Is there altitude limit for GSM(SIM908)?
I often have cell service at 25,000 feet. I have also launched a weather balloon with a GSM tracker and depending on your antenna, you could have service as high as 30k-40k feet. Maybe much higher depending on many variables such as humidity, antenna shape and type, cell tower position and power, local transceiver power, and your circuitry that interprets the signal. I would say 30,000 feet is very doable based on my experiences.