Optical position sensors are the sensors of choice for highly accurate positioning of antennas. There are two broad styles of the encoders viz. incremental and absolute. An incremental encoder is made of a glass disc and a light interrupter. Transparent and opaque markings are put on the outer periphery of the glass disc. Light emitted from a lamp or LED is interrupted by the glass disc and received by a photo diode. As the disk rotates, the light falling on the photo detector is interrupted by the opaque markings, leading to pulses in the photdetector. These pulses are counted to determine the change in position. The disk has an index marker, is used to provide a reference. Though incremental encoders are simple in construction and provide a cheap solution for position sensing, they suffer from one drawback. On the failure of the power to the encoder or the electronic circuit, the electronic counter looses its count value, and hence all information as to the current position. Hence, upon the resumption of the power to the antenna, one would need to move the antenna until the index marker pulse is received, a procedure called ``homing". For large antennas like those at the GMRT, this is unacceptable and hence absolute encoders have to be used.
In an absolute encoder, a pattern corresponding to a gray code is printed on the glass disc. The glass disc moves through a light emitter and a set of light detectors. The number of light detectors are in proportion with the number of bits of the encoders. This enables the encoder to generate a binary word corresponding to the angular position of its shaft. The electronics housed inside the encoder converts the gray code to the natural binary . Also the parallel code gets converted into serial format for transmitting over long distance cable. The encoder is directly mounted on each axis of an antenna.
| Type | Optical, absolute shaft encoder. |
| Resolution | 17 bit (10 arcsec). |
| Max . Shaft speed | 600 rpm. |
| Max. Data rate update | 100 kHz. |
| Illumination | light emitting diode. |
| Input Power | + 5V DC at 300 mA. |
| Output Code | Natural binary. |
| Output data format | Serial. |
| Data transmission | RS  422 differential line driver. |
| Serial output | MSB first, LSB last & then parity bit. |
| Count Direction | CW increasing. |
| Operating temp. | 0 C to +70 C. |