GPS code and carrier multipath mitigation using a multiantennasystem

Multipath is a major source of error in high precision Global Positioning System (GPS) static and kinematic differential positioning. Multipath accounts for most of the total error budget in carrier phase measurements in a spacecraft attitude determination system. It is a major concern in reference stations, such as in Local Area Augmentation Systems (LAAS), whereby corrections generated by a reference station, which are based on multipath corrupted measurements, can significantly influence the position accuracy of differential users. Code range, carrier phase, and signal-to-noise (SNR) measurements are all affected by multipath, and the effect is spatially correlated within a small area. In order to estimate and remove code and carrier phase multipath, a system comprising a cluster of five GPS receivers and antennas is used at a reference station location. The spatial correlation of the receiver data, and the known geometry among the antennas, are exploited to estimate multipath for each satellite in each antenna in the system. Generic receiver code and carrier tracking loop discriminator functions are analyzed, and relationships between receiver data, such as code range, carrier phase, and SNR measurements, are formulated and related to various multipath parameters. A Kalman filter is described which uses a combination of the available information from the antennas (receivers) in the multiantenna cluster to estimate various multipath parameters. From the multipath parameters, the code range and carrier phase multipath is estimated and compensated. The technique is first tested on simulated data in a controlled multipath environment. Results are then presented using field data and show a significant reduction in multipath error     

Hybrid power sources for pulsed current applications

A battery-capacitor hybrid power source was discharged under pulsed current conditions and various ambient temperatures. Significant improvements in voltage drop and run time were obtained with the hybrid device over the battery alone. An equivalent circuit was established to evaluate the voltage behavior of the hybrid device. It was found that at the peak discharge current, as much as 50% of the total energy supplied was provided by a capacitor with only about one-third of the battery's volume     

Superimposed HMM transient detection via target tracking ideas

The quickest detection of superimposed hidden Markov model (HMM) transient signals is addressed. It is assumed that a known HMM is always extant but at an unknown time a second known HMM may also be present, and overlapped with the previous. Two approaches are proposed. The first treats the superimposed HMMs as a unit with an expanded state space, thus converting the problem of detecting superimposed HMMs into detection of a change in HMM, this being readily solved using a previously proposed procedure. Such an approach, though excellent in terms of performance, is not suitable for the superposition of multiple HMMs with large state dimensions due to computational complexity. A second detection scheme (based on multiple target tracking ideas) with much lower computational needs but little loss in terms of performance, is therefore developed     

Adaptive polarimetric target detection with coherent radar. II.Detection against non-Gaussian background

For pt. I see ibid., vol. 37, no. 4, pp. 1194-1206 (2001).This paper presents the derivation of a polarimetric coherent adaptive scheme to detect a radar target against a non-Gaussian background. This completes the results presented in Part I for the Gaussian background. A Texture Free-Generalized Likelihood Ratio Test (TF-GLRT) detector is derived that exploits the polarimetric characteristics of the received radar echoes to improve the detection performance. The proposed polarimetric detector is shown to have Constant False Alarm Rate (CFAR) when operating against compound-Gaussian clutter with unknown parameters. Its performance is fully characterized by both theoretical analysis and simulation. Moreover, the application to recorded radar data demonstrates the performance improvement achievable in practice     

The Huygens Doppler Wind Experiment ' Titan Winds Derived from Probe Radio Frequency Measurements

A Doppler Wind Experiment (DWE) will be performed during the Titan atmospheric descent of the ESA Huygens Probe. The direction and strength of Titan's zonal winds will be determined with an accuracy better than 1 m s⁻¹ from the start of mission at an altitude of ∼160 km down to the surface. The Probe's wind-induced horizontal motion will be derived from the residual Doppler shift of its S-band radio link to the Cassini Orbiter, corrected for all known orbit and propagation effects. It is also planned to record the frequency of the Probe signal using large ground-based antennas, thereby providing an additional component of the horizontal drift. In addition to the winds, DWE will obtain valuable information on the rotation, parachute swing and atmospheric buffeting of the Huygens Probe, as well as its position and attitude after Titan touchdown. The DWE measurement strategy relies on experimenter-supplied Ultra-Stable Oscillators to generate the transmitted signal from the Probe and to extract the frequency of the received signal on the Orbiter. Results of the first in-flight checkout, as well as the DWE Doppler calibrations conducted with simulated Huygens signals uplinked from ground (Probe Relay Tests), are described. Ongoing efforts to measure and model Titan's winds using various Earth-based techniques are briefly reviewed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Power factor correction circuits with robust current control technique

A dynamically robust current control method to synthesize a sinusoidal input current for AC-to-DC converters with boost type topology is presented. Under this control strategy, the inductor current and the diode current of the boost converter are fed back and combined in a special way which makes the input current of the AC-to-DC converter stable and robust. The input current is solely determined by the reference current. When the reference current signal is derived from the sinusoidal input voltage, the input current is sinusoidal and in phase with the input voltage. Theoretical analysis is first provided. Small signal analysis shows that the current loop is inherently stable and has a fast dynamic response. Large signal analysis reveals that the control system is not affected by large disturbances in supply voltage or output load. Computer simulations have been carried out and experimental prototype models have been built to verify the analysis and demonstrate the feasibility of the control strategy. A power factor of 0.998 and a total harmonic distortion (THD) of 3.18% are measured.     

The Soviet/Russian spacesuit history: Part III—The European connection

The European spacesuit system (ESSS) initiated by the European Space Agency (ESA) in the late 1980s had many similarities with the Soviet/Russian ORLAN spacesuit system, due to the Hermes system requirements. First, direct contacts in 1989 permitted closer comparison of the two suit systems, and soon the ORLAN manufacturer Zvezda could be contracted as support to the European spacesuit team. In particular, the suit enclosure design and predevelopment testing and operational analysis were performed in close cooperation between Zvezda and the European team under Dornier.

With the changing system requirements and a closer cooperation between ESA and the new Russian Space Agency (RKA) a new joint spaceplane/stations mission scenario came about. This scenario could be served by one spacesuit system, EVA SUIT 2000, which was to be jointly developed by a team headed by Zvezda and Dornier for ESA and RKA. ORLAN-DMA and ESSS experience and hardware were the initial platforms for these activities to create a new generation spacesuits for the Mir 2 and later the ISSs.

A suit demonstrator was manufactured and tested by the end of 1994 when ESA stopped its spacesuit development activities and the joint EVA SUIT 2000 project was terminated. However, many of the features designed, manufactured and tested for the EVA SUIT 2000 were then implemented by Zvezda in the new Russian spacesuit system ORLAN-M, now in full operation onboard the ISS.     

Infrared signature studies of aerospace vehicles

Infrared (IR) emissions from aircraft are used to detect, track, and lock-on to the target. MAN Portable Air Defence Systems (MANPADS) have emerged as a major cause of aircraft and helicopter loss. Therefore, IR signature studies are important to counter this threat for survivability enhancement, and are an important aspect of stealth technology. This paper reviews contemporary developments in this discipline, with particular emphasis on IR signature prediction from aerospace vehicles. The role of atmosphere in IR signature analysis, and relation between IR signature level and target susceptibility are illustrated. Also, IR signature suppression systems and countermeasure techniques are discussed, to highlight their effectiveness and implications in terms of penalties.     

Composition of Light Solar Wind Noble Gases in the Bulk Metallic Glass flown on the Genesis Mission

We discuss data of light noble gases from the solar wind implanted into a metallic glass target flown on the Genesis mission. Helium and neon isotopic compositions of the bulk solar wind trapped in this target during 887 days of exposure to the solar wind do not deviate significantly from the values in foils of the Apollo Solar Wind Composition experiments, which have been exposed for hours to days. In general, the depth profile of the Ne isotopic composition is similar to those often found in lunar soils, and essentially very well reproduced by ion-implantation modelling, adopting the measured velocity distribution of solar particles during the Genesis exposure and assuming a uniform isotopic composition of solar wind neon. The results confirm that contributions from high-energy particles to the solar wind fluence are negligible, which is consistent with in-situ observations. This makes the enigmatic “SEP-Ne” component, apparently present in lunar grains at relatively large depth, obsolete. ²⁰Ne/ ²²Ne ratios in gas trapped very near the metallic glass surface are up to 10% higher than predicted by ion implantation simulations. We attribute this superficially trapped gas to very low-speed, current-sheet-related solar wind, which has been fractionated in the corona due to inefficient Coulomb drag.