1. Field of the Invention
The present invention generally relates to communication employed in position locating systems and, more particularly, to data compression for reduction of communication delays.
2. Description of the Prior Art
Position locating methodologies and instruments for practicing them have been known and employed throughout the history of navigation. Many early techniques relied on multiple or timed measurements of relative position or direction to visible objects and thus were reliant upon the existence of suitable conditions such as weather to be carried out. More modern systems have exploited other communication media such as radar and radio to avoid such vulnerability. However, until very recently, the accuracy of such systems was relatively low and limited by the accuracy of measurements which could be achieved, often in noisy, long-range or otherwise adverse circumstances such as multiple transmission paths due to signal reflections.
Modern positioning systems exploit triangulation relative to a plurality of spatially separated platforms, such as geo-stationary satellites, which provide redundant coverage of the entire surface of the earth. (Three platforms are necessary to obtain a location relative to the earth's surface (e.g. latitude and longitude) and four platforms are necessary to triangulate a location in three dimensions (e.g. including altitude). Generally at least six platforms are potentially available from any location.)
In such systems, measurements are generally made by analyzing timing and/or frequency of signals transmitted between the location of interest and a suitable number of the platforms rather than measurement of direction or other parameters and results are potentially accurate to within a very few feet. In so-called global positioning systems, signals transmitted from several platforms are analyzed at a receiver which is local to the user. Somewhat similar techniques may be used to determine a position of a source (e.g. transmitter or reflector) of a signal. In these latter systems, it is generally necessary to transmit a signal from the location of interest to be received at plural platforms; the received signal being first evaluated for quality. The best three or four received signals are evaluated in pairs to derive a locus of possible locations for each pair of signals and the region of intersection of the loci corresponding to respective pairs of signals is reported as the location of the transmitter.
While numerous analysis techniques are possible, so-called time difference of arrival (TDOA) and differential doppler (DD) analyses of signals, often in combination (TDOA/DD) to develop a velocity vector if the source is moving or improve location accuracy where the source is stationary and one or more receivers are in motion, are techniques of choice. However, the performance of cross-correlation computations, the details of which are well-understood in the art and, in any case, unimportant to the practice of the invention, to obtain a location of the transmitter requires all data representing the received signals in detail to be present at a single location. Thus communication time to such a location becomes a critical factor in system performance, particularly in regard to fast-moving vehicles such as aircraft. (In this regard, it should be appreciated that the data rate for transmission will often be fixed by the communication link and possibly further limited by conditions thereon.) Specifically, to avoid distortion of signals during communication, the signals received at each platform are usually digitized and a digital representation of the received signals, together with time references (to provide functional coherence of the receivers) is transmitted from each platform to the location (usually a facility corresponding to one of the platforms) where the cross-correlation computation is to be performed.
The digital representation is usually transmitted in a serial coded (e.g. binary) bit stream. Therefore, the transmission time is, at least in part, a function of the number of bits used to represent the signal and the time references and the capacity or bandwidth of the transmission link. By the same token, the fidelity with which the digital signals represent the received signals is a function of the number of bits required to support the desired degree of resolution in representation of the signal. The accuracy of the location derived from the cross-correlation computation is a function of the resolution or fidelity of the digital representation of the signals. Accordingly, it can be readily understood that there is a trade-off between response time and location accuracy as incidents of system performance.
An additional problem is presented by the relative frequency content of the original signal and high sampling rates which may be intended to capture high frequency content. High sampling rates may result in very low or negligible values of coefficients or parameters of data representing such high frequencies if the signal has relatively little energy or power in high frequency components. In other words, a signal having little high frequency content may be "oversampled" at relatively conventional sampling rates (or sampling rates of which the platform may be capable for high resolution), and a burden placed on the communication link to transmit information which is not significant to the result if, in fact, such information exist to more than a negligible degree.
Additionally, it is customary at the present time to carry out transmission of digital information by so-called packet switching in which packets of digital information are provided with digital header information so that the original bit stream can be reassembled even when the packets may not be transmitted over the same communication link or arrive in the original order. Overall, this expedient, while requiring additional bits to be transmitted, increases transmission efficiency and reliability since some communication path can generally be found at any point in time. Nevertheless, the transmission path will generally be imperfect and the loss or corruption of a bit in the header information may defeat or severely compromise reassembly of the data bit stream and, consequently, the entire position location process for that data stream.
The only solution to this potential problem is to provide some degree of redundancy into the digital code which, in view of the numerical nature of the information in the bit string implies that even more bits be transmitted in order to implement correctability and recovery of lost or corrupted bits of the header and numerical information. Signal protocols and formats including such redundancy for signal recovery are referred to as error-correcting codes. Therefore, even though transmission can be made highly reliable in this manner, the number of transmitted bits remains a critical limitation on the speed of system response and often dictates a reduction in the number of bits representing the received signal in order to provide system response with the required timeliness. Accordingly, numerous schemes for data compression to reduce the number of data bits transmitted have been attempted.
A trivially simple method of reducing the number of transmitted bits, currently in use in the art, is to truncate the digitized data in time or amplitude resolution. (That is, fewer bits are transmitted for each sample of the received waveform than are available or fewer samples are represented than are initially quantized or both. In regard to the latter, it is fairly common for a severe mismatch to exist between the data sampling rate and the available communication data rate which, at the state of the art prior to the present invention, must be corrected by additional and often complex data processing.)
However, such truncation may severely impair the accuracy and/or resolution of the location determined from the cross-correlation computation and may increase difficulty or compromise success of the cross-correlation computation. Accordingly, it is known to perform some processing or transformation on the original data to minimize the effect of a data compression for reducing the number of bits transmitted. For example, U.S. Pat. No. 5,570,099 to Gerard Desjardins, assigned to the assignee of the present invention, which is hereby fully incorporated by reference describes a system in which a transformation is performed on data before transmission. (Data compression other than mere truncation, of course, implies that a complementary process must be performed after transmission in a manner similar to coding and decoding of data.) Performing a so-called wavelet transform (discussed, for example, in U.S. Pat. No. 5,561,431 to Larry Peele which, including publications cited therein, is hereby fully incorporated by reference) is another known and well-understood processing technique of choice. The wavelet transform (WT) is a digital signal processing technique that transforms N signal samples into N WT coefficients which describe the distribution of the energy of the signal simultaneously in both time and frequency.
Nevertheless, some degree of degradation of the information will be attributable to the data compression due to both the resolution at which the transformation is computed and the resolution of the data transmitted. A trade-off will thus continue to exist between the amount of compression and the amount of signal degradation that will occur. A common measure for the amount of degradation is the signal-to-quantization ratio (SQR) which is defined as the ratio of the power of the original signal to the power of the difference between the original signal and the compressed form of the signal. A low SQR corresponds to high degradation (due to the relatively larger power of the difference).
It should also be understood that the use of packet switching and error-correcting codes is done at the expense of speed of system response within the limitations thereon imposed by the data link which may also be variable with conditions such as noise. While it is known to dynamically vary data transmission rate and alter coding and decoding techniques (specified in data packet header information) to accommodate available data transmission rates and insure reliable transmission within the available transmission rate, the degree of signal degradation and hence the resolution of location determined by the system will correspondingly vary uncontrollably. Thus, while timely response of the system may generally be assured, there is no corresponding capability at the current state of the art to assure or even evaluate the performance of the system in terms of the quality/fidelity of the data utilized by the system or location resolution derived therefrom.