Most spacecraft utilize a sun sensor to determine the sun orientation relative to the spacecraft. This information serves to point solar panels or help establish the attitude of the spacecraft.

From the Jet Propulsion Laboratory, California Institute of Technology, comes a novel sun sensor on a chip. This chip also has commercial applications besides spacecraft attitude determination.

Fundamentally, the solar compass chip (SCC) is a miniaturized pinhole camera. The focal plane is an active pixel sensor (APS) camera on a chip, and the optics is a small piece of silicon wafer. APS imagers have in recent years become very popular in the low end of the imaging application market.

The APS chip that the SCC is based on has all camera functions integrated on the chip itself.

The optics of the miniaturized camera is a small piece of silicon wafer with an evaporated layer of gold on one side with a number of small pinholes in the gold layer. The silicon wafer is mounted 500 microns from the focal plane, making the system into a pinhole camera.

The sun is so bright that it will penetrate the silicon wafer where there are pinholes, and the rays will form an image. This is basically the same principle upon which a sundial works.

The SCC connects to a processor. The processor reads the image from the camera on a chip and determines the positions of the bright apertures using image processing. The position of the bright apertures can translate to the angle of the sun during the exposure.

The SCC determines the north heading using custom algorithms that mathematically render the following.

Imagine an observer spending an entire day on a flat rooftop. At sunrise, the observer-draws a line on the roof toward the point on the horizon where the sun rose.

At sunset the observer draws another line toward the point on the horizon where the sun sets. Next, the observer uses a protractor and draws a third line, dividing the angle between sunrise and sunset in two. This line represents the direction toward true north.

The SCC does not have to watch the sun for an entire day. It can calculate true north in a fraction of an hour. The specific equations for doing so are in the processor.

Another piece of information that the observer on the rooftop is able to determine is the geographical position. Assume that the observer is carrying an accurate wristwatch set to Greenwich Mean Time (GMT). Also assume that the observer has a collection of current international newspapers stating today's sunrise and sunset times in all major cities in the world.

The observer would take note of the time at sunrise and at sunset. The observer would then browse all the newspapers to find a city that matches the time of the sunrise and sunset. The city that matches the sunrise and sunset is the city where he or she is located. In principle the SCC can do the same in less than an hour.

Yet another example of information that the observer on the rooftop would be able to establish is a celestial coordinate system, that is to say, the location of different stars in the sky. This happens using an accurate wristwatch set to GMT and an astronomical almanac that lists the position of sun as a function of the time.

The observer would then determine the orientation of the sun a number of times during the day. At the end of the day, the observer would have knowledge of the coordinate system in the sky.

The applications for the SCC arc numerous. They include the automotive industry, agriculture, the maritime industry, surveyor equipment, energy conservation and climate control, and the toy industry.