how to handle non-periodic timeseries in bfast (R)

Question

my problem is the following : I have a Landsat NDVI time series that is non-periodic/doesn't have a homogenous frequency. However, the error code I receive is

Error in stl(Yt, "periodic") : series is not periodic or has less than two periods

after having tried to convert my data into a timeseries without explicitely setting a frequency : test_timeseries <-ts(test$nd, start = c(1984,4), end = c(2011,10)). when I try to calculate the frequency or deltat with the help of the functions frequency() or deltat(), it both leads to 1 - which I don't understand , as I have non-periodic data for nearly every month and not only once a year.

So my question is, how to set the frequency in this case and how to deal with this circumstance of non-periodicity ? It seems like, without setting a frequency, I cannot use the function bfast(). sorry if the answer is obvious, I'm very new to timeseries analyses.

Answer 1

Please read the help file. It helps. In this case, it describes the following argument.

season : the seasonal model used to fit the seasonal component and detect seasonal breaks (i.e. significant phenological change). There are three options: "dummy", "harmonic", or "none" where "dummy" is the model proposed in the first Remote Sensing of Environment paper and "harmonic" is the model used in the second Remote Sensing of Environment paper (See paper for more details) and where "none" indicates that no seasonal model will be fitted (i.e. St = 0 ). If there is no seasonal cycle (e.g. frequency of the time series is 1) "none" can be selected to avoid fitting a seasonal model.

So set season = "none" in bfast().