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# Nadir-altimetry datasets

The main geophysical parameter provided by altimeters for sea state is the 
significant wave height (SWH). Three kinds of datasets are delivered, as 
summarized in table {numref}`altimeter_datasets`

```{table} Summary description CCI Sea State Altimeter datasets
:name: altimeter_datasets

| Product             | L2 Pre-Processed {numref}`l2p` | L3 Multi-Mission {numref}`l3` | L4 Multi-Mission {numref}`l4`                                             |
| ------------------- | ------------------------------ | ----------------------------- | ---------------------------- |
| Acronym             | L2P                                                                                                                                                                                                                                                                                        | L3                                                                                 | L4                                                                                 |
| Description         | SWH derived from Level 1 source data (wave forms) for a single mission but averaged at 1 Hz, typically in a satellite projection with geographic information. These expert data form the fundamental basis for higher-level products and require ancillary data and uncertainty estimates. | SWH measurements from L2P of different missions stitched together into daily files | SWH measurements combined from a multiple missions into a monthly space-time grid. | 
| Grid specification  | Along-track observations                                                                                                                                                                                                                                                                   | Along-track observations                                                           | Global regular lat-lon grid, 1°x1°                                                 |                                                                                                                                            
| Temporal resolution | 1 second averages along full or half orbits                                                                                                                                                                                                                                                | 1 second averages into daily files                                                 | Monthly                                                                            |                                                                                                                                    
| Spatial resolution  | 6-7 km |  6-7 km | 1°x1° |
| Coverage            | Mission specific                                                                                                                                                                                                                                                                           | Native to data stream                                                              | Global                                                                             | 
```

## L2P

L2P Along-track files from altimetry missions provide SWH measurements 
separated per satellite and pass (half-orbit), sometimes full orbit or 
sections of orbit (depending on the source provider for the altimeter data), 
including all measurements with flags, corrections and extra parameters
from other sources. These are expert products with rich content and no data loss.

```{figure} images/cci_l2p_altimeter.png
---
height: 250px
name: cci_l2p_altimeter
---
Example of a L2P coverage
```

## L3 

Edited merged daily dataset retaining all valid and good quality 
measurements from all L2P altimeters over one day (one daily file), with 
simplified content (only a few key parameters). This is close to what is 
delivered in NRT or Multi-Year time series by CMEMS project[^footnote1].

```{figure} images/cci_l3_altimeter.png
---
height: 250px
name: cci_l3_altimeter
---
Example of a multi-mission L3 coverage 
```

## L4

Gridded products averaging valid and good measurements from all available
altimeters over a fixed resolution grid (1°x1°) on a monthly basis. This 
dataset is meant for statistics, and visualisation through the CCI toolbox.

```{figure} images/cci_l4_altimeter.png
---
height: 250px
name: cci_l4_altimeter
---
Example of a multi-mission L4 coverage 
```

[^footnote1]: https://doi.org/10.48670/moi-00176