UKHAS Wiki

#include <stdlib.h>
#include <stdio.h>
#include <math.h>
 
float air_density(float alt)					//model from NASA
{    
     float temp,pres;
     if (alt < 11000.0)
     {    // below 11Km - Troposphere
          temp = 15.04 - (0.00649 * alt);
          pres = 101.29 * pow((temp + 273.1) / 288.08, 5.256); 
     }
     else
     {
         if (alt < 25000.0)
         { // between 11Km and 25Km - lower Stratosphere
              temp = -56.46;
              pres = 22.65 * exp(1.73 - ( 0.000157 * alt));
         }
         else
         { // above 25Km - upper Stratosphere
               temp = -131.21 + (0.00299 * alt);
               pres = 2.488 * pow((temp + 273.1) / 216.6, -11.388);
         }
     }
     return(pres / (0.2869 * (temp + 273.1)));
}
 
int main()
{
	int n;
	float a;
	FILE * output;
	output=fopen("sqrt_density.h","w");
	fprintf(output,"u08 sqrt_density[256] PROGMEM={");
	a=air_density(-50.0);	
	for(n=0;n<256;n++)
	{
		fprintf(output,"%d,",(int)(255.0*sqrt(air_density((n*100)-50.0)/a)));
	}
	fprintf(output,"};");
	fclose(output);
}

u08 sqrt_density[256] PROGMEM={255,253,252,251,250,248,247,246,245,244,242,241,240,239,238,237,235,234,233,232,231,230,228,227,226,225,224,223,222,220,219,218,217,216,215,214,213,211,210,209,208,207,206,205,204,203,202,201,199,198,197,196,195,194,193,192,191,190,189,188,187,186,185,184,183,182,181,180,179,178,177,176,175,174,173,172,171,170,169,168,167,166,165,164,163,162,161,160,159,158,157,156,155,154,153,152,151,151,150,149,148,147,146,145,144,143,142,141,141,140,139,138,137,136,135,134,132,131,130,129,128,127,126,125,124,123,122,121,121,120,119,118,117,116,115,114,113,112,111,111,110,109,108,107,106,105,105,104,103,102,101,101,100,99,98,97,97,96,95,94,94,93,92,91,91,90,89,89,88,87,87,86,85,85,84,83,83,82,81,81,80,79,79,78,77,77,76,76,75,74,74,73,73,72,72,71,70,70,69,69,68,68,67,67,66,66,65,65,64,64,63,63,62,62,61,61,60,60,59,59,58,58,57,57,56,56,56,55,55,54,54,53,53,53,52,52,51,51,51,50,50,49,49,49,48,48,47,47,47,46,46,45,45,44,44,44,};

The lookup table was tested by calculating the coefficient used for layer weighting from the transit time over 100m vertical distances during the descent, then dividing by the appropriate lookup table value. This should give a constant, and any errors will be proportional to the resulting error in layer weighting for landing spot prediction or optimal glider waypoint calculation.

As can be seen all the errors are within +- 15%, which is sufficient for most purposes. There is plenty of room for improvement be using GFS data or carrying a MEMS barometer onboard.