GPS World, April 2016

WITH RICHARD B LANGLEY INNOVATION FLYING SAFE By Joshua Stubbs and Dennis M Akos A s the number of unmanned aircraft systems UAS also called unmanned aerial vehicles and drones in use is increasing across many sectors there is an interest in understanding the robustness of the GNSS engine used on UAS With UAS being integrated into the National Airspace System NAS questions arise about what kind of navigation system should be used on UAS and to what degree it should be standardized Conventional aircraft typically use a certified GNSS receiver for navigational purposes and as UAS will share the sky with conventional aircraft in the future it is not unreasonable that UAS will use similar receivers The first part of this article provides background on the status of GNSS standards for UAS In the second part we discuss why radio frequency interference RFI can be expected on some UAS together with what issues the RFI could cause for the GNSS engine A simple experiment to determine the presence of RFI in the GPS L1 band due to proximity of a GPS antenna to electronics is presented in this section as well The third part of the article discusses real time kinematic RTK positioning for UAS purposes In terms of accuracy RTK positioning often provides the best results The robustness of RTK measurements is questionable though because the technique relies on carrier phase measurements We present a case study which shows some of the issues of using RTK positioning for UAS in this part of the article too GNSS STANDARDS FOR UAS GNSS and especially GPS have been used in aviation for quite some time The GPS receivers used for aviation have to guarantee a certain level of performance to be used and are certified by the manufacturer to deliver said performance The Federal Aviation Administration FAA is working on integrating UAS into the NAS The development of UAS has been quick and has led to a lack of standardization for UAS something that does exist for traditional manned aircraft This has led to operators in most cases having to file for exemptions from the existing rules in order to use UAS It is the ambition of the FAA to transition from issuing exemptions to issuing certifications of UAS once an agreement on regulations has been reached There are still a number of challenges associated with a full integration of UAS into the NAS including regulatory procedural and technical challenges The Wide Area Augmentation System WAAS was the first operational space based augmentation system intended to increase the robustness and reliability of GPS for aviation purposes The WAAS Minimum Operational Performance Standards MOPS document see Further Reading specifies what kind of performance GPS plus WAAS provides to aviation users The MOPS requirements have been carefully examined and extended The maximum in band interference levels for aviation have been theoretically analyzed As long as signal and interference levels are within the specified ranges the required performance should be expected These levels combined with the WAAS MOPS provide the aviation community with the standardization required for manned aircraft operations where lives can be at stake if something were to go wrong with a navigation system A Volpe National Transportation Systems Center report see Further Reading recommends the use of certified GPS receivers for applications where GPS is a critical system This is not yet a requirement for UAS and the question remains unanswered as to whether this will be a requirement for UAS in the future Traditional aviation uses required navigation performance RNP a performance based navigation approach to assess what type of navigation systems can be used for different phases of flight For example while an aircraft is en route an RNP of 2 nautical miles is required meaning the actual position of the aircraft cannot deviate more than 2 nautical miles from a reported position It should be noted that RNP takes the entire system into consideration from the spacesegment to the receiver to the capabilities of the aircraft GNSS receivers used on manned aircraft have to be certified to deliver the RNP for each phase of flight for which they are used Receiver autonomous integrity monitoring RAIM is used to ensure that faulty measurements do not affect the position and navigation solution Due to the nature of RAIM more satellites are required than the traditional minimum of four If GNSS supplements other systems on board the aircraft RAIM may be used to only monitor the quality of the system and it will report when performance is below the required minimum This form of RAIM requires a minimum of five satellites However if the aircraft depends on GNSS for navigation RAIM must be able to determine if a particular GNSS Robustness for Unmanned Aircraft Systems 42 GPS WORLD WWW GPSWORLD COM APRIL 2016