Lawrence M. Candell

Design of an Optical Photon Counting Array Receiver System for Deep-Space Communications

Demand for increased capacity in deep-space to Earth communications systems continues to rise as sensor data rates climb and mission requirements expand. Optical free-space laser communications systems offer the potential for operating at data rates 10 to 1000 times that of current radio-frequency systems. A key element in an optical communications system is the Earth receiver. This paper reviews the design of a distributed photon-counting receiver array composed of four meter-class telescopes, developed as a part of the mars laser communications demonstration (MLCD) project. This design offers a cost-effective and adaptable alternative approach to traditional large, single-aperture receive elements while preserving the expected improvement in data rates enabled by free-space laser communications systems. Key challenges in developing distributed receivers and details of the MLCD design are discussed.

An end-to-end demonstration of a receiver array based free-space photon counting communications link

NASA anticipates a significant demand for long-haul communications service from deep-space to Earth in the near future. To address this need, a substantial effort has been invested in developing a free-space laser communications system that can be operated at data rates that are 10-1000 times higher than current RF systems. We have built an end-to-end free-space photon counting testbed to demonstrate many of the key technologies required for a deep space optical receiver. The testbed consists of two independent receivers, each using a Geiger-mode avalanche photodiode detector array. A hardware aggregator combines the photon arrivals from the two receivers and the aggregated photon stream is decoded in real time with a hardware turbo decoder. We have demonstrated signal acquisition, clock synchronization, and error free communications at data rates up to 14 million bits per second while operating within 1 dB of the channel capacity with an efficiency of greater than 1 bit per incident photon.

NPOESS Airborne Sounder Testbed Interferometer (NAST-I) signal processing and initial flight test results

As a testbed to evaluate instrument specifications and data reduction techniques for future satellite atmospheric sounders, an airborne Fourier transform interferometer sounder has been developed for the National Polar-orbiting Operational Environmental Satellite System (NPOESS). The NPOESS Airborne Sounder Testbed Interferometer (NAST-I) collects atmospheric sounding from NASAs high-altitude ER-2 aircraft. Flown at a nominal 20km altitude, collected data is meant to simulate spaceborne atmospheric sounding. The NAST interferometer generates high quality interferograms over the 3.7-16.1 micrometers IR range with a resolution of 0.25cm-1. Interferograms are collected in real-time by an embedded TI TMS320C32 digital signal processing chip housed in an industrial grade 133MHz Pentium PC. Three channels of analog interferogram data and one channel of telemetry data are acquired by four, 16-bit analog to digital converters. Oversampling and decimation are then used to produce high-fidelity samples at a reduced rate and to relax analog filter design requirements. This paper focuses on the signal processing aspects of the instrument and discuss test flight data collected during the instruments initial flight test.

Architectural trades for an advanced geostationary atmospheric sounding instrument

The process of formulating a remote sensing instrument design from a set of observational requirements involves a series of trade studies during which judgments are made between available design options. The outcome of this process is a system architecture which drives the size, weight, power consumption, cost, and technological risk of the instrument. In this paper, a set of trade studies are described which guided the development of a baseline sensor design to provide vertical profiles (soundings) of atmospheric temperature and humidity from future Geostationary Operational Environmental Satellite (GOES) platforms. Detailed trade studies presented include the choice between an interferometric versus a dispersive spectrometer, the optical design of the IR interferometer and visible imaging channel, the optimization of the instrument spatial response, the selection of detector array materials, operating temperatures, and array size, the thermal design for detector and optics cooling, and the electronics required to process detected interferograms into spectral radiance. The trade study process was validated through simulations of the radiometric performance of the instrument, and through simulated retrievals of vertical profiles of atmospheric temperature and humidity. The flexibility of these system trades is emphasized, highlighting the differing outcomes that occur from this process as system requirements evolve. Observations are made with respect to the reliability and readiness of key technologies. The results of this study were disseminated to industry to assist their interpretation of, and responses to, system requirements provided by the U.S. Government.

Signal Acquisition and Timing for a Free Space Laser Communications Receiver

NASA anticipates a significant demand for long-haul communications service from deep-space to Earth in the near future. To address this need, a substantial effort has been invested in developing a novel free-space laser communications system that can be operated at data rates that are 10-1000 times higher than current RF systems. We will focus here on the receiver design which consists of a distributed array of telescopes, each with a Geiger-mode Avalanche Photo Diode (APD) array capable of detecting and timing individual photon arrivals to within a fraction of a nanosecond. Using an array of telescopes has the advantage of providing a large collection area without the cost of constructing a very large monolithic aperture. A key challenge of using a distributed array receiver is combining the detected photons from each of the telescopes so that the combined system appears as a single large collector. This paper will focus on the techniques employed by the receiver to spatially acquire a deep-space downlink laser signal, synchronize the timing of all the photon arrivals at each telescope, and combine the photon detections from each telescope into a single data stream. Results from a hardware testbed utilizing this receiver concept will be shown that demonstrate an efficiency of less than one incident photon per bit at data rates up to 14 Mbps, while operating within 1 dB of the channel capacity.

Electronics considerations for a geostationary interferometer sounder

Future GOES sounders will likely require increased spectral resolution and number of channels in order to meet the sounding vertical resolution and accuracy specifications set forth by the science community. The sensor technology and systems group at MIT Lincoln Laboratory has pursued the use of a high resolution interferometer (GHIS) is order to meet these spectral needs. This paper will focus on the electronics and signal processing necessary for achieving the noise fidelity, dynamic range and data rate constraints for the GHIS instrument. This will include the absolute fringe counting interferometer, analog filtering, A/D conversion, and digital processing. The paper will highlight techniques for reducing the interferometer dynamic range and data rate in order to simplify a GHIS flight design.

Emergency GOES Imager (EGI)

The Emergency GOES Imager study responds to the potential need for a small, back-up imager for weather observations in the event of failure of one or more of the current GOES satellites. The Emergency GOES Imager (EGI) is designed to be compact and lightweight. Minimal spatial resolution is required in the visible and IR band for the purpose of synoptic forecasts. The ground resolution requirement is 16 km for the 10.2 to 11.2 micrometers IR band and 4 km for the 0.5 to 0.7 micrometers visible band. Due to the small size of the instrument, the EGI has the potential to be deployed either alone on a small launcher or as an auxiliary payload on a larger satellite. The overall size of the EGI is dependent on the orientation of the satellite because of the dependence on amount of solar shielding required for the cooler, and the choice of coolers for specific satellite orientations. Although the EGI design is for an emergency system, the design utilizes recent technology in the form of both a linear IR focal plane array, in front of its constant-motion mirror, and a visible CCD array with a staring-format. The IR array has the potential to present a technical challenge to array manufacturers in the area of calibration, assuming a 0.1 K NEDT. We discuss the means by which the emergency requirements are met with this small and simple system, define the limiting technologies in the design, and explore modifications necessary to expand these requirements.

Brassboard tests of a high-resolution interferometer (GHIS) installed in a GOES sounder

Examination of future NOAA Geostationary Operational Environment Satellite (GOES) IR sounder requirement suggested replacing the present GOES I-M series filter wheel instrument with a high-resolution FTIR interferometer. NOAA and MIT Lincoln Laboratory initiated a design and test program replacement feasibility. In collaboration with NASA and ITT Aerospace, a brassboard-version GOES High-Resolution Interferometer Sounder (GHIS) developed under this pathfinder program was installed inside an earlier generation sounder at ITT Aerospace. This paper describes the suite of tests performed while operating the GHIS brassboard at room temperature inside the sounder. Results from the tests effort highlight key issues involved in characterizing FTIR interferometer performance for GOES sounder applications.

Simulation of GOES-8 imager IR striping due to 1/f noise

The present GOES imager exhibits East-West stripes in IR images due to low frequency errors in the calibration of the adjacent North-South detectors. Striping makes delineating boundaries of structures in images difficult, especially in the case of cold scenes. A computer program has been developed that generates simulated IR images using detector noise parameters as inputs. The simulation includes errors due to background drift between space clamps, drift during a space clamp, and errors determining the first order gain during the internal blackbody calibration. The results of the simulation agree well with on orbit measurements of GOES 8 and 9 striping in channels 4 and 5. These simulations can also predict striping performance of future GOES imagers from detector noise parameters allowing for improved detector selection constraints.

GOES imager upgrade design for minimum system impact

An engineering design of an imager upgrade is under consideration for the GOES-N/Q series. The upgrade consist of adding up to three new IR channels at 4 km resolution and doubling the earth coverage rate. The design approach introduces advanced technology as needed to minimize impacts to the sensor optical and mechanical configuration, electronics box layout, data communication links and ground systems. By operating presently redundant portions of detector arrays and developing double rate signal processors, the scanning servo system and electronics modules are largely retained. Bicolor detectors and optical coatings are used to add channels while retaining the present relay optics and radiant cooler layouts. Lossy data compression of visible imagery and lossless data compression of IR imagery are used to preserve the sensor data and precessed data relay communications links. System NEDT and SNR performance and implementation issues for new technologies are addressed.