Adds a FAQ with one entry. (#272)

docs/source/faq.rst

@@ -0,0 +1,36 @@
+.. Copyright 2016 The Cartographer Authors
+
+.. Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+..      http://www.apache.org/licenses/LICENSE-2.0
+
+.. Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+
+==========================
+Frequently asked questions
+==========================
+
+The laser data in the 3D bags is much higher than the maximum reported 20 Hz rotation speed that the VLP-16 can do. Why?
+----------------------------------------------------------------------------------------------------------------------
+
+The VLP-16 in the example bags is configured to rotate at 20 Hz. However, the
+frequency of UDP packets the VLP-16 sends is much higher and independent of
+the rotation frequency. The example bags contain a `sensor_msgs/PointCloud2`__
+per UDP packet, not one per revolution.
+
+__ http://www.ros.org/doc/api/sensor_msgs/html/msg/PointCloud2.html
+
+In the `corresponding Cartographer configuration file`__ you see
+`TRAJECTORY_BUILDER_3D.scans_per_accumulation = 160` which means we accumulate
+160 per-UDP-packet point clouds into one larger point cloud, which
+incorporates motion estimation by combining constant velocity and IMU
+measurements, for matching. Since there are two VLP-16s, 160 UDP packets is
+enough for roughly 2 revolutions, one per VLP-16.
+
+__ https://github.com/googlecartographer/cartographer_ros/blob/master/cartographer_ros/configuration_files/backpack_3d.lua

docs/source/index.rst

@@ -23,6 +23,7 @@ Cartographer ROS Integration
    configuration
    ros_api
    data
+   faq
 
 `Cartographer`_ is a system that provides real-time simultaneous localization
 and mapping (`SLAM`_) in 2D and 3D across multiple platforms and sensor