# Technological Advancement in Tower-Based Canopy Reflectance Monitoring: The AMSPEC-III System
## 1 Introduction  
Measurement and quantification of photosynthesis and other plant physiological processes are crucial to improve our understanding of ecosystem functioning. Tower-based approaches, such as Eddy-Covariance (EC) measurements, are critical to determine the exchange of carbon dioxide (CO\({}_{2}\)) between land surface and atmosphere, and to improve our understanding of cycling of water, nutrients, and carbon [1]. However, EC can only provide spatially discrete observations typically over a few hundred meter or few kilometer footprint, and upscaling these observations to larger areas isdifficult. As a result, the prediction of carbon, water, and energy balances at landscape, regional and global scale is highly uncertain [2; 3; 4; 5].  
As a complementary technique to EC measurements, near surface remote sensing is an effective tool to link carbon fluxes and spectral features of vegetation across various spatial scales, from proximal [6; 7; 8; 9] to airborne [10] and satellite platforms [11; 12]. High-spectral and spatial resolution optical sensors can be utilized to determine physiological processes through the relationship between plant physiological properties and biochemical composition of foliage (e.g., [13]), typically observed through narrow spectral channels, in the 400-2500 nm range [14].  
At the same time, optical remote sensing is also capable of synoptic coverage of the globe via satellite observation, hence providing opportunities for spatially continuous scaling of ecosystem fluxes (e.g., [15; 16; 17; 18; 19; 20]). Network approaches, such as the SpecNet initiative [21], have demonstrated their effectiveness in linking remote measurements to carbon fluxes. Nonetheless, determination of highly dynamic and spatially variable physiological processes remains challenging, as a number of external factors can affect the relationship between biophysical properties and measured reflectance, including sun-view geometry, soil background reflectance, species and canopy characteristics and pigment pool size (e.g., [22; 23; 24; 25; 26; 27]). Spatial and spectral dynamics of some of these processes requires high spatial and temporal resolution [28], and the observation of vegetation status under multiple illumination and viewing conditions (e.g., [29; 30]), which is not easily achieved with existing airborne or spaceborne sensors [31]. Indeed, in order to detect pigment absorption features both high spectral resolution (in the range of few nm) and high signal-to-noise is needed [7].  
Networking multi-angular tower observations based on field spectro-radiometers can help to provide a better understanding of ecosystem dynamics [7; 8]. In particular, low cost spectro-radiometers developed over recent years may help overcome the often significant cost in purchasing these instruments and, thus, limiting the implementation of networks, which would permit more systematic upscaling of tower measured reflectance and fluxes to space. However, differences in data quality between these \"off the shelf\" radiometers compared to well established models is not clear nor is the impact on physiological indices derived from the data. In this paper we address this issue by undertaking a comparison between two portable spectro-radiometers, both installed on a third generation Automated Multiangular SPectro-radiometer for Estimation of Canopy reflectance system (AMSPEC-III). We discuss the system's hardware and software components, as well as the results of the comparison of measured spectra from a three-month field test.