Geosynchronous Research Articles

Ground assisted rendezvous with geosynchronous satellites for the disposal of space debris by means of Earth-oriented tethers

Previous studies have shown that extended length Earth-oriented tethers in the geosynchronous (GEO) region can be used to re-orbit satellites to disposal orbits. One such approach involves the extension of a GEO based tether, collection of a debris object, and retraction of the tether, which transfers the retracted configuration to a higher energy orbit for debris disposal. The re-extension of the tether after debris disposal returns the configuration to the near-GEO altitude. The practical feasibility of such a system depends on the ability to collect GEO debris objects, attach them to a deployed tether system, and retract the tethers for transfer to the disposal orbits. This study addresses the collection and delivery of debris objects to the deployed tether system in GEO. The investigation considers the number, type and the characteristics of the debris objects as well as the collection tug that can be ground controlled to detect, rendezvous and dock with the debris objects for their delivery to the tethers system. A total of more than 400 objects are in drift orbits crossing all longitudes either below or above the geostationary radius. More than 130 objects are also known to librate around the stable points in GEO with periods of libration up to five or more years. A characterization of the position and velocity of the debris objects relative to the collection tug is investigated. Typical rendezvous performance requirements for uncooperative GEO satellites are examined, and the similarities with other approaches such as the ESA's CX-OLEV commercial mission proposal to extend the life of geostationary telecommunication satellites are noted.

HEO Satellite Frame and Differential Charging and SCATHA Low-Level Frame Charging

Significant amounts of satellite charging data have been taken at geosynchronous (GEO) and near-GEO orbit and in low-altitude orbits. There are only limited satellite charging measurements at intermediate altitudes or at middle to high latitudes at high altitudes. In this paper, results of spacecraft frame and differential potentials are shown that were measured over several years on a highly elliptical orbit (HEO2) satellite with an apogee of ~7.2 R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</sub> geocentric and ~63deg inclination. The HEO2 frame and differential charging data were obtained using a plasma spectrometer. Measurements of HEO2 charging levels les -30 V are summarized according to their occurrence in magnetic local time and L. The data show that the HEO2 satellite frame charged to potentials les -30 V, and that the occurrence of satellite frame charging at these high latitudes mimics the charging local time patterns observed by GEO satellites. In addition, the data show that such charging extends from at least a minimum L of les 4 R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</sub> to L > 10 R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</sub> . It was found that the differential charging potential usually exceeded the frame potential and could be estimated by V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">D</sub> ~ 74 + 1.384 V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</sub> . Separately, we show the results of a one-year study of the Spacecraft Charging AT High Altitude (SCATHA) satellite frame potential in the near-GEO region. SCATHA's frame potential was positive les 15% of the time and was ges - 1 V ~ 95% of the time. Large negative frame potentials (< - 1 V) occurred ~ 5% of the time. The data also showed that the large negative frame potentials preferentially occurred on the nightside, as expected, and that they occurred less that 1% of the time on the dayside. These statistics may be used as a reference for estimating the relative attraction of ionized outgassed contaminants for a satellite.

Integrated Connection Admission Control and Bandwidth on Demand Algorithm for a Broadband Satellite Network with Heterogeneous Traffic

There are many system proposals for satellite-based broadband communications that promise high capacity and ease of access. Many of these proposals require advanced switching technology and signal processing on-board the satellite(s). One solution is based on a geosynchronous (GEO) satellite system equipped with on-board processing and on-board switching. An important feature of this system is allowing for a maximum number of simultaneous users, hence, requiring effective medium access control (MAC) layer protocols for connection admission control (CAC) and bandwidth on demand (BoD) algorithms. In this paper, an integrated CAC and BoD algorithm is proposed for a broadband satellite communication system with heterogeneous traffic. A detailed modeling and simulation approach is presented for performance evaluation of the integrated CAC and BoD algorithm based on heterogeneous traffic types. The proposed CAC and BoD scheme is shown to be able to efficiently utilize available bandwidth and to gain high throughput, and also to maintain good Grade of Service (GoS) for all the traffic types. The end-to-end delay for real-time traffic in the system falls well within ITU's Quality of Service (QoS) specification for GEO-based satellite systems.

Multisatellite measurements of electron phase space density gradients in the Earth's inner and outer magnetosphere

The variability of the intensity of energetic (MeV) electrons in the Earth's radiation belts has been a renewed area of interest in recent years, yet a major outstanding question is the origin of these particles. In the current study we attempt to differentiate between local stochastic heating and radial diffusion or transport by investigating whether the plasma sheet contains a sufficient population of energetic electrons to supply relativistic electron enhancements in the Earth's radiation belts by radial transport alone or whether one needs to invoke other more localized acceleration processes. We have ordered the equatorial/central plasma sheet electron data from several satellites (GPS (∼4 RE), LANL geosynchronous (GEO) (∼6.6 RE), POLAR (∼8 RE), and CLUSTER (∼18 RE)) in phase space density (PSD). By combining these measurements at a particular value of the first adiabatic invariant, we are able to examine the radial profile of PSD. We present a case study of data from three successive CLUSTER orbits centered on a small to moderate (DST = −40 nT) geomagnetic storm to investigate the storm dynamics of the electron radial phase space gradient in the magnetotail. Our results indicate a steep outward PSD gradient to near geosynchronous orbit (∼trapping boundary) followed by a fairly flat to moderately rising outward PSD gradient into the plasmasheet near 18 RE, with CLUSTER PSDs highly dependent on the vicinity of the current sheet. We conclude that for this case, there appears to be a sufficient source of electrons to account for the populations of electrons in the inner magnetosphere (i.e., a positive radial gradient). However, evidence of a local PSD peak located between our GPS and POLAR radial positions suggests we cannot rule out local acceleration mechanisms.

Wireless at high altitudes-environmental effects on space-based assets

Marconi sent the first trans-Atlantic signals nearly 100 years ago, marking the advent of both modern wireless communications and the initial perceptions of the effects of solar-related natural phenomena on these signals. Currently, interest is increasing rapidly in the use of communications systems above the Earth's surface for voice and data transport. From large communications satellites (comsats) in geosynchronous (GEO) orbit to numerous proposed low-Earth-orbit (LEO) spacecraft constellations, several initiatives have been undertaken for the expansion of services ranging from paging, inventory tracking, TV, and telecommunications. This extension of wireless concepts to the third dimension — the high upper atmosphere and space — includes such businesses as service providers, launch vehicle builders, satellite manufacturers, ground station equipment providers, and personal communications device manufacturers. Many of the third-dimension wireless concepts, designs, and implementations require consideration of the Earth's atmosphere and space plasma environments that are mostly inconsequential to conventional wireless operations. This paper provides a brief overview of some current satellite-based concepts, focuses on the atmospheric, ionospheric, space, and solar environments that third-dimension wireless faces, and discusses some gaps in our current knowledge base.

Multimedia Satellite Networks and TCP/IP Traffic Transport

To meet an increasing demand for multimedia services and electronic connectivity across the world, satellite networks will play an indispensable role in the deployment of global networks. A number of satellite communication systems have been proposed using geosynchronous (GEO) satellites, medium earth orbit (MEO) and low earth orbit (LEO) constellations operating in the Ka-band and above. At these frequencies, satellite networks are able to provide broadband services requiring wider bandwidth than the current services at C or Ku-band. Most of the next-generation broadband satellite systems will use ATM or "ATM-like" switching with onboard processing to provide full two-way services to and from earth stations. The new services gaining momentum include mobile services, private intranets, and high-data-rate internet access carried over integrated satellite-fiber networks. Several performance issues need to be addressed before a transport layer protocol like TCP can satisfactorily work over satellite ATM for large delay-bandwidth networks. The authors review the proposed satellite systems and discuss challenges such as traffic management and QoS requirements for broadband satellite ATM networks. The performance results of TCP enhancements for Unspecified Bit Rate over ATM (ATM-UBR+) for large bandwidth-delay environments with various end system policies and drop policies for several buffer sizes are presented.

Analysis of a satellite antenna tracking loop driven by BPSK modulated signals

The subject of this paper is an antenna tracking system used to maintain an antenna on board a geosynchronous (GEO) satellite, pointed towards a low earth orbit (LEO) satellite. The tracking system uses the BPSK modulated signals transmitted from the LEO satellite. The signal is received by an electronically switched antenna beam. A suitable processing of the received signal level then generates an error voltage which drives a tracking loop. The measurement of the received signal level is implemented by a power detector. Analysis of the antenna tracking algorithm is carried out and the performance is given in terms of linearized mean-square tracking error. The mean-square tracking error is related to the antenna beam parameters, to the closed loop bandwidth and to the statistics of the power measurement process. In particular, it is shown that a key parameter for the tracking loop performance is the signal-to-noise ratio (SNR) at the power detector output, which is inversely proportional to the variance of the pointing error, and a theory has been developed to find an accurate expression for the SNR. © 1997 John Wiley & Sons, Ltd.

Analysis of a satellite antenna tracking loop driven by BPSK modulated signals

The subject of this paper is an antenna tracking system used to maintain an antenna on board a geosynchronous (GEO) satellite, pointed towards a low earth orbit (LEO) satellite. The tracking system uses the BPSK modulated signals transmitted from the LEO satellite. The signal is received by an electronically switched antenna beam. A suitable processing of the received signal level then generates an error voltage which drives a tracking loop. The measurement of the received signal level is implemented by a power detector. Analysis of the antenna tracking algorithm is carried out and the performance is given in terms of linearized mean-square tracking error. The mean-square tracking error is related to the antenna beam parameters, to the closed loop bandwidth and to the statistics of the power measurement process. In particular, it is shown that a key parameter for the tracking loop performance is the signal-to-noise ratio (SNR) at the power detector output, which is inversely proportional to the variance of the pointing error, and a theory has been developed to find an accurate expression for the SNR. © 1997 John Wiley & Sons, Ltd.

Mutual clock synchronization in global digital communication networks

AbstractThere are numerous synchronization methodologies available for consideration in solving the problem of network synchronization in digital communication networks [1, 2]. They include: independent clocks, pulse stuffing, elastic stores, master‐slave hierarchical, time reference distribution, mutual synchronization, etc. This paper considers the network synchronization performance achievable using the method of mutual clock synchronization observed first by Christiaan Huygens in 1665 [3]. Network synchronization performance metrics studied include: nodal timing accuracy, timing jitter, slip rate, time interval between slips, probability of loss of synchronization. These metrics are shown to depend upon the distance (range) between clocks, ranging error, clock stability, nodal phase error processing bandwidth, data rate, signal‐to‐noise ratio and network connectivity. In this regard, the mutual synchronization performance achievable with long wavelength biological rhythms and electric power system rhythms is compared with the performance achievable using short wavelength rhythms required in wideband and broadband digital communications networks. The results are further applied to the problem of synchronizing a satellite communications network. When intrasatellite communication crosslinks (links between satellites in the same orbital plane) are used in a constellation of communication satellites, it is shown that the maximum data rate, the network connectivity and the constellation altitude drive the achievable network synchronization performance; the latter is set by technological limitations due to clock frequency stability, the maximum range between satellites and the minimum ranging error achievable by the ranging system. In this sense, low Earth orbits (LEO) are preferred over geosynchronous (GEO)Earth orbits. The theory is also applied to the Teledesic and Iridium networks.

Meter-level orbit determination of geosynchronous satellites by an economical tracking system

A high-precision , low-cost geosynchronous (GEO) satellite tracking system is proposed. The main components are two GEO satellites, four automatic ground stations, and a very-long-baseline interferometry (VLBI) system. An accuracy potential of 1-1.5 m in position of GEO satellites has been demonstrated, better than any previous GEO satellite's position accuracy. With a quick and simple initialization algorithm, an efficient and stable two-stage quasi-Newton-Raphson orbit determination program was developed. Based on the anticipated characteristics of the 1990 era VLBI, doubly differenced ranging, and dynamic model, an algorithm-dependent linearized covariance analysis was studied. As a consequence, the major error sources are GM error, station location errors, and systematic measurement errors; the minimum noise variance covariance analysis is not applicable; good geometry and observability are much more important than high data rate and small measurement noises; the errors due to the solar radiation pressure acceleration uncertainties, while secondary, must be considered.