Patent Publication Number: US-7725257-B2

Title: Method and system for navigation of an ummanned aerial vehicle in an urban environment

Description:
The present application is related to U.S. patent application Ser. No. 11/470,152, which is incorporated herein by reference. 
     BACKGROUND 
     Unmanned aerial vehicles (UAVs) are remotely piloted or self-piloted aircraft that can carry sensors, communications equipment, or other payloads. They have been used in a reconnaissance and intelligence-gathering role for many years. More recently, UAVs have been developed for the purpose of surveillance and target tracking. Autonomous surveillance and target tracking performed by UAVs is either military or civilian environments is becoming an important aspect of intelligence-gathering methods. 
     Typically, UAVs use the Global Positioning System (GPS) to provide navigation over various terrains. In many scenarios, UAVs are required to fly in an urban environment, which often have an urban canyon effect. An “urban canyon” is an artifact of an urban environment similar to a natural canyon. The urban canyon is caused by streets cutting through dense blocks of high rise buildings such as skyscrapers. It is known that the urban canyon can cause problems in reception of GPS signals. 
     For example, GPS receivers can suffer from multipath errors which are caused from receiving a composite of direct GPS signals and reflected GPS signals from nearby objects such as buildings. In an urban canyon, a direct line of sight (LOS) GPS signal can be completely blocked by a building structure and a reflected signal can reach the receiver, resulting in position and velocity errors of significant magnitudes. Thus, when UAVs operate in an urban environment, access to the GPS signals needed to navigate without drift is often denied by the urban canyon. 
     Many methods have been proposed for using image-aided navigation of autonomous vehicles such as UAVs. For example, matching or referencing a GPS-denied vehicle&#39;s current image of terrain to a satellite image of the same terrain has been suggested. However, these methods lack the temporal and spatial proximity needed in order to use the images for UAV navigation in an urban canyon. 
     SUMMARY 
     The present invention is related to a method and system for navigation of an unmanned aerial vehicle (UAV) in an urban environment. The method and system comprise capturing a first set of Global Positioning System (GPS)-tagged images in an initial fly-over of the urban environment at a first altitude, with each of the GPS-tagged images being related to respective GPS-aided positions. The captured GPS-tagged images are stitched together into an image mosaic using the GPS-aided positions. A second set of images is captured in a subsequent fly-over of the urban environment at a second altitude that is lower than the first altitude. Image features from the second set of images are matched with image features from the image mosaic during the subsequent fly-over. A current position of the UAV relative to the GPS-aided positions is calculated based on the matched image features from the second set of images and the image mosaic. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features of the present invention will become apparent to those skilled in the art from the following description with reference to the drawings. Understanding that the drawings depict only typical embodiments of the invention and are not therefore to be considered limiting in scope, the invention will be described with additional specificity and detail through the use of the accompanying drawings, in which: 
         FIGS. 1A and 1B  are schematic diagrams depicting a method for navigation of an unmanned aerial vehicle in an urban environment; and 
         FIG. 2  is a processing flow diagram of a method for navigation of an unmanned aerial vehicle in an urban environment. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. It is to be understood that other embodiments may be utilized without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense. 
     The present invention relates to a method and system for navigation of an unmanned aerial vehicle (UAV) in an urban environment. The present method and system are useful in GPS-denied navigation of a UAV. In general, the present method employs an initial UAV fly-over of an urban terrain at a higher altitude that will later be traversed at a lower altitude in the urban canyon. During the initial fly-over, GPS-tagged images are captured, and these images are stitched into a GPS-aided image mosaic. During the subsequent lower altitude flight in the urban canyon, images captured by the UAV camera can be matched to the fly-over image mosaic, and the current position of the UAV relative to GPS-aided positions used to acquire the fly-over image mosaic can be calculated. This results in a determination of the absolute position of the UAV at about the same accuracy as the GPS-aided positions originally used to construct the image mosaic. 
       FIGS. 1A and 1B  are schematic diagrams depicting the method for navigation of a UAV  110  in an urban environment  120  having various buildings  122 . The UAV  110  can be a hover-capable aerial vehicle or a fixed-wing aerial vehicle. One or more image sensors such as a camera can be used by UAV  110 , which provide a means for capturing images during flight. Additional sensors that can be used by the UAV include one or more GPS sensors for obtaining GPS measurements; inertial sensors such as accelerometers or gyroscopes; a radar sensor (e.g., Doppler radar for velocity measurements); and laser detection and ranging (LADAR) or acoustic sensors (for distance measurements). Various combinations of any of the above sensors may also be employed in the UAV. 
     As shown in  FIG. 1A , UAV  110  travels along a first path  130  in an initial fly-over of urban environment  120  at a first altitude A. A GPS satellite  140  transmits GPS signals  150  to a GPS sensor  112  such as a receiver carried onboard UAV  110 . An image sensor  114  such as a camera captures a first set of GPS-tagged images during the initial fly-over, with each of the GPS-tagged images being related to respective GPS-aided positions. Each of the images is captured within a field-of-view (FOY)  160  of the camera. The captured GPS-tagged images are then stitched together into an image mosaic using the GPS-aided positions. 
     As illustrated in  FIG. 1B , UAV  110  makes a subsequent fly-over of urban environment  120  along a second path  170  at a second altitude B that is lower than the first altitude A. A second set of images is captured by the camera within FOV  160  during the subsequent fly-over. Image features in the second set of images are matched with image features from the image mosaic during the subsequent fly-over. A current position of UAV  110  relative to the GPS-aided positions is calculated based on the matched image features from the second set of images and the image mosaic. This aids in the navigation of UAV  110  flying at the lower altitude, since urban environment  120  can interfere with GPS signals  150  because of an urban canyon effect  180 . 
     The present navigation approach differs from referencing to a satellite image in that the UAV acquires its own reference imagery during a preliminary flight. An advantage of the present approach is the temporal and spatial proximity of the acquired fly-over image mosaic to the images the UAV will later capture while navigating in the urban environment. 
       FIG. 2  is a processing flow diagram of the method for navigation of a UAV in an urban environment. As discussed above, a first set of GPS-tagged images are captured during the initial fly-over of the urban environment at a first altitude (block  210 ), with each of the GPS-tagged images being related to respective GPS-aided positions. The captured GPS-tagged images are then stitched together into an image mosaic (block  220 ) using the GPS-aided positions. 
     A second set of images is captured (block  230 ) in a subsequent fly-over of the urban environment at a second altitude that is lower than the first altitude. The second set of images are non-GPS-tagged images. During the subsequent fly-over, image features from the image mosaic are matched with image features in the non-GPS-tagged images (block  240 ). A current position of the UAV relative to the GPS-aided positions is then calculated (block  250 ) based on the matched image features. 
     The present method automatically establishes the correspondences between the images in the fly-over image mosaic and the current images (non-GPS-tagged) of the UAV. This automatic procedure overcomes differences in lighting, view angle, the scene (e.g., cars parked or moved), and particularly scale between the mosaic images and the current images of the UAV. This can be accomplished by combining (1) an inertial position estimate of the UAV to get a prior distribution and initial estimate for possible matches between the mosaic images and the current images, and (2) cueing based on salient image features to “spontaneously” notice matches between the mosaic images and the current images. A variety of salient image feature types can be considered depending on the exact scenario. 
     The present method can be implemented by utilizing computer hardware and/or software, which provide a means for stitching the captured GPS-tagged images together into an image mosaic using the GPS-aided positions, a means for matching image features from the second set of images with image features from the image mosaic, and a means for calculating a current position of the unmanned aerial vehicle relative to the GPS-aided positions based on the matched image features. 
     Instructions for carrying out the various process tasks, calculations, control functions, and the generation of signals and other data used in the operation of the method and systems described herein can be implemented in software, firmware, or other computer readable instructions. These instructions are typically stored on any appropriate computer readable media used for storage of computer readable instructions or data structures. Such computer readable media can be any available media that can be accessed by a general purpose or special purpose computer or processor, or any programmable logic device. 
     Suitable computer readable media may comprise, for example, non-volatile memory devices including semiconductor memory devices such as EPROM, EEPROM, or flash memory devices; magnetic disks such as internal hard disks or removable disks; magneto-optical disks; CDs, DVDs, or other optical storage disks; nonvolatile ROM, RAM, and other like media. Any of the foregoing may be supplemented by, or incorporated in, specially-designed application-specific integrated circuits (ASICs). When information is transferred or provided over a network or another communications connection (either hardwired, wireless, or a combination of hardwired or wireless) to a computer, the computer properly views the connection as a computer readable medium. Thus, any such connection is properly termed a computer readable medium. Combinations of the above are also included within the scope of computer readable media. 
     The method of the invention can be implemented in computer readable instructions, such as program modules or applications, which are executed by a data processor. Generally, program modules or applications include routines, programs, objects, data components, data structures, algorithms, etc. that perform particular tasks or implement particular abstract data types. These represent examples of program code means for executing steps of the method disclosed herein. The particular sequence of such executable instructions or associated data structures represents examples of corresponding acts for implementing the functions described in such steps. 
     The present invention may be embodied in other specific forms without departing from its essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is therefore indicated by the appended claims rather than by the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.