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The Atacama Cosmology Telescope: The Polarization-Sensitive ACTPol Instrument
The Atacama Cosmology Telescope (ACT) makes high angular resolution measurements of anisotropies in the Cosmic Microwave Background (CMB) at millimeter wavelengths. We describe ACTPol, an upgraded receiver for ACT, which uses feedhorn-coupled, polarization-sensitive detector arrays, a 3deg field of view, 100 mK cryogenics with continuous cooling, and meta material antireflection coatings. ACTPol comprises three arrays with separate cryogenic optics: two arrays at a central frequency of 148 GHz and one array operating simultaneously at both 97 GHz and 148 GHz. The combined instrument sensitivity, angular resolution, and sky coverage are optimized for measuring angular power spectra, clusters via the thermal Sunyaev-Zel'dovich (SZ) and kinetic SZ signals, and CMB lensing due to large-scale structure. The receiver was commissioned with its first 148 GHz array in 2013, observed with both 148 GHz arrays in 2014, and has recently completed its first full season of operations with the full suite of three arrays. This paper provides an overview of the design and initial performance of the receiver and related systems.
CALIPSO Lidar Level 3 Aerosol Profile Product: Version 3 Algorithm Design
The CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations) level 3 aerosol profile product reports globally gridded, quality-screened, monthly mean aerosol extinction profiles retrieved by CALIOP (the Cloud-Aerosol Lidar with Orthogonal Polarization). This paper describes the quality screening and averaging methods used to generate the version 3 product. The fundamental input data are CALIOP level 2 aerosol extinction profiles and layer classification information (aerosol, cloud, and clear-air). Prior to aggregation, the extinction profiles are quality-screened by a series of filters to reduce the impact of layer detection errors, layer classification errors, extinction retrieval errors, and biases due to an intermittent signal anomaly at the surface. The relative influence of these filters are compared in terms of sample rejection frequency, mean extinction, and mean aerosol optical depth (AOD). The “extinction QC flag” filter is the most influential in preventing high-biases in level 3 mean extinction, while the “misclassified cirrus fringe” filter is most aggressive at rejecting cirrus misclassified as aerosol. The impact of quality screening on monthly mean aerosol extinction is investigated globally and regionally. After applying quality filters, the level 3 algorithm calculates monthly mean AOD by vertically integrating the monthly mean quality-screened aerosol extinction profile. Calculating monthly mean AOD by integrating the monthly mean extinction profile prevents a low bias that would result from alternately integrating the set of extinction profiles first and then averaging the resultant AOD values together. Ultimately, the quality filters reduce level 3 mean AOD by −24 and −31 % for global ocean and global land, respectively, indicating the importance of quality screening.
Synthesis of Novel All-Dielectric Grating Filters Using Genetic Algorithms
We are concerned with the design of inhomogeneous, all dielectric (lossless) periodic structures which act as filters.
Simultaneous Optical Links with the Inter-Satellite Omnidirectional Optical Communicator
As the onboard data volume for smaller platforms such as CubeSats increases, Radio Frequency (RF) communications systems may be unable to adequately support the required downlink demand. Optical communications (lasercom) systems can relieve the data bottleneck as they can support higher data throughput than RF for comparable Size, Weight, and Power (SWaP). Lasercom crosslinks are of particular interest for spacecraft swarms and constellations because they enable additional mission robustness for distributed science observations and remote sensing applications. The Inter-Satellite Omnidirectional optical Communicator (ISOC) is capable of supporting simultaneous lasercom crosslinks between multiple spacecraft at separations of up to thousands of kilometers in orbit. The ISOC architecture features a truncated dodecahedron chassis containing an array of photodetectors and gimbal-less MEMS mirrors, enabling full-duplex communications. The main objectives of the ISOC terminal include: 1) full sky coverage, 2) Gbps data rates, and 3) the ability to maintain multiple simultaneous links. We show ISOC can support up to 12 simultaneous links with the current architecture at 1-Gbps at 200-km separation with 10-9 BER. Compared to current lasercom transceivers (i.e., point-to-point), ISOC offers increased throughput performance for similar SWaP. A detailed link budget for both scenarios is presented, and the limiting factors for maximum number of simultaneous links are discussed.
High-speed digital fiber optic links for satellite traffic
Large aperture phased array antennas operating at millimeter wave frequencies are designed for space-based communications and imaging platforms. Array elements are comprised of active T/R modules which are linked to the central processing unit through high-speed fiber-optic networks. The system architecture satisfying system requirements at millimeter wave frequency is T/R level data mixing where data and frequency reference signals are distributed independently before mixing at the T/R modules. This paper demonstrates design procedures of a low loss high-speed fiber-optic link used for transmission of data signals over 600-900 MHz bandwidth inside satellite. The fiber-optic link is characterized for transmission of analog and digital data. A dynamic range of 79 dB/MHz was measured for analog data over the bandwidth. On the other hand, for bursted SMSK satellite traffic at 220 Mbps rates, BER of 2 x 10 to the -7th was measured for E(b)/N(o) of 14.3 dB.
The Circumstellar Imager: Direct detection of extra-solar planetary systems
The Astrometric Imaging Telescope (AIT) is designed to probe the circumstellar environment by both direct imaging and indirect astrometric measurements. The Circumstellar Imager (CI) is a coronagraphic camera and is the direct imaging component of the AIT. The CI is designed to obtain high-sensitivity images of the circumstellar region. It provides crucial non-inferential information relating to the frequency, origin, and evolution of planetary systems and all forms of circumstellar matter. Such imaging is usually limited by the scattered and diffracted light halos of the star itself, which are greatly suppressed in the CI by mating a novel high-efficiency coronagraph with a phase-compensated optical system. For faint point sources in the circumstellar region, the CI will have a sensitivity in excess of 5 magnitudes fainter than the as-designed Hubble Space Telescope (HST). Laboratory data are shown for the coronagraph, which, in a diffraction-limited environment, is capable of suppressing the stellar diffraction sidelobes by several orders of magnitude without significant sacrifice of field of view. In order to realize the high rejection levels inherent in the coronagraph design, it is necessary to limit scatter in the optical systems, imposing a mid-spatial frequency figure error requirement an order of magnitude smaller than that of the HST. Experimental data directed toward meeting this requirement are also shown.
Real-time optical laboratory solution of parabolic differential equations
An optical laboratory matrix-vector processor is used to solve parabolic differential equations (the transient diffusion equation with two space variables and time) by an explicit algorithm. This includes optical matrix-vector nonbase-2 encoded laboratory data, the combination of nonbase-2 and frequency-multiplexed data on such processors, a high-accuracy optical laboratory solution of a partial differential equation, new data partitioning techniques, and a discussion of a multiprocessor optical matrix-vector architecture.
Comparative evaluation of optical waveguides as alternative interconnections for high performance packaging
A detailed comparison between optical and electrical interconnections is presented, with emphasis on advantages and drawbacks of optical link utilization. The impact of attenuation, dispersive degradation, and fan out on signal integrity is discussed. Reflections from discontinuities are taken into account in the section on fan out. According to our results, there is no obvious advantage of using optical interconnections for the distribution of digital signals containing significant frequency components in excess of 10 GHz, unless substantial distances are involved, for which low dispersion optical waveguides could provide a solution. The implementation of a polyimide optical waveguide in the MCNC package is discussed.
Generating Submillimeter-Wave Frequencies From Laser Pulses
Semiconductor photoconductive switches generate electrical pulses containing submillimeter-wavelength carrier signals (frequency between 300 and 3,000 GHz) and harmonics thereof when illuminated with short-rise-time pulses from lasers. Device of this type used as local oscilator in heterodyne submillimeter-wave receiver. Electrical output of device coupled via transmission line, waveguide, or antenna to mixer circuitry of receiver. Phase delays between optically activated semiconductor switches determine output carrier frequencies. N electrical pulses generated by each laser pulse. Thus, fundamental output frequency is N times laser-pulse-repetition rate.
The Next Generation of Lightning Mapping
With the removal of the Lightning Imaging Sensor from the International Space Station, a gap has opened in lightning observation from low-Earth orbit. The CubeSpark mission concept aims to fill this role using a constellation of satellites with radio frequency (RF) sensors and bi-spectral optical imagers to observe lightning flashes more completely and with better resolution than is currently possible from space. In this study, we assess the feasibility of multiple methods of not only mapping lightning locations, but also inferring 3D flash and charge structures. This is done primarily by simulating lightning emissions in the very high frequency (VHF) band, modeling their propagation to orbital sensors, and reconstructing their locations using time-of-arrival (TOA) minimization algorithms. Constellation shape, number, and atmospheric conditions are varied in order to quantify the three main sources of geolocation error: geometric, ionospheric, and instrumental effects. The promising results presented here demonstrate 3D resolution better than 1-2 km in many cases, enabling new applications in meteorology and climate sciences.
Whispering Gallery Mode Resonators as Optical Reference Cavities
Highly stabilized lasers are an increasingly valuable tool for metrology. For many applications, however, existing Fabry Perot systems are too bulky and cumbersome. We are investigating the use of miniature monolithic whispering gallery mode resonators as reference cavities for laser stabilization. We seek to exploit the benefit of small size and vibration resistance by suppressing thermally induced frequency fluctuations. We have theoretically investigated the viability of using a thin-film coating to achieve temperature compensation. We have experimentally investigated an active temperature stabilization scheme based on birefringence in a crystalline resonator. We also report progress of laser locking to the resonators.
Fiber-Optic Discriminator Stabilizes Microwave Oscillator
New fiber-optic delay line discriminator enables stabilization of oscillators directly at microwave output frequency, eliminating need for frequency multiplication. Discriminator is wide-band device, capable of stabilizing outputs of frequency-agile microwave sources over multigigahertz tuning ranges. Use of advanced fiber-optic delay line with wider bandwidth and low noise predicted to yield corresponding improvements in phase-noise performance.
Automated frequency standard stability data reduction
A design using a medium sized computer, a fiber optic communication link, and a desk top computer to reduce data collected at a remote site for analysis of frequency standard tests is described. Data generated at the Interim Frequency Standards Test Facility is collected on magnetic tape, which is then transported to a Sigma 5 computer. An Allan variance (two sample variance) is then calculated. The Allan variance (sigma vs tau) is then written to a data file in the Sigma 5. A fiber optic communication link between the Sigma 5 and a desk top computer was designed to transmit data in the Sigma 5 file to the desk top computer. A program was written for the desk top computer that will automatically access the data file in the Sigma 5, process the data and provide a data plot.
LO Peg: Surface Differential Rotation, Flares, and Spot-Topographic Evolution
Using the wealth of ∼24 yr multiband data, we present an in-depth study of the star-spot cycles, surface differential rotations (SDR), optical flares, evolution of star-spot distributions, and coronal activities on the surface of young, single, main-sequence, ultrafast rotator LO Peg. From the long-term V-band photometry, we derive rotational period of LO Peg to be 0.4231 +/- 0.0001 d. Using the seasonal variations on the rotational period, the SDR pattern is investigated, and shows a solar-like pattern of SDR. A cyclic pattern with period of ∼2.7 yr appears to be present in rotational period variation. During the observations, 20 optical flares are detected with a flare frequency of ∼1 flare per two days and with flare energy of ∼10(exp 31-34) erg. The surface coverage of cool spots is found to be in the range of ∼9-26 per cent. It appears that the high- and low-latitude spots are interchanging their positions. Quasi-simultaneous observations in X-ray, UV, and optical photometric bands show a signature of an excess of X-ray and UV activities in spotted regions.
A Ten-Year Global Record of Absorbing Aerosols Above Clouds from OMI's Near-UV Observations
Aerosol-cloud interaction continues to be one of the leading uncertain components of climate models, primarily due to the lack of an adequate knowledge of the complex microphysical and radiative processes associated with the aerosol-cloud system. The situations when aerosols and clouds are found in the same atmospheric column, for instance, when light-absorbing aerosols such as biomass burning generated carbonaceous particles or wind-blown dust overlay low-level cloud decks, are commonly found over several regional of the world. Contrary to the cloud-free scenario over dark surface, for which aerosols are known to produce a net cooling effect (negative radiative forcing) on climate, the overlapping situation of absorbing aerosols over cloud can potentially exert a significant level of atmospheric absorption and produces a positive radiative forcing at top-of-atmosphere. The magnitude of direct radiative effects of aerosols above cloud depends directly on the aerosol loading, microphysical-optical properties of the aerosol layer and the underlying cloud deck, and geometric cloud fraction. We help in addressing this problem by introducing a novel product of optical depth of absorbing aerosols above clouds retrieved from near-UV observations made by the Ozone Monitoring Instrument (OMI) on board NASA's Aura platform. The presence of absorbing aerosols above cloud reduces the upwelling radiation reflected by cloud and produces a strong 'color ratio' effect in the near-UV region, which can be unambiguously detected in the OMI measurements. Physically based on this effect, the OMACA algorithm retrieves the optical depths of aerosols and clouds simultaneously under a prescribed state of atmosphere. The algorithm architecture and results from a ten-year global record including global climatology of frequency of occurrence and above-cloud aerosol optical depth, and a discussion on related future field campaigns are presented.
Stabilized Photonic Links for Deep Space Tracking, Navigation, and Radio Science Applications
Stable fiber optic links have been operational in NASA's DSN for over 20 years providing distribution limited fractional frequency stability below 10-14 at one second and reaching 10-18 at approx 10,000 seconds. We summarize recent stabilized photonic link developments for DSN operation capable of high performance and reliability, and at much lower cost than previous systems. Recent research has focused on two applications, 1) actively stabilized links to transport 100 MHz metrology frequency references to sites up to 30 km from the central atomic frequency standard, and 2) stabilized links transporting broadband S, X, and Ka band signals used in deep space tracking and navigation and antenna arraying applications.
Optical distance measuring instrument
An optical instrument, such as a stability monitor or a target range finder, uses an unstabilized laser to project a composite optical signal of coherent light having two naturally occurring longitudinal mode components. A beamsplitter divides the signal into a reference beam which is directed toward one photodetector and a transmitted beam which illuminates and is reflected from a distant target onto a second photodetector optically isolated from the first photodetector. Both photodetectors are operated on the square law principle to provide electrical signals modulated at a frequency equal to the separation between the frequencies of the two longitudinal mode components of the optical signal projected by the laser. Slight movement of the target may be detected and measured by electrically monitoring the phase difference between the two signals provided by the photodetectors and the range of the target measured with the aid of a microprocessor by changing the separation between the longitudinal modes by shifting the length of the resonator cavity in an iterative series of increments.