Patent Description:
The LOng RAnge Navigation (LORAN) system was developed in the United States during World War II. Subsequent implementations provided for enhancements in accuracy and usefulness, including LORAN-C and later enhanced LORAN (eLORAN) implementations. The eLORAN system is a low frequency radio navigation system that operates in the frequency band of <NUM> to <NUM> and includes transmissions that propagate by ground wave. The eLORAN system transmits LORAN type navigation RF pulses at a center frequency of about <NUM> and differs from LORAN-C because eLORAN transmissions are synchronized to the UTC similar to GPS, and include time-of-transmission control, differential corrections similar to differential GPS, the use of "all-in-view" tracking, and one or more eLORAN data channels that provide low-rate data messaging, differential corrections, and almanac information.

With the rise of satellite-based navigation systems such as Global Positioning System (GPS), there has been relatively little development or investment in terrestrial-based navigation systems, such as the eLORAN system, until recently. A renewed interest in such systems has arisen as a backup to satellite based navigation and timing systems, particularly since low frequency eLORAN signals are less susceptible to jamming or spoofing compared to the relatively higher frequency and lower powered GPS signals.

The bandwidth limited eLORAN data channel is about a fifty to one hundred bits per second data channel. It carries both time perishable and non-perishable data messages to the end user for proper eLORAN system function, but it is relatively slow and results in low data throughput rates. This eLORAN data channel provides multiple messages to the end user to support eLORAN accuracy, integrity, and availability requirements. These messages transmitted along the eLORAN data channel may include UTC time-of-day, differential corrections, system almanac information, broadcast messages, receiver command control and encryption keys, if applicable. Because of the requirement to prioritize on the eLORAN data channel the transmission of time critical messages, such as differential corrections, other message sets, such as the system almanac information, may take several hours to transmit in their entirety. There is, therefore, a need for further developments in the eLORAN system in certain applications to overcome these drawbacks.

<CIT> describes an eLoran system including an eLoran transmitter and a data channel generator to generate system specific data such as transmission site health, UTC messages and differential corrections, and to communicate non-eLoran data such as third party data for e.g. special governmental or commercial services received from outside the transmitter to a client receiver. Messages are broadcast according to message priority.

Specific embodiments are defined by the dependent claims. The invention concerns an enhanced LOng RAnge Navigation (eLORAN) system which comprises a plurality of eLORAN stations, each comprising an eLORAN antenna, and an eLORAN transmitter coupled to the eLORAN antenna configured to transmit data over a eLORAN data channel (LDC) and transmit a series of LORAN navigation RF pulses and an eLORAN control station. According to the invention, the eLORAN control station is configured to cooperate with a plurality of eLORAN stations each comprising an eLORAN antenna, and an eLORAN transmitter coupled to the eLORAN antenna configured to transmit data over a eLORAN data channel, LDC, and transmit a series of LORAN navigation RF pulses. The eLORAN control station comprises a processor and a memory coupled thereto and configured to generate station specific eLORAN data and non-station specific eLORAN data, divide the non-station specific eLORAN data into a plurality of non-station specific eLORAN data subsets between the plurality of eLORAN stations , and cause each eLORAN station to transmit the station specific eLORAN data and its non-station specific eLORAN data subset over the eLORAN data channel in a prioritized manner so that a saving in time for tranmitting the entire non-station specific data compared to transmitting the non-station specific eLORAN data via a single eLORAN station is achieved.

The non-station specific eLORAN data comprises almanac information, which may include reference station latitude, reference station longitude, and a reference station correction list. The station specific eLORAN data may comprise station identification and differential eLORAN corrections as well as station specific integrity flags, health indicators and station status. The eLORAN control station may be configured to cause each eLORAN station to sequentially transmit all of the non-station specific eLORAN data subsets. A plurality of eLORAN receivers may be configured to cooperate with the plurality of eLORAN stations.

In some embodiments, each LORAN station may be configured to implement the eLORAN data channel using a ninth pulse modulation scheme, a ninth-tenth pulse modulation scheme, or each eLORAN station may be configured to implement the eLORAN data channel using a Eurofix modulation scheme or some other modulation scheme may be utilized. At least one eLORAN monitoring station may be coupled to the eLORAN control station.

Another aspect, which does not fall under the scope of the claims, is directed to a method for eLORAN control for a plurality of eLORAN stations, each comprising an eLORAN antenna and an eLORAN transmitter coupled to the eLORAN antenna and configured to transmit data over a eLORAN data channel and transmit a series of LORAN navigation RF pulses. The method may include operating a processor and a memory coupled thereto to generate station specific eLORAN data and non-station specific eLORAN data, divide the non-station specific eLORAN data into a plurality of non-specific eLORAN data subsets, and cause each eLORAN station to transmit the station specific eLORAN data and a corresponding non-station specific eLORAN data subset over the eLORAN data channel.

The present description is made with reference to the accompanying drawings, in which exemplary embodiments are shown. However, many different embodiments may be used, and thus, the description should not be construed as limited to the particular embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete. Like numbers refer to like elements throughout, and prime notation is used to indicate similar elements in different embodiments.

Referring initially to <FIG>, there is illustrated generally at <NUM> an enhanced LOng RAnge Navigation (eLORAN) system that includes a plurality of eLORAN stations <NUM>, each including an eLORAN antenna <NUM> and eLORAN transmitter <NUM> coupled to the eLORAN antenna and configured to transmit data over an eLORAN data channel (LDC) and transmit a series of LORAN navigation RF pulses. Four transmitter stations <NUM> are illustrated and labeled as TX1, TX2, TX3 and TX4, and as shown in the example of the transmitter station <NUM> labeled TX2, each transmitter station <NUM> includes a respective processor <NUM> and memory <NUM>. Each of the transmitter stations <NUM> is operatively connected to an eLORAN control station <NUM> that includes a processor <NUM> and memory <NUM> coupled thereto and configured to generate station specific eLORAN data and non-station specific eLORAN data that are received by users operating eLORAN receivers <NUM>, which could be located in an aircraft, ship or terrestrial land vehicle, and illustrated schematically with Receiver <NUM> in an aircraft and Receiver <NUM> in a ship.

The control station <NUM> is connected to at least one eLORAN Reference Station <NUM>, and at least one operates as a Differential eLORAN reference station. At least one of the Reference Stations <NUM> monitors the LORAN navigation RF pulses and processes to determine differential corrections and transmit information regarding those navigation RF pulses to the eLORAN control station <NUM>. In this example as illustrated, the eLORAN system <NUM> includes three eLORAN Reference Station <NUM>.

The eLORAN system <NUM> is a positioning, navigation and timing (PNT) service used for aviation, maritime and land-mobile vehicle navigation as well a fixed station timing users. As a location and timing system, it may complement the Global Navigation Satellite Systems (GNSS). The eLORAN transmissions are synchronized to an identifiable, publicly-certified source of coordinated universal time (UTC) independent of the GNSS, and thus, may operate independently of the GNSS. The eLORAN system <NUM> includes that additional eLORAN data channel on the transmitted signal, thus distinguishing the eLORAN system from traditional LORAN-C systems. This eLORAN data channel transmits application-specific corrections, warnings and signal integrity information, including station specific eLORAN data such as station identification and differential eLORAN corrections and non-station specific eLORAN data such as almanac information, including reference station latitude, reference station longitude, and a reference station correction list.

Referring now to the Time to Transmit table in <FIG>, examples of the station specific eLORAN data <NUM> and non-station specific eLORAN data <NUM> are identified. Common messages are transmitted among the transmitter stations <NUM> and form the non-station specific eLORAN data <NUM>, which may include encryption keys to encrypt messages. These keys are typically changed frequently. In some systems, the keys are changed daily and in other systems the keys may be changed even for different messages. The common messages forming the non-station specific eLORAN data <NUM> also include receiver control messages, broadcast messages and almanac information, which may include reference station latitude, reference station longitude and reference station correction lists. The station specific eLORAN data <NUM> may include station identification data, differential eLORAN corrections, and the UTC time. The differential corrections may include data related to time differential corrections and position differential corrections.

The Time to Transmit table in <FIG> has values for a medium GRI (Group Repetition Interval), which corresponds to the time duration of the sequence of pulses, and may include a sequence of timing pulses as master and secondary pulses. Generally, the group petition interval corresponds to the specified time interval for all transmitter stations <NUM> of a chain to transmit their pulse groups. For each chain, a minimum group repetition interval is selected of sufficient duration to provide time for each transmitter station <NUM> to transmit its pulse group and an additional time period between each pulse group so that signals from two or more transmitter stations do not overlap in time within the coverage area. The group repetition interval is normally calculated in tens of microseconds and given a whole number designation as a standard. For example, the group repetition interval having <NUM>,<NUM> microseconds corresponds to the standard as "<NUM>," which, for a certain application of the LDC can produce <NUM> messages over two minutes. This eLORAN data channel has a low data rate usually about <NUM> to <NUM> bits per second and provides these multiple messages to a user operating an eLORAN receiver <NUM> to support the accuracy, integrity and availability requirements in the eLORAN system <NUM>.

Referring now to <FIG>, there is illustrated generally how conventional eLORAN stations <NUM> will transmit almanac information as part of their non-station specific eLORAN data <NUM> sequentially from all the associated eLORAN stations. Each eLORAN station <NUM> transmits the same sequence of almanac information in this conventional example. This may cause delay problems in some data transmission because there is a requirement to prioritize transmission of time critical messages such as the differential corrections, while the lower priority message sets, e.g., the almanac information as part of the non-station specific eLORAN data <NUM>, may take one or more hours to transmit in their entirety as shown on the table of <FIG>, which shows a <NUM> hour transmission time for the almanac information for a specific eLoran system comprised of a multitude of reference stations <NUM>. As illustrated in the sequence of <FIG>, all messages are transmitted sequentially from each of the associated eLORAN transmitter stations <NUM>.

These more common system messages as part of the non-station specific eLORAN data <NUM>, however, such as the almanac information, can be multi-cast over many transmitter stations <NUM>. The determination of position requires the receipt of a minimum of three transmissions from three separate transmitter stations <NUM>. Most eLORAN systems <NUM> operate with "all-in-view" eLORAN tracking, where the signals from more than three transmitter stations <NUM> are routinely tracked. Thus, it is possible to leverage bandwidth of all the eLORAN transmitter stations <NUM> for transmission of the non-station specific eLORAN data <NUM> to the end-user.

According to the invention, the eLORAN control station <NUM> is configured to generate the station specific eLORAN data <NUM> and non-station specific eLORAN data <NUM> and divide the non-station specific eLORAN data into a plurality of non-specific eLORAN data subsets and cause each eLORAN station <NUM> to transmit the non-station specific eLORAN data in a corresponding non-station specific eLORAN data subset over the eLORAN data channel. This is a form of alternating message transmission or multiplexing among three or more eLORAN stations <NUM>, where in this example, the almanac information as the almanac messages are multiplexed or "split" between the three eLORAN stations, marked as TX1, TX2 and TX3 in <FIG>. Each transmit the divided non-station specific eLORAN data subsets. Thus, the total time to receive the full non-station specific eLORAN data is split into three. In this example, if only one eLORAN station <NUM> is operable for some reason, such as technical difficulties in the other eLORAN stations, the <NUM> messages would be transmitted from the one eLORAN station in about one hour. If three eLORAN stations <NUM> are used, on the other hand, then the <NUM> messages from each eLORAN station would be transmitted in about <NUM> hours, while five eLORAN stations could transmit the messages in about <NUM> hours, thus achieving considerable savings in time to transmit an entire set. In one simulation, it was determined that one eLORAN station <NUM> could transmit the almanac information in about <NUM> minutes, while three eLORAN stations could transmit the split almanac information in about <NUM> minutes, and five eLORAN stations could transmit the almanac information in about <NUM> minutes.

The eLORAN control station <NUM> is configured such that each eLORAN station <NUM> sequentially transmits all the non-station specific eLORAN data sets as shown in <FIG>, where the sequencing begins at the next subset. For example, a first eLORAN station <NUM> (TX1) would transmit its sequence of <NUM>, <NUM>, <NUM> et al. messages and then start again at <NUM>, <NUM>, <NUM> et al. messages and continue so that the entire set of messages are transmitted. Thus, the sequenced message transmissions repeat and a full message set can still be received from a single transmitter station <NUM> in a nominal one-tower, i.e., one station transmit time.

Before the eLORAN control station <NUM> divides the non-station specific eLORAN data <NUM> into its plurality of non-specific station eLORAN data sets, the eLORAN control station will conduct an analysis of the current eLORAN system <NUM>. Based on the expected propagation coverage, the eLORAN control station <NUM> will determine the best distribution of transmissions from each eLORAN transmitter station <NUM>, and based on the expected propagation coverage, divide the non-station specific eLORAN data <NUM> among <NUM>, <NUM>, <NUM> or more eLORAN transmitter stations <NUM> at the different eLORAN stations <NUM><NUM>.

There now follows further details of the general operation of the eLORAN system <NUM> with its plurality of eLORAN stations <NUM>, which cooperate with the eLORAN Reference Stations <NUM>. The reference stations <NUM> and eLORAN control station <NUM> do not interfere with the timing control of any transmitted signals, and the reference stations <NUM> may provide augmentation data and may provide real-time corrections to published ASF (Additional Secondary Factor) maps for a complete maritime or other terrestrial coverage area, and provide grid data with nominal propagation corrections per eLORAN station <NUM>. Thus, a user operating one or more eLORAN receivers <NUM> may apply both the ASF's from the map and any differential eLORAN corrections received over the eLORAN data channel to improve positioning accuracy to better than <NUM> meters and timing accuracy to better than <NUM> ns. An eLORAN reference station <NUM> will calculate and transmit phase corrections continuously. As noted before, the eLORAN system <NUM> signal structure is between a <NUM> and <NUM> frequency band and with a pulse signal usually at about a <NUM> carrier frequency. The eLORAN signal usually has groups of eight to ten pulses that are spaced about <NUM> millisecond in a TDMA structure. The transmission of groups repeat every group repetition interval. As many as five different eLORAN stations <NUM> may share the same group repetition interval to form a chain with a master and secondary transmission. The eLORAN signal envelope shape identifies a reference of zero-crossing, which is synchronized to the UTC. The transmitted signals may be phase coded <NUM> or <NUM>° for master/secondary identification and mitigation against multiple hop sky waves.

As a non-limiting example, those skilled in the art will understand that different implementations of the eLORAN data channel may exist, such as a three-state pulse position modulation known as the Eurofix modulation system that is standardized by the RTCM and ITU, a 9th pulse modulation system or a <NUM>th-<NUM>th pulse modulation system. The first two modulation systems provide equal data bandwidth of approximately <NUM> to <NUM> BPS, while the third provides approximately <NUM> BPS. All modulation systems are protected by Reed-Solomon forward error correcting code to counter the effects of noise. The Eurofix modulation system has a pulse position modulation of pulses <NUM>. <NUM> by +<NUM>, <NUM>, -<NUM> microseconds, while the <NUM>th and <NUM>th - <NUM>th pulse modulation systems have the additional <NUM>th or <NUM>th and <NUM>th pulses of <NUM> possible values between <NUM> and <NUM> microseconds. In the Eurofix modulation system as noted before, the pulses are pulse position modulated (ppm) plus or minus (one) <NUM> microsecond. There are about <NUM> possible modulation patterns and the pulse position modulated encoding uses about <NUM> of a possible <NUM> balanced patterns to represent <NUM> bits of data per group reputation interval. The data rate may be about <NUM> to <NUM> bits per second based on the group repetition interval and uses the forward error correction as Reed-Solomon encoding. In an example, the Eurofix message length is fixed at about <NUM> bits having <NUM>-bit words.

With the 9th pulse modulation system, the pulse is inserted between the 8th and 9th pulses such as <NUM> microseconds after the 8th pulse and may use <NUM>-state pulse-position modulation to encode the data at a data rate of about <NUM> bits per GRI.

The eLORAN system <NUM> includes the differential eLORAN corrections transmitted within the eLORAN data channel, which may contain ASF phase correction data from the Differential eLORAN Reference Station <NUM>. The almanac information may include the reference station latitude, reference station longitude, and a reference station correction list as noted before. The reference station correction list may have messages that contain the signal ID codes for the corrections from a particular Differential eLORAN Reference Station <NUM>.

In one example, as understood by those skilled in the art, it is possible that the Eurofix modulation system may have <NUM> bits as a DGPS message. A message may have <NUM> bits as a cyclic redundancy check for data link integrity, and <NUM> bits for the Reed-Solomon parity. In addition, <NUM> bits may correspond to <NUM> GRI's of <NUM> bits per message, and thus, correspond to about <NUM> to about <NUM> seconds per message. The 9th pulse modulation system may have a <NUM> state pulse position modulation in another example and <NUM> bits/GRI corresponding to <NUM> bits phase and <NUM> bits envelope and phase.

In operation, the eLORAN receivers <NUM> will have a position calculation based on three or more pseudo ranges to three or more eLORAN stations <NUM>. An eLORAN receiver <NUM> will measure the arrival times, which convert to pseudo ranges by multiplication with the signal's propagation velocity. It should be understood that this propagation velocity is not equal to the speed of light in a vacuum, but depends on the medium the signals travel in and over, such as over sea, over land, or over mountains. The calculations take into effect the true range, a primary factor, a secondary factor, and an additional secondary factor, together with the variation in those factors and any remaining measurement errors. The receiver clock bias may be used in the position calculation. The primary factor delay accounts for the difference between propagation of the signal in the earth's atmosphere as opposed to in free space and the secondary factor delay accounts for signal propagation over water, especially the ocean. The primary factor and secondary factor are known and considered constant and an eLORAN receiver <NUM> uses a model to calculate the delays.

The additional secondary factor may be calculated as the delay caused by signal propagation over land and elevated terrain as opposed to over ocean water. The additional secondary factor (ASF) delay build-up depends on the type of soil and other factors and is the total cumulative delay the signal experiences over sections with different ground conductivity. The ASF map can be provided for the operating area as a grid with a survey nominal ASF's for each eLORAN station <NUM>. By not taking the ASF's into account, this could result in positioning errors of several hundred meters to even kilometers. ASF's are published as a map with an ASF grid for each eLORAN station <NUM>. Corrections are broadcast to users and the eLORAN receivers <NUM> improve their positioning and UTC time accuracy by applying the temporally changing corrections to the current map.

Referring now to <FIG>, there is illustrated a flow diagram generally at <NUM> illustrating method aspects associated with the eLORAN system <NUM> in accordance with an example embodiment, not claimed, for enhanced eLORAN control of the plurality of eLORAN stations <NUM> as described with reference to <FIG>. The method begins (Block <NUM>) and includes generating station specific eLORAN data <NUM> and non-station specific eLORAN data <NUM> (Block <NUM>) and dividing the non-station specific eLORAN data <NUM> into a plurality of non-specific eLORAN data subsets (Block <NUM>). This may further include the steps of a) prioritizing the data transmission, and b) resequencing non-station specific data to permit multicasting and multiplexing. Each eLORAN station <NUM> transmits the station specific eLORAN data <NUM> and a corresponding non-station specific eLORAN data subset over the eLORAN data channel (Block <NUM>). The process ends (Block <NUM>).

Claim 1:
An enhanced LOng RAnge Navigation, eLORAN, control station (<NUM>) configured to cooperate with a plurality of eLORAN stations (<NUM>) each comprising an eLORAN antenna (<NUM>), and an eLORAN transmitter (<NUM>) coupled to the eLORAN antenna (<NUM>) configured to transmit data over a eLORAN data channel, LDC, and transmit a series of LORAN navigation RF pulses, the eLORAN control station (<NUM>) comprising:
a processor (<NUM>) and a memory (<NUM>) coupled thereto and configured to
generate station specific eLORAN data (<NUM>) and non-station specific eLORAN data (<NUM>) and characterised in the processor being further configured to
divide the non-station specific eLORAN data (<NUM>) into a plurality of non-station specific eLORAN data subsets between the plurality of eLORAN stations and
cause each eLORAN station (<NUM>) to transmit the station specific eLORAN data (<NUM>) and its non-station specific eLORAN data subset over the eLORAN data channel in a prioritized manner so that a saving in time for transmitting the entire non-station specific eLORAN data (<NUM>) compared to transmitting the non-station specific eLORAN data via a single eLORAN station is achieved, wherein the non-station specific eLORAN data (<NUM>) comprises almanac information.