# Harmonizing Multi-Source Remote Sensing Images for Summer Corn Growth Monitoring
###### Abstract  
Continuous monitoring of crop growth status using time-series remote sensing image is essential for crop management and yield prediction. The growing season of summer corn in the North China Plain with the period of rain and hot, which makes the acquisition of cloud-free satellite imagery very difficult. Therefore, we focused on developing image datasets with both a high temporal resolution and medium spatial resolution by harmonizing the time-series of MOD09 GA Normalized Difference Vegetation Index (NDVI) images and 30-m-resolution GF-1 WFV images using the improved Kalman filter model. The harmonized images, GF-1 images, and Landsat 8 images were then combined and used to monitor the summer corn growth from 5 th June to 6 th October, 2014, in three counties of Hebei Province, China, in conjunction with meteorological data and MODIS Evapotranspiration Data Set. The prediction residuals (\(\Delta{PR}_{K}\)) in NDVI between the GF-1 observations and the harmonized images was in the range of \(-0.2\) to \(0.2\) with Gauss distribution. Moreover, the obtained phenological curves manifested distinctive growth features for summer corn at field scales. Changes in NDVI over time were more effectively evaluated and represented corn growth trends, when considered in conjunction with meteorological data and MODIS Evapotranspiration Data Set. We observed that the NDVI of summer corn showed a process of first decreasing and then rising in the early growing stage and discuss how the temperature and moisture of the environment changed with the growth stage. The study demonstrated that the synthesized dataset constructed using this methodology was highly accurate, with high temporal resolution and medium spatial resolution and it was possible to harmonize multi-source remote sensing imagery by the improved Kalman filter for long-term field monitoring. harmonizing; Kalman filter; multi-source; growth monitoring; summer corn 11 1266 10.3390/rs11111266 10.  
synthesized vegetation index products have many applications in monitoring global climate change, material and energy cycles and the growth of surface vegetation. [PERSON] et al. [5] integrated the MODIS eight-day and 16-day products to quantitatively estimate green leaf area index (GLAI) over corn fields and evaluate the potential and limitations of the satellite data. [PERSON] et al. [6] evaluated the applicability of the eight-day MODIS composite imagery in monitoring crop growth and yield estimation and the results showed that these images were very reliable. [PERSON] et al. [7; 8] proposed a method which could identify the growth period of maize and soybean with two-step filtering based on MODIS data. However, the low spatial resolution (i.e., 250 m or 500 m) will cause a lot of errors by mixing multiple objects which have great differences in spectral curves and phenological characteristics in one pixel. [PERSON] et al. [9] monitored and evaluated the growth condition of spring maize based on MODIS vegetation index products, MODIS LAI products and Global Land Surface Satellites LAI product. Moreover, the medium-resolution satellites (i.e., Landsat and GF-1) have developed rapidly and can well express the detailed features of the surface [10; 11; 12]. If the time-series is complete enough, the medium-resolution satellites can reflect the growth status of crops over the whole growing period. Due to both the low temporal resolution and because most crops grow during periods of high rainfall and heat, including the summer corn planted in eastern China, the quality of the images obtained during that period can be reduced due to clouds and smog. It also leads to the difficulty of obtaining high quality time series images even for satellites with higher temporal resolution, such as GF-1(four-day and 16 m).