(a) The helicopter must be equipped for IFR operations and equipped with IFR approved GPS navigational units. (b) The operator must obtain prior written approval from the appropriate Flight Standards District Office through a Letter of Authorization or Operations Specification, as appropriate. (c) The operator must be a signatory to the Houston ARTCC Letter of Agreement. 4. Operators who wish to benefit from ADS − B based ATC separation services must meet the following additional requirements: (a) The Operator’s installed ADS − B Out equipment must meet the performance requirements of one of the following FAA Technical Standard Orders (TSO), or later revisions: TSO − C154c, Universal Access Transceiver (UAT) Automatic Dependent Surveillance − Broadcast (ADS − B) Equip- 5. FAA/AIS publishes the grid system way- points on the IFR Gulf of Mexico Vertical Flight Reference Chart. A commercial equivalent is also available. The chart is updated annually and is available from an FAA approved print provider or FAA directly, website address: http://www.faa.gov/air_traffic/flight_info/aeronav. The unique ADS − B architecture in the Gulf of Mexico depends upon reception of an aircraft’s  Mode C in addition to the other message elements described in 14 CFR 91.227. Flight crews must be made aware that loss of Mode C also means that ATC will not receive the aircraft’s ADS − B signal. (b) Flight crews must comply with the procedures prescribed in the Houston ARTCC Letter of Agreement dated December 17, 2009, or later. ment, or TSO − C166b, Extended Squitter Automatic Dependent Surveillance − Broadcast (ADS − B) and Traffic Information. AIM 8/15/19  NOTE − 10 − 1 − 7 Helicopter IFR Operations