###### Abstract
There are two types of retrackers, based on either empirical observations and practical experience, or theoretical knowledge of microwave scattering at nadir, namely empirical retracker and physical-based retracker, respectively. Most notable empirical retrackers include the threshold [46], beta-5 [47], and the Offset Center of Gravity (OCOG) [48; 46], with the OCOG being the core of ice-1 retracker [49]. The physical-based retrackers include the LRM ocean retrackers based on the well-known Brown ocean waveform model [50] (e.g., ice-2 retracker [51; 52]), and the altimetry SAR mode ocean retrackers based on the original work of [PERSON] et al. [53] (e.g., the SAMOSA retracker [53; 54]). Whilst well-developed retrackers have been defined for operation over the oceans and ice surfaces, called ocean retrackers and ice retrackers, respectively, no retracker has been adapted for inland water bodies as of yet. [PERSON] et al. [15] compared the results from different satellite missions for inland water bodies, where most previous studies used ice retrackers [15; 16; 17; 18; 24; 34; 36], and some others included ocean retrackers [15; 16; 24]. Nevertheless, different retrackers need to be developed and compared for inland water bodies. Due to the relatively recent advance towards SAR altimetry operation, started by the CryoSat-2 mission [55], and continued with the Sentinel-3 [56] mission, most of the studies over inland water are quite limited to the classical LRM operation. When trying to retrack waveforms over continental water bodies, the radar altimetric community has widely applied different retrackers including threshold, ice-1 and ice-2 retrackers. The robust OCOG retracker shows a consistent behavior over the different types of waveforms found in [57] with SARAL/Altika mission over different rivers and reservoirs in India. The SAMOSA retracker and its further adaptations have proven well fitting with CryoSat-2 data not only over the open-ocean and coastal zones [58], but also over ice sheets and inland waters where waveforms show specular characteristics [59; 25]. Studies also show the possibilities of retrieving water levels over very small water bodies using techniques that allow precisely monitoring water surfaces. Investigation of individual echoes of Envisat LRM altimetry [60; 61; 62; 63] has shown the possibility of retrieving water levels over water bodies with widths down to 50 m [63]. The SARvatore service provides 80 Hz data dedicated to inland water bodies. Another retracker, \"fully focused SAR\", has shown the potential of monitoring 40 m wide water surfaces by reducing the along-track resolution down to the theoretical limit, equaling half the antenna length [64]. The fully focused processing is a very promising processing technique that can provide very high resolution Level-1B products, and has the ability to open a new paradigm in SAR altimetry when it becomes fully operational. The objective of our study was to develop an operational methodology for water level retrieval from altimetry SAR mode Level-1B waveforms. We focused on small water bodies where continuous clean waveforms rarely exist, and efforts were made to better retrack the contaminated waveforms to improve the accuracy of the water level. First, we modeled three different retrackers including the most widely used threshold retracker, the OCOG retracker, and our in-house SAR physical-based retracker [65] (isardSAT SAR retracker in [65] is an implementation based on the original physical-based model developed by [PERSON] et. al [53], which is also the origin of the sometimes-called SAMOSA model). The latter was integrated in a two-step fitting procedure, exploiting both its normal operation over ocean-like waveforms, and the adaptation to peaky-like waveforms, expected from inland waters.