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10. CANADIAN ADVANCED DIGITAL IONOSONDE (CADI)
The Canadian Advanced Digital Ionosonde (CADI) is a state of the art, low cost, full featured ionosonde ideal for both routine ionospheric monitoring and scientific research.
CADI provides sounding capability using high power radio frequency pulses at vertical incidence. The system integrates phase coding techniques, solid state electronics and PC technology to make CADI a significantly smaller and less expensive ionosonde. The system may be operated with single or multiple receivers. Observables include: echo delay (height) versus frequency; the phase and amplitude of the echo; angle of arrival; and polarisation of the echo. Drifts can also be measured using the spaced-antenna method. This information is used in radio propagation forecasts of the most effective operating frequency for point to point radio communication. The data is also used in scientific research relating to the ionosphere.
Height range up to 512 km (to be increased to 1000 km) with 6 km resolution.
· The digital control system provides high flexibility. Multiple operating modes are available.
· Frequency range from 1 to 20 MHz. Three standard sweeps are provided: low resolution (100 frequencies), medium resolution (200 frequencies), high resolution (400 frequencies). The step size may be selected to be linear or logarithmic.
· System is PC-based with the major units of the ionosonde mounted on plug-in boards. A basic one-receiver system uses two plug-in boards.
Transmitter - The transmitter power required is only 600W. Amplifier units are all solid state and include monitors for forward and reverse power. The use of pulse coding techniques gives an 11 dB signal to noise (S/N) ratio improvement, equivalent to having about thirteen times the transmitter power.
Receiver - The system can incorporate four or more receivers for spaced antenna measurements. The receiver outputs are sampled simultaneously using two microprocessors per channel. The increased data rate over a time-shared antenna system allows further improvements to the S/N ratio using post-detection processing.
Frequency Synthesiser - Frequency generation is provided by a frequency synthesiser based on direct digital synthesis (DDS). The synthesiser has two channels and can produce two output frequencies that can be changed almost instantaneously in frequency or phase. The DDS synthesiser provides the transmitter frequency and the receiver local oscillator frequency.
Noise Suppression - Coherent pulse averaging is used to further improve the S/N. In practice, four pulse averages (at medium resolution) are used, giving a 6 dB S/N improvement. Longer averages tend to obscure real changes in the ionosphere. If the data is processed in a spectral mode, longer averages are possible. This is usually done only for selected data.
Post processing using Fast Fourier Transforms (FFTs) is being evaluated as an alternative. The sample length would no longer be limited; cancellation is avoided as the phase difference between pulses becomes a frequency shift. For a 64 pulse sample FFT, the S/N improvement is 18 dB over a single pulse sample. The use of FFTs on a modern PC offers tremendous benefits without a large delay (a few minutes per ionogram) or expensive special processors.
Operation - The operating software provides a menu of operating modes from which the operator makes a selection.
A complete ionogram requires from a few seconds (at low frequency resolution and averaging two pulses) to several minutes (at high resolution averaging 16 pulses). A medium resolution ionogram with four pulse averaging requires about 45 seconds.
Data is stored to the computer's hard disk. The data is periodically backed up to 120 MB tape cartridges using a standard Colorado Memory Systems backup unit. Remote control and communication is possible using standard PC communication packages.
10.2.1. General Information
|Frequency Range||1 to 20 MHz|
|Frequency Sweeps||100, 200, 400 linear or logarithmic steps|
|Frequency Generation||DDS-based synthesiser|
|Height Range||90 to 512 km|
|Height Resolution||6 km|
|Pulse Coding||13 bit Barker or single pulse|
|Storage||Standard 120 MB tape backups|
|Power Requirements||PC plug-in boards run off standard|
|bus power. Power amplifier units|
|require 110/220V, 50/60Hz, 100VA|
|Dimensions||Power amplifier cabinet 10 x 12 x 8 inches|
|Compute||IBM compatible PC with at least|
|two free 8 bit slots (5 slots fora four-receiver system)|
|Graphics||CGA for single receiver|
|EGA/VGA for multiple receivers|
CADI is operated as a vertical incidence sounder (VIS) providing information on the state of the local ionosphere. CADI provides direct information on: the echo delay (virtual height) as a function of frequency; the phase and amplitude of the reflected signal; and the doppler shift due to motion of the reflector, at one or a series of
frequencies. With multiple receivers the angle of arrival may also be determined from the phase delay at different receivers. From this information the electron density profile and the drift field may be derived.
Software: Data collection and analysis software was developed in close collaboration with researchers at the University of Western Ontario. Scaling software is provided to allow manual scaling of the standard URSI parameters. Advanced analysis software may be obtained on agreement between SIL and UWO. This package utilises Interactive Data Language (IDL) routines and has not yet been integrated in the commercial package.
The CADI Network: The CADI system has already become an integral part of the Canadian Network for Space Research program, one of Canada's Networks of Centres of Excellence. Extensive collaboration among the space and atmospheric research community is providing in depth and effective use of data from the sounders now operating.
Links have also been established with the Canadian military and the Department of Communications.
Locations: The following sites are presently operating -
Rabbit Lake, Sask (58° N, 106° W)
Resolute Bay, NWT. (75° N, 95° W)
Eureka, NWT (80° N, 84° W)
Cambridge Bay, NWT (69° N, 105° W)
London, Ontario (test site)
Planned locations include -
Saskatoon, Sask (52° N, 107° W)
Alert, NWT (82° N, 63° W)
CADI is a full-featured digital ionosonde with a broad range of applications.
Atmospheric research: The combination of portability, low cost and ease of installation make CADI ideal for both permanent installations and campaign-type research programs of shorter duration.
The potential applications of the CADI sounder to atmospheric research is quite large. A short list of current applications includes:
· polar cap studies, including morphology and behaviour of patches and arcs;
· auroral zone studies, stand-alone and in conjunction with optical and VHF/HF radars;
· studies relating ionospheric effects to wind, wave and tidal measurements in the mesosphere;
· equatorial and high latitude electrojet studies.
CADI is a flexible, robust system which can be configured to suit a variety of user applications.
Ionospheric monitoring: CADI provides the full complement of basic information on the state of the ionosphere required for routine monitoring.
· scaling of standard URSI parameters;
· data storage format suitable for World Data Centre archives.
Communications: For communications applications, CADI provides the following capabilities:
· frequency management based on state of local ionosphere;
· diagnostic sounding, suitable for interpreting the known statistical behaviour of various ionospheric parameters;
· cooperative check target for an HFDF system;
10.4. Future Development
The development of the CADI system is an ongoing process. SIL and researchers at the UWO are involved in the joint development of additional features which will enhance the capabilities of the sounder. Our commitment is to the further development of CADI as an accessible, low cost, flexible full-featured instrument with a broad range of uses in scientific and communications research and monitoring of the ionosphere.
S. Gao & J. MacDougall. A dynamic ionosonde design using pulse coding. Can. J. Phys. 68, 1184 (1991)
10.6. For further information:
Scientific Instrumentation Ltd.
2233 Hanselman Avenue
Saskatoon, Saskatchewan S7L 6A7
Telephone: (306) 244-0881
Facsimile: (306) 665-6263
email: cansas::sil (SPAN)
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