The ability to view the details of the surface of the earth from satellites has come a long way, thanks to rapid advancements in technology over the last few decades. Everyone with a smartphone now can “see” an image of their neighborhood through Google Maps.
But applications of fetching images of earth surfaces go way beyond locating a local store. In predicting environmental hazards, in strengthening defense tactics, and in many more applications, it is important to get a satellite view of the earth’s surface.
Primarily, this is accomplished through two different technologies, optical imagery and radar sensing. They are both excellent technologies but depending on the application, one may fare better than the other. Read on to know more about these two techniques and their key pros and cons.
A Brief Look at the Optical Imagery System
Optical imagery, also known as optoelectronics, uses the light spectrum to garner images of the surface of the earth. It works on the principle that when sunlight falls on the earth’s surface, much of it is reflected in the outer spheres.
A satellite equipped with an optical imagery system hovers above the area of the earth whose image needs to be captured. The optical instruments use several spectral channels to capture an image of the said area.
Optoelectronics is a passive mode of capturing earth’s imagery, which means that the satellite does not need to actively transmit any kind of signal to the earth because it simply works using the natural sunlight.
An Overview of the Radar Data Collection System
The radar system of procuring earthly images is more active. In this, a radar sensing unit, typically a synthetic aperture radar (SAR) is placed in a satellite that revolves around the earth. The sensor continuously transmits microwaves towards the surface of the planet and waits for the radar echo (reflected waves) to be generated.
Using the recorded reflection, the radar system creates an image of the earth’s surface that is discernible to the human eye. A typical SAR system on-board a satellite consists of antennae for transmission and reception of microwave signals and complex mechanisms to process the distance between the sensor and the objects on the earth that reflect the waves.
Advantages of Optoelectronics
An optical imagery system provides multiple benefits. Here are a few important ones.
Simplicity
Because the system uses an electromagnetic spectrum that includes the visible light wavelengths, it is easier to use, especially for those who have no or less experience in earth imagery.
Viewing as a Human Eye
Images captured with an optoelectronic system are closer to how a human eye would view the surface of the earth. Hence, even laymen can easily understand the objects that such images portray.
Detailed and High-Resolution Imagery
When used optimally, and that means with higher bandwidth and larger spectral channels, optoelectronics can produce high-resolution imagery that is key for many applications such as defense or agriculture.
Disadvantages of Optoelectronics
With all its simplicity and ease of access, there are certain disadvantages of using optical imagery for scanning the surface of the earth. Listed below are a couple of them.
Cost-Quality Ratio
The resolution of the images is a key parameter against which the usability of an optoelectronic system is measured for a specific application. Naturally, precise details of images captured can be obtained only by high-resolution units.
As a norm, resolutions below 1 meter per pixel is considered acceptable when high-quality images are a requirement. However, such devices come at a huge cost. High levels of accuracy are, thus, dependent on how much an organization is willing to spend.
Weather-Dependent
Optical rays can be blocked by clouds and other weather-related obstacles. When sunlight is obstructed, the optical imagery device cannot obtain sufficient reflected light rays to form a complete image of the surface. In such cases, pictures of the ground may be incomplete or vague, leading to errors in applications.
Benefits of the Radar Sensing Methodology
SAR satellites have some distinct advantages that make them useful in crucial applications. Some of the important benefits are as below.
Continuous Observation
As radar sensing uses microwave rays to read the earth’s surface, it is not hindered by the presence of clouds. Moreover, this technology can be used with equal efficiency regardless of whether it is daytime or night.
Continuous monitoring has several benefits, such as weather prediction, marine pollution detection, gas pipeline observation, and so on.
Fast Monitoring
Radar sensing of surface objects is a lot quicker when compared to optical imagery. The reason behind this goes back to the concept of active monitoring vs passive monitoring. SAR, being an active monitoring method, can cover larger areas in a significantly shorter time.
It has been noted that where optoelectronics may take days to gather data for a particular surface area, SAR can accomplish the same feat within hours.
Drawbacks of the Radar Sensing Methodology
Even though earth surface monitoring is quicker and continuous with radar-sensing, this method has certain limitations as well. Here are a few.
Cost and Complexity
Radar sensing units cannot directly capture images. They collect data in the form of reflected microwave rays that undergo complex processing to lay out a visual representation of the object.
This means that fast processors capable of executing highly complex algorithms must be used, which also increases the cost of the entire unit.
Low Discernibility
Whereas with optical imagery it is easy to interpret the images because they are close to what a human eye can view, images rendered using the radar technology are not so discernible at the first glance.
High-quality images can be obtained using SAR, but that requires expert knowledge of the trajectory and the movement of the satellite that holds the SAR unit. Extremely high precision and accuracy of the satellite’s motion are required to obtain readable images out of a radar-based sensor.
A Combination Approach
Traditionally, survey teams that wanted to capture images of the earth’s surface relied on either optoelectronics of radar units. Based on the application, scientists chose the appropriate technology.
However, with more advancements in the two technologies in recent times, cooperation between the two has been used with great results. The main advantage of using the combination approach is that two data sets can be obtained using two completely different methods and can be compared to generate the best quality images.
While it is true that two data sets used together can produce a huge amount of data, modern technologies such as artificial intelligence and machine learning ensure that this high volume of data can not only be captured but also processed within a short span. The result is a more accurate, more detailed set of images with a wider range of applications.
Takeaway
Optical imagery and radar sensing are both commendable technologies to generate images of the earth’s surface. Applications of such surveying include defense, marine pollution detection, city traffic and parking monitoring, weather change predictions, and many more.
While optical imagery is easy to use and provides images more discernible to the human eye, their usage is often hindered by the absence of clear sunlight. Radar sensing, on the other hand, is a costly and complicated mechanism but delivers continuous monitoring regardless of the time of the day or the season of the year.
The two technologies are often selected based on the application, but a more modern approach entails combining them. The combination generates more accurate imagery owing to large data sets. Through prudent use of these two technologies and the data they generate, surface mapping technologies have the potential to become more precise and distinct in the future.
