Actually noise figure should be as low as possible. But at S-band receiver chain, the 1st noise contributor is the antena follow by a bandpass filter. The antenna noise is quite high and the noise of the bandpass filter is equal to the insertion of the filter. Though you have very low noise amplifier, the overall noise figure is still high. My conclusion is: "if you can design as low noise figure as possible, thumb up. If noise figure of LNA is greater than 1dB but less than 2.5dB, it is still acceptable in LEO receiver"