eLoran and the LDC

In this section, we take a moment to briefly describe Enhanced LORAN (eLoran) and the LORAN Data Channel (LDC). We cannot describe these technologies in great detail because the United States Coast Guard is still developing them. Still, we hope to provide a basic understanding of these standards based on documents published before April 2007.

The LDC is a phase-position modulation scheme that affects the position of an extra pulse that appears after a Secondary pulse chain. In other words, a ninth pulse is inserted after an eight-pulse Secondary chain. This pulse is carefully shifted in time to send a particular message. This pulse is shifted into any one of thirty-two positions; thus, the position of the pulse in time corresponds to a specific five-bit message. Every Secondary transmitter sends its own message over the LDC; thus, in every GRI, between two and four seperate five-bit messages are sent across the LORAN-C signal.

The five-bit message "00000" corresponds to a pulse that appears exactly 100 microseconds after the eighth pulse. This location is called the Zero-Symbol Offset (ZSO). The position of every other five-bit message with respect to the ZSO is defined as: d = 1.25mod(i, 8) + 50.625floor(i / 8), where i is the value of the five-bit message. In practical terms, this equation states that the position of each five-bit message is seperated in time by approximately 1.25 microseconds, whereas each group of eight messages is seperated in time by 50.625 microseconds. Thus, to detect these messages, a LORAN-C receiver must be precise to approximately one microsecond.

A complete message on the LDC is 120 bits. Thus, 24 GRIs must pass - which amounts to approximately 2.4 seconds - before the Secondary transmits a complete message. Each message is seperated into three parts: a four-bit Type, a 41-bit Payload, and a 75-bit Parity. The Type portion specifies the type of message to be sent (i.e., a phase correction, absolute time, or almanac message, etc.). The Payload portion consists of the message's actual data. And, the Parity porition specifies the R(31,16) Reed-Solomon Forward Error Correcting (FEC) Code. This portion assists the receiver in identifying and correcting misinterpreted messages.

The following LORAN-C stations already transmit the LDC (albeit, in an experimental form): Jupiter, FL 7980-Y; Las Cruces, NM 9610-X; Seneca, NY 8970-X; Gillette, WY 8290-X; Middletown, CA 9940-X; Dana, IN 9960-Z; George, WA 5990-Y; and, Grangeville, LA 7980-W.

The eLoran standard is based on this work with the LDC. Indeed, the main difference between eLoran and LORAN-C is that the latter does not contain a data channel. The eLoran data channel will reportedly transmit station identification, almanac, absolute time, leap-second offset, skywave warning, signal failure warning, message authentification, differential LORAN and GPS correction, and official-use messages. In addition, eLoran will apparently be accurate to approximately 50 feet.

The engineers in charge of developing eLoran are making great efforts to seperate their system from any satellite-based navigation system. That is, eLoran will be completely independant from GPS, GLONASS, Gallileo, or any future satellite-based system. This way, eLoran can act a useful backup to any satellite-based navigation system. In other words, if a satellite-based navigation system experiences an outage, eLoran can seamlessly operate in its place. This is an important characteristic of eLoran: recent history has shown that satellite-based communications systems are vulnerable to both intentional and unintentional interference. An independant backup system would ensure the continued operation of global communications and surveillance systems.

References

In this section, we list some LORAN references that we found useful during our research and development process. We hope that you find this information useful during your exploration of LORAN.

LORAN-C Specification Documents