Low Polar Orbit Research Articles

On the wave-particle interaction and removal of energetic particles

There is always a risk of destruction of man-made satellites by the energetic electrons trapped in Van Allen radiation belts in space. These energetic electrons also pose a biological danger to astronauts. The cyclotron resonance interaction is studied between the whistler-mode waves in the frequency range of ELF (Extremely Low Frequency 300 – 3000 HZ) and VLF (Very Low Frequency 3 – 30 kHz) propagating along geomagnetic field line and counter streaming energetic electron. During this process the pitch angle of energetic electrons reduces. This results in the dumping of these electrons into the lower ionosphere. This makes electrons unable to strike the satellites orbiting in low Earth orbit, Geosynchronous, Sun-synchronous or polar orbit. It is shown that the lifetime values of energetic electrons vary from 2.03 to 227.68 hours at low latitudes. It is shown that these waves can remove these energetic electrons from their path and ensure the safety of satellites.

SOS and HOS Performance for Processing Satellite Relayed Signal.(Dept.E)

Signal detection in the presence of additive Gaussian noise can be performed using higher order statistics. This paper deals with a processing of single and multiple emergency signals that relayed from the low polar orbit satellite using parametric spectral estimation techniques with Second and Higher Order Statistics. The detection algorithm based on the Second Order Statistics (SOS) using the parametric techniques, which are the Maximum Entropy Method (MEM), and MUltiple Signal Classification (MUSIC) is designed and examined in this paper. Simulation results illustrate successful performance of signal detection at low signal to noise ratio. These proposed methods suppress additive Gaussian noise of unknown power spectrum, detect and characterize the main properties of the transmitted signals.

The influence of tesseral harmonic $J_{22}$ term on the TLE orbit error of low polar orbit satellite

By expanding the perturbation function of tesseral harmonic $J_{22}$ term of the earth’s non spherical gravitation, it is found that there is a term $\cos 2\Omega _{e}$ related to the rotation of the earth. The period of the term is half a day, and the coefficient includes the factor $\sin ^{2}i$ related to the orbital inclination angle. For the low polar orbit satellite with orbit inclination close to $90^{\circ }$ , $J_{22}$ term has a great impact on the orbit accuracy, which will reach the order of kilometers. Based on the measured orbit data (historical TLEs and GPS precise ephemeris) of the polar satellite GRACE-A/B, the influence of the tesseral harmonic $J_{22}$ term on TLE orbit error is further verified. It is found that the system bias caused by ignoring $J_{22}$ term exists in TLE orbit data for LEO, which shows a periodic change of 180 degrees with the geographic longitude in the in-track direction. In addition, the accuracy of TLE orbit in the northern hemisphere is significantly higher than that in the southern hemisphere, which is related to the distribution of SSN observation network. For GRACE-A satellite, the maximum system bias of TLE orbit can reach close to 1.5 km near the equator of the earth, and 8 km after a 7-day prediction period.

Dynamics of Megaelectron Volt Electrons Observed in the Inner Belt by PROBA‐V/EPT

AbstractUsing the observations of the EPT (Energetic Particle Telescope) onboard the satellite PROBA‐V, we study the dynamics of inner and outer belt electrons from 500 keV to 8 MeV during quiet periods and geomagnetic storms. This high time‐resolution (2‐s) spectrometer operating at the altitude of 820 km on a low polar orbit is providing continuously valuable electrons fluxes for already 5 years. We emphasize especially that some megaeleactron volt electrons are observed in low quantities in the inner belt, even during periods when they are not observed by Van Allen Probes. We show that they are not due to proton contamination but to clear injections of particles from the outer belt during strong geomagnetic storms of March and June 2015, and September 2017. Electrons with lower energy are injected also during less strong storms and the L shell of the electron flux peak in the outer belt shifts inward with a high dependence on the electron energy. With the new high‐resolution EPT instrument, we can study the dynamics of relativistic electrons, including megaelectron volt electrons in the inner radiation belt, revealing how and when such electrons are injected into the inner belt and how long they reside there before being scattered into the Earth's atmosphere or lost by other mechanisms.

Lighthouse: A spacebased mission concept for the surveillance of geosynchronous space debris from low earth orbit

In this paper, a new mission is proposed for space debris surveillance in geosynchronous orbit (GEO). The mission “Lighthouse”, here proposed as a concept study, describes the use of a small satellite in a low polar orbit, equipped with a Schmidt telescope, constantly observing a belt across the geostationary orbit. In this way, a single instrument can sweep the whole orbit everyday regardless the light and weather conditions. Most of observations are nowadays performed by ground telescopes, which are affected by weather conditions and night time duration. Moreover, a single telescope can observe only a portion of the geostationary orbit. The mission concept arose as space application of an ESA ITI (Innovation Triangle Initiative) project designing a Schmidt telescope purposely conceived for the monitoring of NEO (Near Earth Objects) and space debris. A compact version of the telescope (50 cm diameter and 1.61 m length), particularly suitable for space applications, has been designed too. The size and the mass of the telescope enable the use of a small satellite platform, with the related advantages in term of costs and performance. Lighthouse is proposed as a new asset for Space Surveillance and Tracking sensors, complementary to the ground telescopes network.

Simultaneous investigation of magnetospheric plasma and spacecraft charging

The results of magnetospheric plasma fluxes measurement in geosynchronous orbit are presented. The measurements were done onboard the geosynchronous Russian “Electro” spacecraft (SC) in 1995–1997 years. The diurnal variations of the plasma density and temperature were observed at various levels of geomagnetic activity. Distortions of the measured electron and proton spectra caused by negative potential of the charged SC were registered. Typical electron spectra and plasma parameters were determined for various SC charging levels. Similar measurements in low polar orbits at the “Meteor” SC (launched in 2003 year) and the “Universitetskiy–Tatyana” SC (launched in 2005 year) were done.

A Wide Field Auroral Imager (WFAI) for low Earth orbit missions

Abstract. A comprehensive understanding of the solar wind interaction with Earth's coupled magnetosphere-ionosphere system requires an ability to observe the charged particle environment and auroral activity from the same platform, generating particle and photon image data which are matched in time and location. While unambiguous identification of the particles giving rise to the aurora requires a Low Earth Orbit satellite, obtaining adequate spatial coverage of aurorae with the relatively limited field of view of current space bourne auroral imaging systems requires much higher orbits. A goal for future satellite missions, therefore, is the development of compact, wide field-of-view optics permitting high spatial and temporal resolution ultraviolet imaging of the aurora from small spacecraft in low polar orbit. Microchannel plate optics offer a method of achieving the required performance. We describe a new, compact instrument design which can observe a wide field-of-view with the required spatial resolution. We report the focusing of 121.6 nm radiation using a spherically-slumped, square-pore microchannel plate with a focal length of 32 mm and an F number of 0.7. Measurements are compared with detailed ray-trace simulations of imaging performance. The angular resolution is 2.7±0.2° for the prototype, corresponding to a footprint ~33 km in diameter for an aurora altitude of 110 km and a spacecraft altitude of 800 km. In preliminary analysis, a more recent optic has demonstrated a full width at half maximum of 5.0±0.3 arcminutes, corresponding to a footprint of ~1 km from the same spacecraft altitude. We further report the imaging properties of a convex microchannel plate detector with planar resistive anode readout; this detector, whose active surface has a radius of curvature of only 100 mm, is shown to meet the spatial resolution and sensitivity requirements of the new wide field auroral imager (WFAI).

Concepts for a low-cost Mars micro mission

This paper summarises three novel micro-mission concepts for the low-cost exploration of the Martian system. These concepts have been studied under national funding with the aim of defining highly cost-effective options for the delivery of on-board instrumentation into Mars orbit for remote sensing or deployment of lander packages for surface exploration of the Martian system. All three missions use advanced electric propulsion to improve payload mass performance and miniaturised avionics either in the spacecraft bus or payload. Broadly, the three concepts are categorised according to mass: a constellation of micro-satellites (120 kg) in low Mars polar orbit for global atmospheric circulation/climate monitoring using radio occultation techniques; a single mini-satellite (310 kg) for studying composition and origins of Mars’ moons Deimos and Phobos; and a single larger spacecraft (800 kg) based on a standard launcher adapter cone and carrying multiple micro-penetrators for measuring sub-surface water ice composition at pre-determined locations on the surface of Mars.

Autonomous behavioural algorithm for space applications

AbstractThe purpose of this paper is to present a new approach in the concept and implementation of autonomy for autonomous spacecraft. The one true ‘artificial agent’ approach to autonomy requires the spacecraft to interact in a direct manner with the environment through the use of sensors and actuators. Rather than using complex world models, the spacecraft is allowed to exploit the dynamics of its environment for cues as to appropriate actions to take to achieve its mission goals. The particular artificial agent implementation used here has been inspired by studies of biological systems. The so-called ‘cue-deficit’ action selection algorithm considers the spacecraft to be a non-linear dynamical system with a number of observable states. Using optimal control theory a set of rules is derived which determine which of a finite repertoire of behaviours the spacecraft will perform. A simple model of a single imaging spacecraft in low polar Earth orbit is used to demonstrate the algorithm.

Recent Gravity Models as a Result of the Lunar Prospector Mission

The lunar gravity field is determined from the tracking data of previous missions to the Moon with the 1998–1999 Lunar Prospector (LP) mission being the major contributor. LP provided the first measurement of the gravity field in a low polar circular orbit giving complete coverage at high resolution for the entire lunar nearside. However, since there is no direct measurement of the lunar farside from LP or any other mission, gravity details for the farside gravity are greatly limited. Even so, it has become apparent that there are mascons on the farside of the Moon together with the newly identified mascons in the LP data for the lunar nearside. The extended mission low-altitude data (at times less than 10 km above the surface) has gravity information for the nearside to nearly degree and order 180. The 100th-degree lunar gravity models (LP100J and LP100K) extract most of the information from the nominal 100-km altitude. A 165th degree model LP165P attempts to model the extended mission data with some but limited success. This model provides a smooth solution without aliasing when evaluated up to degree 110 allowing for resolution of numerous craters. In addition, a preliminary solution for the lunar Love number is k 2=0.026±0.003.

Multichip Modules — Communications Markets and Materials Opportunities for Space

With the telecommunications market set to grow by some 60% between 1994 and 2001, major opportunities exist for the exploitation of satellite communications. As with other technologies, the challenge is to realise the equipment compactly and cost-effectively. Compact electronics is being achieved with multichip modules (MCMs). Cost-effectiveness in terrestrial fields is being achieved with the use of high reliability plastic encapsulated microelectronics. Evidence shows that plastic encapsulations operated in the tropics are significantly more reliable than hermetic encapsulated devices. Furthermore, the radiation protection afforded by some silicones makes them very attractive even for space applications. The polymers used for multilayer interconnections of MCMs, when exposed to high doses of radiation, also successfully survive with their critical properties unimpaired. Therefore, plastics and polymers are appropriate technologies for use in space applications. THE CHALLENGE AND THE OPPORTUNITY The use of electronics technologies in space poses interesting but manageable challenges and significant opportunities for the use of multichip modules (MCMs) and low-cost packaging technologies, whose properties offer significant advantages in space and locations involving exposure to radiation. Communication via satellites located in space requires performance at high frequencies in the microwave range (delivered by silicon or GaAs MMICs — monolithic microwave integrated circuits — and controlled by MCMs), hardware technologies which are compact and lightweight and will withstand the acceleration forces during launch and manoeuvre into orbit, semiconductor devices and packaging and interconnection technologies which will withstand prolonged exposure to radiation, and an extremely high reliability because of the high telecommunications traffic carried and the enormous cost of repair if a failure should occur. The performance requirements for the communications systems range from low gigahertz proposed for terrestrial mobile communications using low polar orbit satellites to many gigahertz for many channels of international trunk traffic using geostationary satellites. The reliability requirement is to achieve the equivalent of zero failures in 25 years. This would be described statistically as a particularly low probability of failure bounded by confidence limits (governed by the component count and system Fault Tree Analysis), but the practical approach today is to achieve the correct build of components and system, establishing confidence at each level. THE GLOBAL COMMUNICATIONS MARKET Communications is a rapidly expanding market and the total equipment market will grow from $55 billion in 1993 to $86 billion by 2000, subdivided as follows:

Earth radiation balance and climate: Why the moon is the wrong place to observe the Earth

Increasing “greenhouse” gases in the Earth's atmosphere will perturb the Earth's radiation balance, forcing climate change over coming decades. Climate sensitivity depends critically on cloud-radiation feedback: its evaluation requires continual observation of changing patterns of Earth radiation balance and cloud cover. The Moon is the wrong place for such observations, with many disadvantages compared to an observation system combining platforms in low polar, intermediate-inclination and geostationary orbits. From the Moon, active observations are infeasible; thermal infrared observations require very large instruments to reach spatial resolutions obtained at much lower cost from geostationary or lower orbits. The Earth's polar zones are never well observed from the Moon; other zones are invisible more than half the time. The monthly illumination cycleleads to further bias in radiation budget determinations. The Earth will be a pretty sight from the Earth-side of the Moon, but serious Earth observations will be made elsewhere.

SAMPEX measurements of heavy ions trapped in the magnetosphere

New observations of >15-MeV/nuc trapped heavy ions with Z>or=2 have been made by the SAMPEX (Solar, Anomalous, and Magnetospheric Particle Explorer) spacecraft in low polar orbit. The composition of these ions, which are located primarily around L=2, is dominated by He, N, O, and Ne. The N, O, and Ne ions are apparently trapped anomalous cosmic rays. Currently, the origin of the trapped He flux is uncertain. These ions can affect the rate of single-event upsets (SEUs) in spacecraft hardware. >

General Upper Bound On Single-event Upset Rate

A technique of predicting an upper bound on the rate at which single-event upsets due to ionizing radiation occur in semiconducting memory cells is described. The upper bound on the upset rate, which depends on the high-energy particle environment in earth orbit and accelerator cross section data, is given by the product of an upper bound linear energy transfer spectrum I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">∞</sub> and the mean cross section of the memory cell. Plots of the spectrum I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">∞</sub> are given for low inclination and polar orbits. An alternative expression for the "exact" upset rate is also presented. Both methods rely only on experimentally obtained cross section data and are valid for sensitive bit regions having arbitrary shape.

A baseband processor for SARSAT signals

Search and rescue satellite-aided tracking (SARSAT) is a method of employing satellites in low polar orbits to relay the emergency signals of distressed aircraft and marine vessels to an earth station. At the earth station, the signals are processed using spectral estimation techniques, which permit the calculation of the location of the distressed vehicle. In this paper, we describe a baseband implementation for processing distress signals, which employs both the periodogram and the maximum entropy method (MEM). It is shown, theoretically and by computer simulation, that the minimum detectable carrier-to-noise density ratio for a coherent ELT signal is approximately 21 dB-Hz when the 512 complex-point FFT is applied to 50 blocks comprising ls of data. By applying rate-reduction filtering to the data sequence, improved resolution and an increase in carrier-to-noise density ratio are obtained. The baseband implementation also offers flexibility not found in the bandpass processor.

Basic Concepts in the Processing of SARSAT Signals

Search and rescue satellite-aided tracking (SARSAT) involves the use of satellites in low-polar orbits which relay the emergency signals of distressed vehicles to an earth station for signal analysis. In this paper, we present some basic concepts and a theoretical analysis of the spectra produced by coherent and noncoherent emergency locator transmitter signals. It is shown that coherent signals can be easily processed using linear spectral analysis. Noncoherent signals, however, require more advanced methods.

Atmospheric dynamics on Venus and Mars

New results from Pioneer Orbiter observations indicate a continued vortex organization of the cloud level atmosphere in either hemisphere, centered over respective poles. Significant changes in the magnitude of the cloud level zonal circulation over a period of several years have been detected. A strong signature of the solar tidal circulation has been detected in the atmospheric circulation with the lowest speeds occurring in equatorial latitudes about 20° upstream of the sub-solar point. Finally, a solar-locked persistent spatial structure has been discovered in the variance of the ultraviolet brightness measured from brightness normalized images of Venus. Vega balloons (drifting at about 53 km altitude near 7°N and 7°S latitudes) have also provided some unique observations of atmospheric circulation, significant among them being the strong vertical motions, the zonality of their drift speeds as well as a significant temperature difference between the two balloons. The temperature difference which amounts to 6.5°K on average is currently being interpreted as a temperature variation with longitude or time. Diagnostic modelling efforts towards simulating the atmospheric circulation on Venus are continuing and have provided some clues about the processes that maintain them but have not yet been successful in explaining the superrotation of the atmosphere. Knowledge of the Martian atmospheric dynamics on the other hand is still limited by lack of adequate observations. Numerical modelling of the Martian atmosphere continues to provide most of the information about the atmospheric circulation. The situation regarding the paucity of observations should improve with the completion of the proposed Mars Observer mission. The low circular polar orbit planned provides an excellent opportunity to study the Martian atmosphere.

On-Orbit Observations of Single Event Upset in Harris HM-6508 1K RAMS

The SEU and latchup rate of a satellite subsystem containing 384 Harris HM-6508 RAMs was observed over a 2 year period. The satellite was in a low polar orbit. A total of 47 single SEUs, 9 double, 1 triple and 1 quadruple events were observed; the multiple events were 19% of the total. No latchup was observed. The error rate was 7.6 × 10-2 SEUs/chip-year and an upper limit was found of the ratio of latchups/SEUs of 1.7 × 10-2. The observed SEU rate was consistent with predictions based upon laboratory test, using the Lawrence Berkeley Laboratory 88" cyclotron.

Two Methods of Spectral Estimation for SARSAT Signals

Search and rescue satellite aided tracking (SARSAT) depends on the processing of emergency locator transmitter (ELT) signals which are received by a satellite in low polar orbit. Since the signal from a distressed vehicle is normally immersed in a background of other ELT signals (false alarms), interference, and noise, different methods of spectral estimation can provide advantages in estimating carrier frequency. A comparison between average spectrum and minimum spectrum for several real signals is provided here.

High-latitude ionization spikes observed by the POGO Ion, Chamber Experiment

The POGO spacecraft was launched into a low polar orbit October 14, 1965, and carried an ion chamber experiment sensitive to electrons with energies greater than 1 Mev and protons greater than 10 Mev. Ionization ‘spikes’ appearing frequently at high latitudes were studied in detail. These spikes were found to be present near the high-latitude boundary of stably trapped radiation on at least 30% of the satellite passes through this region. Their frequency of occurrence increases markedly with increased magnetic disturbance. Occurrence of those spikes lying just inside the observed boundary of trapping appears to be evenly distributed with respect to local time, but the occurrence of spikes at latitudes outside the region of stable trapping is predominantly in the dawn hemisphere. We argue that these spikes are the result of injection of high-energy electrons (E ∼ 1 Mev) onto field lines near the boundary of trapping and are associated with the electron ‘islands’ reported in the magnetospheric tail by Anderson and others. We show that the spatial distribution of spikes as observed by POGO should be expected as a consequence of electron drift, in the post-midnight quadrant, into a region of pseudotrapping.