Synthetic Waypoint Guidance

Introduction

The development of aircraft guidance, navigation, and control systems has been a long-standing research area. Two common types of conventional aircraft guidance are the direct-to-waypoint (DTW) and track-to-waypoint (TTW) methods in relation to path-following between designated waypoints. The DTW method simply issues heading commands to the vehicle based on the angular difference between the waypoint and vehicle. The TTW method aims to follow the track between waypoints, minimising the lateral offset. However, both methods are far from optimal. This is evident in how the aircraft transitions between flight paths after reaching a waypoint. During these flight-path transitions, the aircraft will often overshoot the desired flight path to correct its track, particularly when the flight-path transition angle is acute. Various control strategies have been investigated to alleviate or minimize flight-path deviations. Such strategies include applying modern control methods such as receding-horizon control and model predictive control to anticipate flight-path changes and take control action before reaching a goal while maintaining adequate vehicle flight performance.

Synthetic Waypoint Guidance is a guidance law fusing the virtual-target concepts with those of pursuit guidance from missile guidance for implementation in to an aircraft guidance system. This guidance law minimizes the waypoint transition effects and produces a smooth flight path without the need of smoothing the waypoint path before hand or the need for complicated path-switching logic or trajectory planning.

Method

Synthetic Waypoint Guidance is a path-following aircraft guidance algorithm that pursues synthetic waypoints using only a small set of guidance parameters, extending the virtual-target concept to complete aircraft guidance. The path is defined by the track between a minimal set of waypoints at specified locations, removing the need for a smooth path to be defined or the need for complicated path-switching logic or trajectory planning when a waypoint is reached. The synthetic waypoint travels along the path between waypoints, with the trailing aircraft traveling a smooth path generated through its own dynamics in following the synthetic waypoint.

The difference between conventional guidiance laws and the synthetic waypoint guidance law at the waypoint transitions can be seen in the following figures. The synthetic waypoint guidance inherently includes flight path prediction due to the tracking of a synthetic target which helps to smooth the resulting flight path of the aircraft.


Conventional Guidance Laws
Conventional Waypoint Methods
(a) Direct-to-Waypoint Method (b) Track-to-Waypoint Method


Synthetic Waypoint Guidance
SWG Method
Synthetic Waypoint Guidance Flight Path for an Acute Angle Transition

Publications