AEP Goes Operational

June 1, 2008 By: Arthur J. Dorsey, Jack Taylor, Major James J. Pace Jr., Philip J. Mendicki GPS World

GPS Control Segment Upgrade Details

The phased transition of the 22-year-old Legacy GPS Master Control Station (MCS) to the Architecture Evolution Plan (AEP) control segment, which became operational in September 2007, constituted a multi-year cooperative effort to achieve a seamless navigation service transition to GPS users. The primary objective of the Enhanced Phase Operations Transition (EPOT) approach was to validate navigation performance before sending or broadcasting any data to the GPS user community. Only after meeting tight performance comparison criteria between the Legacy and AEP control segments did EPOT proceed to the next step.

In 1996, Lockheed Martin received a contract to replace the legacy GPS Operational Control Segment (OCS) with a new control segment, which would move the command and control of GPS into the new millennium. The new system, called the Architecture Evolution Plan (AEP), was designed to be a distributed architecture, primarily using commercial off-the-shelf (COTS) products. The system would incorporate all features of the National Geospatial-Intelligence Agency (NGA) proposed Accuracy Improvement Initiative (AII) and be capable of flying the newest generation of GPS satellites, GPS IIF. AII features included stronger compatibility with the Air Force Satellite Control Network (AFSCN), utilization of measurements from the NGA Monitor Station Network, and improvements in estimation and prediction performance. In addition to upgrades to the New Master Control Station (NMCS) Hardware and Software, AEP would include improvements to the GPS controlled Monitor Stations (MSs) and Ground Antennas (GAs). Improvements in GPS accuracy, integrity, and sustainability were to be substantial. Additionally, a parallel L-Band capability was added, which would simultaneously distribute OCS monitor station measurements to both the Legacy and AEP systems to aid in the development and transition of AEP. AEP would also deliver a new fully functional backup Master Control Station, known as the Alternate Master Control Station (AMCS), at Vandenberg Air Force Base, with a highly automated data transfer capability (the AEP data transfer capability is known as Mission Operation Transfer (MOX)). In 2001, Boeing became the single prime integrator for AEP, with Lockheed Martin subcontracted to continue the development. Originally designed to become operational in 1999, slips in the AEP and GPS IIF design schedules, coupled with budgetary constraints, a GPS constellation which was aging much more gracefully than design specifications called for, and transition activities identifying subtle but complex problems requiring correction pre-transition, led to actual operations start date of September 14, 2007.

OCS Legacy HIstory

Timeframe. This article, given at the ION Technical Meeting, January 2008, presents results from that timeframe. Currently GPS has 16 monitor stations (as opposed to the 14 used as basis here), and correspondingly the Zero Age of Data performance levels have decreased to the 20-centimeter range.

Legacy AII

Protraction of the AEP development effort created opportunity for two significant projects which ultimately led to its highly effective transition. First, the decision was made, and funding secured, to implement AII features on the Legacy system. This effort permitted the early delivery, checkout, and implementation of the advertised accuracy and integrity improvements of AII. Issues with model improvements, inter-operability with the NGA network, and all the operational nuances of controlling a constellation with more than double the monitoring stations, were all tested and corrected independently from the rest of the AEP changes. Additionally, early delivery of AII enabled a low-risk transition concept: Enhanced Phased Operations Transition (EPOT). By enhancing Legacy to have most of the same capabilities as AEP (similar measurement ingestion with parallel L-Band and NGA MS incorporation, similar Kalman filter construction, similar timesteering, minus complete GPS IIF capabilities), the transition team was able to make true "apples-to-apples" performance comparisons between the two systems. Indeed, as will be discussed later, performance comparisons improved to a point where a single measurement difference between the two systems was noticeable in the three-hour performance evaluations.