# Envisat RA-2 Individual Echoes: A Unique Dataset for a Better Understanding of Inland Water
## 1 Introduction  
The Envisat satellite mission, among its several onboard sensors, included a nadir-pointing radar altimeter (RA-2), operating in Ku-band and S-band (the latter being essentially for ionosphere correction). The device was intended by design to downlink pre-processed data (i.e., incoherently averaged) for the extraction of the relevant geophysical parameters in the ground segment. Envisat offered also the option to completely bypass the onboard processing and downlink raw echoes inclusive of the phase information [1]. The RA-2 in its nominal operation mode collected 1795 echoes per second (i.e., one echo every 0.0557 s), however, it averaged over groups of 100 individual echo amplitudes (where phase is discarded) that correspond to a rate of 18 Hz. The resulting data were then transmitted to the ground in packets containing 20 averaged echoes each. More specifically, the instrument was designed to also provide un-averaged Individual Echoes (IEs) including the phase information, which were grouped in sequences (packets) of 1984 complex echo samples (about 1.114 s at a PRF of 1795 Hz), giving approximately 7 km of ground coverage. These sequences were recorded every 3 min, so only a small proportion of the ground track is sampled in this modality. Such mode of operation was intended for research purposes, and it is proposed in this paper as a way to investigate alternatives to the standard on-board incoherent summing. Although the surface is sampled in a spotted way, some IE packets are consistently collected over small water surfaces. This unique feature offers the possibility to study the characteristics of inland water echoes at an unprecedented level of detail. Some groups started analyzing these data to see what capability this enhanced spatial sampling might add for the measurement of the heights in small water bodies. The first attempt to investigate IEs is that of [PERSON] et al. [2; 3] who analyzed two Envisat passes over the Salar d'Uyuni on the Altiplano of southwestern Bolivia, which is a 10,500 km\({}^{2}\) salt lake. Its surface is flat, smooth, and is a specular reflector. The IEs are not noisy (as it normally happens in open ocean) and an amplitude plot reveals the obvious specular structure. More extensive studies were done by [PERSON] et al. [4], e.g., over a small Amazon tributary where inland water waveforms show more coherent features than their ocean counterparts. [PERSON] and [PERSON] [5] made an initial evaluation of IEs over ocean from the SARAL/AltiKa mission. In the conclusions, they state that averaged echoes constructed from just a few IEs can be obtained with a noise level comparable to what was the target at 1 Hz only a decade ago. They also state that IEs are ideal for application across narrow water bodies. [PERSON] et al. [6] show that nadir altimeters can observe a narrow artificial canal (with width below 100 m) using re-tracked SARAL/AltiKa data at 40 Hz and that the target response in the radargram is characterized by high intensity values of the received signal, indicating the typical specular nature of the target. The interest of radar altimetry for hydrological applications is also demonstrated by the joint international effort in developing the SWOT (Surface Water and Ocean Topography) mission, set to launch in 2020 [7]. The aim of this work is to discuss the specular nature of inland waters, with its implications to the processing of the radar returns. In this particular framework, Envisat IE data offer a special opportunity for investigating such specular features. Envisat RA-2, with its continuous and interleaved working mode exhibits enough coherence for Doppler filtering [8]. Moreover, such interleaved working scheme will be offered by future radar platforms for ocean topography applications such as Sentinel-6 (also known as Jason-CS), differently from the current SAR (Synthetic Aperture Radar) missions, such as CryoSat-2 and Sentinel-3, which are based on burst transmissions.