1. A method for navigation of one or more unmanned aerial vehicles in an urban environment, the method comprising: flying a first unmanned aerial vehicle at a first altitude over an urban environment such that the first unmanned aerial vehicle is aided by Global Positioning System (GPS) signals; flying a second unmanned aerial vehicle at a second altitude over the urban environment that is lower than the first altitude such that the second unmanned aerial vehicle is denied access to GPS signals, wherein the first and second unmanned aerial vehicles are in operative communication with each other so that images can be transmitted therebetween; capturing a first set of images of a selected area in the urban environment from the first unmanned aerial vehicle, the first set of images having one or more image features; capturing a second set of images of the selected area in the urban environment from the second unmanned aerial vehicle, the second set of images having one or more image features that correspond to the one or more image features in the first set of images; matching the one or more image features from the second set of images with corresponding image features from the first set of images; and calculating a current position of the second unmanned aerial vehicle based on the matched image features from the first and second sets of images.