A method of calculating the position of an unmanned aerial vehicle

Abstract

System and method of calculating the position of an unmanned aerial vehicle (UAV) 5 within an area utilising differential global positioning system and comprising one or more ground reference stations

Description

Field of Invention The present invention relates to a system and method for the detection of unmanned aerial 5 vehicles (UAV) in a desired area and the calculation of the location of the UAV within that area. Background The Global Positioning System (GPS) (originally called NAVSTAR GPS) was originally 10 developed by the U.S. army in 1978. Currently, GPS uses a constellation of 33 satellites with an orbital height of 20,180 km that each orbit the earth at an altitude of approximately 26,500 km. This constellation of satellites transmits and receives radio waves at three different frequency bands as follows: LI at 1575.42 MHz, L2 at 1227.6 MHz, and L5 at 1176.45 MHz, respectively. Each signal continuously transmits a navigation message at a bit rate of 50 bps 15 towards mobile GPS receivers located at the surface of the earth. These transmissions are precisely sensed in terms of time stamps as both the clocks on the satellite and receiver are synchronised. Then, the mobile GPS receivers process the navigation signals from at least two different satellites, correlating the mobile GPS receiver’s position and time with those of the LO receivers. The position of the mobile GPS receiver is then determined by the determining the £\J 20 intersection of two spheres, defined by the range of the receiver from the two satellites. Processing a signal from a third satellite can improve the accuracy of the determined position. Nevertheless, as the signals are received on the ground from satellites, some error in the determined position is present due to the offsets caused by the ionosphere, troposphere and additional interferences. 25 A relatively new technique known as differential GPS (DGPS) is based on conventional GPS but additionally utilises one or more reference receivers located in a ground station to improve the determination of the position of a mobile GPS receiver. DGPS maintains the employment of a mobile GPS receiver but introduces one or a set of coordinate references located in ground 30 stations which reduces costs and direct interaction with satellite signal processing but keeps the accuracy in the determination of the position of a mobile GPS receiver within a confined network of reference stations. A single reference station checks the satellite metrics and obtains in real time the coordinates of this point with high accuracy. Hence the position of the moving mobile GPS receiver is given by the difference between the fixed reference station and the readings of the mobile GPS receiver itself. The mobile GPS receiver and the reference station compare the metrics and determine a given error with a view to calculate an accurate position. This error can be further corrected by using additional reference stations. 5 There are two methods to correct the determined position in a DGPS system: post-processing and real-time corrections. In a post-processing correction, the reference station receiver needs to have enough computing power to acquire the data from the mobile receiver as well as to carry out algorithms to determine the offsets between reference and mobile receivers. In realtime corrections a real-time station links a reference station with a GPS satellite and applies 10 the correction to the data in a real time basis under specific formats such as RTCM (Radio technical Commission for Maritime Service) or CMR (Compact Measurement Record). This is the most common method used for DGPS. Generally, the accuracy of a DGPS using realtime corrections is proportional to the proximity between the mobile GPS receiver and a reference station. Unmanned aerial vehicles (UAVs), commonly known as drones, are proliferating. They are used currently recreationally, for filming, for surveying, and for other similar uses. There are also plans for UAVs to deliver goods to customers. UAVs also have many military and espionage uses. Although some jurisdictions have restrictions on the use of UAVs, the readily availability and cheap price of UAVs make such restrictions difficult to enforce. For example, National Aviation Authorities all over the world are reporting increasing numbers of cases of near-misses between manned and unmanned aircraft. UAVs are being used recklessly near airports resulting in several near air collisions. As more UAVs take to the skies the likelihood of the mid-air collisions increases. There are also specific issues regarding the use of UAVs 25 over public areas, such as stadia. Further issues arise with the use of UAVs over restricted areas such as prisons and military bases. In light of the above, there is a need to accurately detect the presence and location of UAVs over defined airspaces, such as airports, prisons, sports stadia, and other similar locations. 30 Radar, radio frequency (RF) detection and other currently available detection methods, are not significantly accurate or reliable. Therefore, an improved method of detecting UAVs and accurately determining their location is required. Summary of Invention The present invention provides a method of calculating the position of an unmanned aerial vehicle (UAV) within an area, the method utilising a differential global positioning system 5 according to claim 1. Wherein the claimed method utilises one or more ground reference stations that are in communication with a global positioning system (GPS). Wherein the reference stations are positioned in or on the perimeter of the area wherein the detection occurs. As each of the one 10 or more ground reference stations is in communication with a GPS the position of a UAV entering the area may be calculated using differential GPS. An example of the calculation of the position of a UAV using differential GPS is set out in the description of the embodiments of the invention detailed below. For example, the position of a UAV within the area may be calculated using differential GPS on the basis of the following equation: 20 25 Tu - tr - l2£(k) _ _ *r| + + &T + bu - bR - vu + vR Wherein the suffix U denotes the frame of reference of the ground reference station and the suffix R denotes the frame of reference of the detected UAV and wherein: is a measured pseudo range xW is the satellite position x is the position of the ground reference station or the UAV / is the ionosphere error T is the troposphere error b is a satellite clock offset v is added mixed interferences causing an error in the position In an embodiment of the invention the perimeter of the area can be defined by three or more ground reference stations. That is, each of the three or more ground reference stations will be 30 positioned on the perimeter of the area and the perimeter will be defined by straight lines between adjacent ones of the three or more ground reference stations. In addition, or as an alternative, to having ground reference stations located on the perimeter of the area, one or more ground reference stations may be located within the area. Further, in addition or as an alternative, one or more ground reference stations may be located outside of the area and may be controlled to only detect UAVs that are located within the area. 10 15 The method of the present invention allows the location of a IJ A V within the perimeter of the area to be accurately located. The method of the present invention may further comprise one or more steps of determining whether the UAV is permitted or not-permitted to be in the calculated location within the area and thereby determining whether action is required to be taken against the UAV. This can be done in any manner apparent to the person skilled in the art. In embodiments of the present invention the method may further comprise the step of analysing a radio frequency signal from the UAV to determine what action to take against the UAV. For example, if the radio frequency signal corresponds to the signal of a permitted UAV then a decision can be made that no further action may need to be taken whilst if the radio frequency signal of the UAV does not correspond to a permitted UAV then a decision can be made to take further action against the UAV. The method of the present invention may further comprise the step of taking action to neutralise LO a UAV if it breaches one or more security criterion. Possible security criteria include, but are CM 20 not limited to, unauthorised UAVs located within the area, authorised UAVs travelling erratically, at excessive speed, or at inappropriate altitudes, two or more UAVs in close proximity or any other suitable criteria apparent to the person skilled in the art. The calculation of the position of the UAV within the area by differential GPS may be carried 25 out in a ground reference station, for example the calculation of the position may be carried in out a ground reference station that detects a UAV. Separate calculations may be carried out in each ground reference station that detects a UAV. These calculations may then be communicated to one or more remote control centres that are in communication with the one or more ground reference station as appropriate. The calculation of the position of a UAV within the area by differential GPS may be carried out in a control centre that is remote from the ground reference stations and in communication with the one or more ground reference stations. For example, any ground reference station may transmit the data necessary to calculate the position of the UAV to a remote control centre and the calculation may be carried out in the remote control centre, rather than, or in addition to, in a ground reference station. 5 The method of the present invention may detect a UAV entering the area by a ping signal to a GPS receiver of the UAV. When a UAV is detected within the area a range finding technique will be used by one or more ground reference stations to aid in calculating the position of the UAV utilising differential GPS. This range finding technique comprises a ping signal to a GPS received of the UAV. 10 The area of the method of the present invention may be any area that needs monitoring for encroachment from UAVs. This includes but is not limited to private property such as facilities with sensitive information, buildings with mass gathering events such as stadiums, prisons and country borders, schools, and Universities. 15 The method of the present invention is advantageous in that its operation need not affect any other communication equipment in operation in or near the area. In particular, the method of the present invention may be operated solely in L band frequencies reserved for GPS LO technologies. CM20 The present invention also provides a system for detecting and locating UAVs within an area consisting of one or more ground reference stations located on the perimeter of, or within, the area, wherein: each ground reference stations is in communication with a GPS, is configured to detect a UAV 25 entering the area, and the system is configured to calculate the position of the UAV within the area utilising the method of the present invention. As will be readily understood the system of the present invention can consist of any system comprising one or more ground reference stations that operates according to the method of the 30 present invention. Embodiments of the system may comprise a plurality of ground reference stations positioned on and defining the perimeter of the area. In addition, or as an alternative, one or more ground reference stations may be positioned within the area. As set out above with reference to the method of the present invention, the one or more ground reference stations may be configured to calculate the position of the UAV or a remote control centre may be configured to calculate the position of the UAV on the basis of data received from the one or more ground reference stations. Therefore, in embodiments of the system of 5 the present invention the system may further comprise a control centre that is remote to the one or more ground reference stations that receives signals from the one or more ground reference stations. Further features and advantages of the present invention will be apparent from the preferred 10 embodiment that is shown in the drawings and is described below. Drawings Figure lisa schematic of a system operating according to the method of the present invention; and 15 Figure 2 is a schematic illustrating the operation of the method of the present invention. CM An embodiment of a system operating according to the method of the present invention is shown in Figure 1. The system comprises three ground reference stations 1 that define the LO perimeter of a protected area 2. The protected area 2 contains two buildings 3 that require £\J 20 protecting from unauthorised UAVs 5. A GPS satellite 4 is shown above the area 2 for reference purposes. Three UAVs 5 are present in the area 2. Two operators 6 of the UAVs are shown. The ground reference stations 1 are in communication with one or more GPS satellites 4 and their position is determined on that basis. A UAV 5 is detected by one or more of the ground 25 reference stations 1. The ground reference stations 1 are capable of independently calculating the detected position of a UAV, in the manner set out below, and are also in communication with a control centre (not shown) that can calculate the detected position of a UAV 5 from data provided by one or more ground reference stations 1 as necessary. The provision of the multiple ground reference stations 1 both serves to delimit the protected area 2 and to provide 30 redundancy should one or more ground reference stations cease operating. The calculation of the position of a detected UAV according to an embodiment of the present invention is carried out in either a ground reference station 1 or a central control centre using the following equation: = — Xy | — — %r| + A / + AT + bu — bR — vv + vR This equation is derived as follows: In a conventional standalone GPS system the position of a receiver is given by the difference between a measured pseudo range and a calculated theoretical range (k} The measured pseudo range is determined from the measured position of the receiver modified with external interferences including the troposphere and ionosphere errors, this is defined by the following equation: = |x^ - Xu \ + 1 + T + by - + v Wherein: lx® — Xy| is the difference between the satellite position and the receiver; I is an Ionosphere error; T is a Troposphere error; by is a satellite clock offset, B®is a broadcast offset in synchronisation; and v is added mixed interferences causing an error in the position. The calculated theoretical range is given by range T^and is determined only from calculated figures regarding the position of the receiver. This is defined by the following equation: T® = — xU(o | + / + T + buo — B^k’B^ + v Wherein: \x^ — Xy 0| is a difference between the satellite position and the calculated position; / is a calculated Ionosphere error; T is a calculated Troposphere error; buo is a satellite clock offset with respect to the user; B^k,B\s a calculated broadcast offset in synchronisation; and 5 vis the added mixed interferences. Hence a navigation equation, also known as a residual, is given by St and is described by a difference between the calculated and measured ranges as defined by the following equation: 10 St = -&xUiR ■ + Sx^ • + Sby + SBK + ST + SI +v Wherein: —SxU R is a difference of positions between the calculated theoretical range and the measured pseudo range; Xl 15 is a unit vector; C\| Sx^is a difference between a true location and a broadcast location of the satellite; 8BK, Sby, SI, and ST are a difference (<5) between the relevant parameters of the calculated theoretical range and the measured pseudo range; and C\j v remains the added mixed intereferences. 20 For differential GPS, a navigation equation involves two pseudo ranges, rather than one in the standalone GPS detailed above. Therefore, the calculation of position involves the difference between two pseudo ranges. The two pseudo ranges in the method of the present invention are defined as follows: 25 A pseudo range for the a ground reference station including similar parameters to the standalone GPS pseudo range (difference of distances between reference and user, and errors from Troposphere, Ionosphere, broadcast clock, satellite clock offset and mixed interferences), which is defined by the following equation: 30 Ty ) — |x^ — Xjj | + Iu + Tg + by — B^ + Vy 25 11 24 A pseudo range for the detected UAV which involves the same parameters but from the frame of reference of the UAV, which is defined by the following equation: 5 = + Ir +TR + bR - B(k) + vR Hence the equation for determing the position of a UAV detected by a ground detection station 1 is given by the difference between pseudo ranges as follows: 10 = | — — Xr | + A / + AT + by — bR — Vy + vR Using this equation allows the accurate determination of the position of a UAV 5 within the area delimited by the ground reference station Is. In this manner, the system can accurately determine the position of a UAV 5 that enters the area. The calculation of the position of the 15 UAV 5 can be determined remotely by the control centre in communication with each of the ground reference stations 1 and / or can be determined internally by any individual ground reference station 1. The position of the UAV 5 can then be communicated to any appropriate facility. For example, the position of the UAV may be used as part of a UAV identification and nullification system as part of the process of identifying and neutralising unauthorised UAVs. 20

Claims

1. A method of calculating the position of an unmanned aerial vehicle (UAV) within a desired area utilising a differential global positioning system (GPS), and one or more ground reference stations in communication with a GPS, wherein the ground reference stations are provided within, or on, a perimeter of the desired area; the method comprising the steps of:initially detecting a UAV having a GPS entering the desired area using the one or more ground reference stations to detect a ping signal to the GPS receiver of the UAV signal originating from or received by the UAV; thencalculating the position of the UAV within the desired area by differential GPS utilising the steps:i) the determining the position of the one or more ground reference stations;ii) the detection of said entering of the UAV by a ping of the one or more groundreference stations by means of said ping signal to detect the range of the UAV from each of the one or more ground station; andiii) a range finding technique used by one or more ground reference stations to determine the range of the UAV from the one or more ground reference stations wherein the range finding technique comprises calculating the difference between the pseudo (k}ranges of the GPS of the UAV, . and one of the GPS of the one or more ground reference stations, based on data from their respective GPS; wherein the pseudo position is determined by the following equations:= — *u\ + h + Tu + bu — += |*W - £r| + h + Tr + bR - + vrAnd the position of the UAV is determined by the following equation:= |x w — Xy | — |x W — Xp | + △ / + AT + bv — bR — vv + vRWherein the suffix U denotes the frame of reference of the ground reference station and the suffix R denotes the frame of reference of the detected UAV and wherein:Ty is a relevant u or r measured pseudo range;x^ is the satellite position;xv is the position of the ground reference station, R, or the UAV, U;25 11 24B^is a broadcast offset in synchronisation; / is the ionosphere error;T is the troposphere error;b is a satellite clock offset;v is relevant u or R added mixed interferences causing an error in the position.

2. The method according to claim 1, wherein three or more ground reference stations define a perimeter of the area.

3. The method according to claim 1 or claim 2, wherein one or more ground reference stations are located within the area.

4. The method according to any preceding claim, wherein the UAV entering the area is detected by means of detecting a ping signal to a GPS receiver of the UAV.

5. The method according to any preceding claim, further comprising the step of: analysing a radio frequency signal from the UAV to determine what action to take against the UAV.

6. The method according to any preceding claim, further comprising the step of: taking action to neutralise the UAV if it breaches one or more criterion.

7. The method according to claim 6, wherein the one or more criteria include the UAV being located within a restricted area or the UAV being an unauthorised UAV.

8. A system for detecting and locating UAVs within an area consisting of one or more ground reference stations located on the perimeter of, or within, the area, wherein: each ground reference stations is in communication with a GPS, is configured to detect a UAV entering the area, and the system is configured to calculate the position of the UAV within the area utilising the method of any of claims 1 to 7.

9. The system according to claim 9, comprising a plurality of ground reference stations positioned on and defining the perimeter of the area.

10. The system according to claim 9 or claim 10, further consisting of a control centre that is remote to the one or more ground reference stations that receives signals from the one or more ground reference stations.25 11 24

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