Using UgCS to plan magnetic and other low altitude UAV survey missions
Low altitude UAV survey missions require advanced UAV operating skills, reliable equipment, and professional software for mission planning in order to safely collect high-quality data while flying close to the ground. A magnetometer attached to a moving UAV by a long tow cable makes surveys even more challenging.
Photo 1 © courtesy: Pioneer Aerial Surveys Ltd.
This article is written in partnership with Pioneer Aerial Surveys Ltd. Pioneer employs GEM System’s UAV magnetometer sensor on multiple UAV platforms, such as NOVAerial Procyon and DJI Matrice 600. The majority of Pioneer’s surveys take place in the remote Canadian wilderness, which is characterized by challenging terrain and harsh environmental conditions. Despite these challenges, Pioneer successfully surveyed over a thousand line kilometers in 2017, using UgCS as the mission planning tool.
UAVs equipped with magnetometer sensors represent a technological leap for remote mineral exploration geophysical surveys. They are well-suited for mapping of mineral deposits and pipelines, conducting UXO surveys, and other relevant tasks. Many of these missions take place in remote areas with rough terrain, where limited supply and dangerous environment can compromise conventional aerial or ground survey techniques. Magnetometer-equipped UAVs provide an increased safety for operations, reduced expenditures, and simplified support and logistics, which greatly benefits both the surveyors and their clients.
When conducting a magnetic survey, UAVs carry magnetometer sensors at very low altitudes – usually between 5 to 50m AGL (Above Ground Level). Some manufacturers produce sensors designed specifically for these kinds of operations, such as the GEM GSMP-35U magnetometer by GEM Systems (http://www.gemsys.ca/uav-magnetometers/).
Photo 2 © courtesy: Pioneer Aerial Surveys Ltd.
To carry out a successful and safe low altitude magnetic survey mission, one needs to take into account the following factors:
- The precise altitude above ground level and terrain obstacles;
- The pendulum motion of sensors hanging on a tow cable;
- The sensors’ heading control.
Below you will find a detailed guide on how to plan low altitude magnetic survey missions using UgCS while keeping these issues in mind.
Creating a new mission and setting the survey area’s boundaries
One simple but convenient feature of UgCS is the possibility to create and store multiple missions and routes. You can create an unlimited number of missions; each mission can contain any number of routes.
Firstly, you need to create a new mission:
Figure 1. Create new mission
It is good practice to give meaningful names to your missions – it will allow you to easily find them later:
Figure 2. Set title of mission
Next, press the ‘Add new route’ icon:
Figure 3. Add new route button
Two options are available:
- ‘Create from scratch’ – press this icon to draw your route manually;
- ‘Import from file’ – press this icon to import a KML file or XML file in UgCS format.
Customers that request a survey usually provide KML files with boundaries of the survey area.
To import the file, click on the ‘Import from file’ radio button, select the KML file, and click on the ‘AreaScan’ radio button:
Figure 4. Import route from file
Please note that the KML file must contain ‘LinearRing’ segments in order to import survey area boundaries. To create a ‘LinearRing’ segment in Google Earth, which is the most commonly used tool for this task, you need to click on the ‘Add Polygon’ icon:
Figure 5. Create 'LinearRing' in Google Earth
Then, the KML file can be imported into UgCS to define the boundaries of the survey mission.
Once the boundaries are defined, select the drone that you will use for your magnetic survey and press ‘Next’:
Figure 6. Select vehicle for route
Lastly, but of great importance, is to set parameters in the route parameters window:
Figure 7. Vehicle settings
Most parameters in this window are very important for the mission’s safety. Therefore, we shall consider each parameter in detail:
Home location. As a general rule, we recommend leaving this parameter at the default setting, i.e. ‘1st waypoint (ground level)’. In the event of an emergency situation, your drone will attempt to return to this point. In addition, the ground elevation at this point will be used as a reference to calculate relative altitude for all other waypoints of the route.
Maximum altitude above ground. In most countries, there is a legal limit for the altitude above ground (AGL) for drone flights, unless the operator has a special permission. 120m (or 400ft) is the most common limit. Please don’t set this parameter at higher values without permission from your country’s authorities.
Emergency return altitude. If something goes wrong with the equipment (low battery condition, RC signal loss, etc.), or if the operator presses the ‘Home’ button on the RC remote controller, the drone will ascend to this altitude before flying back to the home location. One must keep in mind that the emergency return altitude is calculated relative to the home position, rather than its current position where the emergency took place. As a general rule, this parameter’s value should be equal to the height of the tallest obstacle (including natural obstacles like trees, hills, and rocks) in your survey area, plus 50 meters away from its perimeter.
Altitude mode. Set this parameter to ‘Above ground’ for all terrain-following low altitude missions.
Trajectory type. For terrain-following missions, this parameter should be set to ‘Straight’. Please note that in this mode your drone will fly between waypoints in a straight line, so the altitude above ground should exceed the height of all trees and other obstacles in your survey area.
Action on loss of RC. This parameter defines what should happen if the communication link between the ground station (RC remote controller) and your drone is lost. By default (option ‘Home’), the drone will abort the mission, ascend to the Emergency return altitude and return to the Home location. Another option for this parameter is ‘Continue mission’. We strongly advise to always use the default option (‘Home’). Firstly, in most countries, the law mandates that a permanent communication link should be maintained between the drone and the operator throughout the flight. Secondly, if something goes wrong (very wrong) after the communication link had been lost, you may never find your drone because the ground station will display only its last known position. Naturally, there are some instances in which maintaining a constant radio link is nearly impossible, but overall you should think twice before selecting the ‘Continue mission’ option.
In addition, please note that if the ‘Continue mission’ option is selected the drone will fly to the last waypoint and hover there. If the radio link is lost, the drone will hover until its batteries are drained, at which point it will most probably (depending on the drone’s settings) land at the last waypoint. To prevent this from happening, the last waypoint of the route should always be placed near the desired landing point.
Double check all settings in the route parameters window and press ‘OK’. The survey area boundaries will be displayed on the map.
Figure 8. Survey area boundaries
Please note that the route is not yet complete and can’t be used for flights – this is indicated by the red exclamation mark in the route’s panel. This is because some AreaScan parameters necessary to calculate the route – ‘Flight Height’ and ‘Side distance’ in the above example – are not set yet.
If you don’t have a KML file with survey area boundaries you can create a route from scratch. Select the respective option during the first step of the route creation. Once all 3 steps of the route creation are complete, you will see the mission editor menu with an empty route. Click the ‘AreaScan’ Icon:
Figure 9. Area Scan tool
Now, it is possible to set the corners of the survey area by double-clicking on the map with the left mouse button (or by pressing Shift and clicking on the map with the left mouse button).
Figure 10. Setting survey area corners on map
The survey area should be a closed polygon – please set the last point over the first one. If your customer has provided you with exact coordinates, you can use the ‘AreaScan’ inspector to enter (or correct) the coordinates of each point.
Next, in order to create a valid route you need to specify two parameters – the flight altitude and the side distance between survey lines.
The altitude is defined primarily by the survey type and the capabilities of the sensor, such as the photo overlap requirements or the client’s specific requirements for the purposes of a geophysical survey.
The distance between survey lines depends on the client’s requirements and the survey type. For magnetic surveys, the distance can be anywhere between 5m and 200m, depending on the geological or environmental target(s) of a survey.
Once these parameters are set, UgCS will calculate the route and an image resembling the one below should appear on the screen:
Figure 11. Calculated route
This route can already be used for real flights, but it is neither optimal nor safe. In the rest of this article we will explain how to make it much better in both regards.
Setting an optimal direction angle of survey lines
By default the survey lines will be aligned from north to south. The survey lines can be aligned parallel to the survey area boundaries in order to reduce the duration of the flight and to minimize the number of waypoints and turns. Minimize the number of fly-outs outside of the survey area boundaries by changing the direction angle, as long as the survey area isn’t a convex polygon. For instance, in the example below we can get a much more optimal route by setting the direction angle to 296 degrees in the ‘AreaScan’ inspector:
Figure 12. Setting an optimal direction angle of survey lines
Ensuring that precise elevation data is used
Most UAVs will fly at the altitude specified in the flight plan, not taking into account the real altitude above terrain features and obstacles. It is a task of the mission planning software to calculate the appropriate altitude at every stage of the flight based on the elevation data in a form of a DEM (digital elevation model).
The DEM elevation data of various geographical locations available in open sources can be outdated or imprecise. This is particularly true for distant and rural areas, where the majority of magnetic surveys are conducted. The difference in precision of elevation data is evident in the example below, where data from an open source (SRTM, Figure 13a) and the more reliable DEM data (Figure 13b) were both imported into UgCS to map the Amphitheater near Red Rocks (Colorado, USA).
Figure 13b: Elevation data imported into UgCS from 2 different sources:
a) Elevation data from an open source b) DEM data
Defining the flight altitude
The image below (Figure 14) shows a typical magnetic survey flight of a UAV (DJI M600) with a magnetometer (GSMP-35U, Gem Systems) managed and controlled by UgCS.
It’s evident from this image that the following factors have to be considered when conducting a magnetic survey with a UAV:
- The required altitude of the sensor above the ground;
- The length of the rope/ tow cable to which the sensor is attached;
- The maximum height of obstacles (e.g. trees) in the survey area.
Figure 14. Magnetic survey flight
Photo © courtesy: Pioneer Aerial Surveys Ltd.
A parameter which is often overlooked but is crucial for safe mission planning is the maximum barometer drift during flight time. Most drones use barometric altimeters to calculate their own altitude relative to the take-off position. Unfortunately, barometric altimeters invariably have some drift depending on the quality of the sensor, air temperature change, and some other variables. In our experience, the maximum drift for a 30-minutes flight can reach up to 5 meters above or below the true altitude.
The key to selecting a safe altitude for the survey is, therefore, to consider the maximum height of obstacles and add 5m to account for the barometer drift. An additional safety margin may be added depending on the terrain, winds and some local site issues like the line of sight and your access to the survey area.
There is a special parameter called ‘Safe height over terrain’ in the UgCS vehicle profile, which enables automated height evaluation during the mission planning. The default value is 5m to compensate for the barometer drift, but it’s not enough for drones with specialized payloads such as a magnetometer on a tow cable. Therefore for magnetic survey missions, it is necessary to increase the ‘Safe height over terrain’ parameter by the length of the tow cable.
Altitude Following Precision
The ‘Altitude tolerance’ parameter in the AreaScan tool allows users to specify how precisely the UAV should follow the desired altitude above ground.
There is a trade-off between following the precise altitude in a certain range and the number of generated waypoints.
The following of more precise altitude allows for a safer flight and better quality data. On the other hand, every autopilot has a predefined limit on the number of waypoints that it can handle at once. For example, DJI M600 drone can only assess 99 waypoints during a single flight.
If a route consists of more than 99 waypoints, intermediate landings are needed to upload the next array of waypoints. (In theory, DJI’s autopilots are able to upload a new route while the aircraft is still in the air, but this operation is less safe and thus should be used with caution.)
For example, if the AGL altitude is set at 40m with a 1m altitude tolerance, UgCS will generate hundreds of waypoints to keep the drone's altitude in the vertical altitudes tolerance corridor between 39 and 41m. The smaller the altitude tolerance value, the more waypoints will be generated. Therefore, the altitude tolerance must be set to a value that would not lead to exceeding the maximum amount of possible waypoints per mission. In the example below, an altitude tolerance increased from 1m to 10m reduces the number of waypoints from 872 to 364 (Figure 15a and 15b).
Figure 15a. An AGL altitude tolerance value of 1m requires 872 waypoints
Figure 15b. An AGL altitude tolerance value of 10m requires 364 waypoints
As a result, a mission that employs the DJI M600 UAV with the altitude tolerance set at 1m (Figure 15a) would require 9 flights compared to only 4 flights required in a mission with the altitude tolerance set at 10m (Figure 15bB).
UgCS provides an option to upload additional arrays of route waypoints into a drone’s autopilot even while it is in the air. This enables the drone to carry out missions with very high precision, but it is nevertheless advised to keep the number of uploaded waypoints reasonable.
Heading of the sensor
For some surveys it is important to maintain a constant heading of the UAV (and the sensor) during the flight - this can be achieved with the ‘Change yaw’ feature of UgCS (Figure 16).
Figure 16: UgCS’s feature of changing the yaw angle
Preventing pendulum motion of the sensor
The standard speed for magnetic surveys is between 5 and 8 m/s. A drone entering a U-turn at the end of a survey line at this speed will cause a pendulum motion of the magnetometer on a tow cable. Adding a 15 to 20m overshoot segment to UgCS’s AreaScan and reducing the speed to around 2 m/s for the overshoot segment in combination with the Adaptive Bank Turn type will effectively prevent the pendulum motion of the sensor. UgCS will add short additional segments to the end of each survey line.
Figure 17. Added overshoot
Optimizing for a safer route
After the initial flight planning, the route can be optimized for a safer flight. This is especially important for magnetic surveys because the survey area can be quite large and may require multiple flights to complete.
Firstly, make sure that the emergency return altitude is set high enough in the route parameters window. This altitude should be high enough to allow a safe way back from any point of the route, taking into account all obstacles and terrain features.
Secondly, the route’s first waypoint should always be set very close to the actual take-off point. This waypoint should be high enough to allow a safe flight towards any other point of the survey mission. This will allow you to set a new starting point (for instance, after a battery swap in the middle of the mission). The drone will ascend to a sufficient altitude set for the first waypoint and then fly straight to the selected “resume point”.
Finally, it is highly recommended to set an additional last waypoint for the route, close to the desired landing point (in most cases it will be the same as the take-off point). This will ensure a safe return of the drone to the home position, even in situations when there are issues with a handheld remote controller (RC) or a ground station computer, and the drone is configured to continue its mission if the radio link with the ground station is lost.
More detailed information about take-off and landing waypoints is published in the article UgCS Photogrammetry Technique for UAV Land Surveying Missions.
Carrying out the mission
A magnetic survey mission with a UAV which carries a sensor attached on a tow cable 5m above the ground is very different from a classic photogrammetry mission (when a UAV flies at 100m altitude). Therefore, the magnetic survey’s UAV operator must control the clearance between the magnetometer sensor and the ground and/or obstacles and be ready to take over the control of their UAV with a remote controller.
Taken together, the modern commercial UAV sensors and software allow for a new, effective and economical way to acquire high-quality magnetic survey data. Effective use of these technologies requires training, good knowledge of all components and very accurate planning. When prepared well, surveyors can greatly benefit from this modern and fast-emerging technology.