How to scan cell towers

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Drone Cell Tower inspection is mainly focused on acquiring as many images as possible to guarantee a full analysis of the different components and geometries during the upcoming stages.

Requirements

Quality of images

In order to design a good data acquisition, it is important to ensure images show the highest possible detail in the structure and antennas:

  • Antenna images from all possible angles. To get the shape of the antenna as well as the rear photo of the antenna.
  • Port configuration of the bottom. The image beneath the antenna is essential for the identification.
  • Tower structure including the foundation and ground cabinets.

Quality of the scene reconstruction

 
Important: Acquired images must be suitable to generate a good quality point cloud. Optimally reconstructed antennas will guarantee good results at the post-processing stage (antenna recognition and measurements).

The final point cloud should meet the following criteria:

  • Low noise level.
  • Surface definition and defined edges.
  • High point density.
  • High relative accuracy, including the correct scale, orientation, and verticality of the tower.

Pix4Dscan_point_cloud.JPG

Figure 1.: Left: Noisy point cloud. Center: Low-density point cloud. Right: (Required) High-quality point cloud

Designing the data acquisition plan

Flight planning depends on the tower structure, the number of points of interest, specific site requirements and tower dimensions.

As a generic example, we can consider a 35m (115 feet) cell tower with 2 antenna nests. Ideally, the missions to perform would be:

  1. One Orbit per antenna nest (green).
  2. One Underneath Orbit per antenna nest (blue).
  3. Cylinder (orange). If the cylinder mission can't be performed due to obstacles (e.g. guided towers), perform multiple single verticals.

Pix4Dscan_35m_tower_example.JPG

Setting references

Cell Tower mission flights are based on certain references that are set based on drone positions while it is flying. 

 
Important: PIX4Dscan does not use the base map for cell tower mission planning. In order to correctly position the mission, characteristic points, for example, the center of the cell tower, need to be identified while flying the drone with the remote controller. The mission is then created using the measured position of these characteristic points.

After tapping the Cell Tower mission in the mission selection menu, follow the guide and determine the Point of interest minimum height, Point of interest center, and the Point of interest maximum height.

Point of interest minimum height

Some locations do not allow users to take-off at the same height as the cell tower base. This includes cases like rooftops or towers located on top of hills. In order to optimize the missions for these use cases, users can set the minimum POI height.

 
Important: This step is optional and should be skipped if the take-off height is at the same level as the cell tower base.

The steps to perform this action are:

  • Place the drone at the same height as the tower base
  • Ensure that the gimbal pitch is -0º (looking forward). It can be set automatically by tapping Set to -0º.
  • Point the camera so that it aims at the base of the tower with the cross-hair.
  • Tap on Next to confirm the current POI minimum height reference.

Pointofinterestminimumheight.jpg

Point of interest center

This is done in order to be as accurate as possible to get the center reference of the Cell Tower to be scanned, warranting that all the flights within the same Cell Tower mission will share the same center position.

If Return to Home (RTH) height is lower than the current drone height when setting the center reference, a pop-up dialog will ask the user to confirm in order to update that value to the current height. This way the RTH will be a safe known height, higher than the tower to avoid potential collisions in case of an RTH.

The steps to perform this action are:

  • Place the drone on top of the tower (at least 10m higher for security reasons).
  • Ensure that the gimbal pitch is -90º (looking down). It can be set automatically by tapping Set to -90º.
  • Point the camera so that it aims at the center of the tower with the cross-hair.
  • Tap on Next to confirm the current center reference.
Pix4Dscan_POI_Center.PNG old_POI_center.jpg

Point of interest maximum height

The next step is to determine the maximum height of the tower. This value is used for planning cylinder missions.

 
Note: Keep in mind that all height values are relative to Home Point Height, so for security reasons ensure to set the home point at the same height as the cell tower base.

To determine the height reference:

  • Align the drone with the highest point of the tower.
  • Ensure that the gimbal pitch is 0º (looking forward). It can be set automatically by tapping Set to 0º.
  • Point the camera so that it aims at the top of the tower with the cross-hair center.
  • Tap Next to confirm the current highest point reference.
Pix4Dcatch_POI_Height.PNG old_POI_altitude.jpg

Sub-mission selection

Once all the references are set, the sub-mission selection panel will be shown. This panel allows the user to select between the different available Cell tower sub-missions.

Cell Tower missions are specially designed to deal with signal lost scenarios, as it can be a common case. If the signal is lost during a mission the drone will continue flying the mission and triggering the images. If once the mission is finished the signal is not back, then the drone will execute the selected signal lost action (RTH, Hover, Land). This guarantees the integrity of the data.

Select the sub-mission to fly by tapping on the sub-mission icon and then tap on Next.

Pix4Dcatch_Cell_Tower.PNG

The number of finished sub-missions and the total number of images for each sub-mission type are shown in case we are currently executing a Cell tower mission. After finishing any of the submissions (except single vertical), the Cell tower mission dialog is displayed. It is then possible to execute another sub-mission selecting it and tapping Set or to exit the mission by tapping on the cross at the top right side of the panel.

Camera parameters

In order to avoid acquiring overexposed or underexposed images, we recommend using the light condition templates (extremely sunny, mostly sunny, mostly cloudy, extremely cloudy) loading the recommended following white balance (WB) and exposure values (EV) for the different mission types.

 
Note: The above exposure values (EV) are reference values, but can be modified within their ranges to adapt the luminosity of the images.

Orbit sub-mission

The main goal of orbit sub-missions is to acquire images with high overlap around the point of interest. In the case of cell towers, this is typically the area of an isolated antenna or of antenna nests, where a group of antennas is located at the same height. Learn which types of antennas are recognized by PIX4Dinspect in AI recognition of antennas.

In order to get an optimally reconstructed point cloud, it is recommended to increase the number of images in the most interesting areas. These areas are normally where the antennas are located, but orbits can be executed at any height where the user wants to have a higher definition/density in the point cloud.

Pix4Dscan_Orbit_Mission.PNG Old_orbit_mission.jpg

Rather than choosing parameters in the panel, values such as gimbal pitch, radius, and height are based on the video feed on the screen, so from the current drone position, which is manually adjusted with the drone's controller. This is to ensure that we are viewing what we are going to scan, so having a correct point of view for the orbit images.

The following parameters can be adjusted.

Parameter Description Recommended value
Flight direction

Flight direction allows users to define the direction of a circular mission

Clockwise or Counterclockwise
Truncate by

Mission truncation allows users to define a truncation angle for the mission, providing flexibility on mission design

Default: 0°, Range: 0° - 315°, Steps: 15° 
Capture mode**

Time interval capture: time optimized continuous flow

Stop at capture point: stability and image centering optimized flow

Time interval capture
Images per orbit The number of images that will be taken along the orbit 80 images
Gimbal pitch Camera pitch at which the images are taken -40º to -35º(min -60º, max -30º)
Radius 2D distance from the previously set reference center 8 m. Based on drone position*
height Orbit height relative to the take-off point Based on drone position, and depending on the POI height

* Recommended value is based on a 2 m radius structure. If the structure is wider then the radius should be increased accordingly.

*Different capture modes are supported for DJI drone

 

Cylinder sub-mission

The purpose of cylinder sub-missions is to guarantee the full scanning of the tower. The overlap between images can be lower as usually there is no need to get a high density/definition point cloud from the tower structure like the one needed for antennas.

Pix4Dscan_Cylinder_Mission_combined.png

 

Parameter Description Recommended value
Flight direction Flight direction allows users to define the direction of a circular mission Clockwise or Counterclockwise
Truncate by Mission truncation allows users to define a truncation angle for the mission, providing flexibility on mission design Default: 0°, Range: 0° - 315°, Steps: 45°
Verticals The number of vertical lines of the cylinder 8
Front overlap The image front overlap in the vertical line 75%
Min height The minimum height of the lowest point in the vertical lines * 15 m (min. 5m above min POI height)
Radius The radius of the cylinder * 8m

The following parameters can be adjusted.

* Recommended value is based on a 2 m radius structure. If the structure is wider then the radius should be increased accordingly.

The following values are automatically calculated:

  • Gimbal pitch: Values will be interpolated along each vertical between -45º at the highest point and the resulting value from looking to the tower base from the minimum height. 
  • The Max flight height: Maximum height for the drone to fly. The drone will start the first line from the top, at a height of Max height = Tower height + Radius.

Single vertical sub-mission

Single vertical sub-missions are meant to replace or complement a cylinder mission in case it can’t be completely performed due to obstacles (e.g. guided towers).

The dashed circle in the map view represents the selected radius. When the drone moves around the point of interest, the starting position of the single vertical is dynamically projected over it. Once a single vertical has been completed, the user remains in the sub-mission view and a green check appears on its location as a visual reference for upcoming single verticals.

Single_Vertical.png image__1_.png

The following parameters can be adjusted.

Parameter Description Recommended value
Front overlap The image front overlap in the vertical line 75%
Min height The minimum height of the lowest point in the vertical lines  15 m (min. 5m above min POI height)
Radius 2D distance from the previously set reference center 8 m*
Flight direction Trajectory of the drone along the single vertical Up to down

* If multiple single vertical sub-missions are performed, it is recommended to use the same radius.

 
Note: We recommend a minimum of 8 verticals per project to guarantee a good reconstruction.

The following values are automatically calculated:

  • Gimbal pitch: Values will be interpolated along each vertical between -45º at the highest point and the resulting value from looking to the tower base from the minimum height. 
  • The Max flight height: Maximum height for the drone to fly. The drone will start the first line from the top, at a height of Max height = Tower height + Radius.

Underneath orbit sub-mission

It is designed to get high-quality images from the lower side of the antennas, connection board, cables, etc. Basically, the strategy is the same as for the orbit sub-mission but with a lower number of images.

Pix4Dcatch_Underneath_orbit_Mission.PNG Old_underneath.jpg

The following parameters can be adjusted.

Parameter Description Recommended value
Flight direction Flight direction allows users to define the direction of a circular mission Clockwise or Counterclockwise
Truncate by Mission truncation allows users to define a truncation angle for the mission, providing flexibility on mission design Default: 0°, Range: 0° - 315°, Steps: 15°
Images per orbit The number of images that will be taken along the orbit 16 images
Gimbal pitch Camera pitch at which the images are taken 25º (min 0, max 30º)
Radius 2D distance from the previously set reference center 8 m
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3 comments

  • Fernando Luís

    The image Underneath orbit sub-mission, similar to the one orbit sub-misson, should have the values used in the example printed in the image (Altitude, Radius, Gimble). Allitude is missing.

     

     

     

     

  • Tomas Bechmann

    I had issues with GPS signal and had to make the radius 18 meters to avoid fall outs. Any issues with a larger radius except for resolution? Any clever tricks to avoid it?

  • Christina (Pix4D)

    Dear Thomas,

    Depending on the drone location, some satellites can be physically hidden leading to a poor GPS signal. Could you eventually try to start the mission in a different location around the cell tower?

    We usually recommend a radius of around 8 meters. A larger value would lead to lower details in the images. Please keep the radius as constant as possible in the different cell tower sub-missions to get a good reconstruction.

    I hope this helps.

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