_error. This is illustrated in FIG. 4. In FIG. 4 it also shown how two spheres with the same time values overlap each other. In this case, the system would consequently generate a warning to the pilot. The possible error d_errorcorresponds to the estimated maximum error of the calculated avoidance manoeuvre trajectory Tr₁, Tr₂. The error of the avoidance manoeuvre trajectory Tr₁, Tr₂increases with the distance from its starting point P₁, P₂and therefore the spheres S_nwill have a larger radius r_nthe further away from the starting point P₁, P₂they are located, as shown in FIG. 3. The respective sphere S_nrepresents a volume of space in which the airplane with a certain probability will be located at a certain moment T_n.

[0035] The calculation of the avoidance manoeuvre trajectories Tr₁, Tr₂is performed continuously at discrete moments t_n. The time distance between two cycles of calculation has to be so short that two spheres S_nwith the same time value t_nbelonging to avoidance manoeuvre trajectories Tr₁, Tr₂of different airplanes cannot “jump over” each other during the time between two cycles of calculation. Otherwise, two avoidance manoeuvre trajectories could overlap each other without being registered by the system. The maximum time distance between two updatings, i.e. between two cycles of calculation, depends on the relative velocity between the airplanes, the radius r_nof the spheres and the maximum accepted overlapping between two spheres s_n. A certain overlapping has to be accepted, since otherwise the time distance between two cycles of calculation would have to be infinitesimal.

[0036] In the above-mentioned calculation of the avoidance manoeuvre directions D₁, it is important that the respective avoidance manoeuvre direction is so chosen that the solution to one critical situation of collision does not cause a new such situation. The avoidance manoeuvre directions D_iof the airplanes therefore have to be co-ordinated with each other in some manner. It has appeared suitable to determine the avoidance manoeuvre direction D_ifor the respective airplane based on the criteria that the completion point C₁(see FIG. 1) of the respective airplane's avoidance manoeuvre trajectory Tr₁is to be at such a large distance from the completion points of each other avoidance manoeuvre trajectory, such as Tr2, as possible. In this connection, it has also appeared that suitable avoidance manoeuvre directions D_iare obtained by a minimization of the function: $f={\sum }_{i,j,i\ne j}\frac{1}{\Vert C_i-C_j\Vert }$

[0037] where C_iis the completion point of the avoidance manoeuvre trajectory Tr_ifor the vehicle i. The function f is consequently the sum of the inverted distance between the completion point C_iof the avoidance manoeuvre trajectories Tr₁, Tr₂of all airplanes. The values of C_iare obtained by means of the formula used for the calculation of the avoidance manoeuvre trajectories Tr₁, Tr₂. In FIG. 5 the possible completion points C_iof the possible avoidance manoeuvre trajectories Tr_iof an airplane based on a certain calculated future position P are represented by an ellipse E.

[0038] An avoidance manoeuvre should be as sharp as possible, and therefore the load factor imposed on the airplane during the avoidance manoeuvre should have as large a value as possible. The largest possible load factor can be applied in a direction in parallel with an axis Z_Bextending straight up through the airplane (see FIG. 5), At the avoidance manoeuvre, the airplane should therefore suitably have a roll angle φ_ibeing such that said axis Z_Bis directed in parallel with the avoidance manoeuvre direction D_i. The completion point C_iis a function of inter alia the roll angle φ_iof the airplane at an avoidance manoeuvre based on the calculated avoidance manoeuvre trajectory Tr_i, and by minimizing the function f with respect to the roll angle φ_i, which roll angle in accordance with the reasoning above determines the avoidance manoeuvre direction D_i, the combination of roll angles φ_iand thereby avoidance manoeuvre directions D_ithat gives the largest possible distance between the completion points C_iof the avoidance manoeuvre trajectories Tr_iof the vehicles is obtained.

[0039] The co-ordination of the avoidance manoeuvre directions D_ishould be independent of the communication between the airplanes and should consequently be performed separately in all airplanes. In order to avoid complications in case one and the same minimum value of the function f is obtained with several different combinations of values of roll angles φ_i, the system can be adapted such that the means 4 for calculation of the avoidance manoeuvre direction in the respective airplane tests the different roll angle combinations in the same order and selects a certain combination of the possible several combinations that minimizes f according to a fixed priority. By utilizing the same priority in all airplanes, the means 4 in the different airplanes can consequently be made to select one and the same roll angle combination that minimizes the function f.

[0040] The above described method for calculation of avoidance manoeuvre directions could give inappropriate results when the completion point C_iof the avoidance manoeuvre trajectory Tr_iof a first airplane passes the flight path of a second airplane at the same time as, in the same way, the completion point of the avoidance manoeuvre trajectory of the second airplane passes the flight path of the first airplane. This is illustrated in FIG. 6, which shows two airplanes flying towards one and the same point in an angle of 90° in relation to each other. C_1,toch C_2,tare the completion points of the avoidance manoeuvre trajectory of the respective airplane at the moment t, whereas C_1,t+dtoch C_2,t+dtare the corresponding completion points at the moment t+dt.

[0041] When the respective calculated completion point has passed the respective calculated flight path, the above described method in this specific case gives avoidance manoeuvre trajectories directed towards each other, with a great risk of collision as a result. This problem can be avoided by the introduction of an auxiliary point C_c,i(see FIG. 7) and the formation of a condition for the geometry concerning the completion point C_iand this auxiliary point C_c,i. The auxiliary point according to the shown example is placed in line with the velocity vector v_iof the airplane calculated for the future position P_iin such a way that a right angle is formed between P_i, C_c,iand C_i.

[0042] The above mentioned problem arises when C_iis located closer than C_c,ito P_jat the same time as C_jis located closer than C_c,jto P_i. For a situation with n involved airplanes, n conditions are required. Each of these conditions comprises the following two inequalities: $\Vert C_{c,j}-P_i\Vert <\Vert C_j-P_i\Vert$ $\Vert C_{c,i}-P_j\Vert <\Vert C_i-P_j\Vert$

[0043] At least one of these inequalities has to be satisfied for each condition. If the method for calculation of avoidance manoeuvre direction gives a solution that does not satisfy these conditions, this solution is rejected and an alternative solution is calculated.

[0044] The collision avoidance system according to the invention requires that each airplane knows the avoidance manoeuvre trajectories calculated for the other airplanes. Furthermore, each airplane needs to have access to data from the other airplanes in order to be able to carry out the co-ordination of the avoidance manoeuvre directions. The calculation of the present and future position, velocity, roll angle etc. for the own airplane is suitably carried out separately in each individual airplane. The values required for subsequent calculations are then communicated to and received from the other airplanes by means of a suitable communication means 16, for instance a radio communication system. The co-ordinated calculation of the avoidance manoeuvre directions is then carried out separately in the respective airplane.

[0045] According to a first alternative, the calculation of the avoidance manoeuvre trajectories of all involved airplanes and the comparison control of these avoidance manoeuvre trajectories are thereafter carried out in each individual airplane without any further data communication between the airplanes. According to a second alternative, the respective airplane only calculates its own avoidance manoeuvre trajectory, whereupon data representing the avoidance manoeuvre trajectory of the respective airplane via the communication means 16 is exchanged between the airplanes for the subsequent comparison control of the avoidance manoeuvre trajectories of all airplanes.

[0046] The first alternative has the advantage that a relatively small data communication capacity is required, since only a relatively low amount of data has to be communicated between the airplanes. However, a large computer power is required in the individual airplane, since each airplane has to calculate the avoidance manoeuvre trajectories for all airplanes. Therefore, the first alternative is most suited for such systems that utilize a relatively simple model with few parameters for the calculation of the avoidance manoeuvre trajectories. If a more complex model is utilized for the calculation of the avoidance manoeuvre trajectories, the other alternative is to be preferred, since the respective airplane in this case only calculates its own avoidance manoeuvre trajectory.

[0047] However, the second alternative requires a larger data communication capacity than the first. The second alternative has the advantage that redundancy is obtained in the system. If an airplane communicates data concerning an avoidance manoeuvre trajectory that turns out to be calculated in another direction than the direction to be expected according to the method for the coordinated calculation of the avoidance manoeuvre directions, an indication is namely obtained that an error has occurred somewhere in the system.

[0048] For registration and calculation of present and future position and other parameters that are required for the calculation of avoidance manoeuvre direction and avoidance manoeuvre trajectory, such as present and future roll angle, velocity etc, suitable means and sensors of know type can be used.

[0049] The collision avoidance system according to the invention has above been exemplified by an application in pilot manoeuvred airplanes. However, the system is not limited to this application, on the contrary it can be adapted for use in other types of aircrafts, for instance unmanned aircrafts, and can within the scope of the invention also be adapted for use in other types of vehicles besides aircraft. Furthermore, it is possible, if considered suitable in any context, to design the system according to the invention in such a way that an automatic avoidance manoeuvre is executed in case of an established risk of collision without any preceding warning or in such a way that a warning is generated without a subsequent automatic avoidance manoeuvre.

[0050] The means 2, 4, 6, 8 and 14 are preferably software modules containing a set of programmable instructions which are capable of being executed by at least one processor for performing the respective functions of each of the means 2, 4, 6, 8 and 14.