I'm something of a beginner to remote sensing.
I've got an NDVI time series, from 2002-2016. I've smoothed the data using an excel spreadsheet with the formula yt = (-2xt-3 + 3xt-2 + 6xt-1 + 7xt + 6xt+1 + 3xt+2 - 2xt+3)/21. Now, I don't understand that - equations baffle me - I trust it works though, because I'm told it does. Using MODIS NDVI 16 day combined data extracted from Google Earth Engine!
Unfortunately, I've got gaps in my NDVI time series, and those gaps correspond with very low or negative NDVI values in the smoothed time series.
I've tried to read about methods to fill gaps in NDVI time series. Chen et al. (2004) proposed a method (https://www.sciencedirect.com/science/article/pii/S003442570400080X). Kandasamy et al. (2013) called that an Adaptive Savitsky Golay filter. But I must be honest, the mathematics and equations serve only to confuse me! I came to Remote Sensing from a background in environmental management rather than science, so to me those papers are sadly about as useful as hieroglyphics.
Given my mathematic incompetence, could somebody please give me either:
a) an equation to paste into Excel which could accomplish Chen et al.'s (2004) method
b) tell me what method I should use to fill the gaps in my NDVI time series?
Essentially - how should somebody who can't turn an equation on a page into a formula in excel fill the gaps in an NDVI time series?