On April 3, 1996, in Dubrovnik, Croatia, a plane crash resulted in
the deaths of U.S. Secretary of Commerce Ronald H. Brown and 33
other passengers and crew. The deadly accident was attributed to
poor weather conditions, a lack of knowledge of the airfield
environment and an off-course approach. As a result of the tragedy,
Congress passed the Ron Brown Airfield Initiative in 2000 to improve
airfield navigation information. Since then, using conventional land
surveying techniques and modern digital photogrammetry, many airport
operators are taking advantage of today’s geospatial and satellite
sensor technology to ensure safer flight paths.
Arrival and
departure paths to and from airports must be kept free of
significant man-made and natural obstacles, including trees,
buildings and other features. The Federal Aviation Administration
(FAA) defines the formal requirements in the United States, as
does the International Civil Aviation Organization (ICAO) for the
rest of the world. In each case, the regulations define a virtual
polygon centered around the ends of the runway, as well as a
series of ever-larger polygons that must be kept free of any
obstacle that an airplane could hit when flying in that space.
As shown below, these virtual polygons, or
“surfaces,” are defined in 3-D space sloping up from the end and
sides of each runway until reaching the edge of the surface. There
are different surfaces for take-off, landing, landing on
nonprecision instruments, landing on precision instruments and
day/night operations. Prior to the Global Positioning System (GPS)
satellite constellation, information regarding obstacles in the
vicinity of each airport could only be collected by extensive
ground survey, with some degree of validation possible with
conventional stereo photogrammetry. Now, with GPS survey equipment
used to locate these obstacles or features precisely, coupled with
the known geometry of satellite stereo imagery or Differential GPS
(DGPS)-controlled aerial stereo imagery, geospatial analysts and
photogrammetrists can take advantage of tools such as ClearFlite
to simplify obstacle identification.
ClearFlite is a digital mapping tool developed for the aviation industry
to help users identify airfield and runway obstructions; export data to
third-party geographic information system (GIS) and 3-D visualization
applications; automatically generate models for single and multiple
runways; and give operators the ability to view 3-D stereo images of
runways and airfields. ClearFlite functionality is available in SOCET
SET, a photogrammetry software package developed by BAE Systems, San
Diego.
Analysts can use ClearFlite to
collect dynamic features such as buildings, hangars, vegetation growth,
and the towers and antennae that accompany today’s explosion in cell
phone growth. Such information is used to automatically generate the
complex airspace surface models defined by the FAA and ICAO. Paper maps
and charts are being phased out in favor of more precise, digital
readouts that can be easily updated and shared via cockpit displays,
laptops and personal digital assistants.
Basic Analysis Procedures
Runway analysis begins with using the known ends of a chosen airport’s
runways to plot the extent of the surfaces applicable for the airport’s
operations. In FAA terminology, these are referred to as the “FAR Part
77” surfaces. Then, using stereo imagery, airport features are collected
in 3-D, using the height and planimetric information for each visible
building or airport feature, and saved as a vector format (Shapefile,
DGN and DXF are all supported by SOCET SET). This allows users to
determine if any of the airport features penetrate the surfaces.
The next step is to model the features in the airport’s vicinity, within
the coverage of the aforementioned surfaces, that may affect an
aircraft’s arrival or departure. Again, using the elevation information
available on the stereo image pair combined with tools such as
ClearFlite, users can develop terrain contours in the airport perimeter.
Stereo imagery clearly defines surface terrain (bald earth), as well as
protruding obstacles (towers, buildings, trees, etc.).
Once all new obstacles have been identified within the surfaces, they
are referenced to existing records to confirm whether they are new or
existing hazards. Depending on the circumstances, a ground survey would
follow or precede this activity to ground validate the obstacles found,
and to ensure that none were missed or had been created since the last
image was acquired. An optional, final step can be performed using 3-D
visualization to demonstrate an airport’s actual arrival and departure
paths, and validate that no potential obstacles penetrate the surfaces.
Benefits for Helicopter Operations
Fortunately, the same type of analysis applies to helicopter navigation.
However, with the increased use of commercial helicopters for news and
traffic monitoring, medical evacuation and police rescue, there’s an
added challenge to clear obstacles in urban areas, as well as remote
areas that aren’t near an airport. Indeed, several recent helicopter
crashes have shown the need for such obstacle studies.
Results are achieved by collecting stereo imagery in the vicinity of the
areas where the helicopter is flying following the steps for runway
analysis, and applying these same principles and additional
specifications for helicopter landing zones to define surfaces and plot
obstacles. This allows safer helicopter operations to hospitals, police
stations, news organizations and other nontraditional landing zones.
Once the analysis is complete, 3-D visualization is used to train pilots
on the correct procedure to avoid potentially hazardous obstacles.
For military operations that occur in mountainous areas such as
Afghanistan and northern Iraq, there are numerous helicopter missions in
search of elusive enemies. The same technology used to develop safe
arrival and departure routes into friendly areas also can be used to
develop arrival and departure paths into hostile territory. Aerial
imagery is scarce in this context, but satellite imagery can be combined
with applications such as ClearFlite to develop more precise terrain and
obstacle models at landing zones and other ad-hoc forward operating
areas. Such applications provide military aviators with a significantly
higher margin of safety from terrain and man-made obstacles in their
paths, particularly when hostile forces preclude the ability to conduct
ground surveys of any kind. Moreover, 3-D visualization of the surface
and potentially hazardous obstacles can be used to provide pilots with
pre-mission training.
Commercial companies such as Dayton, Ohio-based Woolpert Inc. and Delta
Airport Consultants, Charlotte, N.C., have recognized the value of using
ClearFlite to perform more efficient, thorough and cost-effective
flight-obstruction analyses. Clearly, however, the real beneficiaries
are the airport operators, pilots and crew, and passengers, who can
expect safer flight operations.
