# Consolidating ICESat-2 Ocean Wave Characteristics with CryoSat-2 during the CRYO2 ICE Campaign
## 1 Introduction  
Observing ocean surface waves is a key element in several fields, from understanding the underlying mechanics of waves in climatology to risk assessment in marine engineering [1]. The ability to predict the behavior of ocean waves has increased over the past 30 years due to the use of satellite-based observations [1]. The behavior of ocean waves is complex, and most of them would be too inaccessible for monitoring if not for these space-based altimetry missions. Since TOPEX/Poseidon launched in 1992, the majority of the ocean observation satellites have been using radar altimeters, with more modern missions increasing in precision, which has helped to map the ocean's surface. However, while the ocean surface is observed with a high accuracy, the ocean surface waves are by necessity evaluated using the surface variance, as the radar altimeters have footprints ranging between 1 and 10 km in diameter [2]. By utilizing a high-spatial-resolution laser altimeter from the Ice, Cloud, and land Elevation Satellite 2 (ICESat-2), it is possible to observe small-scale wind waves globally, and not just at the locations of wave stations or on the routes of ships [3]. However, to be able to use these observations in addition to the observations from multi-year missions based on radar altimeters, this method must be compared to current methods. Multi-mission comparisons between satellite altimetershave been done for many missions and are essential for continuous time-series observations, as well as for the calibration and verification of measurements [2; 4; 5]. To be able to assess the deviation between CryoSat-2 and ICESat-2, we therefore validate CryoSat-2 with multiple satellites at crossover points, as well as with in situ wave measuring stations, to assess eventual biases in observations when validating ICESat-2 measurements. CryoSat-2 was launched by ESA in 2010 to measure the wind and wave characteristics of the oceans [5]. Because it was launched into a near-polar orbit, with an inclination of 92 degrees and a repeat cycle of 369 days, CryoSat-2 is able to monitor the surface elevation up to latitudes of 88\({}^{\circ}\). Using the SAR (Synthetic Aperture Radar) Interferometric Radar Altimeter (SIRAL), CryoSat-2 observes the significant wave height (SWH) using the leading edge angle of the return power reflected from the ocean surface [6] at a high spatial resolution compared to conventional systems like the Jason satellites. With a nominal footprint of 15 km, SIRAL observes a larger swath of the ocean surface with each pulse and thereby assesses the sea state. ICESat-2 uses a different approach. It carries the Advanced Topographic Laser Altimeter System (ATLAS), a photon-counting laser altimeter, and observes surface elevations at up to 88\({}^{\circ}\) latitude, similar to CryoSat-2. Launched in 2018 by NASA, it follows the successful ICESat mission, which operated from 2003 to 2009, and allows for high-resolution observations of individual return photons, with a nominal 17 m footprint and a 3 \(\times\) 2 beam configuration [7]. Grouping the beams into pairs with a nominal intermediate distance of \(\sim\)3.3 km allows ICESat-2 to measure elevation changes along the ground track. Designed to measure changes in ice thickness as well as vegetation canopy heights, ICESat-2 also provides ocean surface topography data [8]. Studies comparing data from both the radar altimeter on CryoSat-2 and the laser on ICESat-2 have previously been limited to crossover events separated by varying time lags and longer overlaps closer to the poles [9]. As a result, the feasibility and benefits of adjusting the orbit of CryoSat-2 to repeatedly overlap with ICESat-2 were acknowledged [9; 10]. This was achieved in July 2020 with the CRYO2 ICE campaign [11]. CRYO2 ICE is designed to produce repeated overlaps (every 19 th CryoSat-2 and 20 th ICESat-2 revolution) of extended time (\(>\)30 s of overlap), allowing for several near-simultaneous observations of the same surface using both radar and laser techniques [12].