scale a channel to 1000/1500/2000

def scale_channel(self, ch, value):
        '''scale a channel to 1000/1500/2000'''
        v = value/10000.0
        if v < -1:
            v = -1
        elif v > 1:
            v = 1
        if ch == 3 and self.mpstate.vehicle_type != 'rover':
            if v < 0:
                v = 0
            return int(1000 + v*1000)
        return int(1500 + v*500)