A conceptual design of a large aperture microwave radiometer geostationary platform

A conceptual design of a Large Aperture Microwave Radiometer (LAMR) Platform has been developed and technology areas essential to the design and on-orbit viability of the platform have been defined. Those technologies that must be developed to the requirement stated here for the LAMR mission to be viable include: advanced radiation resistant solar cells, integrated complex structures, large segmented reflector panels, sub 3 kg/m(exp 2) areal density large antennas, and electric propulsion systems. Technology areas that require further development to enhance the capabilities of the LAMR platform (but are not essential for viability) include: electrical power storage, on-orbit assembly, and on-orbit systems checkout and correction

Remote Sensing using Signals of Opportunity

Today, there are more than eight thousand satellites in space. Therefore, Radio Frequency (RF) signals broadcast from satellites can be accessed from almost every point on the earth. There will be number of satellites available at most points on earth with different frequency bands. These satellite signals can be used for remote sensing, therefore software that visualizes footprints of satellites and shows characteristics of every satellite available at any point would be useful in determining which signals are available for reflectometry, method where direct satellite signals and reflected satellite signals are received and compared for analysis of certain area. There is a need for a tool which can tell what satellites are available in a given area. International Telecommunication Union (ITU) has database with all the information about satellites which they store in DVDs and partially can be accessed online. Using the ITU database it is possible to create a program with C++ and MATLAB which perform these functions. DVD contains thousands of contours of satellite footprints stored as a binary; therefore, to extract them for formatting the data from that database into a graphical plot needs to be done through C++. In summary, software that can display the information of satellites at any given point is needed in remote sensing

Remote Sensing of Soil Moisture using S-band Signals of Opportunity: Model Development and Experimental Validation

Root zone soil moisture (RZSM) is a vital aspect in meteorology, hydrology, and agriculture. There are currently some methods in passive and active remote sensing at L-band, but these methods are limited to a sensing depth of approximately 10 cm. Observing RZSM (water in the top meter of soil) will require lower frequencies, thus presenting significant difficulties for a spaceborne instrument, because of the required antenna size, the presence of radio-frequency interference (RFI), and competition for spectrum allocations (in the case of active radar). Bistatic radar using Signal of Opportunity (SoOp) (e.g. digital satellite transmitters) provides an opportunity for remote sensing using powerful signals, which already occupies bands allocated for communications. Recently, a tower experiment has been conducted at Purdue Agronomy Center for Research and Education (ACRE). Linearly polarized measurements were made over bare soil, observing a strong reflected signal. Corn is being planted on the field and measurements will be made throughout the growing season. These measurements focus on measuring the soil properties, such as the weight and dielectric constant, as well as the vegetation characteristics

Statistical Orbit Determination using the Particle Filter for Incorporating Non-Gaussian Uncertainties

The tracking of space objects requires frequent and accurate monitoring for collision avoidance. As even collision events with very low probability are important, accurate prediction of collisions require the representation of the full probability density function (PDF) of the random orbit state. Through representing the full PDF of the orbit state for orbit maintenance and collision avoidance, we can take advantage of the statistical information present in the heavy tailed distributions, more accurately representing the orbit states with low probability. The classical methods of orbit determination (i.e. Kalman Filter and its derivatives) provide state estimates based on only the second moments of the state and measurement errors that are captured by assuming a Gaussian distribution. Although the measurement errors can be accurately assumed to have a Gaussian distribution, errors with a non-Gaussian distribution could arise during propagation between observations. Moreover, unmodeled dynamics in the orbit model could introduce non-Gaussian errors into the process noise. A Particle Filter (PF) is proposed as a nonlinear filtering technique that is capable of propagating and estimating a more complete representation of the state distribution as an accurate approximation of a full PDF. The PF uses Monte Carlo runs to generate particles that approximate the full PDF representation. The PF is applied in the estimation and propagation of a highly eccentric orbit and the results are compared to the Extended Kalman Filter and Splitting Gaussian Mixture algorithms to demonstrate its proficiency

Experimental Testing and Validation of P-band Bi-static Remote Sensing of Soil Moisture in 137-138MHz Range

Remote sensing using readily available communication signal transmitted by ORBCOMM satellites at very high frequency (VHF) range (137-138MHz) is a promising method for detecting the root zone soil moisture content. The radio wave reflectivity of soil is strongly correlated to soil moisture content. Therefore, if we were able to measure the reflectivity, we might be able to estimate the soil moisture content. In this preliminary study, we analyze direct signal data from the satellites to investigate and verify communication channels in frequency range of interest and their characteristics (bandwidth, pattern, etc.). The analysis of direct signal data is also used for calibrating signal collection systems and compensating for the subtle differences of systems. After comparing the satellite geometry and spectrum from raw signal, we verified that ORBCOMM has 13 channels in our frequency range of interest. It was also verified that among these 13 channels, the channel with center frequency at 137.56MHz is a public channel shared by all satellites and is not suitable for reflectivity computation in that multiple satellites could be in sight by our antenna and the signal reflecting region cannot be determined. In our long duration (~12 hours) analysis, we observed the visible duration and period of the satellites. Conclusively, using ORBCOMM communication signal for sensing the soil moisture is viable. Further study is needed to build up model that relates soil moisture content to reflectivity and a lot of technical issues need to be resolved

P-Band Satellite Remote Sensing Antenna

Today, there are a huge number of satellites out there in the space orbiting the earth, and there are specific frequency bands allocated for data transmission from these satellites. Signals from these satellites can be accessed at different places on earth, and used for remote sensing. Lower frequency bands are being used in this project, which have not been used earlier for remote sensing. The main idea of this study is to use the properties of two P-band communication satellites to assess their utility for ‘reflectometry’. This remote sensing method is based upon the comparison of the direct and reflected signals, observed along a line of sight to the satellite. P-band is useful since radiation at this frequency will penetrate deep enough to determine the water content in the top 1m of the soil. This root zone soil moisture (RZSM) content is a very important parameter, which has further far reaching applications in monitoring agricultural production and studying changes in climate. The research involves the building and design of antennas for certain specific frequency ranges. Firstly, a communications link budget has to be prepared for the antenna, taking into account factors such as transmitted power, antenna gain, signal modulation, polarization effects, noise, and atmospheric losses. All the antenna parameters are then put into a modelling tool (such as 4NEC2) so as to assess the geometry structures and generate the radiation pattern for our model. After the design is done, the antenna is built according to the specifications. Finally, it is taken out to test if it can get signals from the specific satellite in orbit. Depending on our location and the satellite elevation, the antenna is directed, and the spectrum analyzer is used to get a display of the data from the direct and the reflected signals. These plots and spectrums allow us to compare the power of the direct and reflected signals. The generated cross-ambiguity function is a signal property related to range resolution of the measurement and the ability to separate reflections from different satellites. Analysis of these signals collected using the antenna will determine if satellite transmissions in the P-band are feasible for use in reflectometry remote sensing. This will also immensely contribute to the field of space borne remote sensing

Refinement and Validation of a Real-time Airborne System for Remotely Sensing Ocean Surface using Communication Satellite Signals

The ability to remotely sense ocean wave heights and wind speed by measuring the reflected Radio Frequency (RF) signals from the ocean’s surface has been demonstrated in previous research projects. The recording systems for these research projects collected and stored unmodified RF signals and then analyzed the data through post-processing. Several disadvantages to this approach include large requirements for data storage and lengthy post-processing time. To assist in the creation of a suitable platform for an airplane-based application, a new system was designed which features real-time processing of the RF signals. This system captures two RF signals in the 2.4 GHz regions (direct and reflected), calculates the cross-correlation between the two signals and then outputs the result to a PC. Due to the time-consuming nature of the cross-correlation algorithm, a FPGA based implementation of the system was chosen to conform to the real-time constraints of the system. In this project, previously created Verilog source code for the system was debugged, improved and verified. This project also developed a method to test the system by using several 110-foot sections of RG6 Coaxial Cables. These cables induced a physical delay in the reflected channel, simulating the application’s conditions, in order to cause a shift in the correlation peak. The results are discussed as well as suggestions for future improvements

Remote sensing using I-Band and S-Band signals of opportunity

Measurement of soil moisture, especially the root zone soil moisture, is important in agriculture, meteorology, and hydrology. Root zone soil moisture is concerned with the first meter down the soil. Active and passive remote sensing methods used today utilizing L-band(1-2GHz) are physically limited to a sensing depth of about 5 cm or less. To remotely sense the soil moisture in the deeper parts of the soil, the frequency should be lowered. Lower frequencies cannot be used in active spaceborne instruments because of their need for larger antennas, radio frequency interference (RFI), and frequency spectrum allocations. Ground-based passive remote sensing using I-band(0.1-1GHz) signals of opportunity provides the required sensing depth and solves the problems that come with the spaceborne remote sensing instruments using I-band reflectometry. A dual monopole antenna setup was used with one on the ground for direct signal and one 30m above ground for the reflected signal. The reflectivity and therefore the soil moisture was obtained from the differences between direct and reflected signals. Initially, an S-band (2-3GHz) signal was used as a proof of concept and its ease of implementation because of its higher transmitted power and stationary satellite. This experiment provides conclusions about the root zone soil moisture based on our observation and comparison of direct and reflected satellite signals of two different frequency bands and determination of reflectivity

Prospects for commercialization of SELV-based in-space operations

A workshop was hosted by the Langley Research Center as a part of an activity to assess the commercialization potential of Small Expendible Launch Vehicle-based in-space operations. Representatives of the space launch insurance industry, industrial consultants, producers of spacecraft, launch vehicle manufacturers, and government researchers constituted the participants. The workshop was broken into four sessions: Customers Small Expendible Launch Systems, Representative Missions, and Synthesis-Government role. This publication contains the presentation material, written synopses of the sessions, and conclusions developed at the workshop

Autonomous rendezvous and docking for Space Station Freedom

Viewgraphs on autonomous rendezvous and docking (AR&D) for Space Station Freedom are presented. Topics covered include: requirements for AR&D experience; Comet spacecraft performance; AR&D mission profile; analytical models for approach trajectory, loosely coupled configuration, and contact dynamics; and application to space infrastructure