Summary
U.S. spectrum and aviation regulators have moved in sequence on upper C-band (3.98-4.2 GHz), creating a clearer but time-bound path for expanded wireless use and a defined retrofit window for aircraft altimeters.
The FCC's Notice of Proposed Rulemaking, published in the Federal Register on December 5, 2025, proposes flexible-use licensing in 3.98-4.2 GHz, seeks comment on the licensing framework and incumbency protection, and closed its initial comment cycle on January 5, 2026.
The FAA followed with a proposed airworthiness directive published on January 7, 2026 that would require radio altimeter tolerance upgrades for specified Boeing and Airbus fleets by January 1, 2028, aligning with the current U.S. mitigation horizon.
Signals for Investors
- The regulatory sequencing de-risks upper C-band monetization relative to prior "spectrum first, mitigation later" cycles, but it concentrates execution risk into certification, retrofit capacity, and airline downtime planning through 2027.
- Altimeter retrofit and verification demand should support avionics suppliers, test-equipment vendors, and MRO providers with FAA-compliant validation workflows, while exposing laggards to schedule penalties.
- Wireless operators gain a more legible timeline for additional mid-band capacity planning, but final auction economics will still depend on FCC guardrails, incumbent relocation costs, and aviation compliance progress.
What to Watch Next
Watch for the FCC's post-comment order path in 2026, FAA final-AD language (including any fleet scope changes), airline retrofit progress versus the January 1, 2028 mitigation sunset, and whether international alignment reduces cross-border operational fragmentation.