M- L- To estimate Zenith Total Delay (ZTD) with GNSS (Global Navigation Satellite System) processing software like GNUT/TEFNUT, you typically need to create a configuration file that specifies the settings and parameters for your data processing. Here's a simplified example of a configuration file to estimate ZTD:

```ini

[Processing]

Mode = Precise Point Positioning

ObsFiles = your_observation_file.obs

NavFiles = your_navigation_file.nav

AntennaFile = your_antenna_file.atx

OrbitInterpolation = Nearest

ElevationMask = 10.0

[Estimation]

EstimateZTD = Yes

ZTD = 2.000

[Output]

OutputPath = output_directory

Summary = Yes

Debug = No

```

Explanation of key sections and parameters:

- [Processing]:

- `Mode`: Set the processing mode to "Precise Point Positioning" for ZTD estimation.

- `ObsFiles`: Specify your observation data file (RINEX format, e.g., your_observation_file.obs).

- `NavFiles`: Specify your navigation data file (RINEX format, e.g., your_navigation_file.nav).

- `AntennaFile`: Provide the antenna information file (ANTEX format, e.g., your_antenna_file.atx).

- `OrbitInterpolation`: Choose the orbit interpolation method (e.g., "Nearest").

- `ElevationMask`: Set the elevation mask angle (in degrees) to control which satellites are used.

- [Estimation]:

- `EstimateZTD`: Set to "Yes" to enable ZTD estimation.

- `ZTD`: Initialize ZTD value (e.g., 2.000). You can use an approximate value here.

- [Output]:

- `OutputPath`: Specify the directory where output files will be saved (e.g., output_directory).

- `Summary`: Enable summary output.

- `Debug`: Set to "No" for normal processing. You can change it to "Yes" for debugging purposes.

Please note that this is a basic example, and you should adapt it to your specific dataset and processing requirements. Ensure that your observation and navigation files are correctly referenced, and use the appropriate antenna information.

Additionally, GNUT/TEFNUT may have its own syntax and specific options for configuration files. Be sure to consult the documentation or user guide for the software version you are using to ensure compatibility and accurate configuration.