Former USS Tripoli at Mare Island Shipyard

They were "Liberty" ships.

Now under control of the US Army and used as a floating missile test platform.

http://members.cox.net/stengel/navy.html

From the environmental impact statement filed for the conversion:

"Mobile Launch Platform (MLP) [is] a platform for testing sensors, launching target missiles, and launching interceptor missiles..."

http://www.smdc.army.mil/SMDTC/T_E_Activities/MLP.html

The purpose of the Mobile Launch Platform is to provide a Sea Launch Option for Short To Medium Range Targets and provide a Seaborne Launch Platform for MDA Sensors.

The Mobile Launch Platform uses the former USS Tripoli as its launch platform. It is being utilized to provide the ability to present threat azimuth diversity for BMDS element Tests and it provides the ability to place Surface Sensors in remote ocean areas.Photo of Launching 2 Lance Missiles from the deck

Recent accomplishments for the MLP are successfully Launching 2 Lance Missiles from the deck, completed Operational and Maintenance Modifications, and developed site plan for the Transportable Telemetry System (TTS).