ZP ranger hardware

The principles of operation of the GBT laser rangefinder system have been well documented, so I will present here only a rough overview of the operation of the ranger hardware[8]. The ZP laser rangefinder hardware system is illustrated in figure 2.1[9]. It consists of six main components:

Beam Steering Mirror / Servo System

The dual purpose of this assembly is to aim an outgoing laser beam to a retroreflector target, and to re-direct the returning signal from the retroreflector target to the optical assembly of the ranger. This assembly consists of an altitude-azimuth mounted optical mirror. Motion for the axes is provided by servo motors under computer control. The reference point for any measurements made with the ranger is the exact center of this mirror.

Optical Assembly

This system produces a modulated, optically isolated laser beam to illuminate a target retroreflector, and focuses the return signal from this retroreflector onto a PIN photodiode detector. Both portions of the optical system must be co-axial to work properly, and both must be aligned with the center of the beam steering mirror.

Oscillator System

The oscillator system provides the yardstick that the system uses to measure a range. A 1500 MHz oscillator is locked to a high precision 100 MHz source, and is used to modulate the laser beam. The oscillator system also supplies a 1500.001 MHz signal that is produced using the 1500 MHz and a 1 kHz reference signal produced by the A/D system. When the detected signal (the 1500 MHz laser signal returned by the target retroreflector) is mixed with the 1500.001 MHz signal, the result is a 1 kHz signal whose phase relative to the 1 kHz reference signal is the same as the phase of the return signal relative to the original 1500 MHz output.

A/D Converter

The 1 kHz signal produced by the mixer in the oscillator system is sampled by a 100 kHz, 16 bit A/D converter. This converter uses external sampling clocks generated by the same clock that generates the 1 kHz reference signal, so the sampling of the return signal is locked to the reference signal. The embedded PC then can run code that works out the amplitude and phase of the detected return signal.

Embedded PC

This is an ISA/PC104 compatible single board computer, with an i386 or better processor. It runs DOS, and FTP Software's PC/TCP provides networking capability.

IRIG-B Real Time Clock

Accurate time is provided by a Bancomm bc630AT Real Time Clock module, capable of decoding IRIG-A and IRIG-B time signals, among others.

Figure 2.1: Block diagram of Ranger hardware