HF background noise measurement in HF environments

Details HF background noise measurement in HF environments HF background noise measurement in HF environments

1993-12-08

1995-11-21

Hellner, Mark

Communications: directive radio wave systems and devices (e.g.,

Clutter elimination

342165, 324 7613, 324 7619, H01Q 300

Patent

active

054691710

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

This invention relates to frequency surveillance systems and in particular to a method and apparatus for determining the background atmospheric noise level in unoccupied channels in an operating band. In addition the invention relates to a system of selecting an appropriate operating frequency to minimise the effects of interference.


BACKGROUND

It is well known by communicators using the high frequency (HF) bands that performance depends largely on frequency selection. This is due in large part to the vagaries of the ionosphere. For HF skywave radars, or Over-the-Horizon Radars (OTHR), it is particularly important that the optimum frequency is chosen for the task at hand. This leads to a requirement for real-time frequency advice on a continuous basis.
An HF system will normally consist of one or more antennas connected to one or more receivers. The antennas may be omnidirectional or may be physically arranged to have a degree of directionality. In addition, various analogue techniques can be used to form receiver beams having a defined direction and width. The signals from the antennas are often subjected to some degree of analogue filtering before reaching the receiver. The receiver output can be converted from analogue to digital form before undergoing further processing and analysis. Once in digital form phase weighting techniques can be used to form multiple simultaneous beams.
It is an object of this invention to provide a method and apparatus for monitoring a frequency band and providing an indication of the background atmospheric noise levels in selected unoccupied channels. A method and apparatus for selecting unoccupied channels is disclosd in Patent Application Number PCT/AU92/00201.
It is desirable to operate any HF system at the optimum signal to noise ratio. For an OTHR which relies on backscattered signals this translates to a requirement to select an operating frequency which is based on the clutter to noise ratio. Clutter refers to the signal backscattered from the earth's surface.
A number of sub-systems such as sounders provide information for determining the optimum operating frequency band for a given combination of ionospheric conditions. Once a frequency band has been selected however, an unoccupied channel must be chosen in which to operate. This invention is designed to aid in this choice.


SUMMARY OF THE INVENTION

Therefore, in one form of this invention, there is proposed a frequency surveillance system which monitors an entire frequency band on a continuous basis and provides an indication of the background environmental noise in the band.
In a further form of this invention there is proposed a method of determining background environmental noise in a selected frequency band comprising the steps of: plurality of input sources; (RFI) to produce substantially RFI free averaged signals; visual display means.
In preference the plurality of scans are accumulated from a plurality of antennas such that a block of scans is taken from each antenna.
In preference the plurality of antennas consists of omnidirectional antennas and directional antennas.
In preference the step of transforming the signals includes converting the analogue signals to digital form and applying a Fourier transform to the digital signals to produce the transformed signals.
In preference the step of averaging the transformed signals from a number of scans is applied to a block of scans.
In preference a block of scans consists of four scans.
In preference the digital signals affected by radio frequency interference are rejected by applying an RFI filter.
The RFI free averaged signals are preferably scaled to absolute signals by comparison with signals received from a reference noise source. The reference noise source is preferably noise at -170 dBW/Hz.
In preference the absolute signals are further averaged by averaging over the plurality of antennas. There are preferably two omnidirectional antennas and eight directional antennas in the form of array beams.
In a further form the method i

REFERENCES:
patent: 4754278 (1988-06-01), Fazio et al.
patent: 5056051 (1991-10-01), Tkalcevic

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