Post-processing of NMEA data

Details Post-processing of NMEA data Post-processing of NMEA data

2000-03-07

2002-12-03

Louis-Jacques, Jacques H. (Department: 3661)

Data processing: vehicles, navigation, and relative location

Navigation

Employing position determining equipment

C701S213000, C701S214000, C342S357490, C342S357490, C342S357490

Reexamination Certificate

active

06490524

ABSTRACT:

TECHNICAL FIELD
The present invention relates to the processing of data. In particular, the present invention pertains to a method and system thereof for post-processing Global Positioning System (GPS) data that are in the National Marine Electronics Association (NMEA) format.
BACKGROUND ART
The Global Positioning System (GPS) provides a world-wide navigation and positioning resource for military and civilian use. GPS is based on a constellation of satellites orbiting the earth that act as reference points from which receivers on the ground are able to determine their position. The satellites' orbits are accurately monitored. By measuring the time it takes for signals transmitted from a subset of the satellites to reach a receiver, the receiver can determine its distance from each satellite in the subset. Knowing the distance from each satellite, a roving (mobile) receiver can calculate its position, including its latitude, longitude, and course, for example.
Differential GPS (DGPS) is a method to improve the accuracy of GPS. DGPS works by canceling out the natural and man-made errors that are present in GPS measurements (for example, errors due to imperfect satellite orbits and satellite clocks, multipath reflection, atmospheric distortion, etc.). With DGPS, a second receiver (a reference station or a base station) is used in addition to the first, roving receiver. The base station is placed in a precisely measured and known location. The base station receives signals from each satellite, calculates its position based on the signals, and determines error by comparing the calculated position against the known position. The base station continuously calculates the error because the magnitude of the error can change (for example, because the positions of the satellites change). The base station calculates corrections and transmits them to the roving receiver, which then applies the corrections to its measurements to determine its position more accurately. Typically, the corrections are transmitted from the base station to the roving receiver via radio signals.
A problem with DGPS is that the radio signals from the base stations sometimes are blocked or encounter interference. For example, if the roving receiver passes under a power line, local interference caused by the power line can interrupt the radio signals. Thus, for a period of time, the roving receiver cannot receive correction information from the base station, and consequently the position measurements made by the roving receiver during this period cannot be accurately corrected.
This problem is particularly troublesome in applications where it is desirable, and even necessary, to create an accurate map of a particular area. For example, in agricultural applications, the roving GPS receiver may be coupled with a yield monitor that measures crop yields and other data as the crop is being harvested in a field. The GPS receiver and yield monitor are mounted on a harvesting combine and, as the farmer harvests the crops, yield and uncorrected position information are recorded by a computer system. Using the DGPS correction information, the receiver determines its corrected position in real time (as the position information and yield data are accumulated). The computer system then executes software that correlates the yield and corrected position data to create a map showing yield as a function of position in the field. Clearly, it is necessary to accurately measure position so that an accurate yield map can be made. However, when interference interrupts the radio signal from the base station, correction information cannot be received and used to correct the position measurements made by the roving receiver, and therefore portions of the yield map are inaccurate and unusable. It can be appreciated that accurate position information is desirable for similar types of maps, such as maps showing the distribution of fertilizer, pesticide, etc. It can be further appreciated that accurate position information is desirable for other applications outside of the agricultural applications discussed above, such as marine/hydrographic mapping.
Outside of agricultural applications, one prior art method for addressing the problem created by the interruption of the radio signal from the base station is to store the correction information at the base station, retrieve it later, and then subsequently apply it to the uncorrected information (“post-processing” versus real time processing). For agricultural applications (as well as for some other applications such as marine), NMEA provides a standard data format (e.g., NMEA sentences or NMEA strings) that can be understood by a computer system so that it can process GPS/DGPS information. Most agricultural software has adopted the NMEA format for storing GPS/DGPS information However, the NMEA standard has no provisions for storing data that are required for post-processing. This is problematic because, as described above, in some instances the GPS data cannot be corrected in real time. Thus, in these instances, either inaccurate position data or no data at all will be available to generate a yield map.
Accordingly, what is needed is a system or method to collect, in NMEA format, GPS/DGPS information required for post-processing. Furthermore, what is needed is a system or method that post-processes the NMEA data and can be used with software that generates, for example, a yield map or similar types of maps. The present invention provides a novel solution to these needs.
DISCLOSURE OF THE INVENTION
The present invention provides a method and system thereof that corrects Global Positioning System (GPS) data when Differential GPS (DGPS) correction information cannot be received from a GPS base station in real time. The present invention provides a method and system thereof that can be used with GPS data that are formatted according to the National Marine Electronics Association (NMEA) specifications. Furthermore, the present invention provides a method and system thereof that can be used with the software (current and foreseen) that is used to generate, for example, a yield map or similar types of maps. In other embodiments, the present invention can be applied to other satellite navigation systems, including GLONASS and LORAN-C, for example. The present invention can also be used for non-navigation applications.
The present invention provides a method and system thereof for processing data, including GPS data in the NMEA format. In the GPS embodiment, uncorrected GPS data indicating an estimated position are computed by a receiver via signals from a first broadcaster (e.g., GPS satellites). DGPS data are received by the receiver via a signal from a second broadcaster (e.g., a GPS base station). In the present embodiment, a message is generated by the receiver when, for example, the signal from the second broadcaster is not received. The message contains information that is sufficient to identify the DGPS data stored by the second broadcaster that are needed to correct the uncorrected GPS data.
In one GPS embodiment, the information in the message comprises a time tag for the estimated position fix and, for each GPS satellite used in the position calculation at the same time tag, an identifier tag and IODE (issue of data ephemeris) information.
In one GPS embodiment, the message and the uncorrected GPS data are stored by the receiver for subsequent processing (e.g., post-processing). Alternatively, the message and uncorrected GPS data are stored in an external memory device. The DGPS data are retrieved from the second broadcaster, and the information in the message is used to identify the DGPS data needed to correct the uncorrected information. The uncorrected GPS data are correlated with the DGPS data using time tags, for example, and corrected.
In the present embodiment of the present invention, the GPS data are formatted in the NMEA format, in particular using NMEA sentences such as the GGA sentence, the GSA sentence, the GNS sentence, the GLL sentence, and the RMC sentence. A custom message

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