Issue link: https://read.dmtmag.com/i/62129
multiple systems, the increased L1C signal availability will mitigate navigation outages experienced near tall buildings or other obstacles that block many satellite navigation signals. For average GPS users, this equates to increased navigation accuracy from U.S. as well as international satellite signals. Although the vast majority of applications are for civil purposes, GPS initially was created to coordinate mili- tary operations. To meet our nation's increasing military demands in tough environments, the new GPS III satel- lites can broadcast up to three times the anti-jamming signal power. The intensified signal power increases mission success and personal safety for our troops. To lower the lifetime costs of overall operations, GPS III satellites have an increased design life of 15 years. In addition, each GPS III satellite is designed with flex- ible growth in mind. So if new signals are required, they can be added in future satellites. Similarly, to meet aviation integrity requirements, monitoring can be added for assured navigation in flight-critical takeoff and landing maneuvers. Core Architecture The long life of GPS III satellites is complemented by the GPS III core architecture that enables opportunities for future growth. The GPS III satellite is comprised of five major subsystems: 1. Navigation Payload—The navigation payload per- forms the primary GPS III navigation mission, which is to generate and transmit GPS signals. Along with the atomic clocks, the mission data unit is the core of the navigation payload and is comprised of the navigation processor and signal generators to drive the various GPS signals through transmitters, filters and antenna elements. In addition, the mission data unit employs a high- performance timekeeping system with a control loop to balance the optimal timing performance between the atomic clock and voltage-controlled crystal oscil- lator. The GPS broadcast subsystem is comprised of the transmitters, multiplexers and filters required to amplify, filter and combine the signals for broadcast through the antennas. The GPS III navigation payload is designed to sup- port all the foreseen future capabilities for the entire GPS III mission. The payload provides flexibility from the outset in all critical attributes required for additional features or modifications to existing power levels, waveform types, code quantities and power distribu- tion among codes for the entire GPS III lifecycle. 2. Network Communications Element—This sys- tem performs all non-GPS communications functions required to fly the satellite and relay messages to other GPS satellites. The GPS III network and communications includes the space-ground uplink/ downlink transponder and an enhanced crosslink transponder subsystem. The space-ground uplink/ downlink transponder is required for telemetry, tracking and commanding from satellite operators. The enhanced crosslink transponder subsystem supports a secondary GPS III mission for nuclear- detonation detection, namely to detect nuclear activities should they occur. 3. Hosted Payloads—GPS III satellites can host a range of secondary payloads, including nuclear- detonation detection or for emergency-distress alerting signals and beacons—the latter is a planned future capability. 4. Antennas—The GPS III antenna suite provides The ITT Exelis mission data unit integrates BAE's high-speed processor, Excelitas enhanced atomic clocks and Symmetricom clocks (voltage-controlled crystal oscillators) with ITT Exelis transmitters and Aeroflex di/tri-plexers for broadcast via Lockheed Martin antennas. 16 GEO W ORLD / APRIL 2O12 navigation and communication capabilities that have evolved from previous on-orbit GPS antennas. Navigation payload signals are transmitted through two GPS antenna arrays designed to broadcast over Earth. The 12-element Earth-coverage antenna array trans- mits all GPS signals except for the military M-code signals, which are transmitted through a separate four-element array. 5. Bus—The satellite bus provides the required infrastructure to support all payloads. GPS III is based on the flight-proven A2100 scalable bus to meet cur- rent requirements as well as accommodate future missions. The A2100 bus has the following six subsystems: structural support and deployment mechanisms; thermal Mobility/GPS Special Issue Hardware