This paper presents a development of an open-source flight planning tool for Remotely Piloted Aircraft Systems (RPAS) that is dedicated to high-precision photogrammetric mapping. This tool contains planning functions that are usually available in professional mapping systems for manned aircrafts as well as new features related to GPS signal masking in complex (e.g. mountainous) terrain. The application is based on the open-source Java SDK (Software Development Kit) World Wind from NASA that contains the main geospatial components facilitating the development itself. Besides standard planning functions known from other mission planners, we mainly focus on additional features dealing with safety and accuracy, such as GPS quality assessment. The need for the development came as a response for unifying mission planning across different platforms (e.g. rotary or fixed wing) operating over terrain of different complexity. A special attention is given to the user interface, that is intuitive to use and cost-effective with respect to computer resources.