Just like what you see in the attachment (the first answer). Now, I have a question that how can I conduct two data: AVHRR and MODIS? In fact, AVHRR would be consistent with MODIS in time series. However, owing to that the quality of AVHRR was lower than that of MODIS, the value of AVHRR was bigger than that of MODIS in the same time. For example, (I think that) the value of AVHRR should be 177.5 (unit) in 1999, but now the value of AVHRR is actually 244.0 (unit). Now, I want to know whether or not having a function or method is universal and can be used in my research to help solve the consistent question.
Some of the trends in your AVHRR data look like they are related to sensor or orbital drift.
Here is one paper where the authors discuss this problem and their attempts to correct for it. This may help you find a solution.
Tucker,CJ, Pinzon, JE, Brown, ME, Slayback, DA, Pak, EW, Mahoney, R, Vermote, EF, and Saleous, NE. 2005. An extended AVHRR 8-km NDVI dataset compatible with MODIS and SPOT vegetation NDVI data. International Journal of Remote Sensing 26:4485-4498.
Because of the differences in sensor design, the two sensors can be used together but not put in the same time series. There will always be a difference in the absolute value of the NDVI from the two sensors. Look at this paper - it is in my paper list here in Research Gate (also the Tucker et al 2005 paper above). It describes the impact of sensor design and data processing on the different time series.
Brown, M. E., Pinzon, J. E., Didan, K., Morisette, J. T. & Tucker, C. J. 2006. Evaluation of the consistency of long-term NDVI time series derived from AVHRR, SPOT-Vegetation, SeaWIFS, MODIS and LandSAT ETM+. IEEE Transactions Geoscience and Remote Sensing, 44, 1787-1793.
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