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I'm working in a project where I need the NDVI for some areas from Uruguay for the period 1988-1993, and I found this product "Landsat 5 TM 8-Day NDVI Composite" but I'm not sure of how it was calculated because the landsat frequency is of 16 days and the NDVI value is given for every 8 days. I noticed that several Landsat products have this 8-day frequency though. so maybe this is a general thing of the Landsat products.

Here's the link of the product https://code.earthengine.google.com/dataset/LANDSAT/LT5_L1T_8DAY_NDVI

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Landsat frequency is 16 days, you are right. The product itself did not provide an image over the same place at 8 days frequency, only group images from an 8 days period. It has a similar systematization than MODIS (could be to compare some products). In other words, the only thing that does this collection is to group images between this time period.

For example, if I ask for the first month of 1988:

ImageCollection LANDSAT/LT5_L1T_8DAY_NDVI (4 elements, 1 band)
type: ImageCollection
id: LANDSAT/LT5_L1T_8DAY_NDVI
version: 1502750724877455
bands: List (1 element)
0: "NDVI", float ∈ [-1, 1]
features: List (4 elements)
0: Image LANDSAT/LT5_L1T_8DAY_NDVI/19880101 (1 band)
1: Image LANDSAT/LT5_L1T_8DAY_NDVI/19880109 (1 band)
2: Image LANDSAT/LT5_L1T_8DAY_NDVI/19880117 (1 band)
3: Image LANDSAT/LT5_L1T_8DAY_NDVI/19880125 (1 band)

and the first month of 1989:

ImageCollection LANDSAT/LT5_L1T_8DAY_NDVI (4 elements, 1 band)
type: ImageCollection
id: LANDSAT/LT5_L1T_8DAY_NDVI
version: 1502750724877455
bands: List (1 element)
features: List (4 elements)
0: Image LANDSAT/LT5_L1T_8DAY_NDVI/19890101 (1 band)
1: Image LANDSAT/LT5_L1T_8DAY_NDVI/19890109 (1 band)
2: Image LANDSAT/LT5_L1T_8DAY_NDVI/19890117 (1 band)
3: Image LANDSAT/LT5_L1T_8DAY_NDVI/19890125 (1 band)

the DOY of the scene is the same. Therefore, it's possible to make comparations between each pair of images.

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Landsat's orbit ensures that each location on Earth is imaged at least once every 16 days. Because the imaging path is continuous in its width, the imaging frequency increases towards the poles so that many arctic and antarctic areas are imaged at much higher temporal resolution than once every 16 days. Near the equator, in areas such as Uruguay, the 16-day imaging period is far more prevalent.

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