diff --git a/definitions/mars/grib.ocda.tpa.def b/definitions/mars/grib.ocda.tpa.def
new file mode 100644
index 000000000..ce6ab9ad8
--- /dev/null
+++ b/definitions/mars/grib.ocda.tpa.def
@@ -0,0 +1,8 @@
+# aliasing MARS date and Time to the date and time defined in section 1,
+alias mars.date = dataDate;
+alias mars.time = dataTime;
+
+# dateOfAnalysis  and timeOfAnalysis still exist in this stream but is not used in MARS
+
+unalias mars.step;
+alias mars.step = stepRange;
diff --git a/definitions/mars/grib.oper.fc.def b/definitions/mars/grib.oper.fc.def
index a7a404bc3..1ece3366b 100644
--- a/definitions/mars/grib.oper.fc.def
+++ b/definitions/mars/grib.oper.fc.def
@@ -1 +1,5 @@
-alias mars.step = endStep;
+if (levtype is "o2d" || levtype is "o3d") {
+   alias mars.step = stepRange;
+} else {
+   alias mars.step = endStep;
+}