Telematics is a new vehicle-mounted application that integrates communication and information. In product positioning, it can be divided into two types: portable equipment and vehicle-mounted equipment. GPS navigation and positioning play a key role in remote information processing. In addition to providing navigation information for drivers, vehicle-mounted GPS system can also provide positioning information for vehicle-mounted information service providers in combination with wireless communication technology (such as GPRS/3G). When the service centers of these providers receive the location information of a single car, they can provide road rescue, lost car recovery and other services for the owners. In addition, taxis, buses or tour buses can also use GPS to track and control the fleet. The GPS device of the client is a one-way GPS signal receiver, which can receive the positioning signal of the sky navigation satellite. These more than 20 satellites can emit L 1 and L2 signals at frequencies of 1575.42MHz and 1227.60MHz respectively. General civil GPS receivers only need to receive L 1 at 1578.
GPS positioning system uses the basic triangulation principle of satellites. The GPS receiver first finds the positions of more than three satellites in the air, and then calculates the distance between each satellite and the receiver, so that the coordinate values of the receiver in three-dimensional space can be obtained.
Looking further at the system operation flow of the GPS receiver (see figure 1), the GPS satellite signal is first received by the GPS antenna, and then the high-frequency signal is converted into intermediate-frequency and low-frequency digital signals through the RF front end, and then transmitted to the GPS baseband module. The core technology of this module lies in the design of correlators, that is, the correct satellite number is found by correlator comparison, and then the perpetual calendar, broadcast ephemeris and other data of multiple satellites are obtained by comparison. The more correlators in each channel, the faster the satellite position can be found. At present, the general GPS receiver provides at least 12 correlators, while the high-order receiver has 16 correlators or even 32 correlators.
The control function of GPS receiver is realized by microprocessor or microcontroller, and this processing core can come from outside or be embedded in GPS baseband module. At present, early GPS receiver products often use ARM7 as the core, and high-end models will be upgraded to ARM9 as the core. In addition, these components have microprocessor support functions, such as UART and real-time clock (RTC).
The ephemeris data will be output to the main processor in the format of NMEA0 183 or RTCM, and further integrated with the GIS map engine to display the location of the street, or transmit the location information through the wireless communication interface, so that the remote server can provide further related location services. NMEA0 183 is a standard communication protocol commonly used in GPS, and the simplified ASCII serial communication protocol is used to define the data transmission format. When differential positioning (DGPS) is adopted for GPS, such as WAAS in the United States or EGNOS system in Europe, the protocol format of RTCM or NTRIP 1.0 should be output. In addition, because the original data formats provided by different receivers are usually different, when the data collected by different types of receivers need to be processed uniformly, it is necessary to establish a GPS universal data exchange format. To sum up, in the hardware system architecture of a vehicle-mounted GPS, the main units include antenna, RF front-end, baseband/correlator, processor core, memory and bus interface. These units can be separated to improve the flexibility of design, or integrated into a system on a chip (SoC), a single package (SiP) or a module, thus reducing the design difficulty and cost.
When designing, system engineers must choose among three evaluation factors: efficiency, cost and flexibility. In terms of efficiency, the GPS receiver has four performance indicators, namely, accuracy, sensitivity, time of first positioning and number of channels. When these four performance indicators are required to reach the highest level, it is necessary to emphasize the processor performance, correlator channel number, storage capacity and high-speed external interface of the receiver. In this way, the cost of products will naturally increase substantially, and the mass market may not accept it at this time, so it is often necessary to make some necessary adjustments.
The current technology has been able to integrate RF and baseband in the GPS receiver architecture, and products with high integration can provide better cost performance. Take STA2056 of ST as an example (see Figure 2), which integrates the fundamental frequency and RF functions in a small QFN-68 package. It uses ARM7TDMI as the core in the fundamental frequency part, and the frequency can reach 66MHz；; The RF part is an active antenna system, which includes an interface that is easy to connect with passive antennas. In addition, it also has built-in ROM and SRAM memory. Because only a few external components are needed, the overall material cost can be reduced; Its small size can make the product design lighter and shorter, and it has the advantage of low power consumption. Moreover, this integrated product also saves engineers' research energy in adjusting RF and baseband integration, which can speed up the product market. GPS antenna is also the key to determine the performance of GPS. The background noise of GPS satellite signal is-136dBW. In order to avoid interference, international telecommunication regulations stipulate that the noise of satellite transmission signals should not be greater than-154dBW. The GPS signal is actually quite weak, so the sensitivity of the receiving antenna must be very high. This is closely related to the size and shape of the antenna. The antenna types that can be used for GPS include patch antenna, spiral antenna and planar inverted F antenna (Fapi), among which patch antenna and spiral antenna are most used (see Figure 4). Because GPS signal belongs to circularly polarized wave, GPS receiving antenna must also adopt circularly polarized working mode.
The advantages of flat antenna are durability, relatively easy manufacture and low cost, but it has obvious directivity, and the flat plate must face the sky to get better reception effect. This directionality will bring great restrictions to the use; In addition, although it can successfully receive the satellite signal directly above, if it does not obtain the satellite information with low angle, the error will be large and the accuracy will decrease.
A more advanced method is to use a four-arm spiral antenna, which has the ability of 360-degree omnidirectional reception, so that the antenna has a gain of 3dB in any direction. This allows GPS receivers to be placed at various angles and can receive satellite signals at low angles. In addition, the balun circuit design can be introduced, which can effectively isolate the noise around the antenna, and can accommodate antennas with various functions in a very small space without interfering with each other, which is very suitable for antenna design of handheld devices, but the cost of such antennas is still high. In the use of car navigation, navigation often fails to work normally due to the occlusion factors in the environment. In high-rise roadway, the reception situation is often very poor, and there is no signal available when driving in the tunnel. At this time, dead reckoning (DR) technology can be used as a temporary navigation tool.
The technical principle of DR is to estimate the change of automobile movement position through devices that can sense or measure the change of distance and direction. The distance traveled forward is usually measured by rangefinder or accelerometer; The rotation angle is measured by magnetic compass, gyroscope or differential odometer; The change of height requires the use of a barometer. See Figure 5 for an example of integrated design.
The odometer is a necessary equipment for every car. The GPS receiver can be connected to the odometer through CANBus, but the disadvantage of the odometer is that the accuracy will decrease after a long time of use. The more advanced methods are accelerometers and gyroscopes using MEMS technology, which are small in size and easy to integrate. However, high-precision MEMS elements also require high cost. In addition, in order to improve the accuracy of DR system in practical application, online sensor calibration is often needed, so the positioning signal of GPS is needed to correct the parameter term of DR sensor.
In a short time, the accuracy of DR is quite high, even higher than that of GPS. However, with the increase of service time, the cumulative error effect of DR will become larger and larger, and the navigation accuracy will drop sharply. At this point, we must return to the GPS system, find out the absolute position, and then use DR. DR and GPS are complementary car navigation systems, but there are few commercial products at present. The main bottleneck lies in the accuracy and cost of DR sensor and the algorithm development integrated with navigation system.