RU2654883C2 - Method of produced from spacecraft earth surface images temporal reference determining - Google Patents

Abstract

FIELD: astronautics.

SUBSTANCE: invention relates to space technology and can be used to determine the earth's surface shots from a spacecraft (SC) temporal reference. In the method for determining the produced from SC the earth's surface shots temporal reference, performing on board the time value generation and its transmission with the shots taken in the telemetric data array to the ground receiving point, maintaining onboard the SC a constant temperature for the time values generating equipment stable operation during the shooting, performing the selected image orthorectification, determining the position in the space of the point from which the shooting was performed by orthorectified image. Measuring the SC orbit parameters and determining the moment time of the SC location at the minimum distance from the point from which the selected image was taken. Determining the selected image temporal reference error as the difference between a certain time and the time value generated on board, and then determining the earth's surface temporal reference.

EFFECT: technical result of the invention is enabling an accurate temporal reference of the earth's surface images from the SC.

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Description

The invention relates to space technology and can be used to determine the timing of images of the earth's surface from a spacecraft (SC).

An important role in the process of obtaining scientific information in a space experiment is played by radio telemetry systems. With their help, information about the processes and phenomena under study, as well as the work of scientific equipment and service systems, is transmitted to the Earth. The information and telemetry system used in space experiments consists of an airborne part mounted on a spacecraft and a ground part connected with it by a radio link. The onboard part of the system contains devices for sensing primary information, collecting, converting and subsequently transmitting it to the ground part of the system, which has receiving, decrypting (converting), recording elements, and means for visually displaying the received information.

To transmit a large amount of information received in space flight, multichannel radio telemetry systems (RTS) with various methods of channel separation are used.

The most widespread in the provision of space flights were systems with frequency and time division of channels, due to a number of their technical and operational advantages.

In the frequency division, each channel is allocated a certain frequency band, within which the spectrum of that part of the signal that provides the transmission of information of this channel is practically fit. In time division, each channel is periodically provided with a certain time interval during which the signal of this channel is transmitted.

To transmit information received on board the spacecraft, measurements from sensors are converted into electrical quantities. Electrical signals on board the spacecraft arrive at summing and coding devices forming a telemetric frame (group signal). To separate information from each of the sensors used, special address signs are introduced. The group signal thus formed is emitted into space and received by ground-based points when the spacecraft passes over them.

The simplest way to temporarily coordinate telemetry measurements is implemented in the direct transmission (NP) mode of data to the Earth [1] A. Manovtsev Fundamentals of the theory of radio telemetry. M .: Energy, 1973.

In this case, the information received in the NP mode is automatically tied to the time used at the information receiving point.

However, this method is implemented only when the spacecraft is in the zone of the ground-based measuring point.

During the flight in orbit, the spacecraft periodically turns out to be out of sight of ground-based measuring points (for low-orbit vehicles, mainly sold in our country, most of the spacecraft’s flight does not have direct communication with ground-based points). Therefore, almost all scientific research spacecraft incorporate memory devices (with a capacity of up to 100 Gbps) for recording electrical signals containing information about the phenomena being studied.

To ensure the timing of information in the telemetry frame, special service signals are introduced, generated by the on-board generator of the reference time. Using these signals during ground processing and information analysis, the time moments of the occurrence of an event recorded on board the spacecraft are determined.

A known method, including generating on-board time stamps and transmitting them with the measured parameters of the on-board systems in the formed telemetry frame to the ground receiving point [1] Manovtsev A.P. Fundamentals of the theory of radio telemetry. M .: Energy, 1973. This method is used for most spacecraft having information recording devices.

In this case, the timing of measurements performed when the spacecraft is located at any point in the orbit is provided.

Part of the equipment on the spacecraft has its own time generation devices. For example, at orbital stations, photographic equipment is used that has its own time-generating devices (“built-in clocks”). Such photo equipment was used at the Salyut stations [2] Belyaev M.Yu. "Scientific experiments on spacecraft and orbital stations." M .: Mashinostroenie, 1984. The method for determining the timing of the Earth's surface images taken from the spacecraft, used in this case and taken by the authors as a prototype, included the generation of the time value on board and its transmission with the performed images in the telemetry data array to the ground receiving point.

However, experience shows that almost always there is an error in the formation of the reference time by the generator. This leads to a kind of “departure” of the generated time stamps and the appearance of a temporary error Δt, which, in some cases, can reach 2-3 minutes. The appearance of an error in the timing is facilitated by a change in the temperature of the time generation device.

An error in the timing of images of the Earth’s surface from the spacecraft leads to difficulties in recognizing the information received. If there are no objects in the image that can serve as reference points for decryption, it is often not possible to snap in time and process such an image.

Particularly relevant is the task of determining the exact timing of images of the Earth’s surface for the Russian segment of the International Space Station (ISS). On the ISS, astronauts use various photographic equipment for shooting, including manual. The basic data in the processing of images is the point in time taken automatically captured by the camera. Unfortunately, this parameter is not always accurate, because it depends on the uniformity of the built-in clock of the camera and requires constant adjustment by the crew. Sometimes such time is not enough for such control, especially if it is necessary to quickly remove a newly discovered object, outside the program of planned surveys, i.e. not having enough time to prepare.

But if it is possible to calculate the actual position of the ISS in orbit for a given image using at least one of the shots from a series of surveys with inaccurate time, then it becomes possible to restore the exact time of the surveys, since the time instant for a known position of the ISS is relatively easy to calculate. Then, the obtained time correction can be taken into account in the remaining pictures of this series and even in all subsequent pictures before adjusting the course of the clock, since the speed of “departure” of the camera’s built-in clock is approximately constant and most depends on the temperature of the camera.

The technical result of the proposed method is to ensure accurate timing of images of the earth's surface from the spacecraft.

The technical result is achieved by the fact that in the method for determining the timing of the Earth’s surface images taken from the spacecraft, including generating time values on board and transmitting them with the taken pictures in the telemetry data array to the ground receiving point, a constant temperature is maintained on board the spacecraft for stable operation instruments for generating time values during the shooting, perform orthorectification of the selected image, determine by orthorectified with The position in space of the point from which the survey was taken is measured, the parameters of the orbit of the spacecraft are measured and the moment of time the spacecraft is located at a minimum distance from the point from which the selected image was taken is determined, the error of the time reference of the selected image Δ is determined as the difference between a certain point in time and the time value t ^G generated on board and the timing of the images of the earth’s surface t are determined by the formula t = t ^G + Δ.

The initial data for solving the problem of determining the exact time reference is an identified and coordinate-linked (orthorectified) digital image, i.e. image for which the geographic coordinates of each pixel in the image are calculated.

To calculate the shooting point (the location of the camera), certain pixels are analyzed in the image — the central pixel of the image and the pixels lying on the circle inscribed in the rectangle of the image. Since the coordinates of each pixel are known, many values of the distances from the central pixel of the image to all pixels lying on the circle are calculated and two opposite ones from the center point of the pixel are selected, the distances to which are the largest from the central pixel.

An imaginary circle inscribed in the image is transformed on the earth's surface into a figure close to an ellipse (the intersection of the sphere with a cone), and the desired point of the camera is lying in the plane of the main vertical passing through the center of the image, two points of the semi-major axis of the ellipse (two selected points with maximum distance) and the center of the Earth.

Thus, the spatial problem is reduced to a simpler one, which is reflected in FIG. one.

In FIG. 1 is indicated:

- S is the desired shooting point;

- In - a point on the earth's surface corresponding to the center of the image;

- AB and BC - the greatest distances from point B;

- α is the half-angle of the image (the arc tangent of the angle determined by the known focal length and the size of the matrix with constant charge coupling, known for a given type of camera);

- τ is the angle taking into account the sphericity of the earth's surface and determined from the triangle ABC

From an analysis of the angles α, β, γ, δ, ε shown in FIG. 1 of the triangle system and the sine theorem it follows that:

AB / sinα = BS / sinβ; BC / sinα = BS / sinγ;

2α + β + γ + δ + ε = π; π-2α-δ-ε = τ-2α; β = τ-2α-γ;

From these relations, the angle γ is determined:

tgγ = sin (τ-α-α) / (cos (τ-α-α) + BC / ΛB)

or tgγ = (sinτ * cos2α-cosτ * sin2α) / (cosτ * cos2α + sinτ * sin2α) + BC / AB).

Having determined the angle γ, we can also determine the angle β. After that, the distance from the shooting point S to the center of image B is calculated as: BS = AB * sinβ / sinα.

To go from solving a flat problem back to a spatial one, the fact that the OB and BS vectors lie in the same plane and the angle between them is known is taken into account. If you select a “connected” coordinate system (SC), the Y axis of which passes through the OB, the X axis lies in the plane of the OAB and the Z axis complements the SC to the right or left, then in such a SC the vector BS has only X and Y coordinates as projections BS on the OB and the direction perpendicular to the OB, i.e. as BS * sin ρ and BS * cos ρ, where ρ is the angle between the OB and BS. Since the geographic coordinates of point B are known from the orthorectification of the image, the radius vector of the organic matter in the inertial SC, with its origin in the center of the Earth, is known. Thus, it is possible to compose a transition matrix from such an inertial SC to the “connected” SC selected above. Multiplying the BS vector in the “connected” CS by this transition matrix, we obtain the BS vector already in the inertial CS. And adding this vector with the radius vector OB, we obtain the radius vector OS in the inertial SC, i.e. the desired spatial position of the survey point S. By measuring the parameters of the spacecraft’s orbit, we can determine from them the time instant the spacecraft was at a minimum distance from point S from which the survey was performed.

To eliminate the influence of temperature factors on the error of the timing of the images of the earth’s surface from the spacecraft, it is necessary to maintain a constant temperature on it.

Currently, everything is technically ready for the implementation of the proposed method on the ISS Russian segment. The ISS has a set of cameras for shooting the earth's surface. To generate time values on board, there are time generators and clocks built into the camera equipment. For transmission to a ground station, the existing BITS, BSR-TM telemetry systems can be used. To maintain a constant temperature on the ISS, the existing CTP thermoregulation system can be used. The spacecraft orbit parameters can be determined using the receivers of satellite navigation systems GPS or GLONASS. A computer or other computing devices can be used to determine the position of the point from which the survey was taken, to determine the time when the spacecraft was at a minimum distance from this point, to determine the error Δ and the timing of images t.

The proposed method allows you to determine the timing of the Earth's surface images from the spacecraft, in the case of the presence of time-varying errors in the formation of on-board time stamps. The error obtained by this method of timing is less than 1 s.

Information sources

1. Manovtsev A.P. Fundamentals of the theory of radio telemetry. M .: Energy, 1973.

2. Belyaev M.Yu. "Scientific experiments on spacecraft and orbital stations." M .: Mechanical Engineering, 1984.

Claims (1)

A method for determining the timing of images of the Earth’s surface taken from a spacecraft, including generating time values on board and transmitting them with captured images in a telemetry data array to a ground receiving point, characterized in that they maintain a constant temperature on board the spacecraft for stable operation of the value generation apparatus time during the shooting process, orthorectification of the selected image is performed, the position in open space is determined by the orthorectified image Using the point from which the survey was taken, the orbit parameters of the spacecraft are measured and the moment of time the spacecraft is located at a minimum distance from the point from which the selected image was taken is determined by the error of the time reference of the selected image Δ as the difference between a specific time and the generated board the value of time t ^G and the timing of the images of the earth's surface t is determined by the formula t = t ^G + Δ.

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