Update t265.md: added odometry calibration information

doc/t265.md

@@ -98,7 +98,7 @@ We are still looking into it, please stay tuned.
 
 ### What is wheel odometry and how does it help T265 navigate?
 
-Wheel odometry is the use of sensors to measure how much a wheel turns, it can be used to estimate changes in a wheeled robots position. T265 has wheel odometry support built in, allowing it to use the data from these sensors to refine the position data of the robot. Providing robotic wheel odometer or velocimeter data over USB to TM2 will make the tracking much more robust on wheeled robots, which otherwise can experience many tracking failures. We consider odometer input to be a requirement for robust tracking on wheeled robots.
+Wheel odometry is the use of sensors to measure how much a wheel turns, it can be used to estimate changes in a wheeled robots position. T265 has wheel odometry support built in, allowing it to use the data from these sensors to refine the position data of the robot. Providing robotic wheel odometer or velocimeter data over USB to TM2 and the corresponding calibration data will make the tracking much more robust on wheeled robots, which otherwise can experience many tracking failures. The calibration file format is described in the [section below](#wheel-odometry-calibration-file-format). We consider odometer input to be a requirement for robust tracking on wheeled robots.
 
 ### Can T265 work indoors and outdoors?
 
@@ -107,3 +107,49 @@ Yes, T265 can work both indoors and outdoors. Just like a person, it can be blin
 ### Do multiple T265 devices interfere with each other?
 
 No, you can use as many T265 devices in a space as you like.
+
+## Appendix
+
+### Wheel odometry calibration file format
+Below is a sample calibration file with inline comments (not a valid file format!) that explain each field. Please note that this is an intermediate solution and a more unified interface will be coming.
+```
+{
+    "velocimeters": [ // array of velocity sensors (currently max. 2 supported)
+        {
+            "scale_and_alignment": [ // 3-by-3 matrix, row-major (multiplies measurement)
+                1.0,
+                0.0000000000000000,
+                0.0000000000000000,
+                0.0000000000000000,
+                1.0,
+                0.0000000000000000,
+                0.0000000000000000,
+                0.0000000000000000,
+                1.0
+            ],
+            "noise_variance": 0.004445126050420168, // measurement covariance (to be determined/tuned)
+            "extrinsics": { // relative transformation of sensor frame w.r.t. T265 body frame
+                "T": [ // translation (in meters)
+                    -0.5059,
+                    -0.6294,
+                    -0.6873
+                ],
+                "T_variance": [ // currently not used, please ignore
+                    9.999999974752427e-7, 
+                    9.999999974752427e-7, 
+                    9.999999974752427e-7
+                ],
+                "W": [ // orientation in axis-angle representation, axis x angle (in rad)
+                    -1.1155,
+                    -1.1690,
+                    -1.2115
+                ],
+                "W_variance": [ // currently not used, please ignore
+                    9.999999974752427e-5, 
+                    9.999999974752427e-5, 
+                    9.999999974752427e-5
+                ]
+            }
+        }
+    ]
+}