Widespread in the subtropics and tropics of the Southern Hemisphere, savannas are highly heterogeneous and seasonal natural vegetation types, which makes change detection (natural vs. anthropogenic) a challenging task. The Brazilian Cerrado represents the largest savanna in South America, and the most threatened biome in Brazil owing to agricultural expansion. To assess the native Cerrado vegetation (NV) areas most susceptible to natural and anthropogenic change over time, we classified 33 years (1985–2017) of Landsat imagery available in the Google Earth Engine (GEE) platform. The classification strategy used combined empirical and statistical decision trees to generate reference maps for machine learning classification and a novel annual dataset of the predominant Cerrado NV types (forest, savanna, and grassland). We obtained annual NV maps with an average overall accuracy ranging from 87% (at level 1 NV classification) to 71% over the time series, distinguishing the three main NV types. This time series was then used to generate probability maps for each NV class. The native vegetation in the Cerrado biome declined at an average rate of 0.5% per year (748,687 ha yr−1), mostly affecting forests and savannas. From 1985 to 2017, 24.7 million hectares of NV were lost, and now only 55% of the NV original distribution remains. Of the remnant NV in 2017 (112.6 million hectares), 65% has been stable over the years, while 12% changed among NV types, and 23% was converted to other land uses but is now in some level of secondary NV. Our results were fundamental in indicating areas with higher rates of change in a long time series in the Brazilian Cerrado and to highlight the challenges of mapping distinct NV types in a highly seasonal and heterogeneous savanna biome.

https://www.mdpi.com/2072-4292/12/6/924/pdf

Mapping Three Decades of Changes in the Brazilian Savanna Native Vegetation Using Landsat Data Processed in the Google Earth Engine Platform

To meet the increasing demand from quantum communications and other photon starved applications, we have developed various InP-based single-photon detectors, including discrete single-photon avalanche diodes (SPADs), negative feedback avalanche diodes (NFADs), and Geiger-mode avalanche photodiode (GmAPD) arrays. A large quantity of InP SPADs have been fabricated. Out of 1000 devices with a 25-μm active area diameter, operated under gated mode at temperature of 233 K, with a pulse repetition rate of 1 MHz and pulse width of 1 ns, the average dark count rate and afterpulsing probability are 30 kHz and 8 × 10
 -5
, respectively. Smaller (16-μm active area diameter) and larger (40-μm active area diameter) discrete devices have been fabricated as well, and their performances are presented along with the 25-μm diameter devices. NFAD devices can operate in free running mode and photon detection efficiency of 10-15% can be achieved without applying any hold-off time externally. When the temperature decreases from 240 to 160 K, the noise equivalent power (NEP) decreasesfrom1.9 × 10
 -16
 to 1.8 × 10
 -18
WHz
 -1/2
, with the activation energy being 0.2 eV. The very low NEP at 160 K makes NFAD devices an ideal choice for long distance, entanglement-based quantum key distributions. GmAPD arrays provide an enabling technology for many active optical applications, such as 3-D laser detection and ranging (LADAR) and photon starved optical communications. Both 32 × 32 and 128 × 32 GmAPD arrays have been fabricated with high performance and good uniformity. GmAPD focal plane arrays (FPAs) with framed readout mode have enabled very high-performance flash LADAR systems. GmAPD FPAs with asynchronous readout mode will enable high rate quantum key distributions and other quantum communications applications.

InP-Based Single-Photon Detectors and Geiger-Mode APD Arrays for Quantum Communications Applications

We describe cameras incorporating focal plane arrays of Geiger-mode avalanche photodiodes (GmAPDs) that enable 3-D imaging in laser radar (LADAR) systems operating at wavelengths near 1.0 and 1.5 μm. GmAPDs based on the InGaAsP material system achieve single-photon sensitivity at every pixel of the array and are hybridized to custom CMOS ROICs providing 0.25 ns timing resolution. We present camera-level performance for photon detection efficiency and dark count rate, along with a survey of the evolution of performance for a substantial number of 32 × 32 cameras. We then describe a temporal statistical analysis of the array-level dark count behavior that distinguishes between Poissonian intrinsic dark count rate and non-Poissonian crosstalk counts. We also report the spatial analysis of crosstalk events to complement the statistical temporal analysis. Differences between cameras optimized for the two different wavelengths—1.0 and 1.5 μm—are noted, particularly with regard to crosstalk behavior.

Dark Count Statistics in Geiger-Mode Avalanche Photodiode Cameras for 3-D Imaging LADAR

We describe work on the development of an optical link budget tool for an intensity-modulated direct-detected photon-counting channel utilizing pulse-position-modulation (PPM). We provide new material in several areas. First, we provide an approximation to the channel capacity, which is not known in closed form, to enable efficient search algorithms over the trade space. The expression also illustrates clearly the trade-offs between signal and noise power, and modulation parameters. We provide an approximation to the losses due to log-normal fading, which may be used to model scintillation. We provide approximations for the loss due to photo-detector blocking and jitter. Lastly, we describe a methodology to choose an optimum detector sub-array in the presence of dark noise, blocking, and an arbitrary point spread function.

An optical communications link design tool for long-term mission planning for deep-space missions

There are hefty constraints levied on all space systems, and some of the most fundamental of them are purely physical. Much of the literature published on optical communications for space data systems focuses on the increased capabilities that optical communication systems have to offer, but ignores the practical infusion strategies of the technology into an operable network. Significant resources have been spent over the better half of the last century across multiple governments to establish robust radio frequency (RF) infrastructure around the world, and any new spacecraft will include provisions to utilize that investment. Therefore, every optical communication system flown will be a functional subset of a larger communication architecture dominated by RF. The key is to optimize the architecture and integrated components into a system that can be used to support hybridized RF and optical communications within the same asset, throughout diverse atmospheric (weather) and in-space conditions. The focus of this paper is on the physical realizability of a hybrid optical and RF communications system, and its ability to increase mission capability without imparting extra burden to the host spacecraft in the form of either increased physical mass or power requirements or additional operational requirements. The space applications may feature a hybrid RF/optical aperture this conjunction of a telescope and an antenna has been dubbed the teletenna. The teletenna is a single co-boresighted aperture that combines the radio and optical beams together into one physical package. The tradeoffs associated with the teletenna optimization are discussed, as well as the pointing requirements and strategies. Initial optical terminals on spacecraft have utilized Earth-based beacons Aerospace Technologist in Telecommunications, Optics and Photonics Branch (LCP), 21000 Brookpark Road/Mail Stop 54-1, AIAA and Technical Committee Member Aerospace Technologist in Telecommunications, High Frequency Branch (LCH), 21000 Brookpark Road/Mail Stop 54-1 Senior Aerospace Technologist in Telecommunications, Architectures, Networks and System Integration Branch, (LCA), 21000 Brookpark Road/Mail Stop 54-1 Aerospace Technologist in Data Systems, Information and Signal Processing Branch (LCI), 21000 Brookpark Road/Mail Stop 54-1 Research Aerospace Technologist in Data Systems, Networking and Architectures Branch (LCA), 21000 Brookpark Road/Mail Stop 54-1 Research Aerospace Technologist in Telecommunications, Information and Signal Processing Branch (LCI), 21000 Brookpark Road/Mail Stop 54-1 Aerospace Technologist in Mechanical Components, (LMT), 21000 Brookpark Road/Mail Stop 86-2 to establish beam tracking, but eventually beacon-less pointing will be developed. Investments in technologies to implement autonomous on-board navigation (and maneuvering) will permit a reduction in dependence on ground-based tracking, ranging, trajectory/orbit/attitude determination and maneuver planning support functions resulting in increased intelligence and autonomy of the complete payload. Innovative approaches include exploiting the optical communications terminal to perform navigational measurements such as star sighting, or a star tracker technology developed to process images at very high data rates. This paper discusses how such a beaconless strategy may be incorporated to work with the proposed hybrid communications system, and highlights the synergistic benefit to both communication and navigation functions for the purpose of decreased size, mass and power burden to users. A common constraint between both deep space and near Earth scenarios is mass. As elegant as a proposed optical communications solution may be, it must be shown that the final product is not so cumbersome as to outweigh the benefits, and that includes the necessary RF technology to complete the full communications payload. We establish a stringent goal of compressing the aforementioned highly capable hybrid RF/optical system into a package which installs within a comparable size, mass, and power envelope as the telecommunications subsystem of Mars Reconnaissance Orbiter (MRO). The goal of this paper is to demonstrate that with modern technology our goal is feasible and therefore has a place in today’s space data systems as well as building a bridge to tomorrow’s.

On the Physical Realizability of Hybrid RF and Optical Communications Platforms for Deep Space Applications

The laser beacon power required by a communication terminal for acquisition and tracking in deep space optical link scenarios can be reduced by a factor of 10 to 100 by replacing an integrating array, such as a CCD, with an array of single photon detectors. An additional benefit of the single photon detector array is that each pixel can have MHz bandwidths, allowing simultaneous recovery of photon time-of-arrival information that can be used for uplink data recovery or range measurements.

/pdf/deep-space-acquisition-and-tracking-with-single-photon-tkrx7w7xe3.pdf

Deep space acquisition and tracking with single photon detector arrays

The Flight Laser Transceiver (FLT) is the space-based subsystem of the Deep- space Optical Terminals (DOT) project. The FLT supports retirement of all major risks for deployment of operational deep-space optical communications at ranges out to about 5 AU. Architecture, key requirements, major trades results, selected concept design, required tech- nology maturation, and future scalability are discussed.

https://ipnpr.jpl.nasa.gov/progress_report/42-185/185D.pdf

Overview and Design of the DOT Flight Laser Transceiver

We propose a solution for pointing and tracking an optical terminal using one or more beacons and a slowly
varying background image. The primary application is a deep space optical communication terminal, where
multiple source tracking provides robustness against beacon outage. Our solution uses optical orthogonal codes
modulated on each beacon to separate the signal from each source for centroiding. This technique allows calculation
of the transmit pointing vector from each beacon location as well as from the background image. The
latter can be used to track during beacon outages. We present a simple algorithm for performing this separation,
and apply it to experimental data from a photon-counting detector illuminated by two beacons and one constant
source. Our results show that the photon flux from each source can be accurately estimated even in the low
signal, high background regime. We estimate the variance of the signal estimator due to Poisson fluctuations
and infer the effect on a centroid estimator for tracking.

/pdf/separating-and-tracking-multiple-beacon-sources-for-deep-1xal7fcaav.pdf

Adit Sahasrabudhe

Papers

Deep space acquisition and tracking with single photon detector arrays

Overview and Design of the DOT Flight Laser Transceiver

Separating and Tracking Multiple Beacon Sources for Deep Space Optical Communications

Separating and Tracking Multiple Beacon Sources for Deep Space Optical Communications