What Is Power Doppler?

Doppler velocity-measuring system is a radio tracking measurement system that uses the Doppler effect to determine the radial velocity of an aircraft. From the ground to the aircraft or from the aircraft to the ground, electromagnetic waves of a fixed frequency are emitted. Due to the relative movement between the aircraft and the measurement and control station, the frequency of the received signal and the frequency of the transmitted signal are different from each other. The difference in frequency is the Doppler frequency shift. The Doppler frequency shift is proportional to the velocity component in the direction of the measurement and control station-spacecraft, so the relative radial velocity of the target to the measurement and control station can be obtained by measuring the magnitude of the Doppler frequency shift.

Doppler velocimetry system

Doppler velocity-measuring system is a radio tracking measurement system that uses the Doppler effect to determine the radial velocity of an aircraft. From the ground to the aircraft or from the aircraft to the ground, electromagnetic waves of a fixed frequency are emitted. Due to the relative movement between the aircraft and the measurement and control station, the frequency of the received signal and the frequency of the transmitted signal are different from each other. The difference in frequency is the Doppler frequency shift. The Doppler frequency shift is proportional to the velocity component in the direction of the measurement and control station-spacecraft, so the relative radial velocity of the target to the measurement and control station can be obtained by measuring the magnitude of the Doppler frequency shift.

Chinese name

Doppler velocimetry system

Foreign name

Doppler velocity-measuring system

Types of

Radio tracking measurement system

Classification

Speed measurement system

Make use of

Doppler effect

Function

Determine the radial speed of the aircraft

The Doppler velocimetry system has various forms, and is divided into single-pass and two-pass Doppler velocimetry systems according to the position of the electromagnetic wave radiation source; according to the number of frequencies emitted by the signal source, it is divided into single-frequency and dual-frequency velocimetry systems.

• One-way Doppler velocimetry system

Beacons (including crystal oscillators, frequency doublers, and power amplifiers with thermostats) and ground receiving equipment (including receiving antennas, phase-locked receivers, Doppler frequency shift extractors, and speed measurement terminal equipment) on the aircraft composition. The beacon transmits a radio signal to the ground, which is received by the ground antenna, and directly measures the Doppler frequency shift of the one-way propagation of the radio wave between the aircraft and the measurement and control station, thereby obtaining the distance change rate, that is, the radial velocity data. In order to avoid zero and negative Doppler frequency shifts in the terminal equipment when the aircraft approaches and leaves the measurement and control station, often when artificially extracting the Doppler frequency shift, a value greater than the highest Doppler frequency is artificially added. Shift the offset frequency, and finally subtract this frequency from the measurement result to get the real speed data. The measurement accuracy of this system is mainly determined by the frequency stability of the beacon on the aircraft.

• Two-pass Doppler velocimetry system

It is usually composed of ground transmitter, ground receiver, transmitting antenna, receiving antenna and transponder on the aircraft. It places the transmitting and receiving equipment on the same measuring point, and the transmitter sends a highly stable signal to the aircraft via the antenna, and at the same time sends this signal to the ground receiver as a reference signal. This signal is forwarded by the aircraft transponder or reflected by the aircraft back to the observation point. Due to the relative movement between the spacecraft and the measurement and control station, the frequency of the return signal received by the ground receiving equipment is different from the frequency of the transmitted signal. The return signal can be compared with the reference signal to obtain the Doppler frequency of the round-trip signal. (Corresponding to twice the rate of change of the distance between the aircraft and the observation point), so as to obtain the radial velocity of the aircraft. The speed measurement data of this system is not affected by the frequency drift of the signal source. If the transmitting device and the receiving device are separated, the information extracted by the receiving device is the sum of the Doppler frequency shift of the uplink signal and the Doppler frequency shift of the downlink signal, which corresponds to the uplink and downlink distance and change rate. For example, a multi-station receiving device set at different positions can simultaneously receive signals transmitted or reflected by the aircraft, and multiple distances and change rates can be obtained. Subtracting the two distances and the rate of change yields the rate of change of the distance difference. This system usually uses a multi-station system.

• Dual-frequency speed measurement system

The single-pass dual-frequency speed measurement system is mostly used, and its equipment composition is basically the same as that of the single-pass Doppler speed measurement system. The beacon on the spacecraft simultaneously sends two radio signals with a certain multiple relationship. After receiving by the same receiver on the ground, the two Doppler frequency shifts are extracted and processed accordingly, which can eliminate the ionosphere's electromagnetic wave propagation speed. The effect of improving speed measurement accuracy. This system can also modulate the telemetry signal on the beacon signal, and use the same channel to complete the two functions of tracking and telemetry. It is especially suitable for tracking measurement of near-Earth spacecraft.

What the Doppler velocimetry system obtains is radial velocity information. The distance information can be obtained by integrating the velocity information under the condition of obtaining the initial distance. The accuracy of the distance information mainly depends on the accuracy of the initial distance. The initial distance can be provided by other precision ranging equipment, such as laser ranging equipment. In the 1980s, a photoelectric multi-station measurement system combining laser and Doppler velocimetry was available. The Doppler speed measurement system can be an independent measurement system, or it can be used as a speed measurement sub-system in a continuous wave multi-station system and a microwave unified system.