I'm designing a quadrifilar helical antenna for amateur satellite communications on the 70cm band. This antenna is stationary with an omnidirectional circular polarized pattern. The satellites it communicates with are in low earth orbit around 1000 km high.
I want to use the antenna noise temperature figure alone as comparison measure for determining the best shape for the pattern.
From that graph it look like at at the 70cm band, 440 MHz, the the sky noise temperature is about 10 K minimum. There is still the question of the gradient from ground temperature to sky temperature at zenith.
I found a method in this paper that at first glance seems like what I want, although it's going to take some work to verify that.