CN113503965A

CN113503965A - Method for realizing synchronous imaging of satellite-borne imaging spectrometer by using satellite platform second pulse

Info

Publication number: CN113503965A
Application number: CN202110693725.3A
Authority: CN
Inventors: 王煜; 常振; 林方; 刘国华
Original assignee: Anhui University
Current assignee: Anhui University
Priority date: 2021-06-22
Filing date: 2021-06-22
Publication date: 2021-10-15
Anticipated expiration: 2041-06-22
Also published as: CN113503965B

Abstract

A method for realizing synchronous imaging of spaceborne imaging spectrometers by utilizing satellite platform second pulses of the present invention comprises using FPGAs of two imaging spectrometers to simultaneously detect second pulse signals from satellite platforms, extracting the leading edge of second pulses to generate internal second synchronization signals; The existing second pulse resets the internal timing logic of the FPGA, and the exposure time of the frame transfer CCD is set as its imaging cycle. Two CCDs with different exposure times will complete one or several complete imaging at the arrival of each second pulse. , and start the next round of shooting at the same time; make the imaging spectrometer use the frame transfer CCD, and its pixel readout time and exposure time are overlapped, that is, during each exposure, the readout of the previous image is completed. The invention uses the on-satellite second pulse, and can realize synchronous imaging of multiple imaging spectrometers with different exposure times by designing the exposure sequence and the CCD readout sequence without adding hardware, and the synchronization accuracy is better than 100ns.

Description

Method for realizing synchronous imaging of satellite-borne imaging spectrometer by using satellite platform second pulse

Technical Field

The invention relates to the technical field of satellite-borne imaging spectrometers, in particular to a method for realizing synchronous imaging of a satellite-borne imaging spectrometer by using a satellite platform pulse per second.

Background

The atmospheric observation satellite-borne imaging spectrometer performs spectral imaging on the ground (the area array CCD camera is pushed and swept under multiple spectral lines). And simultaneously acquiring the spatial dimension and spectral dimension information of the target area. And obtaining the distribution of the atmospheric components through the spectral curve of each space dimension point. In order to obtain a wider distribution of gas components, a wider spectrum detection range is required, for example, two imaging spectrometers in the ultraviolet band and the visible band are placed on the same low-orbit satellite platform. Because atmospheric observation has low requirement on geographical resolution, each imaging spectrometer works independently in the past, and geographical coordinates of each line of images are marked by image time scales (on-satellite broadcast time, accurate to seconds). The imaging beats of the two spectrometers depend on respective crystal oscillators, so that the data time scales are staggered due to asynchronous images, and further, the data products, namely an ultraviolet waveband atmospheric distribution graph and a visible waveband atmospheric distribution graph can be staggered by 1 line of images at most and correspond to the geographical position, namely the deviation of 14 kilometers (the flight distance of a low-orbit satellite in two seconds).

Currently, the synchronous control of multiple CCDs is required to be generally in the following forms:

1) homology with the plate: the multi-channel CCD shares one crystal oscillator and one controller. The design is only limited to a small spectrometer, and for the spectrometers with complex light paths and large calibers, focal planes of two spectrometers are arranged in a close range, so that the light paths and mechanical design of the spectrometers are limited, and the spectrometers are not suitable for complex light paths.

2) Using a synchronous controller: an external trigger signal is provided for each camera, and the external trigger signals at different exposure times are synchronized over a long period. Therefore, extra hardware circuits are added to the two existing satellite-borne spectrometers.

3) And synchronizing through a host computer control command: according to the space multi-channel TDICCD camera synchronization method, hardware is not added, and only a load control computer needs to send a starting command through an RS-422 serial port. The two cameras work after receiving the command. However, because the mode is software operation, the satellite main control CPU simultaneously serves a plurality of tasks, and the time delay is uncontrollable; the invention only carries out imaging synchronization when the system is started, is easy to cause error accumulation and is not suitable for synchronization of a continuous photographing type multi-channel imaging spectrometer.

4) Clock phase-locked loop synchronization: the imaging plate timing system circuit uses a high-stability crystal oscillator and a voltage-controlled regulating circuit to construct a phase-locked loop, and uses a tame clock algorithm to obtain a real-time clock. The two spectrometers are operated in a timed photographing mode. This solution also requires additional hardware circuitry.

With the improvement of the requirement of measurement accuracy, the overlap ratio of the gas distribution diagram detected by the ultraviolet band and the gas distribution diagram detected by the visible band is higher, so that the requirement of synchronous shooting of the two imaging spectrometers is provided.

Disclosure of Invention

The invention provides a method for realizing synchronous imaging of a satellite-borne imaging spectrometer by using satellite platform second pulse, which uses satellite second pulse signals, enables two spectrometers to synchronously shoot by setting the exposure time with the second as a period and designing a corresponding CCD chip reading time sequence, obtains an image overlapping error not exceeding 100 nanoseconds, and does not increase more hardware overhead.

In order to achieve the purpose, the invention adopts the following technical scheme:

a method for realizing synchronous imaging of a satellite-borne imaging spectrometer by utilizing a satellite platform pulse per second is based on two imaging spectrometers and a satellite platform, an imaging circuit of the imaging spectrometer comprises an FPGA, a crystal oscillator and a CCD, and comprises the following steps,

FPGA of two imaging spectrometers detect pulse-per-second signals from a satellite platform at the same time, and extract leading edges of the pulse-per-second signals to generate internal second synchronization signals; the satellite-borne imaging spectrometers are arranged to take continuous shots, frame transfer CCDs are adopted, and the two imaging spectrometers are respectively an ultraviolet channel and a visible channel;

resetting the internal sequential logic of the FPGA by utilizing the existing pulse per second, fixing that each second is taken as a CCD operation period, and resetting the value of an exposure time period register by using a second synchronous signal by an exposure and pixel reading module;

defining the exposure time gear of the imaging spectrometer as an integer frequency division value of second, fixing the CCD reading period as an exposure period shorter than the highest frame frequency, and selecting the crystal oscillator frequency according to the total beat number of the reading logic to ensure that the difference value of the reading time and the exposure time meets the redundancy of crystal oscillator errors and system time errors and ensures that the last image is read out earlier than the pulse arrival time of second;

the whole reading time of the CCD is designed to be the exposure time which is less than the highest frame frequency, the error redundancy of the crystal oscillator is met, when an exposure control counter counts a set value, if the exposure control counter is not the last exposure of a second period, resetting and counting again, and starting the next exposure;

if the current time sequence is the last one, stopping counting, waiting for the reset of the pulse per second, resetting the exposure time counter and other corresponding control registers preferentially by the pulse per second in any state, finishing the current time sequence and starting a new time sequence generation period.

Further, the method also comprises integer frequency division defining the exposure time as one second, namely 1/16S, 1/12S, 1/8S, 1/6S, 1/4S, 1/3S, 1/2S and 1S, which are 8 grades.

According to the technical scheme, the method for realizing the synchronous imaging of the satellite-borne imaging spectrometer by using the satellite platform pulse per second realizes the synchronous imaging of 2 or more imaging spectrometers by using the existing pulse per second signal provided by the satellite platform, does not increase extra hardware overhead, and is simple and reliable.

In general, the invention uses satellite second pulse, under the condition of not increasing hardware, the synchronous imaging of a plurality of imaging spectrometers with different exposure time can be realized by designing exposure time sequence and CCD reading time sequence, and the synchronous precision is superior to 100 ns.

Drawings

FIG. 1 is a schematic diagram of an imaging spectrometer imaging system;

FIG. 2 is a push-scan imaging of the imaging spectrometer and the results;

FIG. 3 is a schematic diagram of the deviation between the time scale and the actual exposure under the condition of independent photographing by two spectrometers;

FIG. 4 is a system block diagram of the present invention;

fig. 5 shows the FPGA logic design of the camera imaging circuit of the present invention (when the exposure time is 1/4S, other exposure times are similar).

Detailed Description

In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all, embodiments of the present invention.

The imaging spectrometer is composed of a telescope, a slit, a grating and an area array detector (CCD or CMOS), light passing through the slit is divided into spectra through the grating and is scattered and projected along one direction of the area array CCD, and the other direction of the CCD corresponds to the position of a geometric pixel. Thus, the image obtained by each image shot by the CCD is a distribution of different spectra of a narrow-band target (as shown in FIG. 1).

As shown in fig. 2, an imaging spectrometer loaded with an area array CCD having 1024 × 1024 pixels is mounted on a satellite platform, and when taking a picture of the ground, it is equivalent to that 1024 line arrays CCD scan a map on different spectral lines at the same time, and each image is 1024 spectral lines of a geographically narrow band (1024 pixels).

The satellite-borne atmospheric observation imaging spectrometer performs imaging on the ground, and can obtain the distribution of different gas components according to the characteristic absorption spectrum lines of different gases, thereby providing environmental protection and meteorological data.

In order to observe more gases, imaging spectrometers in different spectral bands can be used and mounted on the same satellite platform, for example, the invention is applied to the case that two imaging spectrometers in ultraviolet and visible spectral bands are mounted on the same satellite platform.

In the past, because the requirement of atmospheric observation on the geographic resolution is not high, two spectrometers shoot independently. And respectively performing map matching according to the data time scales of the two spectrometers. The time scale is the broadcast time provided by the satellite platform, with a precision of 1S. The atmospheric profiles of the two spectrometers are therefore at most likely 7 km out of position (flight distance of the low orbit satellite 1S). As shown in fig. 3, taking one-second exposure time as an example, two spectrometers work with their respective crystal oscillators, and record the imaging time scale (precision 1S) at the initial time of each exposure; the crystal oscillation error of the imaging spectrometer 1 is assumed to be + 30%, and the error of the spectrometer 2 is assumed to be-16.7%, when the time scale of the 4 th image taken by the spectrometer is 4, and the time scale of the 6 th image and the 7 th image taken by the spectrometer 2 is 4, that is, one image with the same time scale in the two spectrometer data is inconsistent due to the real exposure starting time, which may cause the deviation of a line of images (one line of images is the imaging result in time) in the geographic position at most. Note that in one image, corresponding to 1024 spectral lines, there is actually one line in the geographic direction, i.e., a single-line image of 1024 pixels. Thus, the spectrometer geographic coordinates of the two channels are staggered by up to 7 km.

And because of the difference of the respective crystal oscillators, the phase difference is not fixed, but gradually changes periodically. In addition, it should be noted that, in the application of imaging spectrometers, the synchronous deviation of two imaging spectrometers cannot be changed by improving the timing precision, because unlike a frame-type imaging device, the geographic imaging corresponding to one image of the spectrometer is actually a line of pixels, and each pixel is integrated for one second (7 km) in the flight direction, and cannot be subdivided.

With the recent increase in the requirement of users for observation accuracy, the requirement for the coincidence degree of the ultraviolet and visible spectrum atmospheric composition distribution maps is higher. For this purpose, two imaging spectrometers are required to shoot synchronously.

As shown in fig. 4, in the method for implementing synchronous imaging of a satellite-borne imaging spectrometer by using a satellite platform pulse per second according to the embodiment of the present invention, synchronous imaging of 2 or more imaging spectrometers is implemented by using an existing pulse per second signal provided by a satellite platform.

The method comprises the following specific steps:

the method is realized by a camera imaging circuit, wherein a plurality of cameras use the imaging circuit; the imaging circuit comprises core elements such as an FPGA (field programmable gate array) and a crystal oscillator; as shown in fig. 4.

FPGA of the two imaging spectrometers simultaneously detect pulse-per-second signals from the satellite platform, extract the leading edge of the pulse-per-second to generate internal second synchronous signals, and then realize synchronous imaging of the two cameras through FPGA logic design.

The FPGA logic design comprises an imaging time sequence and a combinational logic circuit, the CCD imaging time sequence of the spectrometer is generated by an imaging circuit board, and the clock reference of the logic circuit comes from an on-board crystal oscillator. It is not possible to use the same crystal for both spectrometers. To synchronize the two spectrometers, an external homologous trigger signal is required. Designing a trigger generator separately requires the addition of additional circuitry. The invention uses the existing pulse per second signal of the satellite platform as a synchronous signal source, and completes the photographing synchronization of the two spectrometers on the basis of not increasing hardware.

The second pulse is generated by satellite time system equipment and transmitted to each single machine on the satellite, and a single machine timing information source is provided by matching with the broadcast time packet. The photographing timing of the CCD is controlled by an FPGA (field programmable logic array) of the CCD imaging circuit, using the pulse per second as a synchronization signal, and the timing logic of the FPGA makes the following design:

the exposure time is defined as an integer frequency division of one second, namely 1/16S, 1/12S, 1/8S, 1/6S, 1/4S, 1/3S, 1/2S,1S, and 8 grades. Therefore, different spectrometers can complete the exposure of the whole frame at the whole second time by using different exposure time.

Then, the logic design method of the FPGA is as follows:

resetting the internal sequential logic of the FPGA by utilizing the existing pulse per second on the satellite, fixedly taking each second as a CCD operation period, and resetting the value of an exposure time period register by using a second synchronous signal by an exposure and pixel reading module;

defining the exposure time gear of the imaging spectrometer as an integer frequency division value of second, fixing the CCD reading period as an exposure period (1/16S) which is slightly shorter than the highest frame frequency, and selecting the crystal oscillator frequency according to the total beat number of the reading logic so that the difference value between the reading time and the exposure time meets the redundancy of crystal oscillator errors and system time errors and can ensure that the last image is read out earlier than the pulse arrival time of second;

the whole reading time of the CCD is designed to be slightly less than the exposure time of the highest frame frequency, crystal oscillation error redundancy is met, when an exposure control counter counts a set value, if the exposure control counter is not the last frame of a second period, resetting and counting again, and starting the next exposure. If the last frame is the frame, stopping counting, waiting for the reset of the pulse per second, resetting the exposure time counter and other corresponding control registers preferentially by the pulse per second in any state, ending the current time sequence, and starting a new time sequence generation period.

The specific explanation is as follows:

1) in order to ensure the detection efficiency, the satellite-borne imaging spectrometer continuously shoots and adopts a frame transfer CCD. The two imaging spectrometers are respectively an ultraviolet channel and a visible channel, and the exposure time of the two channels may be set differently for different background brightness of the ground. To achieve second synchronization, the exposure gear needs to be set to an integer fraction of a second. The invention uses 1/16 seconds, 1/12 seconds, 1/8 seconds, 1/6 seconds, 1/4 seconds, 1/3 seconds, 1/2 seconds and 8 exposure time steps of 1 second, namely 16 frames, 12 frames and 8 frames of … 1 frames are continuously shot per second. The frame transfer CCD exposure time is its imaging period,

2) the imaging spectrometer uses a frame transfer CCD with overlapping pixel readout times and exposure times, i.e. during each exposure, the readout of the previous image is completed, as shown in fig. 5. Without synchronization requirement, the CCD operation usually designs the exposure time as the readout time, so as to utilize the exposure period to the maximum extent and reduce the readout rate, so as to obtain greater transfer efficiency and less readout noise. However, since the use of the second pulse synchronization causes an error in the accuracy of the crystal oscillator of the imaging circuit itself, 1S of the crystal oscillator timing may not coincide with one second of the second pulse. The satellite second pulse is a GPS signal or a time service signal, is tamed by a clock source with high stability and is standard second. The crystal oscillator used by the imaging circuit is a common precision aerospace-grade crystal oscillator, the precision error is 50ppm, and the temperature drift and the long-term drift also reach 50ppm, namely, 1 second of the timing of the circuit crystal oscillator can be +/-100 microseconds different from the standard second. The read operation of the CCD chip requires 2950000 beats, and if this beat is used as the crystal clock number, the required crystal frequency is 2950000clocks × 16 beats per second, i.e. 47.2 MHz. On the premise of no need of synchronization, a crystal oscillator of 47.2MHz can be adopted. However, the second pulse synchronization method needs a slightly higher frequency, otherwise, due to the crystal oscillator error, it may cause that when the second pulse arrives, the reading of the last image is not completed (the crystal oscillator is slightly slower than the nominal value), a new period starts, and the pixel data is lost. For this reason, a 48MHz crystal oscillator, slightly larger than the read-out period, can obtain a margin of about 1mS, and can completely eliminate the errors of the crystal oscillator and the second pulse. That is, the shortest exposure time was 1/16 seconds (62.5mS), the operating frequency of the 48MHz crystal oscillator, the exposure period was 3000000 clocks, and the reading time was 2950000clocks (61.5 mS). The end of readout was 1mS earlier than the end of exposure. After the last image read out per second is complete, there are theoretically 50000clocks (1mS) to complete the exposure with the arrival of a second pulse. Due to crystal error, the pulse per second may be earlier or later than this clock number, but the error is not more than 1 mS. Meanwhile, after the exposure period of the last image counts 3000000 clocks, the reset is not actively carried out, but the second pulse reset is waited, and the image of the exposure period starts to be read out after the reset. If no 3000000 clock-second pulses arrive, the reset is forced to start the next cycle, as shown in FIG. 5. Thus, the exposure time of the last image contains the accumulated crystal error within one second (+/-100 microseconds), but is also allowed by the measurement accuracy.

Based on the design, different imaging spectrometers of the same satellite platform can achieve the synchronization precision of 20 nS-100 nS, and the synchronization precision depends on the distance between the two spectrometers and the difference of second pulse signal transmission time delay.

In conclusion, the invention uses the satellite second pulse, and can realize synchronous imaging of a plurality of imaging spectrometers with different exposure times by designing the exposure time sequence and the CCD reading time sequence under the condition of not increasing hardware, and the synchronous precision is superior to 100 ns.

The above examples are only intended to illustrate the technical solution of the present invention, but not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present invention.

Claims

1. a method utilizing satellite platform second pulse to realize the synchronous imaging of spaceborne imaging spectrometer, based on two imaging spectrometers and satellite platform, the imaging circuit of described imaging spectrometer comprises FPGA, crystal oscillator and CCD, it is characterized in that: comprise the following steps,

The FPGA of the two imaging spectrometers simultaneously detects the second pulse signal from the satellite platform, and extracts the leading edge of the second pulse to generate an internal second synchronization signal; the on-board imaging spectrometer is set to shoot continuously, using a frame transfer CCD, and the two imaging spectrometers are ultraviolet and visible, respectively. aisle;

Use the existing second pulse to reset the internal timing logic of the FPGA, and fix one CCD operation cycle per second, and the exposure and pixel readout modules use the second synchronization signal to reset the exposure time period register value;

The exposure time gear of the imaging spectrometer is defined as the integer frequency division value of seconds, and the CCD readout period is fixed to be shorter than the exposure period of the highest frame rate. The difference between time and exposure time satisfies the redundancy of crystal oscillator error and system time error, ensuring that the last image per second is read earlier than the arrival time of the second pulse;

The CCD whole frame readout time is designed to be less than the exposure time of the highest frame rate, which satisfies the crystal oscillator error redundancy. When the exposure control counter reaches the set value, if it is not the last frame of the one-second period, it will reset and count again, and start the next one. frame exposure;

If it is the last picture, stop counting and wait for the reset of the second pulse. In any state, the second pulse will reset the exposure time counter and other corresponding control registers first, end the current sequence, and start a new sequence generation cycle.

2. the method that utilizes the satellite platform second pulse to realize the synchronous imaging of spaceborne imaging spectrometer according to claim 1, is characterized in that: also comprises the integer frequency division that defines exposure time as one second, namely 1/16S, 1/12S, 1/8S, 1/6S, 1/4S, 1/3S, 1/2S, 1S total 8 gears.