Unmanned Aircraft Navigation

March 19, 2012  - By
Unmanned Aerial Vehicles (UAVs), or as they are becoming known Unmanned Aircraft/Aerial Systems (UAS), are in the news now almost every day. As UAV/UAS applications become more and more sophisticated, and operators seek to integrate them into commercial undertakings, the prospect of civil certification and unrestricted access to U.S. airspace is becoming a near-term possibility. This article discusses current UAV/UAS GNSS navigation capabilities and how this could change in the future.


Recently I’ve become involved in looking at civil certification requirements for UAV/UAS GNSS receivers and discovered the prospect that these air vehicles could be operating alongside regular manned aircraft, or at least within sectors of the same airspace. This might sound a little daunting, especially for people who fly civil transport aircraft and helicopters, but the powers that be are intent on insuring the safety and security of our airspace systems with UAS, and it’s taking a lot of brain power to devise ways forward.

image001New systems seem to be advertised on a regular basis, and development has given way to production and fielding for many of them. Manufacturers like Boeing/Insitu, Yamaha, IAI/Malat, and General Atomics are working in the UAV/UAS marketplace alongside other major players like Raytheon, Thales, Sagem, and a host of other companies who have carved out niches in what seems to be an exploding commercial and military business.

So, where exactly are we today with navigation and GNSS on UAVs? Well, you first have to know the vehicle and its applications. A few examples might help. There are a couple of well publicized vehicles which have become quite successful in civil applications. If we first consider the Yamaha RMAX remotely piloted helicopter, the most prevalent application appears to be crop spraying in Japan and elsewhere in Asia. So GPS RTK is a great navigation system for RMAX, especially since this is a commercial application and its unlikely that there would be jamming or spoofing to disrupt the receiver or its radio links. Low-altitude, short-range from the controller — contained and relatively safe, you might think.

RMAX – Crop Spraying

In a relatively short-range application like RMAX, existing high-precision GPS technology seems to work just fine, especially if an operator is nearby and able to take over manual control if anything untoward goes wrong. This is a very successful commercial vehicle and several hundreds have been sold since its introduction in 2003.

Another system which has sold in large numbers is the Boeing/Insitu ScanEagle. No wheels on this baby — launch is via a high-power catapult. This system also navigates with a commercial GPS receiver, which is key to the unique auto-capture system used for retrieving the UAV. The vehicle is flown in low-level horizontal flight to intercept a cable suspended from a portable 50-foot gantry. Precision guidance involving GPS RTK ensures capture when the UAV wing flies into the cable and a hook on the wing snags the cable. Lift is immediately dissipated and the UAV can be recovered by ground crew as it dangles on the cable.

image003   image005

Any summary of “typical” UAS would be lacking without mention of the General Atomics MQ-1 Predator and its bigger brother, the MQ-9 Reaper. These vehicles carry not only INS, but also a high-end commercial GPS receiver used for take-off and landing.

image007 . Credit: Tony Murfin

While these systems are clearly war machines, there are also some in semi-commercial service, including border patrol. If you’ve ever seen one of the many U-tube videos of take-off or landing, it’s hard to see much difference in performance from that of, say, a B-737 or any civil transport. And one of the contributing reasons is that the UAV uses somewhat similar inertial-GPS terminal/landing guidance that civil transports use — a mix of GPS and inertial. We could probably guess that these aircraft also carry a P-code military GPS for remote operations in hostile territory, where enemy jamming and/or spoofing might throw the commercial receiver for a loop. But most UAVs would likely take off and land in a “friendly” environment, so commercial GPS will probably provide the precision needed for these operations.

Now, let’s fast-forward to a time when local law-enforcement wants to patrol or search over populated areas where people are used to seeing manned patrol fixed-wing aircraft, or more likely, police helicopters. Or the local TV station wants to fly cameras overhead for a news story in the making using an unmanned vehicle, or when crop spraying in Kanas or Iowa uses UAS because it has become more economical than using manned aircraft. With the success of UAS in military applications, it’s clear that there is a growing demand to introduce them into civil operations, in a somewhat similar way that GPS transitioned from being a purely military system into extensive commercial use.

How can we make sure these systems operate safely in civilian airspace? Well, that’s an answer the Federal Aviation Administration (FAA) has been seeking for several years. Like most FAA rule-making, the agency turns over a good part of the formulation of technical requirements to the Radio Technical Commission for Aeronautics, more commonly referred to as the RTCA. The RTCA serves as a Federal Advisory Committee, supported by around 400 government, industry, and academic organizations from the United States and around the world. The FAA normally commissions RTCA to develop consensus-based recommendations, which are passed onto the FAA, who normally turn them into regulations which govern aircraft operations in the National Airspace System (NAS).

An RTCA Special Committee (SC) is formed with predetermined terms of reference by asking industry, government, and others if they will provide knowledgeable, contributing members who will meet, discuss the issues, and propose practical technical solutions. These proposals are developed, mulled over, and refined, sometimes over many years and, when found to be acceptable through joint RTCA management and FAA review, they are given to the FAA, which turns them into operational or technical regulations. The game starts for a new system like UAS with the formulation of Minimum Aviation System Performance Standards (MASPS).

For UAS, RTCA “stood up” special committee SC-203, which appears to have been up and running since around 2004. Two guidance documents have been developed and published — RTCA DO-304 “Guidance Material and Considerations for Unmanned Aircraft Systems” and RTCA DO-320 ‘Operational Services and Environmental Definition for Unmanned Aircraft Systems.” Both are essential work the committee had to cover first before they were able to get to MASPS for UAS. And yes, there are lots and lots of acronyms, abbreviations, and special terms in this world of unmanned aircraft.

I attended a recent meeting of SC-203 at the RTCA offices in Washington, D.C. My motivation was to discover what the game plan might be and how it could relate to GNSS avionics and systems. What struck me immediately is that there are a lot of organizations and lots of people who are interested enough to attend — more than 100 government, industry, and other representatives turned up for the opening session.

We listened to a summary of what’s been achieved, what the work plans were for the three-day meeting, and what the target schedule was for release of UAS MASPS. A lot’s been done, but the pressure is high to get to regulations soon so commercial operations can get going. UAS operate in civil airspace today, but largely through temporary Certificates of Authorization (CoA) — often with an observer and even a chase aircraft.

Four Working Groups (WG) have been established: Systems Engineering, Sense and Avoid, Human Factors, and Command and Control. I found what I was looking for in WG1 Systems Engineering when we finally on the last day got to talk about navigation. Navigation is seen as a principle function at the same level as sense and avoid, manage, and command and control, but little work seems to have been done to date in this area. I became a little alarmed at one stage when it seemed that there was a bias against GPS as it “can suffer from jamming and interference.” Sure, but other systems have their failings — like rate of position/velocity drift for inertials — but the beauty of an integrated navigation suite is that one system overcomes another’s potential weaknesses. After all, GPS is THE approved navigation system around which the navigation in the NAS is now built — virtually all manned aircraft use GPS in North America! Admittedly we have back-up systems like VHF Omnidirectional Range (VOR) and Distance Measuring Equipment (DME), which are cornerstones of the NAS airway navigation system, but GPS is the chosen way forward and is key to the FAA NextGen future airspace architecture.

So, jumping to the obvious question — when will we have MASPS and when will UAS be able to fly when they want in U.S. airspace? Not as easy as that, I’m sorry to say. This is a huge task — just consider what it took to build up all the rules and regulations that allow the many thousands of commercial aircraft operations we see every single day in North America and around the world. Years and years of work went into building safeguards and proving their effectiveness. Now we have vehicles that are piloted from remote locations with limited visual “awareness” of their surroundings, by pilots sitting at motionless consoles that appear more like video games than cockpits. But progress is being made, with the objective that no special operational considerations are desired beyond how manned aircraft are currently operated in civil airspace.

So UAS have to step up to this challenge if they want unrestricted airspace access, and that probably means significant changes in vehicle and systems design, qualification, and certification. Given that a large number of UAS use GPS RTK in take-off and landing modes, its likely that we’ll see new qualification standards for these receiver systems. Radio links will have to meet safety requirements, and receivers will probably have to meet more stringent requirements, similar to which aircraft receivers are currently qualified. For your regular land-survey RTK system, there are no requirements which push us to the same software qualification standards as airborne systems. But it’s hard for probabilistic RTK algorithms to meet these current qualification standards that demand 100% repeatability and known outcomes, not likelihood, as in carrier-cycle ambiguity resolution. Things will improve as new constellations come on line, and we can approach certainty with many more measurements from many more satellites.

image009 . Credit: Tony MurfinLeica Viva RTK System

The U.S. government is pushing hard, however, to get to civil standards for UAVs. A recently announced competition is seeking to establish six new test facilities for UAS with one of the objectives to support certification objectives, alongside the ongoing technical committee activities. NASA is also planning a UAS demonstration with a primary objective to qualify an unmanned system for operations in U.S. controlled airspace.

Nevertheless, wherever RTK is essential to operations, it’s quite possible that UAS landing/launch and take-off/recovery could be limited to “company facilities” outside of controlled airspace where regular manned aircraft fly watchfully by.


Tony Murfin
GNSS Aerospace

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About the Author: Tony Murfin

Tony Murfin is managing consultant for GNSS Aerospace LLC, Florida. Murfin provides business development consulting services to companies involved in GNSS products and markets, and writes for GPS World as the OEM Professional contributing editor. Previously, Murfin worked for NovAtel Inc. in Calgary, Canada, as vice president of Business Development; for CMC Electronics in Montreal, Canada, as business development manager, product manager, software manger and software engineer; for CAE in Montreal as simulation software engineer; and for BAe in Warton, UK, as senior avionics engineer. Murfin has a B.Sc. from the University of Manchester Institute of Science and Technology in the UK, and is a UK Chartered Engineer (CEng MIET).