diff --git a/sw/ground_segment/cockpit/saveSettings.ml b/sw/ground_segment/cockpit/saveSettings.ml
index 20463a7259..7c06daa0b1 100644
--- a/sw/ground_segment/cockpit/saveSettings.ml
+++ b/sw/ground_segment/cockpit/saveSettings.ml
@@ -43,21 +43,6 @@ let floats_not_equal = fun f1 f2 ->
   let r = abs_float (f1 /. f2) in
   r < 0.999 || r > 1.001
 
-(* Unit conversions *)
-let scale_of_units = fun u1 u2  ->
-  match (u1, u2) with
-      ("deg", "rad") | ("deg/s", "rad/s") -> 180. /. Latlong.pi
-    | ("rad", "deg") | ("rad/s", "deg/s") -> Latlong.pi /. 180.
-    | ("m", "cm") | ("m/s", "cm/s") -> 100.
-    | ("cm", "m") | ("cm/s", "m/s") -> 0.01
-    | ("m", "mm") | ("m/s", "mm/s") -> 1000.
-    | ("mm", "m") | ("mm/s", "m/s") -> 0.001
-    | ("decideg", "deg") -> 0.1
-    | ("deg", "decideg") -> 10.
-    | u1, u2 when u1 = u2 -> 1.
-    | _ -> invalid_arg (Printf.sprintf "SaveSettings.scale_of_units %s %s" u1 u2)
-
-
 
 (** The save file dialog box *)
 let save_airframe = fun w filename save ->
@@ -145,7 +130,7 @@ let fill_data = fun (model:GTree.tree_store) settings airframe_xml ->
           let unit_setting = attrib "unit"
           and unit_airframe =
             match unit with Some u -> u | None -> raise Exit in
-          scale_of_units unit_setting unit_airframe
+          Pprz.scale_of_units unit_setting unit_airframe
         with
             _ -> 1. in
       let val_list = Str.split (Str.regexp "[ ()]+") airframe_value in
diff --git a/sw/lib/ocaml/pprz.ml b/sw/lib/ocaml/pprz.ml
index 77f51ab82c..02192104db 100644
--- a/sw/lib/ocaml/pprz.ml
+++ b/sw/lib/ocaml/pprz.ml
@@ -73,6 +73,7 @@ let (//) = Filename.concat
 let messages_file = Env.paparazzi_src // "conf" // "messages.xml"
 let lazy_messages_xml = lazy (Xml.parse_file messages_file)
 let messages_xml = fun () -> Lazy.force lazy_messages_xml
+let units_file = Env.paparazzi_src // "conf" // "units.xml"
 
 external float_of_bytes : string -> int -> float = "c_float_of_indexed_bytes"
 external double_of_bytes : string -> int -> float = "c_double_of_indexed_bytes"
@@ -158,21 +159,30 @@ let payload_size_of_message = fun message ->
     2 (** + message id + aircraft id *)
 
 
+let scale_of_units = fun from_unit to_unit ->
+  try
+    let units_xml = Xml.parse_file units_file in
+    (* find the first occurence of matching units or raise Not_found *)
+    let _unit = List.find (fun u ->
+      try
+        if from_unit = (Xml.attrib u "from") && to_unit = (Xml.attrib u "to") then
+          true
+        else
+          false
+      with _ -> false (* not a valid unit declaration *)
+    ) (Xml.children units_xml) in
+    (* return coef *)
+    float_of_string (Xml.attrib _unit "coef")
+  with _ -> invalid_arg "Unit conversion failed"
+
+
 let alt_unit_coef_of_xml = function 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
-    match (u, au) with
-      ("deg", "rad") | ("deg/s", "rad/s") -> string_of_float (pi /. 180.)
-    | ("rad", "deg") | ("rad/s", "deg/s") -> string_of_float (180. /. pi)
-    | ("m", "cm") | ("m/s", "cm/s") -> "100."
-    | ("cm", "m") | ("cm/s", "m/s") -> "0.01"
-    | ("m", "mm") | ("m/s", "mm/s") -> "1000."
-    | ("mm", "m") | ("mm/s", "m/s") -> "0.001"
-    | ("decideg", "deg") -> "0.1"
-    | ("deg", "decideg") -> "10."
-    | (_, _) -> "1."
+    let coef = try string_of_float (scale_of_units u au) with _ -> "1." in
+    coef
 
 let pipe_regexp = Str.regexp "|"
 let field_of_xml = fun xml ->
diff --git a/sw/lib/ocaml/pprz.mli b/sw/lib/ocaml/pprz.mli
index 3ab239c9ca..3eeec6ae4b 100644
--- a/sw/lib/ocaml/pprz.mli
+++ b/sw/lib/ocaml/pprz.mli
@@ -86,21 +86,17 @@ val int32_assoc : string -> values -> Int32.t
 val hex_of_int_array : value -> string
 (** Returns the hexadecimal string of an array of integers *)
 
+val scale_of_units : string -> string -> float
+(** scale_of_units from to
+ *  Returns conversion factor between two units
+ *  The possible conversions are described in conf/units.xml
+ *  May raise Invalid_argument if one of the unit is not valid
+ *  or if units.xml is not valid
+ *)
+
 val alt_unit_coef_of_xml : Xml.xml -> string
 (** Return coef for alternate unit
-    Default possible corrections:
-    deg -> rad
-    rad -> deg
-    m -> cm
-    cm -> m
-    m -> mm
-    mm -> m
-    m/s -> cm/s
-    cm/s -> m/s
-    m/s -> mm/s
-    mm/s -> m/s
-    decideg -> deg
-    *)
+ *)
 
 exception Unknown_msg_name of string * string
 (** [Unknown_msg_name (name, class_name)] Raised if message [name] is not
diff --git a/sw/tools/gen_airframe.ml b/sw/tools/gen_airframe.ml
index 8f656bbe4e..d0ae43979a 100644
--- a/sw/tools/gen_airframe.ml
+++ b/sw/tools/gen_airframe.ml
@@ -26,8 +26,6 @@
 
 let max_pprz = 9600. (* !!!! MAX_PPRZ From paparazzi.h !!!! *)
 
-exception Undefined_scale
-
 open Printf
 open Xml2h
 
@@ -84,18 +82,9 @@ let define_integer name v n =
 
 let code_unit_scale_of_tag = function t ->
   let u = try ExtXml.attrib t "unit" with _ -> "" in
-  let cu = try ExtXml.attrib t "code_unit" with _ -> "" in
-  match (u, cu) with
-      ("deg", "rad") | ("deg/s", "rad/s") -> Latlong.pi /. 180.
-    | ("deg", "") | ("deg/s", "") -> Latlong.pi /. 180.
-    | ("rad", "deg") | ("rad/s", "deg/s") -> 180. /. Latlong.pi
-    | ("m", "cm") | ("m/s", "cm/s") -> 100.
-    | ("cm", "m") | ("cm/s", "m/s") -> 0.01
-    | ("m", "mm") | ("m/s", "mm/s") -> 1000.
-    | ("mm", "m") | ("mm/s", "m/s") -> 0.001
-    | ("decideg", "deg") -> 0.1
-    | ("deg", "decideg") -> 10.
-    | (_, _) -> raise Undefined_scale
+  (* default value for code_unit is rad when unit is deg *)
+  let cu = try ExtXml.attrib t "code_unit" with _ -> if u = "deg" then "rad" else "" in
+  Pprz.scale_of_units u cu
 
 let parse_element = fun prefix s ->
   match Xml.tag s with