Satellite position message fraud detection method and system

Abstract

The embodiment of the present invention discloses a method and system for detecting spoofing of satellite position messages. The embodiment of the present invention can effectively avoid the situation where the device encounters a spoofing signal and fails to generate an alarm when it is started by inputting and configuring the initial longitude and latitude information before the timing protection device is turned on. The satellite receiving module is assisted by the chip atomic clock integrated in the board, which can easily generate more convergent longitude and latitude position change information, thereby generating a high-frequency and stable digital frequency fingerprint. The embodiment of the present invention proposes to utilize the high frequency stability characteristics of the chip atomic clock and the low phase jitter second pulse to convert the longitude and latitude information changes in the NMEA0183GNRMC statement into stable and easy-to-preserve time-frequency fingerprint data, and generate a high-precision and high-reliability simple digital frequency fingerprint at a low cost. The fingerprint itself has relatively good statistical characteristics and can be applied to special time-frequency applications such as time-frequency cryptography, digital timestamps, and auxiliary atomic clock training.

Description

Technical Field

[0001] The embodiments of the present invention relate to the field of digital signal technology, and in particular to a method and system for detecting spoofing of satellite position messages. Background Art

[0002] With the continuous development of science and technology, satellite navigation systems such as GPS and Beidou are becoming more and more perfect. NMEA 0183 is a standard protocol commonly used for satellite receivers to process and transmit position data, and is widely used in the fields of navigation and aviation. It defines a series of data formats and communication rules, allowing different devices and systems to share and interpret information such as position, speed, and time.

[0003] The chip atomic clock is a low-power, high-frequency stable atomic clock based on the coherent layout trapping principle. After being integrated into the satellite receiving chip board, it can provide a more accurate and stable second pulse reference, and can effectively assist the satellite receiving board to generate more reliable and easy-to-use time and location information, which can be integrated into the NMEA 0183 message for effective output.

[0004] In practical applications, when the satellite receiving chip is initially powered on and there are both real and spoofed signals, it is difficult to distinguish the real signal from the spoofed signal through the initial time information. At the same time, the generation and output of existing frequency signals often require high-precision time and frequency test equipment such as frequency counters and phase noise testers to assist in real-time generation. The equipment costs of testing and communication in this generation method are often very expensive, and it is difficult to generate high-precision frequency data on a large scale. Summary of the invention

[0005] To this end, an embodiment of the present invention provides a method and system for detecting spoofing of satellite position messages to solve the technical problem that it is difficult to distinguish real signals from spoofing signals through initial time information in the prior art.

[0006] In order to achieve the above purpose, the embodiment of the present invention provides the following technical solutions:

[0007] According to a first aspect of an embodiment of the present invention, a method for detecting spoofing of a satellite position message is provided. The method is applied to a chip atomic clock and comprises:

[0008] S1, receiving satellite signals in real time and sending a location message containing longitude and latitude information to an MCU, wherein the MCU extracts a preset format statement from the location message and obtains real-time longitude information and real-time latitude information from the preset format statement;

[0009] S2, initialize the clock frequency clock and determine whether the clock value at this time belongs to the preset initial set. If not, the clock is set to 0. If it does, the clock value is increased by 1 after each message update.

[0010] S3, detecting whether the clock value at this time is greater than or equal to the first preset threshold value, if it is greater than or equal to, calculating the longitude and latitude change value according to the real-time message reference value, if it is less than, recording the longitude and latitude reference value;

[0011] S4, after calculating the longitude and latitude change value, determine whether the longitude and latitude change value at this time is greater than or equal to the second preset threshold value, if less than, the satellite signal at this time is a real signal, calculate the longitude and latitude change value according to the next real-time reference value and continue to determine whether the longitude and latitude change value at this time is greater than or equal to the second preset threshold value;

[0012] S5. If the longitude and latitude change values ​​at this time are greater than or equal to the second preset threshold, the satellite signal at this time is a spoofing signal, and continue to judge whether the next longitude and latitude change value is less than or equal to the second preset threshold. If it is less than or equal to the second preset threshold, the satellite is restored to normal at this time, and the longitude and latitude change values ​​are calculated according to the next real-time reference value and continue to judge whether the longitude and latitude change values ​​at this time are greater than or equal to the second preset threshold. If it is greater than the second preset threshold, the satellite signal is still a spoofing signal.

[0013] Preferably, a method for detecting spoofing of a satellite position message further comprises:

[0014] When the satellite signal is a spoofed signal, an alarm message is issued and the satellite signal is temporarily blocked;

[0015] When the satellite returns to normal, the warning message is cancelled and the reception of satellite signals is resumed.

[0016] Furthermore, the preset initial set is [0,3600]; the position message is a NMEA-0183 message, and the preset format statement is a GNRMC format statement.

[0017] Furthermore, the first preset threshold is 10, and the second preset threshold is 80.

[0018] According to a second aspect of an embodiment of the present invention, a method for generating a time-frequency fingerprint of a satellite position message is provided. The method is applied to a chip atomic clock and includes:

[0019] receiving satellite signals in real time and sending a location message containing longitude and latitude information to the MCU, wherein the MCU extracts a preset format statement from the location message and obtains real-time longitude information and real-time latitude information from the preset format statement;

[0020] Calculate the longitude and latitude change value according to the real-time message reference value and convert the longitude and latitude change value into a real-time longitude and latitude change signal;

[0021] The real-time longitude and latitude change signal is combined with a reference frequency to generate a real-time digitized frequency fingerprint signal, and a standard digitized time-frequency fingerprint signal is obtained.

[0022] Furthermore, the real-time latitude and longitude change signal is combined with a reference frequency to generate a real-time digital frequency fingerprint signal to obtain a standard digital time-frequency fingerprint signal, including:

[0023] Converting the real-time longitude and latitude change signal into a position change rate per unit time step;

[0024] Dividing the position change rate by the speed of light to obtain an instantaneous relative frequency deviation;

[0025] The instantaneous relative frequency deviation and the reference frequency are used to generate a real-time standard digital time-frequency fingerprint signal.

[0026] Furthermore, the longitude range is [-π,π] and the latitude range is

[0027] Preferably, a method for generating a time-frequency fingerprint of a satellite position message further includes:

[0028] A stability analysis is performed on the standard digital time-frequency fingerprint to generate a stability analysis result.

[0029] According to a third aspect of an embodiment of the present invention, a system for detecting spoofing of satellite position messages and generating time-frequency fingerprints is provided, the system comprising:

[0030] A satellite receiving module, used for receiving satellite signals in real time and sending a location message containing longitude and latitude information to the MCU, wherein the MCU extracts a preset format statement from the location message and obtains real-time longitude information and real-time latitude information from the preset format statement;

[0031] The clock initialization module is used to initialize the clock frequency clock and determine whether the clock value at this time belongs to the preset initial set. If it does not belong to it, the clock is set to 0. If it does belong to it, the clock value is increased by 1 after each message update.

[0032] A clock frequency judgment module, used to detect whether the clock value at this time is greater than or equal to a first preset threshold value, if it is greater than or equal to, calculate the longitude and latitude change value according to the real-time message reference value, if it is less than, record the longitude and latitude reference value;

[0033] A deception information detection module, used for calculating the longitude and latitude change value and judging whether the longitude and latitude change value at this time is greater than or equal to a second preset threshold value, if less than, the satellite signal at this time is a real signal, calculating the longitude and latitude change value according to the next real-time reference value and continuing to judge whether the longitude and latitude change value at this time is greater than or equal to the second preset threshold value; if the longitude and latitude change value at this time is greater than or equal to the second preset threshold value, the satellite signal at this time is a deception signal, and continuing to judge whether the next longitude and latitude change value is less than or equal to the second preset threshold value, if less than or equal to the second preset threshold value, the satellite is restored to normal at this time, calculating the longitude and latitude change value according to the next real-time reference value and continuing to judge whether the longitude and latitude change value at this time is greater than or equal to the second preset threshold value, if greater than the second preset threshold value, the satellite signal at this time is still a deception signal;

[0034] A real-time longitude and latitude change signal acquisition module is used to calculate the longitude and latitude change value according to the real-time message reference value and convert the longitude and latitude change value into a real-time longitude and latitude change signal;

[0035] A digital frequency conversion output module is used to generate a real-time digital frequency fingerprint signal by combining the real-time longitude and latitude change signal with a reference frequency, and obtain a standard digital time-frequency fingerprint signal;

[0036] The stability analysis module is used to perform stability analysis on the standard digital time-frequency fingerprint and generate a stability analysis result.

[0037] Preferably, a satellite position message spoofing detection and time-frequency fingerprint generation system further includes a satellite spoofing information warning module for executing the following information:

[0038] When the satellite signal is a spoofed signal, an alarm message is issued and the satellite signal is temporarily blocked;

[0039] When the satellite returns to normal, the warning message is cancelled and the reception of satellite signals is resumed.

[0040] The embodiments of the present invention have the following advantages:

[0041] The embodiment of the present invention can effectively avoid the situation where the device encounters a spoofed signal and fails to generate an alarm when it is started by inputting and configuring the initial longitude and latitude information before the timing protection device is turned on. The satellite receiving module, with the assistance of the chip atomic clock integrated in the board, can easily generate more convergent longitude and latitude position change information, thereby generating a high-frequency and stable digital frequency fingerprint. The embodiment of the present invention proposes to utilize the high frequency stability characteristics of the chip atomic clock and the low phase jitter second pulse to convert the longitude and latitude information changes in the NMEA0183 GNRMC sentence into stable and easy-to-preserve time-frequency fingerprint data, and generate a high-precision and high-reliability simple digital frequency fingerprint at a low cost. The fingerprint itself has relatively good statistical characteristics and can be applied to special time-frequency applications such as time-frequency cryptography, digital timestamps, and auxiliary atomic clock training. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] In order to more clearly illustrate the implementation methods of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for the implementation methods or the description of the prior art. Obviously, the drawings in the following description are only exemplary, and for ordinary technicians in this field, other implementation drawings can be derived from the provided drawings without creative work.

[0043] The structures, proportions, sizes, etc. illustrated in this specification are only used to match the contents disclosed in the specification so as to facilitate understanding and reading by persons familiar with the technology. They are not used to limit the conditions under which the present invention can be implemented, and therefore have no substantial technical significance. Any structural modification, change in proportion or adjustment of size shall still fall within the scope of the technical contents disclosed in the present invention without affecting the effects and purposes that can be achieved by the present invention.

[0044] Figure 1 A schematic diagram of the logical structure of a satellite position message spoofing detection and time-frequency fingerprint generation system provided by an embodiment of the present invention;

[0045] Figure 2 A schematic diagram of a flow chart of a method for detecting spoofing of a satellite position message provided by an embodiment of the present invention;

[0046] Figure 3 A schematic diagram of a flow chart of a method for generating a time-frequency fingerprint of a satellite position message provided by an embodiment of the present invention;

[0047] Figure 4 A schematic diagram of a flow chart of generating a real-time digital frequency fingerprint signal in a method for generating a time-frequency fingerprint of a satellite position message provided by an embodiment of the present invention;

[0048] Figure 5A schematic diagram of the architecture of a satellite position message spoofing detection and time-frequency fingerprint generation system provided by an embodiment of the present invention;

[0049] Figure 6 A schematic diagram of a real-time longitude and latitude change signal in a method for generating a time-frequency fingerprint of a satellite position message provided by an embodiment of the present invention;

[0050] Figure 7 A schematic diagram of the position change rate per unit time step in a method for generating a time-frequency fingerprint of a satellite position message provided by an embodiment of the present invention;

[0051] Figure 8 A schematic diagram of a real-time digitized frequency fingerprint signal in a method for generating a time-frequency fingerprint of a satellite position message provided by an embodiment of the present invention;

[0052] Fig. 9 A schematic diagram of generation frequency stability analysis in a method for generating a time-frequency fingerprint of a satellite position message provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0053] The following is a description of the implementation of the present invention by specific embodiments. People familiar with the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0054] In practical applications, when the satellite receiving chip is initially powered on and there are both real and spoofed signals, it is difficult to distinguish the real signal from the spoofed signal through the initial time information. At the same time, the generation and output of existing frequency signals often require high-precision time and frequency test equipment such as frequency counters and phase noise testers to assist in real-time generation. The equipment costs of testing and communication in this generation method are often very expensive, and it is difficult to generate high-precision frequency data on a large scale.

[0055] In order to solve the above technical problems that it is difficult to distinguish real signals and deceptive signals through initial time information and it is impossible to generate high-precision and high-reliability simple digital frequency fingerprints at low cost.

[0056] refer to Figure 1 The embodiment of the present invention discloses a satellite position message deception detection and time-frequency fingerprint generation system, the system comprising: a satellite receiving module 1; a clock initialization module 2; a clock frequency judgment module 3; a deception information detection module 4; a real-time longitude and latitude change signal acquisition module 5; a digital frequency conversion output module 6; and a stability analysis module 7.

[0057] Preferably, a satellite position message spoofing detection and time-frequency fingerprint generation system also includes a satellite spoofing information alarm module 8, which is used to execute the following information: when the satellite signal is a spoofing signal, issue an alarm message and temporarily block the satellite signal; when the satellite returns to normal, cancel the alarm message and resume receiving the satellite signal.

[0058] refer to Figure 5 The architecture consists of a host computer board, a satellite receiving module equipped with a chip atomic clock, a satellite deception information alarm module, and a system power supply module. During the power-on process of the timing equipment, the system power supply module provides a unified DC power supply.

[0059] The satellite receiving module equipped with a chip atomic clock receives satellite signals in real time and transmits the location message containing longitude and latitude information to the host computer MCU for processing, and generates satellite deception alarm information through the satellite position deception information alarm module and generates real-time digital 10MHz frequency fingerprint information through the digital frequency conversion output module.

[0060] Corresponding to the above-disclosed satellite position message deception detection and time-frequency fingerprint generation system, the embodiment of the present invention further discloses a satellite position message deception detection method. The following describes in detail a satellite position message deception detection method disclosed in the embodiment of the present invention in combination with the above-described satellite position message deception detection and time-frequency fingerprint generation system.

[0061] refer to Figure 2 The present invention discloses a method for detecting deception of satellite position messages, which is applied to a chip atomic clock and comprises:

[0062] S1. Receive satellite signals in real time and send a location message containing longitude and latitude information to an MCU, wherein the MCU extracts a preset format statement from the location message and obtains real-time longitude information and real-time latitude information from the preset format statement.

[0063] When the location information is obtained through GPS, the location message at this time is a NMEA-0183 message, and the preset format statement is a GNRMC format statement.

[0064] S2. Initialize the clock frequency clock and determine whether the clock value at this time belongs to the preset initial set. If not, the clock is set to 0. If it does, the clock value is increased by 1 after each message update.

[0065] S3. Detect whether the clock value at this time is greater than or equal to the first preset threshold value. If it is greater than or equal to, calculate the longitude and latitude change value according to the real-time message reference value; if it is less than, record the longitude and latitude reference value.

[0066] S4. After calculating the longitude and latitude change values, determine whether the longitude and latitude change values ​​at this time are greater than or equal to the second preset threshold value. If less than, the satellite signal at this time is a real signal. Calculate the longitude and latitude change values ​​according to the next real-time reference value and continue to determine whether the longitude and latitude change values ​​at this time are greater than or equal to the second preset threshold value.

[0067] S5. If the longitude and latitude change values ​​at this time are greater than or equal to the second preset threshold, the satellite signal at this time is a spoofing signal, and continue to judge whether the next longitude and latitude change value is less than or equal to the second preset threshold. If it is less than or equal to the second preset threshold, the satellite is restored to normal at this time, and the longitude and latitude change values ​​are calculated according to the next real-time reference value and continue to judge whether the longitude and latitude change values ​​at this time are greater than or equal to the second preset threshold. If it is greater than the second preset threshold, the satellite signal is still a spoofing signal.

[0068] The GNRMC satellite message position deception detection assisted by the chip atomic clock is based on the satellite timing protection equipment infrastructure. The equipment architecture consists of a host computer board, a satellite receiving module equipped with a chip atomic clock, a satellite deception information alarm module, and a system power supply module. During the satellite deception detection process, users can input reference values ​​based on the latitude and longitude information measured on the spot. After the device is turned on, it is decided whether to generate a satellite deception alarm message based on whether the real-time latitude and longitude position parsed from the satellite message exceeds the position change experience threshold.

[0069] Preferably, a method for detecting spoofing of satellite position messages also includes: when a satellite signal is a spoofing signal, issuing an alarm message and temporarily shielding the satellite signal; when the satellite returns to normal, canceling the alarm message and resuming receiving the satellite signal.

[0070] Furthermore, the preset initial set is [0,3600];

[0071] The position message is a NMEA-0183 message, and the preset format statement is a GNRMC format statement.

[0072] Furthermore, the first preset threshold is 10, and the second preset threshold is 80.

[0073] With the assistance of the chip atomic clock integrated in the board, the satellite receiving module can easily generate more convergent longitude and latitude position change information. By converting the longitude and latitude information into speed change information and converting it into frequency output changes, a real-time and reliable digital time-frequency output fingerprint can be obtained. The fingerprint itself has excellent statistical characteristics and can be applied to special time-frequency applications such as time-frequency cryptography, digital timestamps, and auxiliary atomic clock training.

[0074] In addition, the embodiment of the present invention also discloses a method for generating a time-frequency fingerprint of a satellite position message, which is applied to a chip atomic clock, referring to Figure 3, which includes: receiving satellite signals in real time and sending a location message containing longitude and latitude information to an MCU, wherein the MCU extracts a preset format statement from the location message and obtains real-time longitude information and real-time latitude information from the preset format statement; calculating longitude and latitude change values ​​according to a real-time message reference value and converting the longitude and latitude change values ​​into real-time longitude and latitude change signals; combining the real-time longitude and latitude change signals with a reference frequency to generate a real-time digital frequency fingerprint signal, and obtaining a standard digital time-frequency fingerprint signal.

[0075] When the location information is obtained through GPS, the location message at this time is a NMEA-0183 message, and the preset format statement is a GNRMC format statement.

[0076] Further, refer to Figure 4 , combining the real-time longitude and latitude change signal with a reference frequency to generate a real-time digital frequency fingerprint signal, and obtaining a standard digital time-frequency fingerprint signal, including: converting the real-time longitude and latitude change signal into a position change rate per unit time step; dividing the position change rate by the speed of light to obtain an instantaneous relative frequency deviation; and using the instantaneous relative frequency deviation and the reference frequency to generate a real-time standard digital time-frequency fingerprint signal.

[0077] refer to Figures 6 to 9 Based on the real-time latitude and longitude position output of GNRMC satellite messages, the real-time latitude and longitude change signals are generated. The real-time latitude and longitude changes are first converted into the position change rate per unit time step (m / s), and then converted into instantaneous relative frequency deviation by dividing by the speed of light. Finally, the real-time digital frequency fingerprint signal is generated in combination with the 10MHz reference frequency, and the frequency stability analysis (Allan Deviation) is generated.

[0078] Convert the longitude and latitude information of NMEA-0183 real-time message into real-time digital frequency information: First, calculate the longitude and latitude change deviation of the current time step by differential method; in the geodetic coordinate system, radians are usually used as units, and the longitude and latitude data are converted into radians; use the formula of the geodetic coordinate system to calculate the change of position, which can be done by calculating the distance between two moments. Assume that time is in seconds; calculate the instantaneous change of position and convert it into a frequency deviation fingerprint; combine the reference frequency output form (for example, take the typical value of 10MHz), and generate a standard digital frequency fingerprint based on the calculated frequency deviation.

[0079] Furthermore, the longitude range is [-π,π] and the latitude range is

[0080] Preferably, reference Fig. 9 A method for generating a time-frequency fingerprint of a satellite position message also includes: performing stability analysis on the standard digital time-frequency fingerprint to generate a stability analysis result.

[0081] Through the analysis of the frequency stability of the conversion generation, it is found that with the assistance of the chip atomic clock, although the frequency fingerprint stability of the longitude and latitude position conversion is slightly inferior to the frequency stability of the chip clock local oscillator, its frequency output short-term stable noise is smaller than that of the satellite receiver, and it has the characteristics of long-term stable convergence. Comprehensively speaking, it can be considered that it has the random application characteristics of long-term stable output.

[0082] Although the present invention has been described in detail above by general description and specific embodiments, it is obvious to those skilled in the art that some modifications or improvements can be made to the present invention. Therefore, these modifications or improvements made without departing from the spirit of the present invention all belong to the scope of protection claimed by the present invention.

Claims

1. A method for detecting satellite position message fraud, characterized in that: The method is applied to a chip atomic clock, and comprises: S1, receiving satellite signals in real time and sending a location message containing longitude and latitude information to an MCU, wherein the MCU extracts a preset format statement from the location message and obtains real-time longitude information and real-time latitude information from the preset format statement; S2, initialize the clock frequency clock and determine whether the clock value at this time belongs to the preset initial set. If not, the clock is set to 0. If it does, the clock value is increased by 1 after each message update. S3, detecting whether the clock value at this time is greater than or equal to the first preset threshold value, if it is greater than or equal to, calculating the longitude and latitude change value according to the real-time message reference value, if it is less than, recording the longitude and latitude reference value; S4, after calculating the longitude and latitude change value, determine whether the longitude and latitude change value at this time is greater than or equal to the second preset threshold value, if less than, the satellite signal at this time is a real signal, calculate the longitude and latitude change value according to the next real-time reference value and continue to determine whether the longitude and latitude change value at this time is greater than or equal to the second preset threshold value; S5. If the longitude and latitude change value at this time is greater than / equal to the second preset threshold, the satellite signal at this time is a spoofing signal, and continue to judge whether the next longitude and latitude change value is less than or equal to the second preset threshold. If it is less than / equal to the second preset threshold, the satellite is restored to normal at this time, and the longitude and latitude change value is calculated according to the next real-time reference value and continue to judge whether the longitude and latitude change value at this time is greater than the second preset threshold. If it is greater than the second preset threshold, the satellite signal at this time is still a spoofing signal.

2. A method for detecting satellite position message fraud as claimed in claim 1, characterized in that: The method further comprises: When the satellite signal is a spoofed signal, an alarm message is issued and the satellite signal is temporarily blocked; When the satellite returns to normal, the warning message is cancelled and the reception of satellite signals is resumed.

3. A method for detecting satellite position message fraud as claimed in claim 2, characterized in that: The preset initial set is [0,3600].

4. A method for detecting satellite position message fraud as claimed in claim 3, characterized in that: The first preset threshold is 10, and the second preset threshold is 80.

5. A satellite position message fraud detection system, characterized in that: The system comprises: A satellite receiving module, used for receiving satellite signals in real time and sending a location message containing longitude and latitude information to the MCU, wherein the MCU extracts a preset format statement from the location message and obtains real-time longitude information and real-time latitude information from the preset format statement; The clock initialization module is used to initialize the clock frequency clock and determine whether the clock value at this time belongs to the preset initial set. If it does not belong to it, the clock is set to 0. If it does belong to it, the clock value is increased by 1 after each message update. A clock frequency judgment module, used to detect whether the clock value at this time is greater than or equal to a first preset threshold value, if it is greater than or equal to, calculate the longitude and latitude change value according to the real-time message reference value, if it is less than, record the longitude and latitude reference value; The deception information detection module is used to calculate the longitude and latitude change values ​​and then determine whether the longitude and latitude change values ​​at this time are greater than or equal to a second preset threshold value. If less than, the satellite signal at this time is a real signal. The longitude and latitude change values ​​are calculated according to the next real-time reference value and continue to determine whether the longitude and latitude change values ​​at this time are greater than or equal to the second preset threshold value; if the longitude and latitude change values ​​at this time are greater than or equal to the second preset threshold value, the satellite signal at this time is a deception signal, and continue to determine whether the next longitude and latitude change values ​​are less than or equal to the second preset threshold value. If less than or equal to the second preset threshold value, the satellite has returned to normal at this time. The longitude and latitude change values ​​are calculated according to the next real-time reference value and continue to determine whether the longitude and latitude change values ​​at this time are greater than or equal to the second preset threshold value. If greater than the second preset threshold, the satellite signal at this time is still a deception signal.

6. A satellite position message fraud detection system as claimed in claim 5, characterized in that: The system further comprises: a satellite deception information warning module, which is used to execute the following information: When the satellite signal is a spoofed signal, an alarm message is issued and the satellite signal is temporarily blocked; When the satellite returns to normal, the warning message is cancelled and the reception of satellite signals is resumed.

Images