Specification and Forecasting of Outages on Satellite Communication and Navigation Systems

Transcription

1 Specification and Forecasting of Outages on Satellite Communication and Navigation Systems S. Basu and K. M. Groves Space Vehicles Directorate, Air Force Research Laboratory, 29 Randolph Road, Hanscom AFB, MA The ionized upper atmosphere often develops electron density irregularities which cause amplitude and phase scintillations of satellite signals. Scintillations, when intense, can cause outages in satellite communication systems and in Global Positioning System (GPS) for navigation. The performance of these systems can be improved by providing global specification and forecast of scintillation. A scintillation specification system, Scintillation Network Decision Aid (SCINDA), has been developed for the South American sector. An equatorial satellite, Communication Navigation Outage Forecasting System (C/NOFS), equipped with suitable sensors, has been planned for the specification and forecast of equatorial scintillation. INTRODUCTION The earth's ionized upper atmosphere often becomes turbulent and develops irregularities of electron density. These irregularities scatter radio waves from satellites in the frequency range of 100 MHz - 4 GHz (Basu et al., 1988; Aarons, 1993; Aarons and Basu, 1994). In the presence of a relative motion between the satellite, the ionosphere and the receiver, the received signal exhibits temporal fluctuations of intensity and phase, called scintillations. Intensity scintillations cause signals to fade below the average level. When the depth of fading exceeds the fade margin of a receiver, the signal becomes buried in noise and signal loss and cycle slips are encountered. Phase scintillations induce frequency shift and, when this shift exceeds the phase lock loop bandwidth, the signal is lost and the receiver spends valuable time to reacquire the signal. Overall, in the presence of scintillations, the performance of communication and navigation systems is degraded. When loss of signal occurs in a satellite based communication or Space Weather Geophysical Monograph 125 This paper is not subject to U.S. copyright Published 2001 by the American Geophysical Union navigation system due to scintillation, the problem is often attributed to an interference or to a failure of equipment at the transmitting or the receiving end, or to the satellite itself. If forecasts of scintillation can be provided, the users will not waste their resources and may instead evolve alternate strategies. Scintillations are strong at high latitudes, weak at middle latitudes and intense in the equatorial region (Basu et al., 1988). Scintillation at all latitudes attains its maximum value during the solar maximum period when the F-region ionization density increases and the irregularities occur in a background of enhanced ionization density. As such, scintillation effects, expected during the upcoming solar maximum period between , are of concern to systems engineers. Scintillation during the solar minimum period is much reduced in magnitude mainly because of decreased background ionization density. The scintillation climatology is well-known but the space-time variability of scintillation and its trigger mechanisms remain unresolved (Basu etal, 1996). An empirical scintillation model, WBMOD, is available (Secan et al, 1995, 1997). The name of the model was derived from the Wideband satellite which provided the multi-frequency scintillation data input to the model. Later, the model was upgraded in the equatorial region with extensive geostationary satellite scintillation database of the 423

2 424 SCINTILLATION EFFECTS ON SYSTEMS "WORST CASE" FADING DEPTHS AT L-BAND SOLAR MAXIMUM SOLAR MINIMUM Figure 1. Schematic of the global morphology of scintillations at L-band frequencies during the solar maximum (left panel) and solar minimum (right panel) conditions. Air Force Research Laboratory at Hanscom AFB, MA. The high latitude upgrade of the model was achieved by using the HiLat and Polar Bear satellite scintillation data. Since scintillation exhibits extreme variability in space and time, such climatological models are useful only for planning purposes. For the support of space based communication and navigation systems, we require weather models, i.e., real-time specification and forecast of scintillation. Such weather models provide, at least, the situational awareness to the user who, under certain circumstances, can switch to other satellites for a better satellite link performance. This paper provides an overview of scintillation at high and low latitudes. The issues related to the specification and forecast of polar scintillation are discussed. A scintillation specification system, developed for the South American sector, is outlined (Groves et al., 1997). It also describes an equatorial satellite, called the Communication/Navigation Outage Forecasting System, C/NOFS, which has been planned for specifying and forecasting scintillation in the equatorial region. RESULTS AND DISCUSSIONS Figure 1 shows an updated schematic of the global distribution of worst case scintillation at L-band frequencies during the solar maximum and the minimum period, originally published by Basu et al., The left hand panel represents the solar maximum condition when scintillation attains its maximum value. The magnetic north and south poles are at the top and the bottom and the magnetic equator is in the middle. The noon meridian is on the left, midnight is on the right and 18 LT is in the middle. It may be noted that L-band scintillation is most intense in the equatorial region, moderate at high latitudes and generally absent at middle latitudes. At high latitudes, scintillations are found to be associated with large scale plasma structures. It has been established that for the interplanetary magnetic field component B z southward, large scale plasma structures, patches and blobs, are observed. The polar cap patches are convected through the dayside auroral oval into the polar cap and then exit into the nightside auroral oval to form auroral blobs (Weber et al., 1984; Tsunoda, 1988; Carlson, 1994; Rodger et al., 1994; Basu and Valladares, 1999; Pedersen et al., 2000). The association of polar cap patches with intermediate scale irregularities (tens of km to tens of meters), responsible for intense scintillations, has been examined in the framework of observations and modeling (Basu et al., 1995; Basu and Valladares, 1999). When the B z component of the interplanetary magnetic field (IMF) is northward, ordered reversals of horizontal plasma flow occur in the polar cap. A soft particle precipitation along the flow reversal forms long sun-aligned polar cap arcs (Kelley, pg. 364, 1989). The arcs become associated with small-scale

3 to a> o ASCENSION ISLAND Fade Depth I > 6 db > 10 db wm > 20 db LT Kp=0-3 BASU AND GROVES GHz Jul 1986 Jan 1987 Jan 1988 Jan 1989 Figure 2. Variation of the occurrence of 1.5 GHz scintillation with the sunspot number observed at Ascension Island near the crest of the equatorial anomaly during the pre-midnight period. The top panel shows the occurrence statistics for magnetically quiet (Kp = 0-3) conditions, the middle panel for disturbed (Kp = ) conditions and the bottom panel shows the variation of the sunspot number. irregularities of electron density due to plasma instabilities in the velocity shear region (Keskinen et al, 1988). Scintillations associated with polar cap patches are most intense owing to the high ionization density of patches and sunaligned arcs, associated with low ionization density, cause only weak scintillations. Polar scintillation is most conspicuous in winter when the solar ionizing radiation is not available to smooth out the irregularities. In the equatorial region, at the time of sunset, the magnetic field line integrated ionospheric conductivity changes rather abruptly across the sunset line or the terminator. The zonal neutral wind and the terminator induced conductivity gradient interact and drive the ionosphere unstable (Kelley, pg. 121, 1989). Large scale plasma bubbles are formed in the bottomside of the ionosphere which rise to great heights and become populated by electron density irregularities (Bernhardt et al, 2000). The tens of meter to kilometer scale irregularities, distributed over a wide range of altitudes, cause intense L-band scintillation. Being associated with relatively smaller scale irregularities, L-band scintillation decays shortly after midnight whereas 250 MHz scintillation may persist till sunrise. Figure 2 shows the occurrence statistics of 1.5 GHz scintillation at Ascension Island during premidnight hours (20-24 LT). This station is located under the crest of the equatorial anomaly in F-region ionization, also known as the Appleton anomaly, where the ionospheric turbulence is encountered in an environment of high background ionization density. The top two panels correspond to magnetically quiet and active periods respectively and the bottom panel shows the variation of the sunspot number. There is no data during June-August, 1988, but past data indicate that during these months scintillation occurrence is minimum in the American-Atlantic longitude sector. During the solar maximum period, L-band scintillations can indeed be intense causing signals to fade below 20 db. It is interesting to note that scintillations in the equatorial region are much enhanced during magnetically quiet period (top panel) as compared to the magnetically disturbed period (middle panel). This may be due to the fact that quiet-time scintillations follow a definite occurrence pattern, starting after sunset and decaying after midnight, whereas the disturbed period scintillation is distributed in time being dictated by the onset time of magnetic storms. The generation

4 426 SCINTILLATION EFFECTS ON SYSTEMS CHILE: 1 OCTOBER 1994 Scintillation of GPS Signals 6300A All Sky Images 00 D Plasma Depletions 01:00 01:05 01:10 Figure 3. The left panels show the nm all sky images of plasma depletions at 0100 UT and 0115 UT obtained at Chile near the crest of the equatorial anomaly on 1 October The numbers 21 and 22 represent 250 km intersections of raypaths of two specific GPS satellites, while the black squares denote additional GPS satellite intersection points. The right hand panels show the scintillation of GPS satellites (PRN 21 and 22) during UT, the tic marks in the ordinate represent 1 db intervals. mechanisms of equatorial scintillation have not been identified but it seems that it is internally driven and cannot be predicted by following the trail of energy from the sun. Figure 3 shows the results of simultaneous GPS scintillation and nm all sky imager observations near the equatorial anomaly region in Chile (Weber et al, 1996). The left hand panels illustrate two nm images at 15 minute intervals. The north-south elongated dark bands represent the footprints of plasma bubbles at 250 km altitude. The positions marked 21 and 22 indicate the intersections of the ray paths from the ground-based GPS receiver to GPS satellites with PRN numbers 21 and 22. Each GPS satellite is identified by the specific pseudorandom noise (PRN) code it transmits. The right hand panels indicate that

5 BASU AND GROVES 427 both satellites scintillate when both raypaths intersect the dark band (see top left image). A few minutes later, at time t=t s, owing to the eastward motion of plasma bubbles as well as the satellite motion, the satellite with PRN 22 emerges from the scintillating region. The other satellite continues to encounter scintillations as its raypath remains within the dark band. It should be pointed out that only weak scintillations of GPS signals, corresponding to a signal excursion of only 5 db, were observed during this solar minimum period. During the solar maximum period, scintillation of GPS signals at this location exceeds 20 db. It should also be mentioned that measurements with GPS satellites offer a unique opportunity to perform multi-point scintillation observations from one station. Figure. 4 illustrates four azimuth-elevation plots of the trajectories of GPS satellites at one hour intervals, as viewed from Antofagasta, Chile, on 26 October The concentric circles indicate elevation angles at 30 intervals, the outermost circle indicating an elevation angle of 0 and the centre of the circles corresponds to the zenith. The azimuth is reckoned with reference to the four cardinal directions, namely, north (N), east (E), south (S) and west (W) as indicated in the diagram. The trajectories of GPS satellites identified by their PRN numbers are shown. The varying diameters of circles along the satellite trajectories indicate the variation of the intensity scintillation index, S 4 which is defined as the ratio of the standard deviation of signal intensity and the average signal intensity. It may be noted that S 4 indices as high as 0.9, corresponding to signal fluctuations exceeding 20 db, were measured in 1999 with the solar maximum period approaching. Scintillation indices are more than a factor five higher than that in Figure 3. Further, at this location, which is equatorward of the anomaly crest, the east-west widths of depletions also increase as the solar maximum is approached. Thus a larger number of GPS satellites will simultaneously suffer strong scintillations in the equatorial region during the solar maximum period. In the Introduction, it has been mentioned that a global climatological model of scintillation, WBMOD, is available. The input to this empirical model corresponds to the sunspot number, magnetic index, day number, universal time, the locations of the satellite and the receiver and the signal frequency. The model output specifies the magnitudes of phase and intensity scintillation and their temporal structures as defined by their spectra. The model is climatological in nature and is useful for planning purposes. It is, however, of limited value to many users in view of the extreme day-to-day variability of scintillation, particularly in the equatorial region (Basu et al, 1996). Since the cause of this variability is not yet resolved, a physics based prediction model is not possible. This has led to the development of a real-time local area scintillation specification system based on two station measurements in the South American sector known as SCINDA, Scintillation Network Decision Aid (Groves et al., 1997). The magnitude of scintillation and the zonal irregularity drift is measured by using spaced receiver scintillation measurements. Such measurements are made at 250 MHz with two geostationary satellites in the east and the west of two stations, one near the magnetic equator (Ancon, Peru) and the other at about 10 magnetic latitude (Antofagasta, Chile) at about the same longitude. The data is brought to the user by the internet and the data drives a scintillation model which considers the upwelling and the zonal motion of irregularities as well as their mapping along the magnetic field to produce three dimensional scintillation structures. Plate 1 shows such structures mapped by the SCINDA system, and the colors, green, yellow and red indicate the increasing levels of scintillation. The SCINDA system is currently incorporating multipoint GPS L-band scintillation measurements at the two stations. It also utilizes the DMSP (Defense Meterological Satellite Program) satellite in-situ measurements of the latitude variation of electron density around the magnetic equator near the longitude of the SCINDA stations. By using the pre-sunset DMSP measurements of the nature of latitude variation of electron density around the magnetic equator, which is dictated by the zonal electric field, it has been possible to predict nighttime scintillation two hours in advance of its onset (Basu et al., 2000). In the near future, a space-based system, capable of specifying and forecasting equatorial scintillation, is expected. Figure 5 shows the conceptual schematic of this satellite, called the Communication/Navigation Outage Forecasting System (C/NOFS) (Bernhardt et al., 2000). The satellite will be launched in an elliptical orbit 400 km x 700 km with an orbital inclination of 13. The satellite insitu sensors will include a Langmuir probe, digital ion drift meter, vector electric field and neutral wind sensors. It will also incorporate a tri-frequency beacon (150, 400, 1067 MHz) and a GPS occultation receiver. These sensors are expected to forecast the occurrence of scintillation by probing the destabilizing and stabilizing forces in the ionosphere, namely the pre-reversal enhancement of zonal electric field from the electric field, and zonal/meridional neutral wind measurements. Real-time data driven electron density models will be derived and validated against electron density profiles obtained by the GPS occultation sensor. Radio wave scattering theory will be combined with electron density irregularity data and electron density profiles to specify amplitude and phase scintillation and their

6 428 SCINTILLATION EFFECTS ON SYSTEMS Antofagasta - 27 October to I UT 2 to 3 UT S o o o O * >1.2 Figure 4. The trajectories of GPS satellites, as viewed from Antofagasta, Chile, are shown in the four sets altitudeazimuth plots at hourly intervals. The four concentric circles indicate elevation angles at 30 intervals, the outermost circle corresponding to 0^ elevation angle and the centre of the circles indicating the zenith. The circles around the GPS satellite trajectories indicate the S4 index of scintillation according to the legend at the bottom of the left panel. temporal structures. Scintillation specifications will be validated by the measurements of scintillation at three beacon frequencies. The space-based C/NOFS and the ground based SCINDA will be able to specify, forecast and validate scintillation products for the users. SUMMARY During the solar maximum period, ionospheric scintillation can affect many space based communication and navigation systems in the VHF/UHF range. The global climatology of scintillation in the VHF/UHF range of frequencies is fairly well established and it can be used in the planning of new communication links. However, in view of the extreme variability of scintillation in space and time, the climate models cannot support operational systems that require real-time scintillation specification. At high latitudes, during solar maximum conditions, scintillation caused by polar cap patches is of concern to many systems. The large scale polar cap patches have been modeled in the framework of convection appropriate for southward IMF. For the development of high latitude scintillation specification and forecast system, we need to determine the trajectories of patches for varying IMF and the saturation amplitude of irregularities need to be established from the standpoint of plasma instabilities. In the equatorial region, scintillation specification system is a reality. Systems, such as SCINDA, could be developed primarily because the large scale irregularity motion is ordered and the irregularities causing scintillations can be mapped between the northern and southern crests of the equatorial anomaly. Although theoretical studies, intensive observations and refined modeling have continued for many decades, the forecasting of equatorial scintillation remains a challenging task. It is hoped that satellites, such as C/NOFS, will be able to track simultaneously the stabilizing and the destabilizing forces and thereby advance our capability to forecast equatorial scintillation.

7 BASU AND GROVES 429 Scintillation Network Decision Aid (SCINDA) Plate 1. Illustrates the three-dimensional irregularity structures mapped by the Scintillation Network Decision Aid, SCINDA, in South America, which is based on an irregularity model and 250 MHz scintillation measurements made at Ancon, Peru, and Antofagasta, Chile.

8 430 SCINTILLATION EFFECTS ON SYSTEMS Velocity RPA Driftmeter Neutral Wind C/NOFS Langmuir Probe Beacon i Nadir GPS Receiver Photometer Figure 5. Schematic of the Communication/Navigation Outage Forecasting System (C/NOFS), a proposed equatorial satellite with its suite of sensors. Acknowledgments. The work at Air Force Research Laboratory was partially supported by the Air Force Office of Scientific Research task 2310G9. REFERENCES Aarons, J., The longitudinal morphology of equatorial F-layer irregularities relevant to their occurrence, Space Sci. Rev., 63, 209, Aarons, J., and S. Basu, Ionospheric amplitude and phase fluctuations at the GPS frequencies. In: Proceedings of ION GPS- 94, Arlington, VA, pp. 1569, Basu, S., E. MacKenzie, and Su. Basu, Ionospheric constraints on VHF/UHF communication links during solar maximum and minimum periods, Radio Sci., 23, 363, Basu, S., Su. Basu, J.J. Sojka, R.W. Schunk, and E. MacKenzie, Macroscale modeling and mesoscale observations of plasma density structures in the polar cap, Geophys. Res. Lett., 22, 881, Basu, S, E. Kudeki, Su. Basu, C.E. Valladares, E.J. Weber, H.P. Zengingonul, S. Bhattacharyya, R. Sheehan, J.W, Meriwether, M.A. Biondi, H. Kuenzler, and J. Espinoza, Scintillations, plasma drifts, and neutral winds in the equatorial ionosphere after sunset, J. Geophys. Res., 101, 26795, Basu, Su., and C.E. Valladares, Global aspects of plasma structures, J. Atmos. Solar Terr. Phys., 61, 127, Basu, S., K.M. Groves, Su. Basu, and P.J. Sultan, Specification and forecasting of scintillations in communication and navigation systems: current status and future plans, J. Atmos. Solar Terr. Phys., 2000 (in press). Bernhardt, P.A, J.D. Huba, CA. Selcher, K.F. Dymond, G.R. Carruthers, G. Bust, C. Rocken, and T.L. Beach, New systems for space-based monitoring of ionospheric irregularities and radio wave scintillation, this issue, Carlson, H.C, Jr., The dark polar ionosphere: Progress and future challenges, Radio Sci., 29, 157, Groves, K.M., S. Basu, E.J. Weber, M. Smitham, H. Kuenzler, C.E. Valladares, R. Sheehan, E. MacKenzie, J.A. Secan, P. Ning, WJ. McNeill, D.W. Moonan, and M.J. Kendra, Equatorial scintillation and systems support, Radio Sci., 32, 2407, Kelley, M.C, The Earth's Ionosphere, Academic Press, San Diego, CA, Keskinen, M.J, H.G. Mitchell, J.A. Fedder, P. Satyanarayana, S.T. Zalesak, and J.D. Huba, Nonlinear evolution of the Kelvin-Helmholtz instability in the high latitude ionosphere, J. Geophys. Res., 93, 137, Pedersen, T.R, B.G. Fejer, R.A. Doe, and E.J. Weber, An incoherent scatter radar technique for determining two-dimensional horizontal ionization structure in polar cap F region patches, J. Geophys. Res., 105, 10,637, Rodger, A.S, M. Pinnock, J.R. Dudney, K.B. Baker, and R.A. Greenwald, A new mechanism for polar patch formation. J. Geophys. Res., 99, 6425, Secan, J.A., R.M. Bussey, E.J. Fremouw, and S. Basu, An improved model of equatorial scintillation, Radio Sci., 30, 607, Secan, J.A., R.M. Bussey, E.J. Fremouw, and S. Basu, High latitude upgrade to the Wideband ionospheric scintillation model, Radio Sci., 32, 1567, Tsunoda, R.T, High latitude F region irregularities: a review and synthesis, Rev. Geophys., 26, 719, Weber, E.J., J. Buchau, J.G. Moore, J.R. Sharber, R.C. Livingston, J.D. Winningham, and B.W. Reinisch, F-layer ionization patches in the polar cap, J. Geophys. Res., 89, 1683, Weber, E.J., S. Basu, G. Bishop, T. Bullett, H. Kuenzler, P. Ning, P. Sultan, C.E. Valladares, and J. Araye, Equatorial plasma depletion precursor signatures and onset observed at 11 south of the magnetic equator, J. Geophys. Res., 101, 26,829, S. Basu and K.M Groves, Air Force Research Laboratory, 29 Randolph Road, Hanscom AFB, MA ( santimay@aol.com; Keith.Groves@hanscom.af.mil).