check heading discrepancy

def check_heading(self, m):
        '''check heading discrepancy'''
        if 'GPS_RAW_INT' in self.status.msgs:
            gps = self.status.msgs['GPS_RAW_INT']
            if gps.vel < 300:
                return
            diff = math.fabs(angle_diff(m.heading, gps.cog / 100.0))
        else:
            return
        self.report('heading', diff < 20, 'heading error %u' % diff)