## 1 Introduction  
Sun-Induced chlorophyll Fluorescence (SIF) measured from remote sensing platforms provides a new optical means to track photosynthesis and gross primary production (GPP) of terrestrial ecosystems [1]. SIF consists of photons of red and near infrared (NIR) light that are emitted by chlorophyll (Chl) pigments as part of the de-excitation mechanisms in response to absorption of Photosynthetically Active Radiation (PAR). Since plants respond actively and continuously to different environmental conditions, continuous and long term observations become crucial in vegetation monitoring to understand terrestrial biosphere processes [2]. With this aim, in recent years many ground-based field spectroscopy systems have been developed and mounted on towers [3; 4; 5]. In addition, with production of global SIF maps from atmospheric satellites, e.g., [6; 7; 8], and with the forthcoming new ESA mission, the FLuorescence Explorer (FLEX) [9], the use of tower-based SIF continuous measurements will play a key role in supporting validation and calibration activities for satellite and airborne measurements. Many of the currently existing systems mounted on towers dedicated to detecting SIF typically measure both (1) the up-welling radiance coming from the surface, and (2) the incoming down-welling solar irradiance; at sensor level for practical reasons. The height of towers for measuring SIF mainly depends on the vegetation type to be monitored, ranging from 5-25 m to measure agricultural fields, to 20-50 m to monitor SIF over forests. Examples include: (1) the NASA FUSION tower ([[ftp://fusionftp.gsfc.nasa.gov/FUSION](ftp://fusionftp.gsfc.nasa.gov/FUSION)]([ftp://fusionftp.gsfc.nasa.gov/FUSION](ftp://fusionftp.gsfc.nasa.gov/FUSION))); (2) the TriFLEX instrument [10]; (3) the UNEDI system [11] at the FluxNet Hyytiala site ([[http://fluxnet.ornl.gov](http://fluxnet.ornl.gov)]([http://fluxnet.ornl.gov](http://fluxnet.ornl.gov))); and two systems developed by Universita degli Studi Milano-Bicocca: (4) the Multiplexer Radiometer Irradiometer (MRI) [12; 13] and (5) the HyperSpectral Irradiometer (HSI) [14; 15; 16]. Although novel statistical techniques were recently developed to estimate SIF at both satellite and at proximal-sensing scales [17; 18], when measuring SIF at Top Of the Canopy (TOC) the two state-of-the-art ground-based family techniques still relied on are based on the Fraunhofer Line Discriminator (FLD) principle [19; 20; 21; 22] or on the Spectral Fitting Methods (SFM) [23; 24; 25]. Both FLD and SFM have been extensively used to measure SIF at the oxygen absorption features [26; 27; 28; 29; 30; 31]. Notice that even though the FLD family of techniques was originally formulated to measure the SIF in-filling in the Fraunhofer solar lines [20; 32], they are now commonly used for measurements in the terrestrial O\({}_{2}\) bands (see references above). In particular, the O\({}_{2}\)-A band becomes advantageous when measuring SIF close to the target (\(\sim\)cm) since it provides a higher fractional depth than the Fraunhofer bands while neglecting atmospheric effects [17]. Despite this advantage, when increasing the optical path between the target and the sensor, if atmospheric effects are not corrected in the oxygen absorption regions, the estimated SIF can rapidly become prone to errors [33]. Because of that, past field campaign experiments dedicated to measuring SIF from hundreds of meters using airborne platforms always counted on atmospheric correction strategies as part of their retrieval processing [29; 33; 34; 35]. Conversely, for instruments mounted on towers, the need to correct radiances acquired at the oxygen regions for atmospheric effects when aiming to estimate SIF is not so obvious. Notice that FLD and SFM families of SIF retrievals have been formulated to be applied at TOC level, assuming that: (a) the atmospheric path between the target and the sensor is short enough to be neglected; and (b) the solar irradiance is acquired directly at the same height as the target.