What are Telemetry Signals?

Telemetry system refers to a system that has the functions of measuring, transmitting, and processing certain parameters of a measured object at a certain distance, that is, a system that transmits short-range measurement values of object parameters to a long-distance measurement station to achieve long-distance measurement . ^[1]

As shown in Figure 9-12, the telemetry system consists of an input device, a transmission device, and a processing device. Among them, input equipment includes sensors and signal conditioners; transmission equipment includes multiplexing modulators, transmitters, transmitting antennas at the transmitting end, and receiving antennas, receivers, and multiplexing demodulator at the receiving end; processing equipment includes recording and Display equipment and data processing computer systems. ^[2]

The working process of the telemetry system is divided into two parts: the transmitting end on the spacecraft and the receiving end on the ground. At the transmitting end, the sensor first converts the parameter signal into a weak current signal, and then the signal conditioner amplifies it into a normalized voltage signal suitable for collection. Then, the multi-channel signal modulator combines the multiple standardized signals according to a certain system to form It is suitable for the group signal transmitted by a single channel to modulate the carrier of the transmitter. The final modulated signal is amplified by the transmitter power and sent to the transmitting antenna for transmission, as shown in Figure 9-12 (a). At the receiving end, after the signal is transmitted to the receiving end via the radio link, the receiving antenna is first sent to the receiver for carrier demodulation to obtain the group signal, and then the various telemetry signals are restored by the multiplexing demodulator and sent to the processing equipment for processing. Record, display and data processing, as shown in Figure 9-12 (b). ^[2]

According to the type of transmitted signal, telemetry systems can be divided into analog telemetry systems and digital telemetry systems. The signal transmitted by the analog telemetry system is an analog signal. Therefore, when there is a digital parameter in the telemetry parameter, it is not necessary to transform it, and it can only be regarded as an analog parameter transmission. When the telemetry signal has an analog signal, the digital telemetry system needs to first sample and encode the analog signal into a digital signal before transmitting.

According to the channel multiplexing mode or signal division parameter mode, the telemetry system can be divided into frequency division system, time division system and packet division system. Among them, frequency division system and time division system have a long history and are widely used; packet division production is a new telemetry method, which has been gradually adopted in recent years.

Frequency division system modulates each signal to subcarriers of different frequencies, and the parameter of division is the frequency of the signal. To prevent aliasing, the frequency spacing of each subcarrier should be made sufficiently large. In the frequency division system, the signal is first modulated by a multi-channel subcarrier modulator, and then synthesized into a group signal by an adder; at the receiving end, the group signal is first divided into a group of band-pass filters. Multiple modulated signals, and then demodulate each signal with the corresponding subcarrier demodulator. In the frequency division system telemetry system, the multiplexed modulator is each subcarrier modulator and the addition after it. The multi-channel signal demodulator is a branched band-pass filter and a subcarrier demodulator for each channel. The advantages of frequency division system are simple equipment, but small capacity, low accuracy, and poor anti-interference ability. It is suitable for applications with fewer test parameters (less than 30). ^[2]

For example, the baseband electrical signal output by the encoder can be used to modulate the carrier wave, and the transmitter transmits it to the receiving end in a wired or wireless manner. The receiver, also known as a receiver, is used to amplify, convert, and detect the received electrical signal into a baseband signal and send it to the decoder. Decoder

The telemetry system is widely used. Here we take the aerospace as an example to introduce the function of the telemetry system.

Telemetry systems on rockets are an important part of rockets and spacecraft. In the test work of the rocket in the technical area and the launch area, the telemetry system participated in the test work before the launch of the rocket. The test is not only a test of the telemetry system itself, but also a test of the working conditions of the parts of the whole rocket related to telemetry. This test plays an important role in improving the reliability of rocket launch and flight reliability. Automatic procedures are used in the telemetry system detection process to ensure the safety and reliability of the launch. ^[3]

Frequently occurring faults in telemetry systems on microcomputer telecontrol equipment are: data failures and crashes and large amounts of bad data. The data failure crash is that the telemetry data is reflected as fixed dead data at the master station, and the graph is a straight line. This failure is generally similar to the cause of a large number of bad data, mainly due to the harmonics and spikes of the measured line. Stabilizing or disturbing voltage and current impacts the normal operation of the measurement circuit, causing the measurement circuit and the processing program to confuse and cause a crash. If the interference does not reach the worst case that causes the crash, a large number of absolute values will appear in the collected data Big data, that is, maximum and minimum values appear. This makes telemetry data unusable. Often, such transient impacts have large differences and are not durable, so according to the age of the equipment and the impact resistance of the equipment, the performance will be different: A large amount of data that exceeds the normal range appears momentarily, and then returns to normal; It appears for a long time Bad data; crash. ^[4]

The current telemetry system and computer technology are combined to achieve the standardization, modularization, and automatic management of the entire system of software and hardware; the single data stream code rate reaches 2 Mbit / s or higher; the telemetry subcarrier, code rate, and frame format All of them are programmable; all telemetry parameters can be stored in real time, and some parameters can be processed and displayed in real time; the recording uses a double density magnetic recorder, 8 mm digital magnetic recorder and magneto-optical disc in accordance with international standards. Future telemetry systems have the following development trends. ^[5]

(1) Standardization, serialization, modularization and miniaturization of telemetry systems. The telemetry system conforms to international standards and national military standards on the main parameters such as radio frequency band, transmission system, code pattern, multi-channel signal format, and records. The equipment unit has greater versatility and maintainability, and the station deployment is more flexible. ^[5]

(2) Engineering of telemetry software. With the rapid development of computer technology, the status of software in telemetry systems is becoming more and more important. From the programmable setting of parameters, storage, display, pre-processing to intelligent control management and testing of equipment, all rely on software to complete and can be reorganized. Technology allows programming of functions and protocols even in a set of hardware loads. Therefore, the efficiency of system operation depends to a large extent on the software's capabilities, and software is a complex task in the system development process. Only by implementing software engineering can the flexibility, operability, and reliability of telemetry equipment be effectively improved. .

(3) Telemetry standards are in line with international standards. The establishment and implementation of telemetry standards is an important sign reflecting the development of telemetry technology. At present, the CCSDS standard, which is a technical standard formulated by the Space Data System Advisory Committee (CCSDS), is basically unused in China's telemetry field. In order to be in line with international standards, it is imperative to develop and promote the CCSDS standard. CCSDS subcontracting telemetry technology and channel coding and decoding technology are the key technologies of the CCSDS standard. It is inevitable to develop a universal serial telemetry product that complies with the CCSDS standard. ^[5]