Using the RTL-SDR as a Software GPS Receiver
Dr. Carles Fernandez-Prades, Dr. Javier Arribas and Dr. Pau Closas have published an academic paper showing how they were able to implement an RTL-SDR based GNSS (Global Navigation Satellite System) receiver in software.
What they have done is use their open source GNSS software receiver program with a RTL-SDR connected to an active GPS antenna. An active GPS antenna requires DC power to be passed to the LNA in the GPS antenna through the antenna connection, so a Bias-T network is required to ensure DC power does not enter the RTL-SDR dongle.
More information can be found on their webpage here.
Could a gps antenna be configured for inmarsat?