This paper presents a uGPS (Underground GPS), which is the first SDR (Software Defined Radio)-based GNSS service to commodity GNSS receivers, including the latest iPhone, Android, and Car navigation. The uGPS is a system consisting of SDRs for GNSS signal generation, a GNSS D.O. for nano-second level timing synchronization's economic selection, fiber optic, and leaky feeder for a minimal environmental effect in tunnel environments. The proposed uGPS provides consistent 8 GPS satellites and 4 GLONASS satellites signal through a 1.5-kilometer-long tunnel. The uGPS equipped tunnel is highly compatible with commodity GNSS/GPS receivers. Therefore, it does not require any APP or special hardware. Our benchmark tested package was on top of the FPGA, in which modifications and upgrades were demonstrated via evaluation on three popular GNSS/GPS devices including Android phones, iPhones, and Car navigation systems. Further extensive evaluations demonstrated that the uGPS consistently achieved average location accuracy of 10 meters in 30~70km/h speed driving tests (without a map-matching algorithm), and a seamless handover between live GNSS and the uGPS system.