A compact and portable target assembly (Fig. 1) has been developed to measure the boresite alignment of LRO’s Lunar Orbiter Laser Altimeter (LOLA) instrument at the spacecraft level. The concept for this target assembly has evolved over many years with earlier versions used to test the Mars Observer Laser Altimeter (MOLA), the Geoscience Laser Altimeter System (GLAS), and the Mercury Laser Altimeter (MLA) space-based instruments. These earlier laser altimeters were single ranging channel instruments, but as demonstrated with the five-channel LOLA instrument, the target assembly can simultaneously measure the co-boresite alignment of multiple transmitter laser beams and receiver channels (Fig. 2).
The target assembly flips the transmitter laser beam into the optical receiver aperture and measures their co-alignment error by scanning the laser far-field image across the receiver field-of-view (FOV) in two orthogonal axes. Plotting the receiver response as a function of the transmitter beam deviation angle yields the effective laser altimeter transceiver alignment. The target assembly components include a Laser Beam Dump (LBD) to attenuate the input laser energy by an order of magnitude, a lateral transfer retro-reflector (LTR) to flip the transmitter laser beam by exactly 180° into the receiver telescope, a set of motorized and computer-controlled Risley prisms to scan the transmitter laser beam across the receiver FOV in a controlled and measurable fashion, and a set of parallel plate neutral density (ND) filters to attenuate the output laser beam by several orders of magnitude to a receiver-safe level.
The target assembly is compact, modular, accurate, and easy to use. The target assembly itself is alignment insensitive, so no special care needs to be taken in placing the target assembly in front of the instrument under test other than making sure that the transmitter laser beam is not vignetted at the target assembly entrance aperture. In addition, sealing the transmitter path to the target assembly input aperture is usually required to prevent stray light from saturating or damaging the sensitive receiver detector(s) of the instrument under test. The target assembly components can be easily changed to customize the target assembly for any required active sensor co-alignment measurement task.
This work was performed by Luis Ramos-Izquierdo, V. Stanley Scott, Haris Riris, and John Cavanaugh of Goddard Space Flight Center, and Peter Liiva and Michael Rodriguez of Sigma Space Corporation. GSC-15789-1