Shinhak Lee

Optimization of intensity modulated beams with volume constraints using two methods: cost function minimization and projections onto convex sets.

For accurate prediction of normal tissue tolerance, it is important that the volumetric information of dose distribution be considered. However, in dosimetric optimization of intensity modulated beams, the dose-volume factor is usually neglected. In this paper we describe two methods of volume-dependent optimization for intensity modulated beams such as those generated by computer-controlled multileaf collimators. The first method uses a volume sensitive penalty function in which fast simulated annealing is used for cost function minimization (CFM). The second technique is based on the theory of projections onto convex sets (POCS) in which the dose-volume constraint is replaced by a limit on integral dose. The ability of the methods to respect the dose-volume relationship was demonstrated by using a prostate example involving partial volume constraints to the bladder and the rectum. The volume sensitive penalty function used in the CFM method can be easily adopted by existing optimization programs. The convex projection method can find solutions in much shorter time with minimal user interaction.

Earth-image tracking in the IR for deep space optical communications

Sub-microradian level laser beam pointing to an Earth-based receiver is required for deep space optical communications. This requires a beacon emanated from Earth towards the spacecraft. The beacon could be a laser or reflected sunlight from Earth. Earth image tracking in the visible is hampered by significant albedo variations and/or crescent Earth image yielding large central errors. Here, we report results of Earth-image tracking in the infrared (8 to 13 micron) region of the spectrum with the aim of substantially alleviating the two challenges mentioned earlier.

Accelerometer-assisted tracking and pointing for deep space optical communications

NASA/JPL has been developing acquisition, tracking and pointing (ATP) technologies for deep space tracking and pointing of an optical communication beam using linear accelerometers to enhance pointing. Linear accelerometers provide excellent accuracy in sensing the vehicles acceleration with the advantage of small size, low power, low cost, and a broad range of well developed products. We present the concept of accelerometer-assisted tracking, error analysis, and progress made on its implementation.

Inertial sensor assisted acquisition, tracking, and pointing for high data rate free space optical communications

We discuss use of inertial sensors to facilitate deep space optical communications. Implementation of this concept requires accurate and wide bandwidth inertial sensors. In this presentation, the principal concept and algorithm using linear accelerometers will be given along with the simulation and experimental results.

Acquisition, tracking, and pointing using Earth thermal images for deep space optical communications

The feasibility of using long wavelength Earth thermal (infrared) images for telescope tracking/pointing applications for both deep space free-space optical communications has been investigated and is reported here. The advantage of this technology rests on using full Earth images in this band, which yield more accurate estimates of geometric centroids than that of Earth images in the visible band. Another major advantage is that these images are nearly independent of Earth phase angle. The results of the study show that at a Mars range, with currently available sensors, a noise equivalent angle of 10 to 150 nanoradians and a bias error of better than 80 nanoradians can be obtained. This enables precise pointing of the optical communications beam for high data rate links.

Low-noise detector with RFI mitigation capability for the Aquarius L-band scatterometer

The upcoming Aquarius sea-surface salinity mission has tight requirements on backscatter measurement accuracy and stability at L-band frequencies (1.26 GHz). These requirements have driven the development of new capabilities in the scatterometers backend detector electronics, which are the focus of this paper. Topics include the development of flight-grade hardware aboard the scatterometer for radio frequency interference (RFI) detection and mitigation, and analog/digital electronics design techniques used to reduce system noise and achieve highly linear power detection over a wide dynamic range. We also summarize the approach taken to test the scatterometers processing and control functions at the level of the integrated Aquarius flight instrument, and present some recent results from the integrated testing campaign.

Optical communication demonstration and high-rate link facility

Motivated by demands for faster, better, cheaper spacecraft, NASA is developing deep-space optical communication technology which promises reduced mass, volume, and power consumption compared to radio-frequency technology. While earth-orbiting optical receivers may eventually be employed, initial deep-space optical communication links are expected to utilize terrestrial telescope receivers. As the communication beam passes through the atmosphere, atmospheric turbulence causes the beam to scintillate, dramatically impacting its temporal and transverse nature. The statistics of these effects must be measured extensively if optical deep-space communication links are to be fully modeled and the design of deep-space communication links optimized. Sponsored by the Engineering Research and Technology Development program, the purposes of the Optical Communication Demonstration and High-Rate Link Facility are to demonstrate a Gbps-class optical downlink, gather extensive link statistics, and provide high-rate down...