Fix links (#24)

Author: m-mohr

README.md

@@ -70,8 +70,8 @@ This list is not exhaustive and can be extended with the processing level specif
 | RAW        | Data in their original packets, as received from the instrument. | [Sentinel-1 RAW](https://sentinel.esa.int/web/sentinel/technical-guides/sentinel-1-sar/products-algorithms/level-0-products/raw) |
 | L0         | Reconstructed unprocessed instrument data at full space time resolution with all available supplemental information to be used in subsequent processing (e.g., ephemeris, health and safety) appended. | [Landsat Level 0](https://www.usgs.gov/media/files/landsat-8-level-0-reformatted-data-format-control-book) |
 | L1         | Unpacked, reformatted level 0 data, with all supplemental information to be used in subsequent processing appended. Optional radiometric and geometric correction applied to produce parameters in physical units. Data generally presented as full time/space resolution. A wide variety of sub level products are possible (see below). | [Sentinel-1 Level 1](https://sentinel.esa.int/web/sentinel/user-guides/sentinel-1-sar/product-types-processing-levels/level-1) [Sentinel-2 L1A](https://sentinel.esa.int/web/sentinel/user-guides/sentinel-2-msi/product-types/level-1a) |
-| L2         | Retrieved environmental variables (e.g., ocean wave height, soil-moisture, ice concentration) at the same resolution and location as the level 1 source data. A wide variety of sub-level products are possible (see below). | [Sentinel-2 L2A](https://earth.esa.int/web/sentinel/technical-guides/sentinel-2-msi/level-2a-processing) |
-| L3         | Data or retrieved environmental variables which have been spatiallyand/or temporally re-sampled (i.e., derived from level 1 or 2 products). Such re-sampling may include averaging and compositing.  A wide variety of sub-level products are possible (see below). | [ENVISAT Level-3](http://envisat.esa.int/level3/), [Sentinel-2 L3](https://s2gm.sentinel-hub.com/) |
+| L2         | Retrieved environmental variables (e.g., ocean wave height, soil-moisture, ice concentration) at the same resolution and location as the level 1 source data. A wide variety of sub-level products are possible (see below). | [Sentinel-2 L2A](https://sentinel.esa.int/web/sentinel/technical-guides/sentinel-2-msi/level-2a-processing) |
+| L3         | Data or retrieved environmental variables which have been spatiallyand/or temporally re-sampled (i.e., derived from level 1 or 2 products). Such re-sampling may include averaging and compositing.  A wide variety of sub-level products are possible (see below). | [ENVISAT Level-3](https://earth.esa.int/eogateway/documents/20142/37627/mkl3_product_handbook_is1r0.pdf), [Sentinel-2 L3](https://s2gm.land.copernicus.eu) |
 | L4         | Model output or results from analyses of lower level data (i.e.,variables that are not directly measured by the instruments, but are derived from these measurements) | |
 
 ### Expression Object