Andrew O'Brien

New Ocean Winds Satellite Mission to Probe Hurricanes and Tropical Convection

AbstractThe Cyclone Global Navigation Satellite System (CYGNSS) is a new NASA earth science mission scheduled to be launched in 2016 that focuses on tropical cyclones (TCs) and tropical convection. The mission’s two primary objectives are the measurement of ocean surface wind speed with sufficient temporal resolution to resolve short-time-scale processes such as the rapid intensification phase of TC development and the ability of the surface measurements to penetrate through the extremely high precipitation rates typically encountered in the TC inner core. The mission’s goal is to support significant improvements in our ability to forecast TC track, intensity, and storm surge through better observations and, ultimately, better understanding of inner-core processes. CYGNSS meets its temporal sampling objective by deploying a constellation of eight satellites. Its ability to see through heavy precipitation is enabled by its operation as a bistatic radar using low-frequency GPS signals. The mission will depl...

Mitigation of Adaptive Antenna Induced Bias Errors in GNSS Receivers

Antenna arrays equipped with adaptive filtering allow global navigation satellite system (GNSS) receivers to operate in environments with harsh, sustained interference. However since these antennas utilize spatial and temporal filters, they have the potential to introduce significant bias errors into the code and carrier phase measurements made by GNSS receivers. For precision navigation applications, these biases must be mitigated. A novel bias estimation and correction technique is described in which additional logic is added to the receiver in order to provide runtime compensation for these antenna-induced biases. The technique is general in the sense that it can be applied to a wide variety of adaptive antenna and receiver implementations. It utilizes a computationally efficient bias estimation equation that incorporates stored antenna data, the adaptive filter weights, and the receiver discriminator function. After estimating the bias, the receiver can compensate for the bias errors either in the navigation processor or in the tracking loop of the GNSS receiver. Simulations demonstrate centimeter-level bias correction accuracy for a variety of signals using an adaptive antenna array in an environment with interference.

Calibration and Unwrapping of the Normalized Scattering Cross Section for the Cyclone Global Navigation Satellite System

This paper develops and characterizes the algorithms used to generate the Level 1 (L1) science data products of the Cyclone Global Navigation Satellite System (CYGNSS) mission. The L1 calibration consists of two parts: the Level 1a (L1a) calibration converts the raw Level 0 delay-Doppler maps (DDMs) of processed counts into received power in units of watts. The L1a DDMs are then converted to Level 1b DDMs of bistatic radar cross section values by unwrapping the forward scattering model and generating two additional DDMs: one of unnormalized bistatic radar cross section values (in units of square meters) and a second of bin-by-bin effective scattering areas. The L1 data products are generated in such a way as to allow for flexible processing of variable areas of the DDM (which correspond to different regions on the surface). The application of the L1 data products to the generation of input observables for the CYGNSS Level 2 (L2) wind retrievals is also presented. This includes a demonstration of using only near-specular DDM bins to calculate a normalized bistatic radar cross section (unitless, i.e., m2/m2) over a subset of DDM pixels, or DDM area. Additionally, an extensive term-by-term error analysis has been performed using this example extent of the DDM to help quantify the sensitivity of the L1 calibration as a function of key internal instrument and external parameters in the near-specular region.

Comparison of Output SINR and Receiver C/N_0 for GNSS Adaptive Antennas

Antenna arrays with space-time adaptive processing (STAP) are commonly utilized to allow the reception of signals in harsh interference environments. The primary applications of STAP have been radar and communications systems, where the STAP weights are optimized for output signal-to-interference-plus-noise ratio (SINR); however, there is increasing demand for STAP in time-of-arrival (TOA) estimation applications, such as global navigation satellite system (GNSS) receivers. It is understood that TOA estimation performance is primarily dependent on receiver postcorrelation carrier-to-noise ratio (C/N0). In this study, the distinction between SINR and C/N0 is established, and a performance analysis of the common STAP algorithms is performed in the presence of interference. It is demonstrated that output SINR is an inadequate indication of GNSS receiver C/N0 performance. As a result, caution should be used when measuring the relative performance of different STAP algorithms in GNSS applications. Consequently, the present work develops a novel STAP algorithm which maximizes C/N0. Additionally, the performance of common STAP algorithms is analyzed in the context of this performance bound, and it is demonstrated that, in many cases, their performance is near optimal.

Small and Adaptive Antennas and Arrays for GNSS Applications

This paper reviews Global Navigation Satellite Systems (GNSS) antennas and arrays for robust coverage in presence of interference signals with a focus on antenna design aspects. A number of small antenna designs are presented that cover multiple current and future GNSS frequency bands (1150-1610 MHz) with sizes as small as 1 in (25 mm) in diameter. Several arrays are presented with a size as small as 3.5 in (8.9 cm) in diameter and approximately 0.5 in (13 mm) thick as compared to commercially available apertures 14 in (35.6 cm) and 5.5 in (14.0 cm) in diameter. These small arrays are shown to satisfy the gain requirements of GNSS receivers while simultaneously offering four to six antenna elements for adaptive interference suppression.

High power coherent-on-receive radar for marine surveillance

In this paper we describe the development of a low cost, high power coherent-on-receiver radar. The unit has a 25kW peak transmit power and is capable of accurately measuring velocity as well as range. The design of the radar is optimized for marine surveillance, although the techniques developed have general application. The radar system is validated through a series of tests that culminate with sea surface measurements.

The CubeSat Radiometer Radio Frequency Interference Technology Validation (CubeRRT) mission

The CubeSat Radiometer Radio Frequency Interference Technology Validation (CubeRRT) mission is developing a 6U CubeSat system to demonstrate radio frequency interference (RFI) detection and mitigation technologies for future microwave radiometer remote sensing missions. CubeRRT will perform observations of Earth brightness temperatures from 6-40 GHz using a 1 GHz bandwidth tuned channel, and will demonstrate on-board real-time RFI processing. The system is currently under development, with launch readiness expected in 2018 followed by a one year period of on-orbit operations. Project plans and status are reported in this paper.

Real-time estimation of ocean wave fields from marine radar data

This paper discusses methods for using measurements of the backscattered power and the Doppler shift of radar signals scattered from the ocean surface to compute maps of phase resolved ocean wave fields. Results are compared with buoy and lidar measurements of ocean surface waves off the coast of southern California.

Demonstration of space-time adaptive processing for DSI suppression in a passive radar

Passive radar uses illuminators of opportunity instead of a dedicated radar transmitter. A number of already transmitted signals can be used, however, their nature is mostly continuous and their utilization is a complicated task. One of the key limiting factors of passive radar is the strong direct signal interference (DSI), even though many physical and signal processing countermeasures have already been developed. In the case of using terrestrial signals for airborne target detection, elimination of a certain elevation angle of arrival is a possible solution. As the real-world environment is non-stationary, adaptive methods are expected to give better results. The paper shows and compares two space-time adaptive vertical antenna array processing methods for DSI mitigation, applied in terrestrial navigation signal based radar. In addition, both methods are verified by processing signals captured under real-world conditions. Both methods offer more than 10 dB DSI suppression.

Calibration and validation of the cygnss level 1 data products

This presentation will include an overview of the recently launched NASA CYGNSS mission Level 1 calibration algorithms and their on-orbit validation [1], [2]. The validation of the Level 1 calibration will be performed in several steps, including a) a detailed noise floor analysis to assess the observed on-orbit noise power levels over the open ocean, b) multiple consistency checks using a forward model and co-located ocean wind and wave truth reference data and c) a term by term error analysis of all the non-ocean corrections applied to the final sigma0 estimates. An outline of the Level 1a (calibration from raw Level 0 instrument counts to units of watts for the received power) and the Level 1b (calibration from watts to bistatic scattering cross section) algorithms are each shown below. Three key components of the Level 1a calibration will be presented, namely, an analysis of the instrument (alone) and antenna noise characteristics over the ocean, a study of the range of received power levels from the surface, and comparisons with a forward model. The key components of the Level 1b calibration presented here will include validation of the main corrections applied to arrive at a surface sigma0 estimate, including receiver antenna gain, GPS transmitter and scattering area corrections.