From 00bdb17034c2a016401780bf2e7ea281b13be356 Mon Sep 17 00:00:00 2001
From: Arnaud Taffanel <arnaud@taffanel.org>
Date: Wed, 1 Aug 2018 09:37:45 +0200
Subject: [PATCH] Add output interpretation section to Readme.md

Closes #2.
---
 readme.md | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/readme.md b/readme.md
index 8174f50..8999ecf 100644
--- a/readme.md
+++ b/readme.md
@@ -34,3 +34,9 @@ As soon as it is initialized, the sensor will start accumulating motion count. C
   // Get motion count since last call
   flow.readMotionCount(&deltaX, &deltaY);
 ```
+
+## Output interpretation
+
+The sensor outputs unitless ticks that corresponds to the amount of movement that has been observed in front of the sensor. These ticks can be converted to a distance if we know the distance to the tracked surface and we assume the tracked surface is flat.
+
+Parameters of the PMW3901 sensor have been devised experimentally in order to implement it in the Crazyflie firmware. The measurement equation are described in  [Marcus Greiff's Master thesis](http://lup.lub.lu.se/luur/download?func=downloadFile&recordOId=8905295&fileOId=8905299) section 6.5. The implementation is part or the [Crazyflie kalman filter](https://github.com/bitcraze/crazyflie-firmware/blob/6308ff47ff4d4691f9b7f6f991564244c76d7910/src/modules/src/estimator_kalman.c#L1034-L1098).