SGDrape
SGL´s Computer Aided Drape Flying System

High resolution geophysical data require surveys to be flown at a consistent height above the ground. It is also important that the survey aircraft maintains a consistent and safe altitude on the two orthogonal survey line directions.  SGL has designed and implemented a computer-assisted system, SGDrape, to enable the company's flight crews to maintain an optimal flight altitude (drape surface) during surveying while at the same time ensuring that primary and control lines intersect at the same altitude.  The result has been a vast improvement in the consistency of SGL´s high resolution data acquired, particularly in hilly or mountainous terrain.

To create a drape surface, a digital terrain model of the survey area is modified to reduce all slopes to within the capabilities of the survey aircraft to safely climb and descend.  The drape surface is then loaded into the survey aircraft´s navigation system, SGNav, together with the planned flight lines.  As the aircraft flies along the survey lines the navigation system provides guidance to the flight crew using a modified Instrument Landing System (ILS) indicator mounted in the cockpit, and the flight crew matches the aircraft´s altitude to the planned altitude for that location.  Altitude information comes from Real-time Differential GPS (RDGPS), or the aircraft´s radar or laser altimeter.

A typical application for SGDrape is shown in the diagram below, where the mountains rise out of the ocean and reach an elevation of about 2000 m in only a few kilometres.  Without the drape flying program it is difficult for the survey crew to fly adjacent and orthogonal lines at a consistent altitude.  Decisions must be made as to when to start climbing while approaching a hill or ridge, how deep into a valley to descend, how steeply to descend off a ridge, and what altitude to fly when flying parallel to a ridge to enable control lines to intersect with all of the primary lines.  The task of defining the correct drape altitude is much better performed by a computer using a digital terrain model, leaving the flight crew more time to fly the aircraft, watch for other aircraft and obstacles, and monitor the survey instruments.  The SGDrape system removes the guesswork from the selection of survey altitudes in rough topography, to enhance the quality of the final data, and to improve the safety of the survey operation.

Rugged terrain with draped flight lines - Nominal survey altitude of 300 m AGL

System Advantages

The main advantages of the SGDrape system are:

• Preparation of the drape surface allows for pre-survey analysis and optimization of the survey line direction, as well as a quantification of the achievable flying height over the entire survey area.  This results in more efficient use of exploration dollars.
• Prior to survey flying clients can see what the drape surface looks like and can examine what altitude the sensors will be at over the entire survey area.
• Control line and traverse lines intersect at the same altitude, which facilitates data levelling.
• The drape system is a tool which makes flying a smooth drape much easier by extending the advantages of electronic navigation to the third dimension, which allows the flight crew to fly a better, safer survey.  The system effectively takes the guess work out of maintaining a safe and consistent survey altitude.
• The drape surface removes significant differences in the altitude between subsequent flights due to changing weather conditions (wind, visibility, temperature, etc), changing aircraft fuel load, and different flight crews.  Differences in the altitude of adjacent survey lines flown in opposite directions are also minimized.
• The system is not an autopilot, so the pilot is always in full control of the aircraft.

A digital terrain model is essential for the process.  In areas of steep topography, locations and altitudes of the highest points are most important.  Topography below the highest points, which is beyond the climb/descent rate of the aircraft need only be modelled in a very general manner. 

In many cases, surveys are flown in areas where only small scale maps, sometimes of questionable accuracy, are available.  SGL endeavours to find, or create, the best digital terrain model possible.  We have found that SGDrape is not particularly sensitive to minor inaccuracies in the original terrain model, because the desired result is to drape over the topography, not to follow the terrain exactly, especially over very steep terrain. 

The system also offers a significant safety advantage because the rate of climb is limited before the start of the survey operations to that which can be safely maintained by the aircraft.  SGDrape reduces pilot workload and fatigue, leaving more time for the crew to attend to other flight tasks.  The company continues to limit survey operations to good VFR conditions under daylight hours allowing the flight crew to maintain a careful lookout for uncharted obstacles and avoid situations where the aircraft may be unable to maintain a safe altitude and airspeed.