Sentera PHX - Terrain Following User Guide

Sentera PHX - Terrain Following User Guide



PHX® Terrain Following

Feature User Guide

Engineered in Minneapolis, MN

May 3, 2021

Doc # 17030 Rev A


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1          Terrain Following Feature
            1.1           When to Use Terrain Following Feature
                        1.1.1         Analytics Data Collection
                        1.1.2         Flight Safety
                        1.1.3         Other Data Collection
            1.2           Limits
                        1.2.1         Obstacles
                        1.2.2         Accuracy
                        1.2.3         Mission Size
            2.1           Enabling the Terrain following Feature
            2.2           Recommended Settings
                        2.2.1      Defaults
                        2.2.2      Tolerance
                        2.2.3      Max Climb Rate
                        2.2.4      Max Descent Rate
            2.3           Best Practices
                        2.3.1         Planning
                        2.3.2         Ground Setup and Area of Operation
                        2.3.3         In-Flight Operation
                        2.3.4         Landing
            2.4           Using a Terrain Following Enabled Mission Plan
                        2.4.1         Uploading the Mission to the Drone
                        2.4.2         The Terrain Bar
            2.5           Offline Maps
                        2.5.1         Downloading Terrain Elevation Data
3          Glossary
4          Support
5         Revision History

0 Terrain Following Video Guide

The Terrain Following Feature for PHX can be used to help maintain a more consistent GSD and can help improve Sentera Analytics results. The Terrain Following Feature is useful when flying over terrain with 30ft or more of elevation change.

The user guide contains the information to safely and properly use this feature. The video is only a tutorial of how to enable the feature in a mission.
WARNING: The video does not contain all of the information needed to safely and properly use the terrain following feature. Refer to the user guide for complete information.

1 Terrain Following Feature

The terrain following feature uses published terrain data to plan changes in altitude during flight to maintain a more constant height above ground.


1.1 When to Use Terrain Following Feature

The terrain following feature should be enabled when flying PHX in areas with terrain variation. The terrain following feature can be useful in several situations:


1.1.1 Analytics Data Collection

Using the terrain following feature can improve FieldAgent processing and analytics results by maintaining a more consistent GSD value during the data collection.


1.1.2 Flight Safety

Some areas have changes in terrain altitude that can be hazardous during low altitude (150-200ft AGL) flights. Using the terrain following feature can help compensate for the changes in elevation that may lead to unplanned obstacle interactions.


1.1.2 Other Data Collection

If the data being collected is sensitive to changes in GSD the terrain following feature can be enabled for improved results.


1.2 Limits

The terrain following feature has limits in its total capability. These limits are as follows.


1.2.1 Obstacles

The terrain following feature is not an obstacle avoidance feature. It will not account for, detect, or avoid obstacles such as trees, buildings, and powerlines.

New construction or recent elevation changes may not be captured in the terrain elevation database. Use caution when using the terrain following feature near recent construction (or similar) areas.


1.2.2 Accuracy

The terrain following feature is limited in accuracy by the supplied terrain elevation data and the accuracy of the aircraft navigational sensors. RTX

The accuracy of the terrain following feature cannot be improved with RTK GPS. The PHX does not currently use RTK GPS information for navigation. RTK GPS information may improve the height above ground information in the image metadata but will not improve GSD consistency.

1.2.3 Mission Size

When the terrain following feature is enabled more waypoints are added to the mission plan to command the changes in altitude. The number of added waypoints is dependent on the tolerance setting described in Defaults section and the terrain elevation variation of the area being flown over. See the example mission plans below.

It is recommended not to exceed 2000 waypoints when planning a mission for the PHX. No Terrain Following Enabled

The mission plan contains 272 waypoints, as shown in Figure 1.         

        Figure 1: Mission Plan Terrain Following Enable and Tolerance Set to 30 Feet

The mission contains 317 waypoints, as shown in Figure 2.

        Figure 2: Waypoints Terrain Following Enabled and Tolerance Set to 0 Feet

The mission contains 916 waypoints, as shown in Figure 3.


Figure 3: Waypoints

2 Using the Terrain Following Feature

This section contains information on how to enable terrain following and the associated best practices.


2.1 Enabling the Terrain following Feature

Fly PHX does not use terrain/elevation data by default. The feature must be enabled during mission planning.

 1.      Plan the take off and survey mission items, as shown in Figure 4.

Figure 4: Survey Mission Items

2.      Press Done Adjusting, as shown in Figure 5.

3.     Then reselect the Survey mission item to open the survey details menu.

Figure 5: Done Adjusting

4.      Select the Terrain menu. Then check the Vehicle follows terrain checkbox, as shown in Figure 6.


        Figure 6: Checkbox

Note: Terrain following is not enabled by default for the survey planned in the mission item. This process will need to be for each survey mission item in the planned mission. This process will also need to be done for each unique mission plan.

2.2 Recommended Settings

A description of how to use the terrain following settings.


2.2.1 Defaults

The default settings for terrain following, as shown in Figure 7.
        Figure 7: Defaults

2.2.2 Tolerance

The tolerance value is used to set the threshold that determines if there is a large enough change in reported elevation between adjacent terrain tiles to command a change in flight altitude.

A 15-foot tolerance is recommended for use with PHX. 

2.2.3 Max Climb Rate

The default setting is recommended

2.2.3 Max Descent Rate

The default setting is recommended.

2.3 Best Practices

The best practices for using the terrain following feature.

2.3.1 Planning

When planning a mission with the terrain following feature enabled, do not assume that all terrain elevation and obstacles are accounted for automatically (see Limits section for information on the limits of the terrain following feature).

Trees, power lines, and fences are not accounted for in the terrain elevation database.

Check the backside of hills and tree lines for obstacles that may be hidden from sight.

Plan accordingly and check the area(s) that will be flown over during the mission to ensure there are not obstacles that may interfere with the drone while in flight.

2.3.2 Ground Setup and Area of Operation

Take into consideration where the drone will be set up, launched, and operated from. Varying terrain can cause loss of visual line of sight and reduced or lost communications between the drone and the CommsBox. For example, the drone may fly over the back side of a hill where it cannot be seen.

Set up the area of operation in a location that gives the most consistent line of sight for visual observation and communications signal. This location is often an area at the highest terrain elevation, such as the top of a hill.

2.3.3 In-Flight Operation

Always maintain visual line of sight. Ensure the drone is ascending and descending as expected.

Deviation from nominal flight may be grounds for using emergency procedures to land the drone or terminate flight. Prioritize the safety of the operator and others in the surrounding areas.

2.3.4 Landing

The landing portion of the flight plan does not use terrain elevation data during execution. Always plan the landing on a flat stretch of ground. Never land into the face of a hill. Landing on the back side of hill may cause the drone to overshoot the planned landing zone.


2.4 Using a Terrain Following Enabled Mission Plan

A description of using a mission with the terrain following feature enabled.

2.4.1 Uploading the Mission to the Drone

Press Upload Required at the top of the screen to send the mission to the drone, as shown in Figure 8.

Figure 8: Upload Required

Because the terrain following feature adds waypoints to the mission plan, the mission may take longer to upload the drone.

Once the mission is uploaded to the drone, the waypoints will appear on the fly screen.

2.4.2 The Terrain Bar

The mission planning screen shows how the terrain data and flight altitude vary during the mission using the terrain bar at the bottom of the screen. The black rectangles represent the reported terrain elevation variation, and the orange circles represent the planned aircraft flight altitude at each waypoint. When the terrain following feature is enabled the orange circles will include altitude variation, as shown in Figure 9.

Figure 9: Orange Dots Terrain Bar with Terrain Following Disabled 

To see the terrain elevation variation the mission must be downloaded from the autopilot after it has been uploaded, as shown in Figure 10.

Figure 10: Download

This converts the mission items into individual waypoints, allowing the user to see the terrain elevation and flight altitude at each waypoint, as shown in Figure 11.

Figure 11: Waypoints Terrain Bar with Terrain Following Enabled

Sometimes all the waypoints cannot be shown at once. Grab anywhere inside of the terrain bar and scroll to the right or left as needed to see all the terrain and waypoint information., as shown in Figure 12.

Figure 12: Scrollbar

Note: It is important to check that the orange circle that represents flight altitude never intersects or goes below the black rectangle that represents the reported terrain elevation. CHECK ALL POINTS FOR THIS BEFORE EVERY MISSION.

Negative terrain interaction may cause the aircraft to impact with the ground, as shown in Figure 13.

Figure 13: Impact

When this occurs and the mission is uploaded, the mission checker may report a warning stating that a waypoint is below the home point altitude, as shown in Figure 14.


Figure 14: Warning

Manually checking for negative terrain interaction needs to be done even if no warning message appears.

This warning can also be present when operating from the top of a large elevation change. For example, if you plan to set up and launch the drone from on top of a hill then survey over a field below the hill. If the flight altitude over the field is lower than the elevation of the field, then the warning will be sent by the mission checker, as shown in Figure 15.


Figure 15: Warning

2.5 Offline Maps

A description of the use of offline maps and terrain elevation data.

2.5.1 Downloading Terrain Elevation Data

Use the offline maps feature as described in the Sentera PHX User Guide to prepare to download map tiles with terrain elevation data. Offline Map Download Menu with Fetch Elevation Data Enabled   

Once the map download settings have been chosen, select Fetch elevation data option in the download menu, as shown in Figure 16.
         Figure 16: Fetch elevation data

Verify that all map and elevation tiles have been downloaded in the offline maps screen. Offline Map Download Verification

Figure 17 displays example of map and elevation tiles that have successfully downloaded.
            Figure 17: Elevation Tiles

3 Glossary

Definitions of used terms.

Above Mean Sea Level
Above Ground Level
Ground Sample Distance

4 Support

Need Support? Email us at or call us 844-736-8372.

5 Revision History

This table list the revision history.


Release No.


Revision Description

Rev -

May 3, 2021

Initial document.