This is a Sources Sought/RFI for market research only; the USSF/SSC SATCOM and PNT office is not seeking proposals and will not award a contract based on responses. It seeks industry statements of capability for a secure, safety-certified Monitor Station Service (MSS) to modernize the GPS Ground Control Segment’s global PNT receiver network and replace obsolescing AEP monitor-station data feeds while preserving Kalman Filter continuity. Required capabilities include simultaneous multi-frequency/multi-code tracking of legacy, CNAV, and M-Code/MNAV signals across up to 63 space vehicles, plus MOSA/API integration, DAL D safety assurance, NSA cryptographic handling, IL5/IL6 security, FSET certification, DevSecOps/T&E, sustainment, and obsolescence management. The Government envisions phased delivery: Phase 2.1 replaces operational P(Y) data feeds by late 2028, Phase 2.2 delivers remaining core capabilities by late 2030, and Phase 2.3 additional capabilities are desired by CY2032, with sustainment through at least CY2035. Performance/support involves globally distributed remote monitoring sites and Secret/collateral environments; respondents should address technical approach, schedule/IMS, ROM cost, delivery model, data rights, GFE/GFI, contract type, and secondary capabilities.