1. Field of the Invention
The present invention relates generally to methods and apparatuses for determining and accurately predicting high frequency radio communication performance. More specifically, the invention relates to creating a real time Ionospheric map and forecasting based on a climatological model and updated data from sounders and other resources.
2. Description of the Related Art
For most of this century, communications with ships at sea has been carried out primarily in the high-frequency (HF) portion of the radio spectrum. Signals in this frequency range have the property of being reflected by the ionosphere, and therefore can reach very long distances without the use of satellites or repeaters.
The main difficulty encountered in using HF radio waves to communicate is dealing with the variability of the radio propagation mechanism, specifically the variability of the reflection of radio waves by the ionosphere. The ionosphere is created by the chaotic process of solar radiation and the interaction of that radiation with the earth's magnetic field. The prediction models of ionospheric propagation used today are generally effective in predicting long-term trends, but prediction of short-term phenomena has not been successful.
Programs have been written to predict propagation of HF radio waves. These include IONCAP, VOACAP and ICEPAC. Each of these programs uses an ionospheric prediction model based on limited historical data. In each case the election density of the ionosphere, which determines the reflective properties of the ionosphere, is modeled as a function of certain input data. The input data may include time, date, sunspot number, and geographic location. The modeling function predicts the median behavior of the ionosphere as well as HF propagation for the conditions specified by the input data. In each case, however, the modeling function prediction is subject to inaccuracy and no adequate method has been devised to correct the inaccuracies in a self consistent manner.
Maintaining an HF communication circuit requires managing many alternate communication paths and frequencies. Because of the variability of the ionosphere, skilled operators are required to maintain the links in HF communication circuits. Certain improvements have been made over the years. For example, HF communications systems now use an instrument which tests the ionospheric path one wishes to use and displays or ranks the frequencies available in order of their efficacy. Such an instrument is typified by the BR Communications, Inc. linear FM ionospheric sounder.
Another difficulty encountered in using HF communications is that the HF spectrum is generally congested. This congestion increases the likelihood that interference between different signals. Ambient radio noise impacts receiver efficiency. In planning an HF communications network, it would therefore be desirable to have knowledge about propagation conditions over an extended area so that a large number of usable frequencies and paths could be chosen consistent with FCC and ITU-R rules and regulations. This would enable the system to reroute signals to paths in regions with less traffic and expand the capacity of the network. The key to a truly reliable automatic HF communications network is therefore accurate knowledge of current propagation and radio interference conditions.
Because of the lack of reliable forecasts, the most accurate and reliable existing method for fixed platforms or mobile platforms such as ships or airplanes to obtain knowledge of propagation and radio interference is through direct real-time measurement and evaluation of ionospheric conditions between the desired communication nodes using ionospheric sounders. The sounders may be coupled with computers to generate an HF frequency management system that controls the operation of the HF communications radios so that the radios may be automatically tuned to a channel on which the HF waves are currently being reflected by the ionosphere along the desired communication path and on which there is not excessive traffic. One example of a HF Frequency Management System used by the military is the Chirpsounder.RTM.-based system.
The Maritime ARCS (Automatic Radio Calling System) is illustrative of a Chirpsounder-based automatic frequency management system combined with an automatic reporting and linking system for data and single side band (SSB) voice communications between ship and shore. The basis for the enhanced performance and ease of use of ARCS is automatic tuning of the ship's HF radio based on real-time knowledge of propagation and interference.
ARCS uses Chirpsounders to periodically measure ship-shore propagation conditions (signal-to-noise ratio, multipath, distortion, etc.) over the HF band so that the radio frequency of the HF communications channel can be changed in real time to the frequency that can best maintain reliable communications. To accomplish this, a Chirpsounder transmitter at the shore station transmits a low power (10 watt) sweeping RF test signal from 2 to 30 Mhz, typically four times per hour. On each ship, a Chirpsounder receiver automatically synchronizes and demodulates the transmitted Chirp.TM. waveform by tuning the receiver in synchronization with the transmitted signal. The receiver analyzes the received Chirp signal to determine propagation quality as a function of frequency. Based on this measurement, the Chirpsounder automatically ranks the propagation quality of all the assigned operating frequencies to each station. Those frequencies or channels with a high quality ranking are recommended for use, while those with a low quality ranking are rejected. In addition, the Chirpsounder receiver routinely samples the highly ranked channels to determine if they are free of interference from other users. This occupancy measurement is used to further refine the individual channel rankings, so that the highest ranked channels will support reliable HF propagation and at the same time are unoccupied.
A large worldwide network of operating Chirpsounder transmitters is currently in use by various government and military organizations. Most of these organizations maintain their Chirpsounder transmissions on a published schedule, thereby allowing commercial users to use these transmissions free of charge and without restrictions. Thus, ARCS is able to take advantage of the existing worldwide Chirpsounder network and infrastructure to economically provide the benefits of Chirpsounder-based automatic frequency management to the commercial maritime community.
The Chirpsounder system also incorporates Chirpcomm.TM., an automatic spread spectrum order-wire feature, which allows brief emergency or system management messages to be transmitted from shore to ship with very high reliability. The Chirpcomm message data is superimposed on the basic Chirpsounder sweep, and therefore is transmitted on all frequencies 2-30 Mhz. Because Chirpcomm uses the complete HF band, messages can be sent with a very high degree of confidence without needing to select a "good" frequency (i.e., a high quality narrow band channel). In addition, the signal processing used to demodulate the Chirpcomm signal provides protection from interference and signal distortion. Because the Chirpcomm message automatically modulates a standard Chirpsounder sweep, no additional transmissions are required--a new 40-character message can be sent on each Chirpsounder sweep.
Normally, Chirpcomm is only used to transmit a two-character shore station ID. However, if the shore station wants to contact the ship, the service provider can generate a "call-in" command by placing the ship's ARCS ID in a Chirpcomm message. The ARCS ID is an alphanumeric code unique to each ARCS controller, much like the ship's radio call sign. When the ARCS receiver/controller decodes the Chirpcomm message and matches the ARCS ID in the message to the ship's own ID, the ship's ARCS controller will automatically place a call to the service provider in response to the Chirpcomm "call-in" command. The shore station then transfers whatever message traffic is waiting. By placing multiple ARCS IDs in a Chirpcomm message, the shore can initiate call-in commands for up to eleven ships for each Chirpcomm message sent.
The use of Chirpsounders and ARCS has made it possible for individual ships or planes to determine propagation data to individual base HF communication stations, but there is no real-time ionospheric mapping system in existence today to support large HF communications networks over large regions. Individual Communicators may empirically test various communication paths using Chirpsounders and ARCS, but no system level network propagation management system exists. A large area real-time map would be desirable because it would enable management of communication network resources at a system level.
The lack of an appropriate real-time data input in prior art systems for application to remote areas of interest is even more limiting that any lack of model flexibility or generality. Prior art systems rely on land-based Vertical Incidence Sounders (VIS). Vertical Incidence Sounders include a transmitter and receiver located at the same point and obtain data from the ionosphere directly over that point. Data points are thus obtained only for ionospheric regions overlying the land masses where the VIS are located. This is problematic because most of the important region for communications may overlay ocean regions, and ionospheric characteristics may vary significantly in those regions.
Prior art real-time ionospheric mapping systems are inadequate because they are based upon models which are updated by instruments which sample ionospheric parameters either directly (i.e., VIS or nearly directly (i.e., Quasi-VIS) overhead. Prior art systems do not use OIS methods for developing independent estimates of the various ionospheric layers for application in mapping. Specifically, prior art does not include any scheme for real-time modeling of the sporadic E layer.
It would be desirable if a real-time mapping and prediction system could be developed which could use data obtained over ocean regions using Oblique Incidence Sounders (OIS). Oblique Incidence Sounders include a transmitter and a receiver at a location remote from the transmitter. Data obtained from Oblique Incidence Sounders is from ionosphere regions overlying points between the transmitter and receiver, where no equipment may necessarily be located. It would also be desirable if data other than sounder data could be used. It would also be beneficial if such a mapping system could process data in a manner that allows data obtained from various locations to influence the state of the model in different locations according to the expected correlation of the conditions between the locations. Additionally, it would be desirable if the system could adapt based on the availability of data points in different regions.
In view of the foregoing, there is a needed for methods and apparatuses for gathering current propagation data over a large network both for real time communication and for making accurate short-term predictions of propagation conditions over the network. Dynamic forecasting based on a real-time data map would for the first time provide truly accurate short-term propagation forecasting for the vast oceanic regions now unsampled by prior art systems. The problems of HF networks with the peakiness (viz., temporal clustering) of traffic could be addressed. A large area real-time map of ionospheric propagation characteristics would enable an HF network to reroute traffic to alternate paths during rush hour periods to take advantage of unused capacity when certain paths are full.
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