By John Keller
GREENBELT, Md. - NASA optoelectronics experts are making plans to build a new ground-based global network that uses green laser beams to track orbiting satellites and to study Earth.
The network is called the Satellite Laser Ranging (SLR) System Network Replacement Project, which is to take the place of the existing third-generation Mobile Laser Ranging System, otherwise known as MOBLAS.
While MOBLAS today does its job with one operator, the SLR System Network is to be designed for continuous autonomous operation. Although MOBLAS is accurate to a few millimeters, NASA officials say the existing system has reached performance limits for data quality and quantity, while demand for precision tracking data continues to grow.
Further compounding NASA’s design challenge is the long-term decline in federal operational and research budgets for satellite laser ranging. Officials insist the new SLR System Network must have relatively low costs to build and operate, and are emphasizing the need for commercial off-the-shelf (COTS) components.
Experts at the NASA Goddard Space Flight Center in Greenbelt, Md., are evaluating a prototype system called the SLR2000, and say the contractors ultimately chosen to design and build the future SLR System Network may use technology from the prototype, or propose a whole new system design.
The SLR2000 is a single-photon detection satellite laser-ranging system with a newly developed quadrant microchannel plate-based photomultiplier tube receiver, and a diode-pumped microchip laser. The pulsed laser operates at 2 kHz, and the transmitted laser pulse is spread to cover the 40-centimeter exit aperture of a telescope, and limited in pulse energy to render the system safe to human eyes.It also has signal-recognition software algorithms to identify signals buried in noise, NASA officials say.
Although the SLR2000 prototype represents a substantial improvement over the outdated MOBLAS network, the prototype today is too complex, is not completely automated, and lacks hardware and software for security, as well as for adequate numbers of temperature, current, and voltage sensors, NASA officials say.
Instead, officials are looking for the future SLR System Network to be even more technologically advanced, less expensive, modular, easily upgradeable, and able to communicate in real time to a central facility, which will coordinate and monitor the network’s performance.
The SLR System Network must range to corner-cube retroreflector-equipped satellites as high as geosynchronous orbit - 22,000 miles above the Earth’s surface. In addition, the new network must be able to track all orbiting satellites as high as 12,000 miles in daylight and at night, with the possible exception of the Global Positioning System (GPS) satellites.
The SLR System Network will use 532‑nm green lasers, efficient detectors, and bandpass filters, which are comparable with most of the world’s satellite laser-ranging systems. This laser wavelength is the same as that used in relatively inexpensive handheld laser pointers that FBI agents suspect were used in recent incidents where lasers hit the cockpits of commercial aircraft.
The SLR System Network lasers, however, should not pose a danger to humans or animals on the ground, or to aircraft in flight, because the laser is relatively wide at ground level, and focuses to a fine point only at satellite altitudes.
The future laser-based satellite tracking network also must have automated satellite scheduling and data processing via the Internet, the ability for authorized operators to log onto the system, operate it, and override most of the automated features, and failsafe automated shutdown for serious problems.
NASA officials also want the SLNR System Network to be built for easy upgrades to accommodate the next generation of satellite laser-ranging systems that can operate in transponder mode with other lunar, planetary, or deep-space mission transponder systems. The system also must be upgradeable to be used as a ground terminal in a space-to-ground laser communications link.
For more general information contact NASA’s Regina Williams at 301-286-5063, or at [email protected]. For technical information contact NASA’s Jan McGarry at [email protected], NASA’s Thomas Zagwodzki at [email protected], or NASA’s David Carter at [email protected].
