The Solar Beacon installation consists of two Heliostats initially atop the two towers of the Golden Gate Bridge, but now on the balcony level of the Berkeley Campanile (Sather Tower). A Heliostat is a device that will reflect the Sun’s light to a fixed position and keep it there while the Sun moves through the sky (due to the rotation of the Earth). This is done by moving the position of the mirrors slightly to track the Sun, keeping the vector normal to the mirror surface exactly half way between the Sun’s position and the observers position, since for a mirror the angle of incidence equals the angle of reflection.
Moving the mirror is accomplished by using a computer controlled Pan-Tilt mount, so called because you can change the azimuthal angle about the zenith vector (“pan”) and the elevation angle from the zenith (“tilt”). Since the Sun is approximately 0.5 degrees in diameter, the accuracy of this movement must be better than this to keep the observer in the reflected beam. For Solar Hills, we are using the Pan-Tilt mounts made by FLIR Motion Control Systems PTU-D48 series. The PTU-D48 accepts ascii or binary commands through an RS-232 serial interface. These commands originate in software at the Space Sciences Laboratory and are sent through the internet to a cell phone wireless connection to a cellular modem atop the tower. Commands are sent every second to move the mirror to a series of fixed positions, which mimics to high accuracy continuous tracking of the Sun.
The mirror panels (two per Heliostat) are made of aluminum and are stiff enough to hold them parallel and flat. Each panel has four mirrors, two on each side. When both panels are in place, there are 4 mirrors co-aligned on the front side. The co-aligned mirrors on the front side of the panels will be used to project the beam to distant observers.
To reflect the solar light to a specific observer, you need to know the position of the Sun and position of the observer with respect to the Heliostat. Since the Sun moves in the sky throughout the day and the season, you also need to know the exact date and time that the observer is at that location. Knowing where the Sun is in the sky, given a date and time, was figured out centuries ago and can be calculated using standard algorithms and formula. The observer’s spatial and temporal position must be entered in a database: latitude, longitude, elevation (above sea level) and exact time the observer will be at that position. Not many people know where they are on the surface of the Earth to the accuracy required, so we can use online tools like Google Maps or global positioning coordinates (GPS) found in many cell phones.
People who would like to see the solar light from Solar Beacon must schedule a “performance” by entering their future location. The online scheduler application software will come back with a choice of available time slots for that particular location, and the observer can reserve a specific time. Once this transaction is complete, the performance database is updated with the new reservation. Unavailable times for a fixed position are due to non-optimum position of the Sun or conflicting previous reservations. The Schedule software will do its best to filter out impossible projections (e.g. observers behind Mt. Tamalpais), but a good rule is that if you can’t see the top of the Berkeley Campanile western or southern sides, then Solar Beacon cannot beam to you.
In parallel and running continuously, the Heliostat control software is interrogating the database for the current scheduled performance, sending pan and tilt commands to both the West and South Heliostats. The code can also apply minor angular adjustments to the mirrors that can “turn on and off” the light to the observer, though the reflected light is still there, just projected somewhere else. A “performance” consists of a fixed period of time where the two beams from the Tower point towards (and away) from the observer in a specific temporal pattern. There will exist pre-existing performance patterns and possibly user-defined patterns.