Abstract
The aviation community has been using radio signals for long-range navigation for quite some time. However, with the decommissioning of Loran-A, alternative long-range radio navigation methods are being investigated. The current candidates appear to be Loran-C, VLF comm nav (the VLF communications stations), and Omega. While these signals have seen some use for long-range navigation, there does not appear to be any systematic evaluation of the “service segments’ (ground stations’) ability to function reliably for the duration of a given mission. The authors believe that the often quoted availability ratio (station time available to total time) does not provide sufficient information to judge a signal’s usefulness in aircraft navigation. For example, a signal availability of 98 percent could characterize either a transmitter with a mean time to repair (MTTR) of 2 minutes and a mean time between failure (MTBF) of 98 minutes, or a transmitter with MTTR of 2 hours and MTBF of 98 hours. If this particular signal were required throughout a particular flight, the first signal’s reliability might be acceptable while the second’s would probably be unacceptable unless some form of backup navigation system is provided. Due to lane ambiguity problems, the accuracy of the backup system required for the Omega and VLF comm nav receivers is highly dependent upon the length of time that the primary signal is unavailable. This information is not apparent when only the availability ratio is given. To obtain more meaningful statistics on radio navigation signal reliability, the authors have maintained a file of Loran-C, Omega and VLF comm nav station failures reported since late 1973. From this failure data, the MTBF and MTTR statistics for individual transmitters as well as averages for transmitter types have been calculated. This basic information has also made it possible to characterize the reliability of each transmitter type in various ground station redundancy situations. Many of the stated reliabilities are given in terms of mission duration. The mission duration of interest depends of course on the application. Of particular concern is the suitability of the signals to support oceanic or other remote area flights where there are no short-range navaids. Flight times of interest are therefore assumed to be from 2 to 10 hours. Portions of the analysis have been limited to these values.
Published Version
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