The temporal and spatial variation in ionospheric structures have often frustrated the efforts of communications and radar system operators who base their frequency management decisions on monthly mean predictions of radio propagation in the high frequency (short-wave) band. The University of Massachusetts Lowell’s Center for Atmospheric Research (UMLCAR) has produced a low power miniature version of its DigisondeTM sounders, the DigisondeTM Portable Sounder (DPS), capable of making measurements of the overhead ionosphere and providing real-time on-site processing and analysis to characterize radio signal propagation to support communications or surveillance operations.
The system compensates for a low power transmitter (300 W vs. 10 kW for previous systems) by employing intrapulse phase coding, digital pulse compression and Doppler integration. The data acquisition, control, signal processing, display, storage and automatic data analysis functions have been condensed into a single multi-tasking, multiple processor computer system, while the analog circuitry has been condensed and simplified by the use of reduced transmitter power, wide bandwidth devices, and commercially available PC expansion boards. The DPS is shown in the composite Figure 1-1 (with the integrated transceiver package shown in Figure1-1A, and one of the four crossed magnetic dipole receive antennas in Figure 1-1B).
Figure 1-17 shows the physical layout of the four receiving antennas. The various separation distances of 17.3, 34.6, 30 and 60 m are repeated in six different azimuthal planes (i.e., there is six way symmetry in this array) and therefore, the Df’s computed for one direction also apply to five other directions. This six-way symmetry is exploited by defining the six azimuthal beam directions along the six axes of symmetry of the array, making the beamforming computations very efficient. Section 3 of this manual contains detailed information for the installation of receive antenna arrays.
New technology involved in this system includes:
- Electronically switched active crossed loop receiving antenna
- Commercially sourced 10 MIPS TMS 320C25 digital signal processor (DSP)
- 4 million sample DSP buffer memory
- 71 to 110 MHz digital synthesizer on a 4"x5" card
- Compact DC-DC converters allowing operation on one battery
- Four-channel high speed (1 million 12-bit samples/sec) digitizer board
- A 160 Mbits/sec parallel data bus between the digitizer and the DSP
- A proprietary multi-tasking operating system for remote interaction via a modem connection without suspending system operation
- Direct digital synthesized coherent oscillators
- 21 dB signal processing gain from phase coded pulse compression
- 21 dB additional signal processing gain from coherent Doppler integration
- Automatic ionospheric layer identification and parameter scaling by an embedded expert system
The availability of a small low power ionosonde that could be operated on-site wherever a high frequency (HF) radio or radar was in use, would greatly increase the value of the information produced by the instrument since it would become available to the end user immediately.
One of the chief applications for the real-time data currently provided by digital ionospheric sounders is to manage the operation of HF radio channels and networks. Since many HF radios are operated at remote locations (i.e., aircraft, boats, land vehicles of all sorts, and remote sites where telephone service is unreliable) the major obstacle to making practical use of the ionospheric sounder data and associated computed propagation information is the dissemination of this data to a data processing and analysis site. Since HF is often used where no alternative communications link exists, or is held in reserve in case primary communication is lost, it is not practical to assume that a communications link exists to make centrally tabulated real-time ionospheric data available to the user. Furthermore, local measurements are superior to measurements at sites of opportunity in the user’s general region of the globe since extreme variations in ionospheric properties are possible even over short distances, especially at high latitudes [Buchau et al., 1985; Buchau and Reinisch, 1991] or near the sunset or sunrise terminator.
However, for most applications, the size, weight, power consumption and cost of a conventional ionospheric sounder have made local measurements impractical. Therefore the availability of a small, low cost sounder is a major improvement in the usefulness of ionospheric sounder data. Shrinking the conventional 1 to 50 kW pulse sounders to a portable, battery operated 100 to 500 W system requires the application of substantial signal processing gain to compensate for the 20 dB reduction in transmitter power. Furthermore, a compact portable package requires the use of highly integrated control, data acquisition, timing, data processing, display and storage hardware.
The objective of the DPS development project was to develop a small vertical incidence (i.e., monostatic) ionospheric sounder which could automatically collect and analyze ionospheric measurements at remote operating sites for the purpose of selecting optimum operating frequencies for obliquely propagated communication or radar propagation paths. Intermediate objectives assumed to be necessary to produce such a capability were the development of optimally efficient waveforms and of functionally dense signal generation, processing and ancillary circuitry. Since the need for an embedded general purpose computer was a given imperative, real-time control software was developed to incorporate as many functions as was feasible into this computer rather than having to provide additional circuitry and components to perform these functions. The DPS duplicates all of the functions of its predecessor the DigisondeTM 256 [Bibl et al., 1981] and [Reinisch, 1987] in a much smaller, low power package. These include the simultaneous measurement of seven observable parameters of reflected (or in oblique incidence, refracted) signals received from the ionosphere:
Because the physical parameters of the ionospheric plasma affect the way radio waves reflect from or pass through the ionosphere, it is possible by measuring all of these observable parameters at a number of discrete heights and discrete frequencies to map out and characterize the structure of the plasma in the ionosphere. Both the height and frequency dimensions of this measurement require hundreds of individual measurements to approximate the underlying continuous functions. The resulting measurement is called an ionogram and comprises a seven dimensional measurement of signal amplitude vs. frequency and vs. height as shown in Figure 1-2 (due to the limitations of current software only five may be displayed at a time). Figure 1-2 is a five-dimensional display, with sounding frequency as the abscissa, virtual reflection height (simple conversion of time delay to range assuming propagation at 3x108 m/sec) as the ordinate, signal amplitude as the spot (or pixel) intensity, Doppler shift as the color shade and wave polarization as the color group (the blue-green-grey scale or "cool" colors showing extraordinary polarization, the red-yellow-white scale or "hot" colors showing ordinary polarization).
- Range (or height for vertical incidence measurements)
- Doppler Shift and Spread
- Angle of Arrival
- Wave Polarization
Figure 1-2 Five-Dimensional Ionogram
Another objective of the DPS development was to store the data created by the system in an easily accessible format (e.g., DOS formatted personal computer files), while maintaining compatibility with the existing base of DigisondeTM sounder analysis software in use at the UMLCAR and at over 40 research institutes around the world. This objective often competed with the additional objective of providing an easily accessible and simply understood standard data format to facilitate the development of novel post-processing analysis and display programs.
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