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docs/building/sources/constants.rst

@@ -11,9 +11,23 @@
 Although this source is called `constants`, its purpose is to provide
 fields with values that only depend on the grid cell and/or the time.
 
+Because the source needs to generate fields on the same grids as the
+other, it requires a template field. This is provided in the recipe with
+the `template` keyword:
+
+.. literalinclude:: constants.yaml
+   :language: yaml
+
+The value ``${input.join.0.source1}`` is the "path" to the first source,
+starting from the root of the recipe. The path is composed of the of the
+nodes in the recipe, separated by dots. If a node contains a list, the
+index of the next node is added after the node name, starting from 0.
+
 The is a mean to provide the model with with space and time information
 during training and inference.
 
+The following fields are available:
+
 latitude
    Each grid point has the value of its latitude in degrees.
 

docs/building/sources/constants.yaml

@@ -0,0 +1,12 @@
+input:
+  join:
+    - source1:
+      args1: value1
+      args2: value2
+
+    - constants:
+      template: ${input.join.0.source1}
+      params:
+        - insolation
+        - cos_julian_day
+        - sin_julian_day