# Spaceborne LiDAR Systems: Evolution, Capabilities, and Challenges
## 2 Basic Principle and Functionality
### Instrument Configuration and Signal Detection
#### 2.1.2 Receiver Subsystem  
The receiver typically consists of a telescope equipped with spatial and spectral filters and an electronic photodetector [24]. Receiver sensitivity is a primary concern, as transmitted laser power can be on the order of megawatts (MW), while the received power is typically reduced to nanowatts (nW). Furthermore, the sensitivity of the photodetector strongly correlates with the overall performance of the system, directly affecting the capability [47]. LiDAR receivers generally operate in either analog (linear/full-waveform) mode or digital (photon-counting) mode [48]. Photodetectors commonly used in LiDAR systems include photomultiplier tubes (PMTs) and avalanche photodiodes (APDs) [49; 50; 48], both of which can function in either analog or digital mode. When operated in Geiger mode (digital mode), APDs are referred to as Single-Photon Avalanche Diodes (SPADs) [48]. The choice of detector type depends on the mission requirements and the wavelength range of interest. PMTs have historically been preferred for ultraviolet (UV) and visible wavelengths, such as 355 nm and 532 nm, due to their high gain and reasonable quantum efficiency at those wavelengths [48; 51]. On the other hand, APD offers better performance at the 1064 nm, where PMTs exhibit poor quantum efficiency [51]. Photon counting sensors are capable of registering individual photon events, allowing for high vertical resolution, enhanced sensitivity, and reduced susceptibility to electronic noise, including dark counts and analog-to-digital converter (ADC) noise [30; 52]. In addition, they often provide higher signal-to-noise ratios (SNRs) for a given signal energy. However, a key limitation of photon-counting detectors is dead time, which refers to an interval following a photon detection during which the sensor cannot register another event. This reset period, which typically lasts tens of nanoseconds, restricts the maximum achievable count rate and can cause a signal loss in high-flow environments [30; 53]. Several techniques have been developed to mitigate dead-time effects, such as the following: (1) using low photon rates and (2) using an array of detectors to reduce the chance that multiple photons are hitting the same detector at the same time [30]. Various examples can include multidetector arrays and multichannel signal preprocessing circuits that have been developed to independently preprocess each detection channel, significantly reducing noise counts and suppressing dead-time effects [54]. Furthermore, advanced dead-time correction algorithms have been proposed to improve the accuracy and efficiency of photon-counting LiDAR systems [55; 56]. Expanding on photon-counting capabilities, SPAD arrays have been developed with a smaller pitch, higher pixel density, and integrated processing electronics, enabling better resolution and accuracy in 3D imaging [57]. Unlike single-element SPADs, Silicon Photodet multipliers (SiPMs) integrate multiple avalanche diodes to form a high-efficiency photon detection device. SiPMs are promising candidates for future LiDAR missions. SiPMs are arrays of SPADs connected in parallel, acting as a solid-state alternative to PMTs [58]. They are known for their high photon detection efficiency and low noise characteristics. They are instrumental in applications requiring high sensitivity and low light detection. Recent studies have shown that SiPMs can maintain their performance even in harsh space environments. For example, GRBAlpha and VZLUSAT-2 CubeSats have demonstrated the durability of SiPM arrays over three years of in-orbit operation, highlighting their potential for long-term space missions [59]. A review of recent missions reveals how the aforementioned preferences and considerations have translated into real-world detector configurations. For instance, the LITE mission utilized PMT detectors operated in analog mode on their 532 nm and 355 nm channels. At the same time, CALIOP used a PMT operated in analog mode on both of their 532 nm channels [16; 51].