Instrumentation & Measurement Magazine 24-4 - 93

object in the surveyed area. For the further design steps, only
the height measurement will be considered for the definition
of the UAV-based instrument, since it is the measurement that
usually exhibits the highest uncertainty [7].
Step 2 - For the considered application, the measurement
range, which depends on the maximum height of the object in
the surveyed area, has been considered equal to 3 m.
Step 3 - This step requires the analysis of the measurement
uncertainty related to the height measurements. Thus, the
measurement procedure and the technology to obtain this
kind of measurement has to be chosen. For the height measurements,
the most commonly used technologies are: the aerial
photogrammetry, which is based on the acquisition of several
RGB images at several waypoints during the UAV flight; and
the aerial laser scanning, which is based on the acquisition of
the 3D point clouds provided by a LIDAR embedded on the
UAV at several waypoints.
In the case of aerial photogrammetry, the uncertainty
model reported in [7] has been adopted. This model considers
the case of measuring the height of objects by processing two
images acquired by the UAV at two waypoints. In Fig. 4a, the
height measurement uncertainty, uh
, against flight altitude and
the uncertainty related to the measurement of the UAV orientation,
uθ
, is depicted. From this analysis, to achieve a target
uncertainty in the order of 20 cm for a maximum flight altitude
of 14 m: the maximum position measurement uncertainty
has to be 20 cm; and the maximum allowed orientation uncertainty
must be 10°.
On the other hand, in case of aerial laser scanning, the LIDAR
provides the distance of each point respect to its z-axis.
This measurement has to be aligned to the UAV coordinate
reference system for providing the height of the object. The uncertainty
sources affecting the height measurements are: the
uncertainty related to the distance measurement provided by
the laser; and the uncertainty related to the measurements of
the laser beam pitch and roll angles. In Fig. 4b, the height measurement
uncertainty against flight altitude and uncertainty
related to the orientation measurements uθ is depicted. To
achieve a target uncertainty in the order of 5 cm for a maximum
flight altitude of 50 m: the maximum uncertainty related
to the distance measurements has to be 10 mm; and the maximum
orientation uncertainty must be 0.2°.
Step 4 - According to the uncertainty analyses, it is possible to
choose the type of sensors for navigation and sensors for a mission
that will be embedded on UAV for both the techniques.
In the case of aerial photogrammetry, the distance between
the two waypoints, where the images are acquired, and the orientation
of the second waypoint with respect to the first one,
are estimated by means of image processing algorithms [7] applied
on the two images acquired by the RGB camera. In this
case, the payload will include the RGB camera with a gimbal,
called Zenmuse X4S, and the sensors for navigation will consist
of the GPS receiver and the INS embedded in the N3 DJI
module. The measurements provided by the sensors for navigation
are used for the autonomous navigation of the UAV,
only. As reported in [7], the estimated measurement uncertainty
will be 16 cm at the flight altitude of 14 m.
In the case of aerial laser scanning, the payload will consist
of the LIDAR mini VUX-UAV, and the sensors used for navigation
are the GPS receiver and the INS embedded on the N3
DJI module. Since the chosen LIDAR exhibits a distance uncertainty
of 10 mm at 50 m of flight altitude, and an orientation
uncertainty of 0.2°, the estimated uncertainty for the height
measurement will be 5 cm.
Step 5 - For both the solutions coming from the previous steps,
since the application requires the survey of wide archaeological
sites, and all the remote control, telemetry and payload
data, has to be sent and/or received by the ground control station,
the COFDM standard has been selected.
Step 6 - In order to define the UAV platforms for both the solutions,
the UAV weight budgets have been estimated. The
weight of the payload for the aerial laser scanning solution
is higher than in the case of aerial photogrammetry; consequently,
this solution presents a higher maximum total weight
Fig. 4. The height measurement uncertainty uh is depicted against flight altitude and the uncertainty related to the measurement of the UAV orientation uθ
aerial photogrammetry; b) For aerial laser scanning.
June 2021
IEEE Instrumentation & Measurement Magazine
. (a) For
93

Instrumentation & Measurement Magazine 24-4

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