update README

Signed-off-by: Kento Yabuuchi <kento.yabuuchi.2@tier4.jp>

sensing/gnss_poser/README.md

@@ -4,8 +4,15 @@
 
 The `gnss_poser` is a node that subscribes gnss sensing messages and calculates vehicle pose with covariance.
 
+This node subscribes to NavSatFix to publish the pose of **base_link**. The data in NavSatFix represents the antenna's position. Therefore, it performs a coordinate transformation using the tf from `base_link` to the antenna's position. The frame_id of the antenna's position refers to NavSatFix's `header.frame_id`.
+(**Note that `header.frame_id` in NavSatFix indicates the antenna's frame_id, not the Earth or reference ellipsoid.** [See also NavSatFix definition.](https://docs.ros.org/en/noetic/api/sensor_msgs/html/msg/NavSatFix.html))
+
+If the transformation from `base_link` to the antenna cannot be obtained, it outputs the pose of the antenna position without performing coordinate transformation.
+
 ## Inner-workings / Algorithms
 
+
+
 ## Inputs / Outputs
 
 ### Input