Generally speaking, in IT most devices are programmed to communicate with ground devices (on earth) but what methods are used to be able to communicate with satellites?
What is the common language to write for a handheld gps?
Non in particular. When a company makes a product like that, they'll generally base it around a chipset (PIC, AVR, ARM, ...) and a provided (ish) tool chain. If it's advanced enough, it might be able to run a small kernal (QNX, linux, ...) and then the language choice is really up in the air. If I had to guess, C/Assembly might be more popular amoung the older units.
How are they written to communicate with satellites?
What do you mean by that? If your talking about how the software interracts with the hardware, that depends on the chipset as well as how it was wired up. I would guess that most of them just wire the receivers up to some ports, or maybe map it to memory.
If you're talking about the method they use, take a look here.
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