Pretty good indeed
You've made a nice little game, and fun too.
The only reason it's so low here is because the replay ability is next to none. You kill, get more money, buy better satellites, kill more, repeat. It was fun untill about wave 15, then started to get boring. At a couple more past that, I had four super satellites, and decided to exhaust all my money in little ones for a big splodey at the end. I must say, it was very entertaining. I stopped a wave after -that, which I easily recovered from by the way.
You've got a great concept, but that's exactly what it is. A great concept. I love what you have so far, but it just isn't enough. I would love to see you put a little extra in there. And no, I don't think there should be a health bar. The ammo bar is one thing that makes it different and more challenging then the others.
What I suggest:
-More enemies, definitely
-Something that does not include more satellites
I would instead like it if you could be able to upgrade the ones you have currently instead, like more ammo, or different type of shots. Weather it be a one time fee, up the price of the satellite, or both, it's your choice. It IS your game, and you have complete control on how it goes.
-Powerups? This, I'm not sure if it will work. Although maybe a health up or limited time unlimited ammunition might come in handy. Not sure if they would work well for this game.
What you have is good, but I know you can do better. Maybe even just make a second that's got a whole bunch more features on it and a slightly different and sleak look then the first, while building off of it.
~BBB