Equatorial Scintillation and the SCINDA System

by Quirino Sugon Jr.

The Manila Observatory operates three SCINDA systems deployed in Manila, Baguio, and Davao.  Dr. Keith Groves, AFRL is the PI of the SCINDA project.  Fr. Daniel J. McNamara, SJ is the PI of the Philippine SCINDA project.  I found a useful description of the SCINDA system from a publicly available report of the Institute for Scientific Research at Boston College on ionosphere modelling:

Patricia H. Doherty, Leo F. McNamara, William J. Burke, William J. MNeil, Louise C. Gentil, “Ionospheric Modeling: Development, Verificaton and Validation,” Institute for Scientific Research, Boston College (2007).

The SCINDA part is pages 11-12:

5.1. Scintillation Effects on GPS

Equatorial scintillation is a phenomenon in which small-scale irregularities in the F region of the ionosphere interrupt radio communications between the Earth and orbiting satellites. The process occurs only at night and is limited to a region from the magnetic equator to about 20˚ to the North and South. The process results in large-scale scintillation bubbles which obliterate some regions of the sky to communication. The disruptions are greatest at low frequency and decrease in severity as frequency increases, but during solar moderate and maximum years, they are significant at the 1.5-GHz range at which the Global Positioning System operates. The end result, insofar as positioning with GPS is concerned, is loss of information from some or all satellites. It has been documented that under certain conditions, complete loss of GPS capability can result from scintillations. Therefore, equatorial scintillations are extremely important for systems that rely upon GPS, which, in this day and age, encompasses the better part of both civilian and military systems.

The Scintillation Network Decision Aid (SCINDA) is a nowcasting tool that provides a specification of the current scintillation conditions over a chosen theater. The system works like this. First, sensors placed at various strategic points around the globe monitor the power levels of signals from Earth-orbiting satellites. When scintillation along these links is detected, the measured scintillation intensity level is used along with empirical models of the scintillation bubble structure and evolution to generate regions of predicted communication outages which are then projected onto maps for chosen theatres. The current number of SCINDA stations at this writing is fourteen. Since the previous version of WBMod relied on only three stations in the equatorial region, it is clear that the addition of the SCINDA data constitutes a major improvement in WBMod reliability.

5.2. Scintillation and Frequency

There are two basic frequencies monitored by the SCINDA network. The first is the region around 250 MHz, which will be called the UHF here. This is monitored by listening to beacons from geostationary satellites using standard receivers. The second frequency monitored is around 1.5 GHz, which we will call the L-Band frequency. These data are obtained by NOVATEL single-frequency GPS receivers which have been specially modified to produce scintillation parameters. Both of these are used in the production of the SCINDA outage maps, although outage maps are currently produced only for the UHF. The GPS data are downshifted in frequency through an ad hoc algorithm to compliment the UHF data. There are, however, two stations that listen to fixed geostationary satellites at the L-Band. These are Ascension Island and Antofagasta. What is important, though, is that scintillation data from both systems is collected and archived continuously. It is therefore all available for incorporation into WBMod.

The climatology of UHF and L-Band scintillation differ substantially. First, LBand scintillation essentially goes away at solar minimum while UHF scintillation stays strong. Second, the UHF scintillation extends pretty much uniformly over the magnetic equator while the L-Band scintillation peaks at the Appleton anomaly crests at about 12˚ to 15˚ geomagnetic. This behavior is shown in Figure 4 for the Atlantic sector. Also of note is that the seasonal behavior of UHF and L-band scintillation are somewhat different. UHF scintillation persists pretty much unabated through the winter months while L-Band scintillation is much more peaked around the equinoxes.

Read more on the SCINDA and the Wide Band Scintillation Model.


About ateneophysicsnews
Physics News and Features from Ateneo de Manila University

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