

First Fix: Two PNTs are better than one

March 22, 2021  - By

With a very good PNT device already installed for flying the aircraft, why not just tap into that one for the payload, right? This might not be a good idea, for several reasons.

By John Fischer
Vice president, Advanced R&D, Orolia

Photo: Orolia

John Fischer. (Photo: Orolia)

The navigation device in a UAV is very important, precisely because there is no pilot. It must navigate autonomously. It must also be optimally suited for the airframe, either fixed or rotary wing, providing the accuracy and reliability for all modes of flight, from takeoff to landing. A lot of engineering goes into the design and certification of each UAV’s navigation system to qualify it for flight.

UAVs can have multiple missions with interchangeable payloads: cameras for observation and inspection; communication equipment for relaying links or supplying emergency cellular base stations; or sensing equipment such as radar, lidar, spectrometers, etc. These payloads also need positioning, navigation and timing (PNT) sources for their missions, for example, to accurately geo-timestamp the collected data.

With a very good PNT device already installed for flying the aircraft, why not just tap into that one for the payload, right? Actually, this might not be a good idea, for several reasons.

Recertification. Modifying the navigation device, which is part of the flight control system, risks having to re-certify the aircraft for flight safety. Though a UAV has less severe restrictions on safety than a manned aircraft, it can still cause property damage or even injury and loss of life if it crashes in a populated area. The Federal Aviation Administration has numerous standards — DO-178 for software, DO-254 for hardware, DO-160 for testing — to ensure avionics are designed and tested for safe operation. Every modification, regardless of how small, must follow these standards and may require expensive re-certification of the aircraft’s airworthiness.

Performance Requirements. These vary with each mission. The flight control system includes a navigation device that was selected based on the aircraft’s special requirements. These will not necessarily match the needs of the payload. For example, consider pitch, roll, and yaw sensing accuracy. The accuracy required to determine the pointing angle of a camera might not be the same as what is needed for level flight.

Interchangeability. A particular UAV can have multiple payloads for different missions. Conversely, a particular mission payload can be adapted and installed on several different UAVs. Having a second PNT device matched to the payload allows it to stay with the payload as it is moved to different UAVs. This can lower the total cost of ownership and operation, since the extra cost of a second device is small compared to the adaption work and design changes necessary to make a single PNT device be suitable for all situations.

Missing the T in PNT. Typically, the navigation device for flying the aircraft doesn’t have a precise internal oscillator for supplying time and/or frequency — it doesn’t need it. However, most payloads can benefit from the time/frequency component to enhance mission performance. A low phase noise oscillator with low g-sensitivity that is disciplined by the precise time supplied by a GNSS receiver can substantially improve the performance of any payload radar or communication system.

A second device does not impact SWAP or cost significantly — GNSS receivers and inertial navigation systems are no longer large, expensive items. A second PNT device is typically small, weighing less than a kilogram and consuming only a few watts of power. There are also fewer connectors and cable harnesses when a removable payload is not sharing the aircraft’s PNT data, so the weight differential might be zero. PNT devices can share antennas on the aircraft via splitters, so there is no need to place additional antennas.

Technology upgrades. Micro-electromechanical systems (MEMS), inertial sensors, cameras, lidars, radars and other sensors are all evolving at a rapid pace with better technology available with each passing year. Flight control systems evolve at a different pace — mostly because of the flight certification process, but also for lack of a driving need. UAVs navigate just fine with the equipment they have today. A separate payload PNT device allows the system designer to keep pace with evolving technology, choosing the latest and best for the mission without disrupting the navigation system.

Just as “two heads are better than one” for problem solving, having two PNT devices in a UAV is often the better solution.

John Fischer is vice president, Advanced R&D, Orolia, and a member of GPS World’s Editorial Advisory Board.

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