## 5 Discussion
If no valid observation occurred in the time window for compositing, residual clouds will be included in the results. The other undesired result was the unsmooth transition in the composites. The unsmooth transitions were mainly derived from the variation of atmosphere conditions ion different dates in the compositing period. As the TOA reflectance rather than surface reflectance was used in the current study, the atmosphere condition that contributed differently to the surface reflectance was the main source of error that will introduce bias to the composites. The high accuracy atmosphere correction can be used to further improve the radiometric consistency of the proposed composites [15, 49]. However, due to the lack of a shortwave infrared band which was usually needed for aerosol optical depth estimation using the dense dark vegetation approach, the atmosphere correction method for HJ-1A/B imagery is still in the exploratory stage [23, 24, 34, 57]. Atmosphere correction using MODIS atmospheric parameters as the auxiliary data would be helpful for the quality improvement HJ-1A/B composites. In future, the atmosphere corrected surface reflectance will be conducted in the research. The radiometric calibration error is another important error source that will influence the accuracy of compositing. Some studies have evaluated the radiometric performance of HJ using well-verified TM images or MODIS datasets [58, 59], and the results indicated that the image quality of HJ-1A/B CCDs was stable and possessed good consistency. A recent study monitored the radiometric responses of the HJ-1A/B CCD using all available HJ-1A/B images over the pseudo-invariant Dunhuang gobi calibration ground site from 2008-2013 [60]. It indicated that the degradation occurred for all bands of the sensors at different rates and levels. The degradation of sensors might be an important error source which influences the distribution of points in Figure 7. Therefore, a more symmetrically radiometric calibration accuracy evaluation should be performed to provide a more reliable and quantitative radiometric performance knowledge of HJ CCD sensors. The dense in time series and fine spatial resolution composites can play a key role in providing spatially and temporally explicit data for land surface dynamics monitoring as well as bio-physical parameter retrieving. With the efforts being put into compositing monthly, seasonal or annual imagery products, a broader range of applications have been conducted using these composites [11, 20, 21, 61, 62]. Typically, for some more time-sensitive applications such as monitoring rapid disturbance (e.g., insect defoliation, fire, wetland inundation), a smaller compositing time window was required. Currently, the capacity of fine spatial resolution images to generate products for such applications is still limited by the satellite revisit frequency [11, 21]. Fortunately, opportunities from new satellite missions or virtual constellations will address these limitations. In the future research, dense temporal and high spatial resolution imagery provided by small constellations is a likely possibility for the earth observation satellites [63, 64]. With new satellite missions such as Sentinel-2A/B or Landsat 8 Operational Land Imager (OLI) execution [65, 66, 67], more constellations or virtual constellations would be developed and the integrated measures from different satellite programs will make the temporal repeat frequencies much denser than present. Take the Sentinel-2 for instance. Similar to HJ-1A/B, the Sentinel-2 mission is also based on a pair of satellites configuration (Sentinel-2A and 2B) and has a relatively large swath, allowing a two to three-day revisit time at mid-latitudes. A virtual constellation of HJ-1A/B, Landsat-8 with Sentinel-2 would increase the observation frequency considerably. Such dense data will be particularly beneficial for many time-sensitive application studies [68].