diff --git a/sw/ground_segment/tmtc/messages.ml b/sw/ground_segment/tmtc/messages.ml
index 0661e986aa..209c785423 100644
--- a/sw/ground_segment/tmtc/messages.ml
+++ b/sw/ground_segment/tmtc/messages.ml
@@ -67,7 +67,7 @@ let one_page = fun sender class_name (notebook:GPack.notebook) bind m ->
           let literal_values = values_of_field f in
           let alt_value =
             try
-              let coeff = float_of_string (Pprz.alt_unit_coef_of_xml f)
+              let coeff = float_of_string (Pprz.alt_unit_coef_of_xml ~auto:"display" f)
               and unit = Xml.attrib f "alt_unit" in
               fun value -> sprintf "%s (%f%s)" value (coeff*.float_of_string value) unit
             with
@@ -86,7 +86,7 @@ let one_page = fun sender class_name (notebook:GPack.notebook) bind m ->
           (* box dragger *)
           field_label#drag#source_set dnd_targets ~modi:[`BUTTON1] ~actions:[`COPY];
           let data_get = fun _ (sel:GObj.selection_context) ~info ~time ->
-            let scale = Pprz.alt_unit_coef_of_xml f in
+            let scale = Pprz.alt_unit_coef_of_xml ~auto:"display" f in
             let field_descr =
               if Pprz.is_array_type type_ then
                 match GToolbox.input_string ~title:"Index of value to drag" ~text:"0" "Index in the array ?" with
diff --git a/sw/lib/ocaml/pprz.ml b/sw/lib/ocaml/pprz.ml
index 1302dc4167..00892d003e 100644
--- a/sw/lib/ocaml/pprz.ml
+++ b/sw/lib/ocaml/pprz.ml
@@ -162,7 +162,7 @@ exception Unit_conversion_error of string
 exception Unknown_conversion of string * string
 exception No_automatic_conversion of string * string
 
-let scale_of_units = fun from_unit to_unit ->
+let scale_of_units = fun ?auto from_unit to_unit ->
   if (from_unit = to_unit) then
     1.0
   else
@@ -173,7 +173,9 @@ let scale_of_units = fun from_unit to_unit ->
           (* will raise Xml.No_attribute if not a valid attribute *)
           let f = Xml.attrib u "from"
           and t = Xml.attrib u "to"
-          and a = String.lowercase (ExtXml.attrib_or_default u "auto" "") in
+          and a = match auto with
+            | Some a -> a
+            | None -> String.lowercase (ExtXml.attrib_or_default u "auto" "") in
           if (f = from_unit || a = "display") && (t = to_unit || a = "code") then true else false
         ) (Xml.children units_xml) in
       (* return coef, raise Failure if coef is not a numerical value *)
@@ -187,13 +189,16 @@ let scale_of_units = fun from_unit to_unit ->
       | _ -> raise (Unknown_conversion (from_unit, to_unit))
 
 
-let alt_unit_coef_of_xml = function xml ->
+let alt_unit_coef_of_xml = fun ?auto xml ->
   try Xml.attrib xml "alt_unit_coef"
   with _ ->
     let u = try Xml.attrib xml "unit" with _ -> "" in
     let au = try Xml.attrib xml "alt_unit" with _ -> "" in
-    let coef = try string_of_float (scale_of_units u au) with
-      Unit_conversion_error s -> prerr_endline (sprintf "Unit conversion error: %s" s); flush stderr; "1." (* Use coef 1. *)
+    let coef = try string_of_float (match auto with
+      | None -> scale_of_units u au
+      | Some a -> scale_of_units u au ~auto:a)
+    with
+    | Unit_conversion_error s -> prerr_endline (sprintf "Unit conversion error: %s" s); flush stderr; "1." (* Use coef 1. *)
     | Unknown_conversion _ -> "1." (* Use coef 1. *)
     | _ -> "1."
     in
diff --git a/sw/lib/ocaml/pprz.mli b/sw/lib/ocaml/pprz.mli
index b7ec04c53f..5338be84fa 100644
--- a/sw/lib/ocaml/pprz.mli
+++ b/sw/lib/ocaml/pprz.mli
@@ -95,7 +95,7 @@ exception No_automatic_conversion of string * string
  *  and from_unit or to_unit are empty string
  *)
 
-val scale_of_units : string -> string -> float
+val scale_of_units : ?auto:string -> string -> string -> float
 (** scale_of_units from to
  *  Returns conversion factor between two units
  *  The possible conversions are described in conf/units.xml
@@ -103,7 +103,7 @@ val scale_of_units : string -> string -> float
  *  or if units.xml is not valid
  *)
 
-val alt_unit_coef_of_xml : Xml.xml -> string
+val alt_unit_coef_of_xml : ?auto:string -> Xml.xml -> string
 (** Return coef for alternate unit
  *)