3.05 LIDAR mods, docs, FSPL optimisation

models/fspl.cc

@@ -1,30 +1,33 @@
 /*****************************************************************************
 *  ITU-R P.525 Free Space Path Loss model for Signal Server by Alex Farrant  *
-*  15 January 2014							     *
+*  15 January 2014                                                           *
+*  optimised G6DTX April 2017                                                *
 *  This program is free software; you can redistribute it and/or modify it   *
 *  under the terms of the GNU General Public License as published by the     *
 *  Free Software Foundation; either version 2 of the License or any later    *
-*  version.								     *
-*									     *
+*  version.                                                                  *
+*                                                                            *
 *  This program is distributed in the hope that it will useful, but WITHOUT  *
 *  ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or     *
 *  FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License     *
-*  for more details.							     *
-*									     */
+*  for more details.                                                         *
+*
+* https://www.itu.int/rec/R-REC-P.525/en
+* Free Space Path Loss model
+* Frequency: Any
+* Distance: Any
+*/
 
 #include <math.h>
 
+// use call with log/ln as this may be faster
+// use constant of value 20.0/log(10.0)
+static __inline float _20log10f(float x)
+{
+  return(8.685889f*logf(x));
+}
+
 double FSPLpathLoss(float f, float d)
 {
-/*
-Free Space Path Loss model
-Frequency: Any
-Distance: Any 
-*/
-	//MHz to GHz
-	f = f / 1000;
-
-	double dbloss = (20 * log10(d)) + (20 * log10(f)) + 92.45;
-
-	return dbloss;
+  return(32.44 + _20log10f(f) + _20log10f(d));
 }