E A Essex, P A Webb, I Horvath, C. McKinnon, N. Shilo and B. Tate
Cooperative Research Centre for Satellite Systems
Department of Physics, La Trobe University, Bundoora, Victoria 3083, Australia
Ph: +613 9479 1485
Recently, with the escalating cost of large satellite missions, attention has turned to smaller satellites. Their advantages of low overall cost in construction and launch, and short time span between conception and launch has given a new impetus to the further study of the geosphere. By using a combination of space-based and ground-based receivers, it is possible to undertake new and exciting experiments directed towards furthering our knowledge of the ionosphere. Combinations of high earth orbit satellites, such as the Global Positioning System (GPS), and low earth orbit (LEO) microsatellites are providing the capability for satellite to satellite occultation experiments to reconstruct the vertical profile of the ionosphere. The topside ionospheric and plasmaspheric ionisation content may also be explored with satellite to satellite experiments. This paper describes some of the experiments proposed for future microsatellites, such as those planned for the Australian satellite FedSat.
The ionised part of the atmosphere, the ionosphere, causes distorting effects or errors in satellite communications, navigation and altimetry. In order to understand these effects and improve our knowledge of the ionosphere, the various regions of the ionosphere have been monitored to different degrees on a long-term basis. Even instruments such as incoherent scatter radars, while providing profiles of both the topside and bottom side ionosphere, do not operate on a continuous basis and provide poor resolution for the lower ionosphere. The ground-based ionosondes provide non-homogeneous coverage of the bottom side ionosphere, as they are limited to observations from land. This is particularly true in the Southern Hemisphere where the oceans cover most of the Hemisphere. The topside ionosphere and the region above, the plasmasphere are not easily monitored on a long-term basis. Only a few instruments such as incoherent scatter radars, topside sounders and in situ satellite-based diagnostics are able to provide details on the structure, composition and dynamics of the topside ionosphere. The properties of the plasmasphere are even less well known, and techniques to explore it are very limited. The advent of the Global Positioning System (GPS) with 24 satellites in 12 hour orbits at 20,000 kilometres, provides the opportunity to monitor on a global basis the variation of the ionisation content of the ionosphere and plasmasphere. For the GPS navigation and timing systems, the ionosphere produces the largest source of errors. These errors can be measured and utilised to improve models of the global distribution of ionisation, and hence be used to improve error corrections in future satellite applications. The GPS receiver network on the ground, together with GPS receivers in orbit, provide the unique opportunity to perform this task. One of the payloads on the Australian scientific microsatellite, FedSat, planned for launch in November 2000, is a GPS receiver. The launch coincides with the expected peak of the sunspot cycle, and hence at this time the disruptions to satellite reception and error corrections due to space weather are predicted to be at maximum levels. The recent deployment of space-based GPS receivers has demonstrated the feasibility of using satellite to satellite experiments for monitoring the ionosphere. In this paper we discuss some of the satellite to satellite experiments proposed for the Australian satellite FedSat.
FedSat, the first Australian satellite to be launched in over 30 years, will be a microsatellite of around 58 kg launch mass. It is planned to launch into a sun synchronous polar orbit at a height of 800 km. FedSat's mission is basically a science and engineering one. The planned scientific payloads include a fluxgate magnetometer for monitoring the Earth's magnetic field and solar -terrestrial interactions as well as a dual frequency GPS receiver for ionospheric and atmospheric research. The aims of the program are to conduct basic research on the structure and dynamics of the ionosphere, plasmasphere and magnetosphere. The results will be applied to the forecasting of space weather. The space- qualified dual-frequency GPS receiver and patch antenna, supplied by NASA, are being built by Spectum Astro of Arizonia, USA. Using the two coherently connected frequencies f1 = 1575.42 MHz and f2 = 1227.60 MHz, high precision measurements will be made of the group and phase delay between the receiver on board of FedSat and the transmitters on the GPS satellites visible to FedSat. Two types of measurements are planned. In the occultation mode (Hajj et al 1994), the ray path from the GPS satellites will pass horizontally through the ionosphere and hence provide scans through the ionosphere as FedSat rises and sets with respect to a GPS satellite (Hajj and Romans, 1998). The second type of measurement is for the overhead mode where the ray paths from the GPS satellites in a vertical cone above FedSat pass through the plasmasphere. Figure 1 illustrates the two modes. As the FedSat orbit is planned to be at 800 km, most of the ionisation measured by this technique is located in the plasmasphere and topside ionosphere. The following sections detail some of the proposed experiments to be carried out using the on board GPS receiver.
The application of limb sounding in planetary occultation experiments has been used repeatedly by NASA to study the atmospheres of planets in our solar system (Tyler, 1987). The application of the technique to studies of the Earth's atmosphere had to wait until a suitable radio source, namely GPS, became available. Observing the GPS constellation of satellites in space from a low earth orbit (LEO) satellite such as FedSat, orbiting at 800km provides a powerful means of imaging the ionosphere and the inner magnetosphere, the plasmasphere. The provision of occulting geometry from a space based receiver enables effectively a horizontal scan through the ionosphere to be obtained (Hajj et al 1994). The lack of horizontal scans from ground based observations of GPS satellites has limited the current development of computerised ionospheric tomography. Because of the geometry, there is a lack of horizontal total electron content (TEC) information that causes a lack of information along the vertical direction in the reconstructed tomographic image. To overcome this problem most researchers use some form of a priori information (Villani and Essex, 1996). A review of ionospheric tomography algorithms may be found in Raymund (1994). In the planned experiment for FedSat, this problem will be overcome by using a combination of ground based as well as space based GPS occultation data which will provide both vertical and horizontal information for the application of the tomographic reconstruction, (Horvath and Essex, 1998). Although in one day there will be many hundreds of occultations of FedSat with GPS satellites, only those close to the track of the FedSat satellite will be used. Off track measurements are difficult to interpret as they move through a large range of latitude and longitude. As FedSat will be in a sun synchronous polar orbit, it will be possible to study not only the low and mid latitude ionosphere, but also the higher latitude ionosphere especially over the vast expanses of the oceans in our region. The principal advantages of the tomographic reconstruction techniques lie in the large geographic coverage and the cost effectiveness. Incoherent scatter radars are only able to provide the information from a smaller geographical area at the cost of millions of dollars to build and operate. Recent interest in the development of space-based tomography has seen the launch of the first GPSMET LEO, in the proof of concept for Earth radio occultation experiments. Various researchers have recently performed ionospheric reconstruction from the GPS occultation data (Ruis et al, 1997; Hajj and Romans, 1998).
A further area of interest is the electron content of, as well as the irregularities in, the plasmasphere between FedSat and the GPS satellites orbiting above FedSat. With a GPS antenna located on the top of the FedSat satellite, it will be possible to undertake TEC measurements vertically above FedSat to the GPS satellites for coincidence occurrences in the orbits. For antenna reception within a cone of 5 degrees above FedSat these coincidences would enable information to be obtained on the topside ionosphere and the plasmasphere. Figure 2 shows model calculations of the TEC content for three cases: below 400km, between 400km and 800km and above 800 km. The model plasmasphere used in these calculations is based on diffuse equilibrium, and in combination with the International Reference Ionosphere (IRI), forms a global ionospheric-plasmasphere model (Webb and Essex, 1997). At low latitudes at sunspot maximum, there is clearly significant ionisation above 400km. For the high latitudes in the Australian region of the Southern Hemisphere, where the magnetic pole is offset from the geographic pole toward Australia, the plasmapause is located at lower geographic latitudes. Hence for coincidences at higher latitudes, it will be possible to estimate the location of the plasmapause. This is because, at these locations, the ray paths from the GPS satellites to the FedSat satellite will not intersect the plasmasphere, and there would normally be only a negligible amount of ionisation above 800km, so that the TEC should approach zero (Klobuchar et al, 1994). Recent analysis of ground-based GPS TEC data has shown the existence of a large enhancement of ionisation equatorward of the mid-latitude trough in the plasmapause region. This increase in ionisation may be related to heating of the topside ionosphere by turbulent dissipation of ring current energy at the plasmapause (Horvath and Essex, 1999; Titheridge, 1976). FedSat may also be used to investigate this ionisation buildup, especially over the Southern Hemisphere where the ground-based measurement are sparse.
Irregularities in the plasma between FedSat and the GPS satellites should also be detectable as scintillation in the phase path data. These irregularities may be located in either the ionosphere or the plasmasphere. Jacobson et al (1996) has used sensitive ground- based techniques to investigate field-aligned irregularities in the plasmasphere. Scintillation activity in the nighttime equatorial anomaly regions of the low latitude ionosphere, and in the high latitude ionosphere, is known to peak at sunspot maximum, the next maximum being around 2000. Figure 3 is an example of GPS amplitude and phase scintillation indices S4 and s f recorded at the southern high latitude ground station of Casey on 23 September, 1998. Hence the incidence of scintillation in the L band signals from GPS measured in both the occultation experiments and in the vertical direction is another area for study.
There is some indication that the occurrence of well-developed equatorial anomaly crests and their location is a precursor of the development of nighttime equatorial irregularities (Jayachandran et al, 1997). The combination of both ground based and satellite-based measurements would provide an excellent opportunity to further our understanding of the low latitude ionosphere.
LEO microsatellites such as the planned Australian microsatellite FedSat provide the opportunity for researchers in Space Sciences to further investigate those regions of the ionosphere for which existing equipment and techniques are either difficult or impossible to apply. In the future, constellations of microsatellites may provide a global coverage of the ionosphere, previously not possible with ground based equipment.
This work is supported by the Australian Cooperative Research Centre for Satellite Systems and by an Australian Research Committee grant. C.McK. and N.S. are holders of Australian Postgraduate Awards and P.W. and I.H. are holders of La Trobe University Postgraduate Awards.
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