Design and Implementation of GPS Radiosonde Communication System
introduction
Among the many high-altitude detection methods, GPS radiosonde, as a new type of detection method, has the characteristics of global coverage, low cost, high detection accuracy and high vertical resolution. It is more and more widely used by meteorological, water conservancy, civil aviation and other business departments. Local application has huge market demand. Therefore, the development of GPS radiosonde has good economic and social benefits.
The design of the communication system of the GPS radiosonde is to encode and digitally modulate the temperature, humidity, pressure and GPS positioning data detected by the GPS radiosonde in real time, then amplify it into a digital radio frequency signal by the power amplifier, and then transmit it back to the receiver from the air. After demodulating and decoding the received signal, it is sent to the computer for data processing.
Design and implementation of communication system
The communication system is composed of a transmitting part and a receiving part, and the working mode is simplex. The transmitting part is composed of a transmitting module and a transmitting antenna. The receiving part is composed of a receiving antenna, an outdoor gimbal, a cable protector, a low-noise amplifier, a 30-meter feeder and a receiving module. The system block diagram is shown in Figure 1.
Antenna design
The antenna is an important part of the communication system. In high altitude, due to the large wind speed and uncertain wind direction, the transmitting antenna uses an omnidirectional antenna made of 1 / 4λ flexible cable, the reception level fluctuation is small, the gain is about -3 dBi, and the length is about 17.7cm (before use The standing wave ratio must be strictly tested, and the length should be appropriately increased or decreased according to the standing wave.) It has been proved by GPS radiosonde's multiple flying tests that the transmitting antenna made of 1 / 4λ flexible cable can not only receive a good receiving effect, but also save a lot of costs.
The receiving antenna uses a Yagi directional antenna with a relatively wide beam width (horizontal lobe width of 65o and vertical lobe width of 53o) and a gain of 10dBi. Because the GPS radiosonde has azimuth uncertainty during the lift-off, the receiving antenna must be equipped with a servo tracking mechanism. The purpose of using Yagi antenna is to reduce the complexity of servo tracking and reduce the amount of equipment.
In practical applications, the PTZ is used as the servo tracking mechanism because the receiving antenna is lighter and the beam width is wider. The gimbal decoder controls the rotation of the outdoor gimbal, and the communication with the computer adopts RS-485 level, supports long-line transmission, and adopts the PELCO-D protocol. Figure 2 is a schematic diagram of the control process of the outdoor gimbal.
Transmitting and receiving module
In the design of transmitting and receiving modules, the use of wireless transceiver chips for data transmission is a good choice. It has the characteristics of low cost, high integration, and easy implementation. From the aspects of working frequency and receiving sensitivity, this design uses Chipcon's CC1020 chip, and the modulation method is GFSK. CC1020 is an ideal single-chip programmable RF transceiver chip, dedicated to low-power and low-voltage radio products, especially used in narrow-band systems. It is programmed to work at 300 ~ 1000MHz. It integrates multiple functions such as radio frequency transmission, radio frequency reception, PLL synthesis, GFSK modulation and demodulation, programmable control, etc. The main working parameters can be changed through serial bus interface programming.
In the transmit mode, the synthesized RF signal is directly fed to the power amplifier PA, and the GFSK signal output by the radio frequency is generated by GFSK modulation from the baseband signal fed to the DIO pin. To get 200mW output power from the transmitting module, a low power amplifier must be used to drive the output. Considering the convenience of design and debugging, the integrated monolithic amplifier ECG003 of WJ company is adopted. ECG003 is a highly dynamic general-purpose amplifier, using InGaP / GaAS HBT technology, ECG003 P1dB is 24dBm, Gain is 20dB, CC1020 output only needs 3dBm. The software is divided into CC1020 launch mode configuration software and data frame encoding software. The launch mode configuration software is completed by the microcontroller through the configuration of the CC1020 register. The data frame encoding software is to assemble the transmitted data plus the synchronization header and end character into a frame Transmitted into the transmitter becomes radio frequency data and transmitted. Since the data format of the sync header is asynchronous UART mode, AAAA33CC should be used for non-connected 0 or 1 bytes to receive a stable DC level. The single-chip computer uses Atmel's ATmega8L. Its chip integrates a large-capacity memory and a rich and powerful hardware interface circuit. It has high performance and low price. It can be programmed with C language. The block diagram of the transmitting module is shown in Figure 3.
In the receiving mode, CC1020 can be regarded as a traditional superheterodyne receiver. The RF input signal is amplified by low-noise amplifiers (LNA and LNA2), and then flipped through an integrator (I and Q) to generate an intermediate frequency IF signal. In the intermediate frequency processing stage, the I / Q signal is mixed and filtered, amplified and converted into a digital signal by A / D, and then automatically acquired control, channel filtering, demodulation and binary synchronization processing, output demodulated data on the DIO pin. The receiving module configures the CC1020 register through the microcontroller to complete the receiving mode. CC1020 is set to automatic power-up sequence state, and it will be automatically processed by a wake-up signal for power signal processing and carrier signal detection. The single-chip microcomputer sets CC1020 to enter the receiving mode. The built-in bit synchronizer synchronizes the synchronization clock with the incoming data and decodes the data. After data filtering, bit synchronizer, data decoding and other processing, valid data is output. After removing the synchronization header and terminator, the MCU transfers the data to the computer through the serial port. Set the bandwidth of the IF filter to 50kHz, and the look-up table shows that the GFSK demodulation requires a signal-to-noise ratio of 9 dB at a bit error rate of 10-2. According to the calculation, the noise figure is 1.6 dB, and the receiving sensitivity is calculated as follows:
Smin = -114 + NF + 10lgB + S / N (1)
After calculation, the receiving sensitivity is -116 dBm, which meets the overall design requirements.
The single chip microcomputer can also read the received signal strength indicator (RSSI) register data, convert it into the receiving level and send it to the digital tube for display, which can directly reflect the current receiving level. The block diagram of the receiving module is shown in Figure 4.
Selection of feeder and low noise amplifier
A 30-meter SYV-50-5 feeder is used between the receiving antenna and the receiver, and the attenuation is about 10 dB.
The low-noise amplifier has a gain of 20 dB and a noise figure of 1.0 dB. Considering that the placement of the low-noise amplifier at the rear stage of the feeder will affect the sensitivity of the receiver, it is necessary to install the low-noise amplifier at the front stage of the feeder.
In order to avoid the risk of lightning strikes and protect the safety of personnel and equipment, we have connected a coaxial cable protector in series between the antenna and the feeder, and its insertion loss is 0.3 dB.
Design and calculation of key indicators
Transmission distance
The total gain of the receiving channel is 20dB, the gain of the transmitting antenna is Gt = -3dBi, the transmitting power is Pt = 200mW = 23dBm, the radio wave is transmitted in space, and its free space loss is calculated as follows:
Ls = 32.45 + 20logf (MHz) + 20logD (km) (2)
Radio waves are emitted from the transmitter module, radiated through the antenna, and propagated through the air. The signal is attenuated. When it reaches the receiver, the received field strength level will be calculated as follows:
Pr = Pt + Gt + Gr-Ls (3)
According to the requirements of the overall index, when the maximum transmission distance is 200km, the free space loss is Ls = 130.6dB, and the reception level to the receiver is -90 dBm. The receiving sensitivity is -115dBm, and the technical fading reserve is 25dB. The more technical fading reserves, the stronger the anti-jamming capability and the fewer bit errors.
Assuming that the closest distance of the GPS radiosonde to the receiving antenna is 20m, the free space loss is Ls = 50.6dB, the receiving level to the receiver is -10dBm, and the saturated receiving level of the receiver is 10dBm, which can satisfy Distance transmission requirements.
Line of sight
The transmission distance of radio waves depends not only on the power, but also requires the receiver to have a certain technical fading reserve, but is also affected by the height of the transmitting and receiving antennas, that is, the effect of the line of sight.
Due to the influence of the curvature of the earth, the maximum visible distance (line of sight) D (km) between two points (antenna heights Hm and hm) can be calculated as follows:
The above formula has considered that the atmosphere is in a standard refraction state, and the equivalent earth radius is 8493km.
In this system, because the antenna of the receiving part is erected on the ground, and the antenna of the transmitting part is in the air with the GPS radiosonde, it is assumed that the receiving antenna is erected at a height of h = 4m. The minimum height of the radiosonde must not be less than 2167m.
Conclusion
The prototype of the GPS radiosonde communication system discussed in this article has been tested and verified by the test, and all technical indicators are in line with the design requirements. After several times of GPS radiosonde release and comparison test, the data transmission efficiency is more than 98%, and the maximum transmission distance is more than 200km, which can fully meet the GPS radiosonde communication requirements.
In order to improve the reliability of the communication system, appropriate error correction coding techniques can be considered to reduce bit errors, which will be reflected in the next step of improved design.
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