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Loran has provided navigation service since 1958. Though not originally designed with data broadcast capabilities, Loran's versatility has enabled data to be broadcast with great benefits. Research in the last two decades has resulted in a tremendous increase in the data capacity of Loran thereby increasing its utility. Currently, a modernized Loran is being evaluated for its capability to backup GPS and data modulation is an integral part of this Loran design. This paper details some recent Loran modulation designs and ideas.

Marine radiobeacon networks are being used to broadcast differential Global Positioning System (GPS) corrections to marine users. The correction data digitally modulates signals from some of the existing marine radiobeacons, which operate in the 285 to 325 kHz band, creating 'DGPS/radiobeacons.' The corrections improve the accuracy of the GPS fix from 100 m to 5-10 m, and provides position fixing service for many marine applications which are too demanding for the normal GPS service. A DGPS/radiobeacon broadcast network is attractive for many reasons. First, the marine radiobeacons are already widespread and well located. Moreover, the broadcast of DGPS data from an existing radiobeacon only requires the addition of a GPS reference station and an inexpensive modulator. Consequently, the DGPS/radiobeacon signal can be reliably received at ranges which are well beyond the radio horizon, where the DGPS corrections themselves are still valid. Furthermore, forward error correction can be used to improve the reliability or range of the DGPS/radiobeacon signal. The improvements made possible by channel coding are analyzed here and a code for DGPS/radiobeacons is recommended.

Atmospheric noise is caused by lightning and dominates other natural radio noise sources below 30 MHz. A wide-band atmospheric noise process appears as low-power Gaussian background process with bursts of high-power impulses. The impulses are due to nearby lightning and cause the first-order density of atmospheric noise to have longer tails than the Gaussian distribution. They also tend to occur in groups which means that the atmospheric noise channel has memory. Digital radio systems which operate on the atmospheric noise channel can use hard decisions to desensitize the receiver to the bursts, and forward error correction to recover those symbols which are received incorrectly. If these systems use a high degree of interleaving, then the channel can be modelled as a binary symmetric channel. However, many radio systems which operate below 30 Mhz must minimize data delay and therefore interleaving depth must be chosen carefully. This paper uses two Markov chains to model the memory of the atmospheric noise channel. It derives the transition probabilities for these chains from atmospheric noise error processes which were recorded at 306 kHz. Then, it uses the models to estimate the probability of codeword error, and compares these estimates to codeword error rates which are obtained directly from the recorded error processes. These comparisons are made for the Golay code with a variety of bit interleaving depths, and for a Reed-Solomon code with a variety of symbol interleaving depths. Both Markov channel models predict the actual performance of the codes with much greater accuracy than the binary symmetric channel. This paper also includes some results for binary convolutional codes.

Enhanced Loran (eLoran) is currently being implemented to provide back up to global navigation satellite systems (GNSS) in many critical and essential applications. In order to accomplish this, eLoran needs to provide a high level of availability throughout its desired coverage area. While the current Loran system is generally capable of accomplishing this, worldwide, there remain a number of known areas where improved coverage is desirable or necessary. One example is in the middle of the continental United States where the transmitter density is not adequate for providing the desired availability for applications such as aviation in some parts. This paper examines the use of lower power, existing assets such as differential GPS (DGPS) and Ground Wave Emergency Network (GWEN) stations to enhance coverage and fill these gaps. Two areas covered by the paper are the feasibility and performance benefits of using the antennas at these sites. Using DGPS, GWEN or other existing low frequency (LF) broadcast towers requires the consideration of several factors. The first is the ability of the transmitting equipment to efficiently broadcast on these antennas, which are significantly shorter than those at a Loran station. Recent tests at the US Coast Guard Loran Support Unit (LSU) demonstrated the performance of a more efficient transmitter. This technology allows for the effective use of smaller antennas at lower power levels. Second is the ability to broadcast a navigation signal that is compatible with the Loran system and the potential DPGS broadcast (when using a DGPS antenna). The paper examines some possibilities for navigation signals. The goal is to develop a suitable low power signal that enhances navigation and is feasible for the transmission system. The second part of the paper examines the benefits of using these stations. The benefits depend on the location of the stations and the ability seamlessly to integrate them within the existing Loran infrastructure. Analysis of these factors is presented and the coverage benefits are examined. [ABSTRACT FROM AUTHOR]