## 1 Introduction
### Precursor Autonomous Systems  
A few autonomous systems with pointable radiometers already exist and are briefly described here. The most used autonomous system for measurement of water reflectance is the Seaprism version of the CIMEL CE318 multiband photometer system. This system was originally commercialized in the 1990s for the measurement of aerosol properties and includes direct sun, near sun, and principal plane and almucantar sky radiance measurements, and it became the unique instrument of the AERONET network [9]. The Seaprism version was developed in the early 2000s [10] by reprogramming of the pointing system, especially to perform downward pointing measurements of upwelling radiance from water. The system evolved over the years to include improvements of the optical and electronic components, as well as development of new versions, e.g., including the possibility of nighttime lunar measurements [11]. The system consists essentially of an optical head with two fore-optics protected by collimators, containing a filter wheel for multiband optical measurements with a very wide dynamic range (from direct sun to dark water), a robotic pointing system, a control box providing pointing and measurement instructions and managing power and data, and associated auxiliary equipment for power generation (solar panels) and data transmission (METEOSAT satellite network uplink or cellular link). The system is extremely robust, giving reliable maintenance-free performance for long-term (\(\sim\)1 year) deployments in a very wide range of environments including land and water sites from the tropics to the polar regions. The OSPREY system [12] was designed to be commercialized by Biospherical Instruments as a very-high-performance modular system of radiometers on pointing systems. The radiometers include a hyperspectral spectrometer and numerous single-band microradiometers within a single thermally controlled casing and a filter wheel in front of the spectrometer allowing polarimetric, direct sun, straylight-corrected, and dark measurements. The system, described in detail in Reference [12], has a very high dynamic range and high accuracy pointing, and is capable of water, sun, sky, and moon radiance measurements. The RFLEX system [13] consists of three hyperspectral TRIOS/RAMSES radiometers measuring downwelling irradiance, downwelling sky radiance, and upwelling water radiance mounted at fixed zenith angles (0\({}^{\circ}\), 40\({}^{\circ}\), and 140\({}^{\circ}\), respectively) on an azimuthally rotating platform with associated control software. This system was designed principally for deployment on moving ships, where the azimuthally rotating platform allows achieving an optimal 90\({}^{\circ}\) or 135\({}^{\circ}\) relative azimuth to sun for any ship heading. Instructions for system hardware construction using low-cost components (except for the COTS radiometers) were made publicly available via SourceForge ([[https://sourceforge.net/projects/rflex/](https://sourceforge.net/projects/rflex/)]([https://sourceforge.net/projects/rflex/](https://sourceforge.net/projects/rflex/))) and source code for communication with TRIOS/RAMSES was made available via GitHub ([[https://github.com/[PERSON]/PyTRIOS](https://github.com/[PERSON]/PyTRIOS)]([https://github.com/[PERSON]/PyTRIOS](https://github.com/[PERSON]/PyTRIOS))). The system was used operationally on two \"ferryboxes\" mounted on ships of opportunity in the Baltic Sea. The DALEC system [14] is commercially available from Insitu Marine Optics and consists of three hyperspectral spectrometers measuring downwelling irradiance, and downwelling (sky) and upwelling (water) radiance embedded at fixed zenith angles (180\({}^{\circ}\), 140\({}^{\circ}\), and 40\({}^{\circ}\), respectively) within a compact azimuthally rotating body on a gimballed mount. Control and data are managed by a standard personal computer (PC) using software supplied by the manufacturer. This system was designed principally for deployment on moving ships but can also be used from fixed platforms.