# GNSS Signal Distortion Estimation: A Comparative Analysis of L5 Signal from GPS II and GPS III
## 1 Introduction  
The navigation signal is the sole link between a satellite and a user, and its quality is particularly important to the user [1]. With the rapid development of global satellite systems (GNSSs), more and more wideband GNSS signals have been designed to meet diverse precise and robustness positioning, navigation, and timing (PNT) services for users [1]. However, even normally operating satellites in orbit will produce a slight signal distortion from the navigation payload transmission channel. The nonlinear effect of a high-power amplifier (HPA) has the greatest influence on signal distortion; it causes the signal quality of the GNSSs to not meet the official interface control document (ICD) requirements [2, 3, 4]. Considering the limited frequency resources, the diversity services, and the challenges of the satellite's payload size, weight, and imperfect analog components, new-generation GNSS signals employ constant envelope multiplexing (CEM) techniques to multiplex signals. The multiplexed signal has a constant envelope, and the advantage of CEM signal is that multiple signal components can share the transmission link from the modulator to the antenna in the satellite transmitter. Furthermore, the CEM signal allows the HPA to operate at its full saturation to maximize amplification efficiency without any distortion. However, due to the bandwidth limitations of D/A and pre-HPA harmonic filtering, the CEM signals are inevitably destroyed by the band-limiting filter before the HPA. Hence, its amplitude to-amplitude (AM-AM) and amplitude-to-phase (AM-PM) features can produce different gain and phase shift due to the non-CEM signals when the HPA works in full-saturation mode. Several studies on the HPA nonlinear effects in the GNSS field have been reported in recent years [5, 6, 7, 8, 9]. [PERSON] presented phase-lock loop (PLL) tracking jitter caused by HPA nonlinear effects and demonstrated the PLL trackingjitter performance under different signal systems, different filter bandwidths, and different power amplifiers models through numerical simulations [5]. [PERSON] et al. observed that HPA leads to out-of-band regeneration of the signal spectrum, asymmetry of the correlation curve, and related power loss [6]. Taking the correlation loss, code loop zero-crossing point bias, and carrier tracking error as the evaluation index, [PERSON] evaluated the HPA nonlinear on different GNSSs signals [7]. [PERSON] et al., analyzed the influence of different HPA group delay orders on signal ranging deviation through theoretical modeling and concluded that nonlinear distortions are mainly affected by the second-order group delay [8]. [PERSON] et al. analyzed the influence of different component nonlinearities of navigation payloads on the signal [9]. As shown in the aforementioned studies, the distortion issues induced by the HPA nonlinearity in space can have serious impacts on the quality of GNSS nominal signals, which inevitably degrades the GNSS ranging performance. Most importantly, the signal distortion introduced by the transmission channel of the navigation payload varies with satellites, which is particularly disadvantageous for differential positioning [10]. In addition, some related studies have shown that the nonlinearity of the navigation payload transmission channel is a complex combination of multiple distortions, rather than a simple linear superposition [11]. The evaluation of the navigation signal quality can detect the abnormality of the signal in time, and it is an important means to judge the normal operation of the satellite in orbit. The received signal is often polluted by various transmission channel distortions. However, the current quality assessment and monitoring of navigation signals mainly focus on the state of the \"received signal\", rather than the cause of signal distortion; in other words, the characteristics of the transmission channel have not been studied adequately. The navigation signal is an intuitive reflection of the characteristics of the transmission channel.