US-12618979-B1 - Precise wide area ionosphere correction solution for multi-spectrum alternative sources of space based PNT signals

Abstract

An Ionosphere Correction Extrapolation (ICE) solution for correcting ionosphere-induced errors in a space-based positioning, navigation, and timing (PNT) signal uses a reference receiver to create a local ionosphere model. The ICE solution is able to accurately correct range measurements for single frequency or signals of opportunity user equipment (UE). The ICE solution produces a Vertical Total Electronic Content (VTEC) contour map that is centered with respect to a single dual-frequency reference receiver, such as a Global Positioning System (GPS) receiver and extending out to the visible horizon with respect to that reference receiver. The generated VTEC map enables the implementation of ionosphere correction without the need for a direct dual-frequency measurement.

Inventors

• Paul Manz
• Thomas Blenk, Jr.
• Ekta Patel
• Kevin Schaal

Assignees

• U.S. GOVERNMENT AS REPRESENTED BY THE SECRETARY OF THE ARMY

Dates

Publication Date

20260505

Application Date

20230421

Claims (19)

1. 1 . A method for correcting for ionosphere-induced errors in a positioning, navigation, and timing signal received at a single frequency, the method comprising the step of producing a reference Vertical Total Electronic Content contour map centered with respect to a single dual-frequency reference receiver and extending out to the visible horizon with respect to that reference receiver and wherein the step of producing a Vertical Total Electronic Content contour map centered with respect to a single dual-frequency reference receiver and extending out to the visible horizon with respect to that reference receiver further comprises the steps of: identifying a reference receiver; from a plurality of space vehicles, receiving a reference positioning, navigation, and timing signal from each space vehicle of the plurality of space vehicles at least two frequencies; for each space vehicle of the plurality of space vehicles, measuring an ionosphere delay from a dual-frequency measurement; determining the inter-frequency bias of the reference receiver; for each space vehicle of the plurality of space vehicles, calculating a Vertical Total Electronic Content at an Ionosphere Pierce Point of the space vehicle relative to the reference receiver; determining a reference Vertical Total Electronic Content contour map comprising thirteen points using a three-stage extrapolation process through the use of Delauney triangulation wherein the thirteen points correspond to the reference receiver, a location of each of four space vehicles, four cardinal points on the visible horizon and four intercardinal points on the visible horizon and wherein the three-stage extrapolation process comprises a first Delauney triangulation of reference receiver location and space vehicle location, an extrapolation of Vertical Total Electronic Content to the visible horizon and a second Delauney triangulation including a plurality of extrapolated points on the horizon and employing the reference Vertical Total Electronic Content contour map at a user receiver to correct for ionosphere induced errors in a positioning, navigation, and timing signal received at a single frequency.
2. 2 . The method for correcting for ionosphere-induced errors in a positioning, navigation, and timing signal received at a single frequency of claim 1 further comprising the steps of: receiving a positioning, navigation, and timing signal from a space vehicle at a single frequency; and employing the reference Vertical Total Electronic Content contour map by a user receiver to correct the positioning, navigation, and timing signal received at the single frequency.
3. 3 . The method of claim 2 wherein the positioning, navigation, and timing signal is received from a low-earth orbit space vehicle.
4. 4 . The method of claim 2 wherein the positioning, navigation, and timing signal is received from a medium-earth orbit space vehicle.
5. 5 . The method of claim 2 wherein the positioning, navigation, and timing signal is received from a geosynchronous orbit space vehicle.
6. 6 . The method of claim 2 wherein the step of employing the Vertical Total Electronic Content contour map by a user receiver to correct the positioning, navigation, and timing signal received at the single frequency further comprises the step of receiving the Vertical Total Electronic Content contour reference map at the user receiver.
7. 7 . The method of claim 6 wherein the step of employing the Vertical Total Electronic Content contour map by a user receiver to correct the positioning, navigation, and timing signal received at the single frequency further comprising the steps of: determining the Ionosphere Pierce Point of a source space vehicle on the reference Vertical Total Electronic Content contour map; calculating a Vertical Total Electronic Content at the Ionosphere Pierce Point of the source space vehicle using the reference Vertical Total Electronic Content contour map; determining an ionosphere delay based on the Vertical Total Electronic Content at the Ionosphere Pierce Point of the source space vehicle.
8. 8 . The method of claim 7 wherein the step of determining an ionosphere delay based on the Vertical Total Electronic Content at the Ionosphere Pierce Point of the source space vehicle further comprises the step of multiplying the Vertical Total Electronic Content at the Ionosphere Pierce Point of the source space vehicle by a conversion factor and an obliquity factor.
9. 9 . The method of claim 8 further comprising the steps of: receiving the Vertical Total Electronic Content contour map at the user equipment determining the Ionosphere Pierce Point of a source space vehicle on the reference Vertical Total Electronic Content contour map; calculating a Vertical Total Electronic Content at the Ionosphere Pierce Point of the source space vehicle using the reference Vertical Total Electronic Content contour map; determining an ionosphere delay based on the Vertical Total Electronic Content at the Ionosphere Pierce Point of the source space vehicle by multiplying the Vertical Total Electronic Content at the Ionosphere Pierce Point of the source space vehicle by a conversion factor and an obliquity factor.
10. 10 . The method of claim 9 wherein the step of calculating a Vertical Total Electronic Content at the Ionosphere Pierce Point of the source space vehicle using the reference Vertical Total Electronic Content contour map further comprises the steps of: determining a three surrounding points on the reference Vertical Total Electronic Content contour map which surround the Ionosphere Pierce Point of the source space vehicle; calculating the Vertical Total Electronic Content at the Ionosphere Pierce Point of the source space vehicle using the Vertical Total Electronic Content of the three surrounding points.
11. 11 . The method of claim 1 wherein the step of determining the inter-frequency bias of the reference receiver comprises performing a first order linear fit of a plurality of ionosphere measurements for a first frequency.
12. 12 . The method of claim 11 wherein the first frequency is the Global Positioning System L1 frequency.
13. 13 . The method of claim 12 wherein the third stage of the three-stage extrapolation process comprises the step of calculating a Delauney triangulation using the measured Vertical Total Electronic Content values extrapolated to the horizon.
14. 14 . The method of claim 1 wherein the step of calculating a Vertical Total Electronic Content at an Ionosphere Pierce Point of each space vehicle relative to the reference receiver further comprises the steps of: for each space vehicle of the plurality of space vehicles, calculating a difference by subtracting the inter-frequency bias from each ionosphere delay; for each difference, calculating a quotient by dividing the difference by the obliquity factor; and multiplying each quotient by a conversion factor to convert from nanoseconds of delay at a first frequency to Total Electronic Content Units.
15. 15 . The method of claim 1 wherein the first stage of the three-stage extrapolation process comprises the step of calculating a Delauney triangulation using locations for each space vehicle of the plurality of space vehicles.
16. 16 . The method of claim 1 wherein the second stage of the three-stage extrapolation process comprises the step of extrapolating the measured Vertical Total Electronic Content values to the horizon.
17. 17 . The method of claim 16 wherein the step of extrapolating the measured Vertical Total Electronic Content values to the horizon further comprises the steps of: calculating the position of four cardinal points at the horizon; interpolating along each of the four cardinal directions until reaching a last point that falls inside of the Delauney triangulation; calculating coefficients for four separate straight-line fits starting from the reference receiver and extending along each cardinal direction; extrapolating Vertical Total Electronic Content values to the horizon; and calculating four additional horizon points between the four cardinal points by averaging Vertical Total Electronic Content values of two neighboring points.
18. 18 . The method of claim 17 wherein the four additional horizon points are northeast, southeast, northwest and southwest.
19. 19 . A method for correcting for ionosphere-induced errors in a positioning, navigation, and timing signal received at a single frequency, the method comprising the steps of: identifying a reference receiver; from a plurality of space vehicles, receiving a reference positioning, navigation, and timing signal from each space vehicle of the plurality of space vehicles at least two frequencies; for each space vehicle of the plurality of space vehicles, measuring an ionosphere delay from a dual-frequency measurement; determining the inter-frequency bias of the reference receiver; for each space vehicle of the plurality of space vehicles, calculating a Vertical Total Electronic Content at an Ionosphere Pierce Point of the space vehicle relative to the reference receiver; and determining a reference Vertical Total Electronic Content contour map comprising thirteen points using a three-stage extrapolation process through the use of Delauney triangulation wherein the thirteen points correspond to the reference receiver, a location of four space vehicles, four cardinal points on the visible horizon and four intercardinal points on the visible horizon and wherein the three-stage extrapolation process comprises a first Delauney triangulation of reference receiver location and space vehicle location, an extrapolation of Vertical Total Electronic Content to the visible horizon and a second Delauney triangulation including a plurality of extrapolated points on the horizon; employing the reference Vertical Total Electronic Content contour map at a user receiver to correct for ionosphere induced errors in a positioning, navigation, and timing, signal received at a single frequency by receiving a positioning, navigation, and timing signal from a space vehicle at a single frequency; and user receiver correcting the positioning, navigation, and timing signal received at the user receiver at the single frequency for ionosphere induced error.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit under 35 USC § 119(e) of U.S. provisional patent application 63/375,801 filed on Sep. 15, 2022. STATEMENT OF GOVERNMENT INTEREST The inventions described herein may be manufactured, used and licensed by or for the United States Government. FIELD OF THE INVENTION The invention relates in general to positioning, navigation, and timing signals and in particular to error correction for positioning, navigation, and timing signals. BACKGROUND OF THE INVENTION The world is increasingly reliant on space-based systems for navigation and communication. Space-based systems, also known as Global Navigation Satellite Systems (GNSS), are an essential element of the global information infrastructure with impacts in numerous government and civilian applications. These include applications in the military, agriculture, aviation, disaster response, shipping and transportation sectors. In addition, major communications networks, banking systems, financial markets, and power grids depend heavily on space-based systems for precise time synchronization. The Global Positioning System (GPS) system is the most well-known of these systems. GPS is a U.S.-owned utility that provides users with positioning, navigation, and timing (PNT) services. This system consists of a constellation of satellites, or space vehicles (SV), that transmit PNT signals to earth-based receivers. These receivers use the PNT signal to calculate the user's position and time. The ionosphere is one of the single greatest error sources for space-based PNT signals. The ionosphere is part of Earth's upper atmosphere, and it reflects or modifies radio waves passing though it, such as PNT signals. For example, when left unaccounted for, the ionosphere can induce range errors of up to 100 meters at the Global Positioning Satellite (GPS) L1 frequency. Thankfully, for many of today's GPS or other GNSS receivers, the ionosphere can be accounted for, to minimize overall PNT errors. Multiple methods exist to account for ionosphere effects including global models and direct measurement observations. The most prevalent method employed in high precision applications is performing a dual-frequency measurement to make a direct ionosphere correction. GPS satellites broadcast PNT signals on two frequencies, the L1 and L2 frequencies. By observing the relative delay of signals broadcast from the same source but at different frequencies, user equipment (UE) can directly determine the range error induced by the ionosphere. Unfortunately, many US Army and civilian applications are single-frequency solutions to limit size, power and cost. These single-frequency solutions are unable to make direct ionosphere observations using the dual-frequency method. In addition, several other recently introduced factors and challenges have rendered current high precision ionosphere solutions inadequate. Often, high precision is required in Radio Frequency (RF) contested environments, or in dense urban terrain. In both conditions, it is low likelihood that dual-frequency measurements are achievable due to corruption or denial of one ore more signals originating from a single source. In addition, this problem space compounds with the proliferation of Low Earth Orbit satellites broadcasting PNT signals from space. Current and future LEO based PNT SVs transmit one frequency at a time to save on power consumption. A new solution is needed to provide accurate ionosphere corrections addressing these challenges that is frequency and SV agnostic. A work-around method exists in which a dual-frequency U E can measure ionosphere effects for each space vehicle (SV) transmitting PNT signals and pass this information to single frequency UE. This approach works well when the UE has a clear view of all possible SVs. However, this “work around” does not work well when the UE view of SVs being skewed by terrain, obstacles or jamming. US Army precision weapons and munitions will likely operate from protective postures such as behind buildings, next to protective terrain, or under dense foliage, all while being exposed to adversary jamming attacks. Accordingly, precision guided munitions are forced to operate in sub-optimal conditions. Another well-known approach to correcting ionosphere induced range errors is by applying the global Klobuchar model. The Klobuchar model, or global model, provides an estimate of the ionosphere delay as a function of local time at the Ionosphere Pierce Point (not the user's location). It is typically represented with up to eight adjustable parameters which are updated periodically depending on the season and solar activity. While the global Klobuchar model is extremely beneficial, it is not suitable for high-precision applications. The Klobuchar model is estimated to reduce the root mean square (RMS) range measurement by approximately 50% with zenith delay errors up to 10 meters during the day. The is further exa