4.8. Laser AO Operations Safety

Operating a laser guide star facility has several implications on the overall operation of the GMT observatory. They include:

• Safety and regulations. To prevent propagation of the laser beacons onto flying aircrafts, satellites, the International Space Station or other non-disclosed objects.
• Observatory policies. To avoid laser light contaminating neighboring telescopes field of view.

The SWCS Laser AO Safety Requirement is given by SWC-6881 Safety Standards and Codes: to comply with safety standards and codes as defined in GMT-SE-REF-00229.

Laser AO Operation Safety involves aircraft, laser traffic, and spacecraft safety elements, the relations between which are shown in the Figure below. The subsystems are discussed in the following subsections.

Fig. 4.17 Integration of the Aircraft, Laser Traffic, and Spacecraft, Safety subsystems. (1) LGS targets stored in the Observing database are sent to the Laser Clearing house for approval a few days before observation, (2) the Scheduler updates the night plan based on LGH clear windows, (3) the Sequencer retrieves observation for execution by the TCS, (4) the All-sky camera and VITRO systems monitor in real- time safe propagation while the Laser Traffic Control System checks the collision likelihood with other telescopes and feeds back predictions, (5) alarm shutters the lasers and orderly open the AO loops; the sequencer is notified to pause the execution until observations can be resumed.

4.8.1. Aircraft Safety

VITRO

The VITRO system (Visualizacion de Transito Aereo Oceanico) shows in real-time a feed of aircraft locations, with their altitude, velocity and flight direction, similar the one used by air traffic controllers. The VITRO system is available from accredited vendors by the Chilean Direccion General de Aeronautica Civil (DGAC), the Chilean equivalent of the Federal Aviation Administration (FAA) in US. GMTO will contract with VITRO system vendors to provide the system, and to explore the possibility of interfacing with the data feed. This could allow for automation of the telescope control system to trigger appropriate alarms and shuttering of the laser beacons in the event of an impending collision between a laser beam and an aircraft.

All Sky Cameras, Boresight Cameras

Complementary systems (e.g., all-sky and bore-sight cameras) have been successfully deployed by other observatories to provide additional aircraft detection redundancy. GMTO will explore deploying similar systems for its laser guide star operations. An all-sky camera system takes continuous images of the night sky to search for aircrafts, using exposure times that are sufficiently long (typically 3s.) for airplanes to leave a trail on the sensor image. A data pipeline then processes the images to detect streaks left by bright moving objects over consecutive frames. Fitting trajectories to the streaks can then help to discard false-positives. The resulting detections will be assigned a level of severity depending on the aircraft distance to the laser constellation and time to collision, estimated from velocity vectors. Alarm severity levels will be handled accordingly by the telescope control system or interlock control system, e.g., NOTICE on detection but no collision (typically airplanes flying at horizon) WARNING (airplane on course to collision) SHUTTER (airplane entered the safety collision cone around the laser constellation). An additional possibility is to use a boresight camera system, which would work in a similar way, using an IR camera on a smaller field of view, and centered on the vicinity of the LGS constellation.

These auxiliary systems send detection events and alarms to the logging system and the telescope control system and/or interlock system for appropriate action. For example, audible and visual signals may alert the telescope operator when there is a detection, or an automated command may trigger laser shuttering when an aircraft is in the critical zone.

4.8.2. Laser Traffic Safety

GMT laser guide stars operate at 590 nm and may hinder neighboring sites observing in that regime. The laser traffic control system (LTCS), originally deployed by Keck circa 2001, was developed to address the potential for telescopes to “cross beams” while propagating laser guide stars. The system is currently deployed at several major observing sites including Mauna Kea, Roque de los Muchachos, Cerro Pachon, Cerro Paranal, and Haleakala [Summ12]. The system is a straightforward way to adapt, configure, deploy and operate [DOrg12].

Distributed among participating telescopes, the LTCS collects pointing data, laser propagating and laser impact status for all the telescopes. The information is made available by all the telescopes at predefined URLs. The LTCS predicts whether beam collision will occur between observatories based on a geometric model of the telescopes and their pointing field of view. While the specific pointing data of the telescopes may be proprietary and are thus not available to the operators, the LTCS has a GUI that informs telescope operators at once if a beam collision will occur. If crossing is deemed imminent, the LTCS provides a visual countdown via a “Status and Alarm Summary” screen and will shutter the laser via the interlock system at the crucial moment. However, if the observations are not laser sensitive, operators can override the LTCS (with a phone call confirmation), allowing laser operations to proceed in the area. The system provides additional capabilities, e.g., time to collision, slew prediction, etc., allowing tight integration into the TCS and early contingency planning.

4.8.3. Spacecraft Safety

Rules governing satellite avoidance have been described in several publications10. Basic compliance requires the observatory to send a list of LGS targets a few days in advance, and to obtain clearance, from the Laser Clearinghouse (LCH). The process is handled by e-mail using a data format that can seamlessly integrate into the observatory database, planning tool, and sequencer.

The LGS targets are stored in the observing database. A few days before an LGS run an automated process retrieves a list of candidate targets and their observability window from the database, and then sends a Predictive Avoidance Request Message (PRM) by email to LCH for approval, following a pre-established format. The list of targets, processed by the LCH, returns to GMT ahead of the run in a Predictive Avoidance Message file (PAM) showing the allowed times of propagation for each requested telescope position. The satellite’s position should not fall within the laser avoidance cone, which factors in a 0.25 degree uncertainty (half-angle) for laser pointing plus an additional margin defined by the LCH. When a satellite passes within the laser avoidance cone, a shutter window is shown in the PAM. A software tool that updates the observing database with clearance information ingests this file. The scheduling system accesses the list of authorized targets and time window to make proper adjustments to the schedule.

During an observation, the sequencer and the status human-machine interface (HMI) implement a real-time alarm and a timer that counts down toward laser shuttering. The Interlock and Safety System would safely shutter the laser system 10 seconds before an actual shuttering event. At laser shutter, the sequencer should gracefully stop an LGS observation and, even long before, inform the observer of alternative observation strategies, e.g., wait if the window is short, move to another clear target, consult the scheduler for a better alternative, etc..