Fix link to PhD thesis

README.md

@@ -12,7 +12,7 @@ Fusion is a sensor fusion library for Inertial Measurement Units (IMUs), optimis
 
 The Attitude And Heading Reference System (AHRS) algorithm combines gyroscope, accelerometer, and magnetometer data into a single measurement of orientation relative to the Earth.  The algorithm also supports systems that use only a gyroscope and accelerometer, and systems that use a gyroscope and accelerometer combined with an external source of heading measurement such as GPS.
 
-The algorithm is based on the revised AHRS algorithm presented in chapter 7 of [Madgwick's PhD thesis](https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.681552).  This is a different algorithm to the better-known initial AHRS algorithm presented in chapter 3, commonly referred to as the *Madgwick algorithm*.
+The algorithm is based on the revised AHRS algorithm presented in chapter 7 of [Madgwick's PhD thesis](https://x-io.co.uk/downloads/madgwick-phd-thesis.pdf).  This is a different algorithm to the better-known initial AHRS algorithm presented in chapter 3, commonly referred to as the *Madgwick algorithm*.
 
 The algorithm calculates the orientation as the integration of the gyroscope summed with a feedback term.  The feedback term is equal to the error in the current measurement of orientation as determined by the other sensors, multiplied by a gain.  The algorithm therefore functions as a complementary filter that combines high-pass filtered gyroscope measurements with low-pass filtered measurements from other sensors with a corner frequency determined by the gain.  A low gain will 'trust' the gyroscope more and so be more susceptible to drift.  A high gain will increase the influence of other sensors and the errors that result from accelerations and magnetic distortions.  A gain of zero will ignore the other sensors so that the measurement of orientation is determined by only the gyroscope.