# Crowded Space: A Review on Radar Measurements for Space Debris Monitoring and Tracking
## 2 Radar Configurations for Space Debris Monitoring and Tracking  
Radar systems are based on the simple principle of echo: a signal is sent to a specific target, which reflects part of it, and the echo travels back to the source [8]. This operation can be undertaken by a single system or by multiple systems. When a single system is used for both transmission and reception, it is defined as monostatic. On the other hand, when two or more systems are involved in the process, the system is defined as bistatic or multistatic, respectively [8]. The type of transmission is crucial as well, being a radar-defining feature. Generally, the transmission signal can be divided into two categories: continuous signals (continuous wave, CW) and pulsed signals. A continuous signal is employed to retrieve the frequency shift between the carrier and the reflected signal, giving information about the Doppler frequency and the velocity of the target. A pulsed signal is used to retrieve the distance of the target from the radar, giving information about the range and the position of the target [8]. Obviously, this is a simple categorization that does not take into full account the nuances that could characterize a complex radar transmission; nonetheless it is satisfactory for the purpose of this paper.  
The most suitable radar configurations for debris monitoring and tracking consist of parabolic reflectors, phased arrays, and interferometers. Parabolic reflectors are much appreciated for their ability to convey high power towards a specific direction [8]. The canonic large dish, which is several wavelengths in diameter, in fact guarantees a high gain, which is essential to allow the signal scattered from the debris to be detected despite the very long distances. A remarkable key feature is the ability to mechanically point the beam at the target with great accuracy, allowing the tracking of the object whenever the azimuth and elevation maximum rotation speeds allow it [8]. For this reason, large parabolic reflectors are usually preferred when the primary purpose of the measurement is to follow the target and retrieve accurate information about the orbit (i.e., orbit determination).  
Phased arrays consist of a combination of single radiating elements such as dipoles, slot or patch antennas [8]. Each constituent element is controlled in phase and amplitude, allowing the creation of an electronic beam that can be manipulated without relying on a mechanical apparatus. This feature enables a simultaneous coverage of great portions of the sky, outperforming, in this sense, the parabolic reflector (which is typically characterized by a narrow beam). In this respect, phased arrays are particularly suitable for surveys on space debris and useful for the detection of uncatalogued objects. Finally, radar interferometers are multiple-antenna systems based on the interference of electromagnetic waves. The target is located by measuring the phase difference of the signal received by different antennas. They are usually composed of several antennas separated by a wide-baseline (with different distances in the order of tens, hundreds or thousands of kilometers, e.g., Very Long Baseline Interferometer--VLBI). This technique emulates a very large antenna with size comparable to the maximum baseline adopted, allowing the synthesis of a wide aperture. However, the reduction of the data is much more complicated with respect to the other configurations [9; 10].