# Deep Learning Methods for Space Situational Awareness
## 2 Related Works
### Non-AI-Based Methods  
In the past years, algorithms for the detection of small space objects have been extensively studied and can be categorized as follows:  
1. Background subtraction methods [83; 84; 85] are one of the most popular approaches for detecting objects. This algorithm works by comparing the moving parts of a video with a background and a foreground image. 2. Optical flow analysis method [86] is a technique used to describe the movement of objects within a sequence of images that have a small-time gap between them, such as video frames. Optical flow calculates a motion vector for the points within the images and provides an estimate of where the points might be in the next image. 3. Track-before-detect methods [87; 88] is a concept where a target is tracked before it is detected. Usually, target detection is performed by means of thresholding of the input signal, and the output is passed to a tracker. In this paradigm, sensor data on a provisional target are integrated over time, and the target is detected without the use of any threshold. This approach is able to track targets even with low SNRs (signal-to-noise ratios). 4. Frame difference methods [89; 90] are similar to the background subtraction methods. This is a technique where the system checks, pixel-by-pixel, the difference between two consecutive video frames. If two corresponding pixels have changed their appearance, motion is detected. Most of these techniques work well with a little blur and thresholds to distinguish real motion from noise because frames could significantly differ from each other as, for example, lightning and conditions change. Other types of object-tracking methods have recently been developed. In [91], the goal was to evaluate the performance of a multi-static radar for the SSA mission, also presenting the advantages and disadvantages of the use of telescopes or radars. The advantage of using optical telescopes allows you to observe objects at very long distances compared to radar observations. On the other hand, radar systems can operate 24/7 regardless of weather conditions and do not depend on sunlight as a source of illumination. They can fine-tune the transmitted signal to perform proper processing and estimate the physical and dynamic properties of the target, providing highly accurate measurements essential for orbit determination techniques. However, one of the major disadvantages of using radar for SSA purposes is its high cost, as such systems require very high transmitting power. Figure 3: Space debris detection as a sub-group of SSA: enabling technologies. We can see another type of object tracking experiment in [92] where a camera is used for events. When a pixel emits an event, the current brightness is stored and continuously monitored for changes in brightness. If the modified value exceeds a threshold, the camera generates an event. When the changed value exceeds a threshold, the camera generates an event containing the pixel coordinates (x, y), time (t), and the changed polarity (p) (+1 for brightness increasing, \(-1\) for brightness decreasing). There are many benefits to using this method, including the following:  
* High temporal resolution: unlike conventional cameras, event cameras can catch extremely fast motion without experiencing motion blur
* Low Latency: there is no need to wait for the frame's overall exposure time because each pixel operates independently. An event is notified as soon as the change is discovered. * Low Bandwidth: by broadcasting brightness changes, event cameras eliminate duplicate data. * Low Power: in event cameras, power is only utilized to process the shifting pixels. * Despite the advantages listed above, there are also significant disadvantages:
* High Noise: because of how the sensors are built, event cameras are particularly susceptible to background activity noise brought on by transient noise and leakage currents from semiconductor PN junctions. * Large Pixel Size: compared to a regular camera, the modern event camera has larger pixels.