*3.1. Remote Sensing Platforms for Canopy N Monitoring*

#### 3.1.1. Ground-Based Platform

Spectral data collected by ground-based sensors can be divided into non-imaging spectral data and imaging spectral data, with a spectral range primarily in the VIS–NIR, and in several studies involving the SWIR [18,45]. This close-range spectral information has ultra-high spectral resolution and can respond to subtle changes in N. It has been widely used in N monitoring for both leaf and canopy scale. Non-imaging spectral data are point spectral data, and lack spatial information for N estimation at regional scale [46]. Imaging spectral data, on the other hand, combine spatial and spectral features and allow the estimation of canopy parameters from faceted data. However, due to being restricted by data volume and acquisition method, it is generally used for ground study and rarely used directly for diagnosis and applications of large area. Current ground-based spectrometers used commonly include ASD FieldSpec (Analytical Spectral Devices, Boulder, CO, USA), RS-5400 (Spectral Evolution, Haverhill, MA, USA), HR-1024i (Spectra Vista Corporation, Poughkeepsie, NY, USA), SOC710 (Surface Optics Co. Ltd., San Diego, CA, USA), and FISS (Institute of Remote Sensing and Digital Earth, Chinese Academy of Sciences, Beijing, China) [47–50], etc. These instruments provide a stable and high spectral resolution, but some of them are heavy and are generally measured in a backpack or mounted on a tripod in research, with a single angle and way of acquiring data. The advent of handheld instruments such as the RapidSCAN (Holland Scientific Inc., Lincoln, NE, USA), 4300 Handheld FTIR (Agilent Technologies Inc., Santa Clara, CA, USA), and Crop Sense (Beijing Academy of Agriculture and Forestry Sciences, China) [51,52] symbolizes the development of spectral sensors towards lightweight and flexibility. The portable spectrometer can acquire data on a wide range of measurement scales flexibly and efficiently, and it is easy to install on a variety of platforms such as lab benches, black boxes, and rocker arms. In addition, the multi-angle spectral acquisition device [53–55] consists of several moving parts to adjust the observation position and direction, where a goniometer is often used to control the observation of zenith angle changes. The sensor is placed on the goniometer to obtain spectral data in the viewing zenith angles (VZA) ranged from −60◦ to 60◦ [53,54]. It is a convenient platform for obtaining multi-angle crop spectral data, which has many applications in the study of the vertical distribution of N in the plant canopy.

#### 3.1.2. UAV-Based Platform

With the development of lighter and smaller sensors and the increased carrying capacity of UAVs, the UAVs carrying sensors for data acquisition have become mainstream platforms in crop N monitoring. It is possible to rapidly acquire ground data with high spatial, temporal, and spectral resolution, facilitating the research at small and medium scales. Compared to airborne platforms (operating at kilometers of altitude), UAVs have the benefit of low cost, low operating altitude, and greater flexibility in terms of data collection arrangements. The sensors currently on UAVs specifically include digital cameras, multispectral/hyperspectral sensors, infrared thermal imagers, chlorophyll fluorescence sensors and LIDAR sensors [56–61]. The hyperspectral imaging spectrometer perfectly combines the advantages of spectroscopic and imaging technology for use in a large area. The payload of UAVs has made lightweight, low-cost sensors a research focus, and airborne hyperspectral imaging spectrometers such as the UHD185-Firefly and Cubert S185 (Cubert GmbH, Ulm, Baden-Württemberg, Germany), and Micro-Hyperspec (Headwall Photonics Inc., Boston, MA, USA) [62–64] are already being used for N monitoring. The PIS112 hyperspectral imaging spectrometer (Beijing Academy of Agriculture and Forestry Sciences, China), GaiaSky-mini hyperspectral imaging camera (Sichuan Dualix Spectral Imaging Technology Co., Ltd., Chengdu, China) [65] and other sensors, as well as the eightrotor unmanned aircraft system based on RGB and 25-band small multispectral cameras (Zhejiang University, China) [66,67] developed by multiple teams in China, have also been used for agricultural monitoring with good results. Among the image data acquired by the UAV platform, hyperspectral can show subtle changes in crop spectral reflectance features due to its narrow bandwidth and wide continuous spectral range, which is conducive to the fine monitoring of crop N. Most of the multispectral data are small in volume and the spectral range encompasses the N response sensitive VIS–NIR bands. However, the low spectral resolution tends to result in "missing" spectral information while overcoming the high redundancy of hyperspectral information. Studies using multispectral data sources must consider whether the 'missing' spectral information contains sensitive bands and how it can be modeled. UAV remote sensing is not affected by the external environment, such as the atmosphere, and provides better access to high-quality spectral information, making it a common data source for crop N research.
