UAS Geospatial Data Science Compared to Satellites
Tyler Lewandowski & Matt Watson
Using geospatial science, images of the earth are taken everyday and used to observe and display people and the world around them. Geospatial technologies allow the analysis of human interactions with their surrounding environment. Geospatial science uses information technology to understand people, places, and the processes of the Earth. Unmanned Aerial Systems (UAS) are relatively new to geospatial science but are already proving they have the potential to make imagery more accessible and increase the possibilities. While UAS are increasing in popularity the main image collection tool in geospatial science is still satellites. Satellites and unmanned aerial systems have different characteristics and capabilities which makes them both unique when collecting geospatial data.Satellites have been used to capture images since the late 1950’s. Since then they have been the main component in geospatial data science. Initially this technology was only available to the military, but has been available commercially for many years now. Companies like Planet Labs Inc. provide satellite imagery, from their own network of satellites, to anyone who requests it. This company allows other companies to request access to high resolution images of a specific point on the planet. This technology is not easy or cheap to operate however.
In order to start an operation like this, a company needs to have the resources to design and build a satellite capable of taking images of the earth. After the satellite is built, getting it into space can be difficult as well. First, the company must find another organization that is willing and capable of sending something into space. Secondly, the company must contact the Federal government about what they intend to launch into space. From there, the government will either disapprove or approve the launch of the satellite. Before launching, the designers of the satellite must be sure that the technology can withstand the launch and the environment of space. If anything goes wrong, maintenance and repairs cannot be done once in space. Along with the satellite in orbit, operations need to be established on the ground as well. The ground portion supports the space portion of this operation. Ground stations will have control over the payload to allow operators to adjust the sensors on the satellite. The health of the satellite is monitored in case something onboard goes wrong. Sensor information relays the attitude to the ground stations as well. These stations will also be tracking the location of the spacecraft. Since the satellite is in orbit around the Earth, the ground station will not always be able to receive data from the sensors. Tracking the satellite in its orbit can let the organization know when this is happening and where to implement another ground system if necessary. This design and infrastructure is needed to acquire geospatial data using satellites.
The use of satellites within geospatial data have different types of resolutions that affect the images generated. Temporal resolution is how much time passes between each data collection period. Spatial resolution is how big a pixel is on the ground and this can be measured in centimeters per pixel (Hassler & Baysal-Gurel, 2019). Each outcome can be scaled for different users and uses, with the most popular being recreational, mapping, and surveying. The main purpose of using recreational is to show the area without the need of being accurate to a point, making it the lowest quality and the least accurate. Mapping has more of a use for mapping out
areas; they are a lot more accurate but have more room for improvement. Survey is the highest quality that can be created and is the most accurate out of the three. The application of these maps are to create an accurate survey of an area because they are mostly used for urban development or building plans. The use of GPS for recreational use is accurate within twenty-five feet, mapping grade would be around a meter, and a survey grade map would be within a centimeter.
While obtaining geospatial data, it is important to take into account the different errors and the sources of them. The main sources of error are ephemeris, atmospheric refraction, multipath, and geometric dilution of precision (GDOP). Ephemeris, the movement of the satellite around the Earth, can generate errors when the direct location of the satellite’s atomic clock is off even by a nanosecond; if the positioning is off, it can create images that are off by meters. Atmospheric refraction occurs when the signal is redirected in transmission. As it moves towards Earth’s surface, the signal follows a zig-zag shape. Multipath errors occur when a signal is reflected or delayed due to the interference of geographical features, creating a messy signal. When too many satellites are in the same relative location GDOP can occur and downgrade the precision of GPS (Gupta, et al., 2019). Along with the common errors of using satellites for geospatial data, governments have gone certain lengths to restrict them.
Geospatial is regulated. Given the potentially sensitive nature of geospatial information governments have limited the accuracy available commercially. When in use by the government geospatial information is kept as accurate as possible. Geospatial data can be used for counterintelligence. The accuracy of these satellites without limitation can be within centimeters. With over four thousand working satellites orbiting the earth, it is very easy for other nations to pick up on top secret projects and operations being performed. An agreement was made to limit the accuracy for the purpose of keeping some of those a secret. The Many nations have actually gone to figuring out when certain satellites are in position to observe their own and to hide what they want to keep undiscovered. Although a great majority, if not all, nations have resorted to this practice, the agreement remains intact to limit the resolution capabilities of commercial satellites. The commercial use of geospatial data has been reduced to the same limitations for the purpose of security for its own nation and others. Unmanned aerial systems provide an alternative to satellites, and currently are not regulated in terms of accuracy. The way a company can obtain this data through UAS is by hiring someone to fly a mission. Using a UAS required three different components which are the aircraft, the ground control station (GCS), and global positioning system (GPS). The aircraft would be the platform used to carry the sensor for the mission. The types of aircraft that can be used are fixed wings for long distances and flight duration, or multirotors which can takeoff vertically and hover. Multirotors can also be less expensive than fixed wings as well. The GCS is the base of operations for the mission. This is where commands are inputted and sent to the aircraft. This is done with omni and directional antennas. Omni antennas send out signals in all directions so the GCS can stay connected to the UAS. Directional antennas can be pointed at the UAS to extend the range the aircraft can fly and acts as a redundancy as well. The antenna will have to be moved in order to stay connected with the aircraft. At the GCS the pilot in command will be flying the UAS as well as monitoring the flight instruments. The sensor operator will be operating the sensor and assisting the pilot in command during takeoff and landing. The final component in operating a UAS is GPS. GPS is what allows the UAS to know where it is at any given time. This allows the aircraft to follow autonomous flight plans and perform autonomous functions. It even makes it possible for the UAS to hold its position in winds. The GPS also allows the pilot and sensor operator to see the aircraft and the aircraft’s movement on a map in real time.
The use of UAS for collecting geospatial data has many different factors that affect the quality of the data. The resolution of the images taken is an important part of why companies would want to use UAS for some operations. The temporal and spatial resolution depends on what the flight crew is wanting to achieve. UAS can have a high spatial resolution if desired. Data taken with UAS can have a spatial resolution of less than ten centimeters (Jagt, Lucieer, Wallace, Turner, & Durand, 2015). As of right now there are no legal limits to how high UAS resolutions can go.
Another characteristic of UAS that makes it good for collecting geospatial data is how adaptable and nimble they can be. Sensors on the aircraft can be switched and replaced as needed. If a sensor breaks during flight the UAS would land and the sensor would be taken off and replaced with no alterations to the actual airframe. The same process can be done if the flight crew wants a different type of sensor on the aircraft. Different sensors can include a color sensor, infrared sensor, or a multispectral sensor. This variability allows for UAS to be equipped for many different types of operations. In addition to the sensors, a large part of what makes UAS a viable option for data collection is how nimble they are. UAS have the ability to be deployed anywhere and can move freely in the air. This allows for companies to pick a subject anywhere they would like, within legal limits, to be observed. The vertical takeoff capabilities of multirotors or catapult launching a fixed wing allows for UAS to be deployed in many different environments.The aircraft does not require a set straight and narrow flight path as well. The flight crew can perform maneuvers and adjust the autonomous flight path as needed. Being able to control the aircraft in flight also gives the operator the option to change the area of operation in the middle of the mission. If the flight crew decides to change the location being observed the UAS can be maneuvered to the desired location at the time.
The commercial use of geospatial data is limited within the satellite realm but is not as heavily regulated for the use of UAS. As stated before, the quality of commercial satellites is limited to sixty centimeters at least. Since the UAS industry is still in its infancy, the Federal Aviation Administration (FAA) has not restricted the quality of the sensors used. In a published article, UAS was used to survey the North Altyn Tagh Fault line (Gao, Xu, Klinger, Woerd, & Tapponnier, 2017). The main purpose of the study was to compare results of UAS generated geospatial images to satellites. Below, Figure 1 shows the image acquired from a satellite.  |
Figure 1: The image was acquired by the Pleiades satellite (No specification on first or second generation).
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Although it is identifiable, the fault line is definitely grainy. The accuracy of the Pleiades image is around 0.7 meters (Gao, Xu, Klinger, Woerd, & Tapponnier, 2017). Satellites are usually hundreds, up to thousands of miles away from the surface of the Earth. The Pleiades satellite is about 431 miles away from the surface ("PLÉIADES 1"). The high altitude along with the restricted accuracy contributes to a lower-quality image. With satellites, they are coursed for a set path around the Earth with sensors being on a fixed orientation. This allows the satellites to have high spatial resolution with little to no temporal resolution since it is improbable for them to observe one point. Since the two have different resolution specialties, it creates a divide for each to be used in respective situations. Comparing the satellite images to UAS, Figure 2 shows the same area using a drone.
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| Figure 2: Above is an image acquired by a drone of the North Altyn Tagh Fault line. |
UAS, as briefly mentioned, has high temporal resolution which correlates to smaller applications. It would make the drone’s geospatial data more applicable to surveying. Since surveying involves a specific area, it would allow UAS to be proficient in this use of geospatial data. Satellites would be used more for mapping and recreation. There would not be a huge need for total accuracy for both of these uses and because satellites are already implemented, it would be reasonable to use the older systems for lower quality images that still serve the purpose.
Geospatial data has high applicability that can be utilized by many different types of industries. Commercial satellite and UAS industries utilize this type of data regularly. The use of satellites dates back to the late 1950’s; the infrastructure to obtain data has been here for many years. The satellites yield images with an accuracy up to 60 centimeters for commercial use, due to government regulations. A lot of the data is strong in spatial resolution but lacks in temporal
resolution, which makes the satellite images ideal for recreational and mapping uses. UAS has a higher temporal resolution that can be used for surveying buildings or ground areas. The FAA does not regulate the accuracy of the images obtained which means the images can have a resolution of whatever is needed for the user. Even though the infrastructure for geospatial data relating to satellites is still there, there is room for UAS to be used. The use of UAVs for surveying and obtaining data about a geographical location can be used for smaller-grade projects such as surveying a plot of land, inspecting buildings, etc.. UAS will never replace satellites because of the different proficiencies and deficiencies for both types of systems; the use of the data and what is trying to be studied ultimately determines what is the best fit for the project.
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