The System — September 2007

September 1, 2007  - By

AEP on the Ground

Advance to a New Architecture

The U.S. Air Force Space and Missile Systems Center (SMC) announced in late August that the long-planned upgrade of the GPS command and control system will occur during the second week of September, fulfilling a major program commitment for 2007.

Dubbed the Architecture Evolution Plan (AEP), the upgrade will replace the entire GPS master control station, including both software and hardware, some of which dates back to GPS’s inception in the 1970s. The upgrade will, among other things, begin preparing the master control station to work with the latest generation of Block IIF satellites when they go into orbit; further work with the next-generation OCX, however, will be necessary for managing M-code and the new L5.

“The delivery of SMC’s new GPS ground segment to the 50th Space Wing [will enable] transition of satellite operations from a 1970s-era GPS mainframe computer to a new server-based AEP ground segment,” said Lt. Gen. Michael Hamel, SMC Commander. “I am very proud of the team that has thoroughly tested the new system to ensure no change to the GPS signal during the changeover to the new system. The best analogy I could make is that this is like changing the engine on a car while traveling 50 miles an hour down the road.”

Col. David Madden, GPS Wing Commander, added “The replacement of the legacy system to AEP is a benefit to both the warfighter and the civil community. AEP is designed to improve operations, increase efficiency, and provide a foundation for new capabilities as they become available. The replacement from the legacy mainframe system to a distributed architecture provides the capability to command and control the next generation of GPS satellites and lays the foundation for a new security architecture to support the warfighter in the field.”

The AEP transition will take place over a period of four to six days; the total cost of the new ground control system amounts to approximately $800 million. The Air Force will not announce the exact date and time of transition, but will inform users 48 hours after completion.

The upgrade features a new satellite control foundation that replaces the legacy system and adds new digital communications. This means installing and activating a new master control station at Schriever Air Force Base which includes new hardware and software to generate navigation messages, a new system for controlling satellites, and new capability to command satellites through the Air Force Satellite Control Network (AFSCN). Adding the AFSCN will increase the number of available antennas for contacting satellites. The upgrade also involves installing and activating the alternate Master Control Station at Vandenberg AFB in California, and upgrading the current GPS ground antennas.

Ground control at Schriever AFB will phase in a few satellites in the GPS constellation at a time; the process will be completely reversible if it encounters any problems. Before it begins, both old and new ground control systems will be synchronized in terms of positioning data, namely their reception of satellites’ timing and navigation signals, down to the millimeter level. GPS users should not notice the transition, according to the Air Force.

The AEP will retain all of the legacy monitoring stations around the world currently utilized by the U.S. Air Force, as well as the National Geospatial-Intelligence Agency (NGA) monitoring stations around the globe originally added as part of the Legacy Accuracy Improvement Initiative. Additional NGA monitoring station sites are in the process of being brought online for future inclusion within the AEP.

Galileo Tests; Rescue Role


The antenna dish at Chilbolton Observatory.

The test campaign using the large antenna at Chilbolton Observatory to analyze the navigation signals transmitted by GIOVE-A, the first Galileo satellite, has been successfully completed, the European Space Agency (ESA) announced in mid-August.

Analysis of the satellite’s signals since January 2006, to verify their conformance with the Galileo system specification, has involved the Navigation Laboratory at ESA’s European Space Research and Technology Centre (ESTEC) in the Netherlands, the ESA ground station at Redu, Belgium, and the Rutherford Appleton Laboratory (RAL) Chilbolton Observatory in the United Kingdom.

Following signal analysis, operators have made some adjustments, re-programming the spacecraft’s navigation signal generation unit to compensate for changes to the signals introduced by the amplifier that boosts them for transmission to Earth and by a filter that protects adjacent frequency bands from interference.

To achieve the correct solution, the calibrated Chilbolton station was used to receive the signals from GIOVE-A. ESA’s Navigation Laboratory processed the resulting data. The signal generator manufacturer, TAS (France), calculated the new settings for the unit. Finally, the satellite manufacturer, Surrey Space Technology Limited (SSTL), uploaded the new values to the payload using their ground station at Guildford, in the United Kingdom.

According to ESA, “GIOVE-A is now transmitting optimized signals. Research and testing continues, and manufacturers are using the signals as they develop the receivers that users will need when Galileo enters operational service.”

Global Search and Rescue. Once operational sometime after 2012, Galileo will improve the detection of emergency beacons, according to program representatives whoattended the annual Joint Committee Meeting of COSPAS-SARSAT, the international program for satellite-aided search and rescue. Galileo satellites will carry transponders to relay distress signals to search and rescue organizations. Galileo partners have committed to developing a search and rescue component as an integral part of Medium Earth Orbit Search And Rescue (MEOSAR), the future worldwide search and rescue satellite system.

COSPAS-SARSAT already has systems operating in low-Earth orbit and geostationary orbit. The low-Earth orbit satellites can determine the location of emergency beacons using the Doppler effect as they pass overhead. However, there is a delay in relaying the distress signal because the satellites can only “see” a part of the Earth’s surface at any given time and a beacon is only detected when the satellite passes nearly overhead. Also, the satellites must store the location of the emergency and transmit it to a ground station once one comes into range, creating further delay.

Search and rescue transponders on geostationary satellites can constantly view a large, fixed area of the Earth, eliminating the time delay in detecting distress signals. However, they cannot automatically determine the location of the distress beacon as the low-Earth orbit system does, but must rely on the beacon to use a navigation system to find its position and include it in the distress call.

Emergency beacons require a direct line-of-sight to the geostationary satellites. There are some situations where this is impossible, such as near the Earth’s poles, where the satellites are too low in the sky, or when an accident occurs where surrounding terrain obscures the satellite.

Future Improvements. To improve performance of the overall COSPAS-SARSAT system, plans are now being made to fly search and rescue payloads on future navigation satellites. The various navigation satellite constellations will each have about 20 to 30 satellites in medium-Earth orbit, providing global coverage, including at the Earth’s poles, and with multiple viewing angles to the satellites, eliminating terrain blocking.

The Galileo search and rescue component will provide two services. The Forward Link Alert Service, backward-compatible with current COSPAS-SARSAT components and interoperable with all other planned MEOSAR elements, detects activated distress beacons and notifies the appropriate rescue body. A new Return Link Service will send a return message to the emergency beacon, notifying the emergency victims that their distress signal has been received and help is on its way.

The Galileo In-Orbit Validation Programme, which will have four satellites fitted with search and rescue transponders, will demonstrate the Galileo MEOSAR services — although its flight timetable has yet to be finalized or announced.

DOT Weighs NDGPS Future, Asks Public Input

The U.S. Department of Transportation’s (DOT) Research and Innovative Technology Administration (RITA) is preparing an assessment on the inland component of the Nationwide Differential Global Positioning System (NDGPS) that will determine its future.

As part of that assessment, it is seeking public input from users of the system.

The current expansion of the NDGPS has been placed on hold pending congressional review of the system’s funding; RITA’s assessment is part of that review. Differential GPS uses the fixed location of a reference station on the ground to improve the positioning resolution provided by civilian GPS satellite signals down to 1–3 meters. NDGPS facilities also monitor GPS satellites for anomalous behavior and issue integrity warnings when necessary.

The NDGPS program is operated jointly with the DOT’s Federal Highway Administration, Federal Railroad Administration, and Office of the Secretary of Transportation; the Department of Homeland Security’s U.S. Coast Guard; the Departmentof Commerce’s National Geodetic Survey and Forecast Systems Laboratory; and the Department of Defense’s Air Force and Army Corps of Engineers. Begun in 1997, to date there are 37 operational NDGPS sites. Two additional sites are ready for construction and could be operational in a matter of months, according to the Coast Guard.

As part of the assessment, RITA published a notice in the Federal Register addressing the current user requirements for the inland or terrestrial component of the NDGPS. This assessment is in preparation for making a recommendation to the National Space-Based Positioning, Navigation and Timing (PNT) Executive Committee, which oversees the entire GPS, on the need to continue to operate inland NDGPS and to make a decision on its future funding.

If no transportation requirements or other federal user requirements are identified as a result of the needs assessment, and if there are no other federal or other funding sources willing to sponsor or partner in sponsoring NDGPS, the DOT will develop a decommissioning plan for NDGPS, according to RITA.

The deadline for public comment is October 1, 2007. Comments may be submitted via the Internet at the Department of Transportation Web site. Instructions for other methods of submitting comments, including via postal service and fax, can also be found there in the docket management portion of the site.

The Robots of DARPA

The U.S. Defense Advanced Research Projects Agency (DARPA) has named 36 teams as semifinalists for its Urban Challenge to take place later this year.

The DARPA Urban Challenge will feature autonomous ground vehicles executing simulated military supply missions in a mock urban area. It will take place November 3at an urban military training facility located on the former George Air Force Base in Victorville, California.

The 36 semifinalists will compete in the Urban Challenge National Qualification Event (NQE), October 26–31. The top 20 teams from the NQE will move on to the Urban Challenge final event on November 3, and compete for cash prizes worth $2 million for first, $1 million for second, and $500,000 for third place.

At the NQE and the final event, the robots must operate entirely autonomously, without human intervention, and obey California traffic laws while performing maneuvers such as merging into moving traffic, navigating traffic circles, and avoiding moving obstacles. DARPA conducted competitive site visits across the United States to select the semi-finalists.

“The depth and quality of this year’s field of competitors is a testimony to how far the technology has advanced since the first Grand Challenge in 2004,” said DARPAdirector Tony Tether.

Stanford University’s winning robot vehicle from the last DARPA Challenge in 2005, which ran across the Mojave Desert, consisted of a stock Volkswagen Touareg R5 thatincorporatesd measurements from GPS, a 6DOF inertial measurement unit, and wheel speed for pose estimation.

Click here for the list of semi-finalists, along with other race information.


This article is tagged with , , , , , and posted in Defense, From the Magazine, GNSS