URGENCY BASED SCHEDULING

The present invention relates to a method of scheduling for multi-function radars. Specifically, the present invention relates to an efficient urgency-based scheduling method.

The most common known method of scheduling for radars comprises the following steps and is shown in Figure 1. First, the scheduler makes a list of tasks that need to be carried out by the radar. Second, the scheduler uses a search algorithm to iteratively work out the best order for the tasks to be performed in for every possible combination of the tasks that need to be carried out. The best order criteria may vary depending on the use or situation of the radar. For example, the search algorithm may primarily look for the quickest possible sequence of tasks, perhaps factoring in whether certain tasks need to be performed in certain orders and whether certain tasks can't be performed at certain times.

A situation in which creating the best order list of tasks is particularly complicated is in naval situations, as shown in Figure 2, where the platform 210 on which the radar is mounted moves (one position shown as 210a and one position shown as 210b) due to the motion of the sea 200 and radar visibility 220 changes (one position shown as 220a and one position shown as 220b) is limited for certain areas of sky, e.g. point A, at certain points in time.

The major problem with this known scheduling method is that it becomes exponentially harder to compute as the number of tasks increases. Another problem with this known scheduling method is that every time a new task is added to the list of tasks to be performed, the scheduler has to carry out another search to create a new best order list.

The present invention provides a method of scheduling tasks in a radar apparatus comprising the steps of: receiving one or more tasks to schedule; calculating an urgency function for each said task; and storing the said tasks using said urgency function to order each said task relative to the other said

tasks; wherein when a task is to be performed, the task having the highest value of urgency function is located.

The solution of the present invention provides a more optimal approach to scheduling tasks to be performed by a radar. Specific embodiments of the invention will now be described, by way of example only and with reference to the accompanying drawings that have like reference numerals, wherein :-

Figure 1 is a diagram illustrating the method of the most common known scheduling scheme for radars; Figure 2 is a diagram showing a naval situation illustrating how the radar visibility changes with movement of the naval platform upon which it is mounted;

Figure 3 is a diagram showing an embodiment of the scheduler of the present invention;

Figure 4 is a graph plotting urgency functions against time for two example tasks;

Figure 5 is a graph plotting the simplified urgency functions against time for the two example tasks of Figure 4; and

Figure 6 is a graph plotting a simplified urgency function against time for one of the example tasks of Figures 4 and 5 and also showing when the target is not visible.

The specific embodiment of the present invention will now be described with reference to Figures 3 to 6.

The scheduler arrangement 300 is shown in Figure 3. The scheduler 310 receives requests for tasks that need to be performed from various sources, for example surveillance search sectors, radar tracking, BITE etc. (not shown) and receives radar availability information from the radar antenna. It also connected provides the next job for execution to the antenna when the radar is says it will next be free.

When the scheduler 310 receives a new job, the job has an urgency function calculated for the job and is added to the task and function store 320.

The urgency function of a task is a function based on the respective drop dead time of the task, desired time for completion of the task; and relative importance of the task. The function has to be monotonically increasing with time, such that the urgency value never decreases as time elapses.

In the specific embodiment of the present invention, the urgency function used is a linear function with different gradients before and after the desired time for completion of the task. This greatly reduces the mathematical complexity in subsequent processing. The urgency functions in this embodiment are given by:

U(t)=uO+u1 ^* (t - td) V t < td; and Equation 1

U(t)=uO+u2*(t - td) V t > td Equation 2 where u is the urgency function, t is time, td is the desired time of execution of the look, U(t) is the urgency function and t is time. Equation 1 thus deals with the urgency before the desired time and Equation 2 deals with the urgency after the desired time.

It will be understood that non-linear functions can be used but that this increases the complexity of any subsequent processing. Only certain types of non-linear functions can be ordered in a similar way to linear functions. By using the properties of linear functions, looks with the same values of u1 and u2 as their before and after desired time urgency gradient can be ordered into two sets, one before desired time and one after at some reference time. This ordering of urgency will then not change with time. This enables the most urgent look of any given type to be selected without having to determine the actual urgencies of all the looks of that type at any given time. This ordering ability is true of only those functions that can be represented by a polynomial in positive powers with positive coefficients. For such urgency functions the ordering then becomes desired time ordered with earliest desired time first.

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For each task that is received for scheduling by the scheduler 310, an urgency function is computed for that task and then it is stored in the task and function store 320. When each urgency function is stored in the task and function store 320, it is placed in order of urgency. Each different type of look, including different 'priorities' of a given look type (for instance different surveillance sectors) will have its own pair of before and after desired time urgency ordered lists with the most urgent look within each list placed at the front of the list.

The process of placing a new look onto the relevant list involves searching the list to find the correct location for the look on the list and then

'slotting' the look into the list at this location. This is done with a search and slot-in routine, based on a combination of a double linked list and pointers maintained for use in a 'binary chop' search, in this embodiment. A wide number of other routines well known to the experts in software could be used instead.

At regular intervals, looks in these lists that have 'expired' are removed from the lists as a housekeeping exercise to minimise the search times.

When a look for execution is requested the lists are searched in order to find the most urgent look by starting with the first look on the list (most urgent in that list. If that look is not currently visible (executable), the visibility of the next entry on its list is determined. This process continues until a visible look is found (which, by virtue of the ordering of the list, is of necessity the most urgent visible look on the list) or the list exhausted. If a visible look is found, its urgency is then calculated and, if it is more urgent than the previously found 'most urgent' look, it becomes the most urgent look so far and the search moves on to the next list.

When all the lists have been 'searched', the look that has been found to be the most urgent is passed on for execution and also removed from its urgency list and any other lists it might appear on. A refinement that can be added to the process in this embodiment is to remove from the list any look found to have 'expired' at the time of the urgency

search. The process of removing a look from its urgency lists can, where relevant, be used to signal to the look requestor that the look has been sent for execution or failed as the case may be.

It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.