title: Using greenbrown package with Landsat data
body: <p>I have been trying to use the 'PhenologyRaster' function of the <code>greenbrown</code> package to model the growing season of my study area. However, everytime I run the function, I get empty outputs (e.g. the SOS.2016 layer will show as NA). My question is the following: am I having issues because I am running the function on one single year of data, or because Landsat time series are somewhat irregular (i.e. frequency of ~30 scenes per year)?</p>
  
  <p>I am using the following piece of code to run the PhenologyRatser function:</p>
  
  <pre><code>PhenoTest = PhenologyRaster(landsat2016,start=c(2016,1,3),end=c(2016,12,20),freq=24,approach="Deriv",min.mean=-0.5,tsgf='TSGFspline',interpolate=TRUE)
  </code></pre>
  
  <p>The function is applied on a raster stack with the following characteristics:</p>
  
  <pre><code>class       : RasterBrick 
  dimensions  : 526, 591, 310866, 18  (nrow, ncol, ncell, nlayers)
  resolution  : 30, 30  (x, y)
  extent      : 604965, 622695, 4208175, 4223955  (xmin, xmax, ymin, ymax)
  coord. ref. : +proj=utm +zone=10 +datum=WGS84 +units=m +no_defs +ellps=WGS84 +towgs84=0,0,0 
  data source : in memory
  names       : X2016.01.03, X2016.01.19, X2016.02.04, X2016.03.07, X2016.03.23, X2016.04.24, X2016.05.10, X2016.05.26, X2016.06.27, X2016.07.13, X2016.07.29, X2016.08.14, X2016.08.30, X2016.09.15, X2016.10.01, ... 
  min values  :     -0.1964,          NA,     -0.5382,          NA,     -0.4696,     -0.2197,     -0.2803,     -0.4274,     -0.4827,     -0.2631,     -0.5256,     -0.4856,     -0.5631,     -0.3204,     -0.5512, ... 
  max values  :      0.1714,          NA,      0.2425,          NA,      0.2061,      0.5173,      0.4583,      0.2470,      0.3629,      0.5165,      0.2981,      0.2802,      1.6199,      0.5016,      0.3007, ...
  </code></pre>