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GPS chip principle and market status in Shenzhen

  • Categories:News Center
  • Time of issue:2021-01-08 11:28
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(Summary description)The Global Positioning System is a set of satellite positioning system constructed by the US Department of Defense (Air Force). Its basic principle is to use the signals emitted by 24 satellites orbiting the earth and perform geometric calculations. To get the Cartesian Coordinate of the receiver, another equivalent system is the Global Navigation Satellite System (GLONASS) constructed by the former Soviet Union.

GPS chip principle and market status in Shenzhen

(Summary description)The Global Positioning System is a set of satellite positioning system constructed by the US Department of Defense (Air Force). Its basic principle is to use the signals emitted by 24 satellites orbiting the earth and perform geometric calculations. To get the Cartesian Coordinate of the receiver, another equivalent system is the Global Navigation Satellite System (GLONASS) constructed by the former Soviet Union.

  • Categories:News Center
  • Author:Wang Yuehua
  • Origin:
  • Time of issue:2021-01-08 11:28
  • Views:

The Global Positioning System is a set of satellite positioning system constructed by the US Department of Defense (Air Force). Its basic principle is to use the signals emitted by 24 satellites orbiting the earth and perform geometric calculations. To get the Cartesian Coordinate of the receiver, another equivalent system is the Global Navigation Satellite System (GLONASS) constructed by the former Soviet Union.

"Figure 1 Complete Data Frame Icon"

Basic concepts of GPS

Basically, this system is also composed of 24 satellites. The 24 satellites are distributed on 3 orbital planes. Each orbital plane is composed of 8 satellites evenly distributed with a difference of 45 degrees (Latitude). The altitude is about 19100Km, and it takes 11 hours and 15 minutes to run the satellite a week. Because this article mainly introduces the positioning system (Navstar GPS) built by the United States, so there is not much to describe about GLONASS.

Simply put, the Global Positioning System is a space-based system that uses radio waves and time differences to measure distance. It can provide accurate location data, speed, and time. The whole system can be divided into three parts, which we call Space Segment, Control Segment and User Segment. The first Space Segment is mainly composed of 24 satellites operating in a semi-synchronous orbit. The so-called semi-synchronous orbit refers to the fact that it takes about 12 hours (11 hours and 58 minutes) for a satellite to travel a week. So in a day, the same satellite happens to be Appear on top of a stationary user's head once (23 hours and 56 minutes). The 24 satellites are evenly distributed on 6 orbital planes with 4 satellites in each orbit, and each orbital plane is 55 different from the equator. The inclination angle of these satellites is about 20,200Km on average.

The ranging signals emitted by these 24 satellites have two channels (D-Band), one of which is called L1, and the transmission frequency is 1575.42 MHz. The second is L2, the transmitting frequency is 1227.6MHz. These two carrier frequencies are responsible for transmitting the spread spectrum signal with a high frequency carrier. The spread spectrum communication (Spread-Spectrum) technology used by GPS will be introduced in detail in the article.

"Figure 2 Simple Block Diagram"
Communication technology adopted by GPS

The receiver’s position calculation principle is basically based on the ranging code broadcasted by the satellite, the GPS receiver measures the transmission time of its satellite signal, and then compares it with the receiving time on the receiver to use the time difference And the transmission speed calculates the distance between the satellite and the receiver.

C/A code generation

The 50Hz data information code can further provide the satellite position at the time when the receiver receives the satellite signal, and then use the simultaneous equation established by these data to find the coordinate axis position of the receiver and correct the timing deviation on the receiver. the amount. A 3D position solution requires at least 4 satellite data to be locked.

To clearly understand the principle of GPS communication technology, it is necessary to know the ranging code (C/A code) broadcast by the satellite. The C/A code is composed of 1023 pseudo-random codes at a clock rate of 1.023MHz, so a set of C/A code frames (1023 bits) will be repeated every 1ms, so The short C/A code is designed to enable the receiver to track the satellite signal in a short time. Each satellite has its own set of fixed C/A codes for identification. These C/A codes are all Gold Codes, P codes are a set of pseudo-garbled codes with a clock rate of 10.23MHz. Since P codes are only open to military use, we will not discuss them here.

In addition to the C/A code, the most important thing is the 50Hz satellite data information code. On the satellite, this information code will be convolution with the C/A code and then modulated to the L1 carrier. This information code contains The data includes the precise orbit data of the satellite itself that transmitted the information code, as well as the basic orbit data of other satellites. In addition to these, there are also the transmission time of the information code and a conversion character (Hand Over Word, HOW) used in C/A code is switched to P code, timing calibration data, data on whether the satellite itself and other satellites are operating normally, ionospheric propagation delay parameters, and coefficients for calculating UTC.

Information code

A complete data information code is composed of 25 data frames (Frame), and each data frame contains 1,500 bits. Each data frame is divided into 5 sub-data frames each with 300 bits. Yuan, (Figure 1) is an icon of a complete data frame. In terms of a data transmission rate of 50 Hz, a sub-data frame takes 6 seconds, a data frame takes 30 seconds, and a complete 25 data frame takes 12.5 minutes.

It is worth noting that in a complete set of 25 data frames, the data of the first 3 sub-data frames of each data frame is fixed. Since the data in the first 3 sub-data frames are the precise orbit data (ephemeris) of the satellite itself that transmitted this information code, and timing calibration data, etc., this means that the data related to the satellite itself will be updated every 30 seconds, and then 2 The data of each sub-data frame is the basic orbital data of other satellites. Since this is larger than the data volume, it takes 25 data frames to transmit all the data.

Spread spectrum action

After understanding the C/A code and the information code, we will convolution these two codes, which is also equivalent to doing a spreading action here, because the bandwidth of the information code is small (50Hz) and the bandwidth of the C/A code is relatively small. (1.023MHz), so it is equivalent to expanding the bandwidth of the information code, and then in the BPSK modulation mode (when the bit changes from logic 0 to 1 or from 1 to 0, the phase of the carrier must change 180 (Degree) The frequency that carries the signal to L1, (Figure 1) is a simple block diagram.

The task of the satellite receiver

All the signals mentioned above belong to the signal of the satellite transmitter, and the work flow done on the receiver side is: find and lock the C/A code, C/A code compensation, carrier compensation, information symbol synchronization, Data frame byte synchronization, search and identify other unlocked satellites, calculate the distance between each satellite (pseudorange) and the position of the disjoint cube program.

In a typical satellite tracking method, the receiver first determines which number of satellites are in the sky of the receiver, and then the receiver locks on a certain satellite for the acquisition process. Of course, the reason why it can determine which number is The number of satellites in the sky of the receiver depends on the satellite orbit data stored in the receiver and the time and location of the receiver.

In the receiver's specifications, this kind of boot method is called warm boot or hot boot. In contrast, the method of turning on when there is no data in the receiver is called cold boot. At this time, the receiver software must plan a more systematic method to search for satellites. Once a satellite is locked, the receiver can use the decoded information code to know which satellites are in the receiver. 

Special technology of GPS

Generally speaking, the strength of the satellite signal received on the ground is already lower than the thermal noise of the receiver itself. This is due to the spread spectrum and the longer propagation distance. In order to intercept satellite signals from these noises, GPS uses a code correlation technology (code correlation), and the receiver itself will first generate a set of codes. Comparing with the signal received from the satellite, when the two sets of codes are the same, the correlation value is the largest. At this time, the original signal spectrum can be obtained after de-spreading. The process can be referred to (Figure 2).

"Figure 3 Spectral Layer and Its Reconstruction"
GPS chip

"Figure 4 Common CMOS Architecture for Mixer Design"
The general commercial GPS chipset consists of a radio frequency IC and a baseband IC. The main function of the RF IC is to down-convert the GPS 1.57542GHz carrier to the base frequency. It is mainly formed by low noise amplifier, mixer, automatic gain control, phase locked loop and analog/digital converter. Generally, such chip design will use Bi-CMOS process to take into account the advantages of Bipolar's fast speed, high gain, high driving force, low broadband noise, and CMOS's low power consumption and high density.
The CMOS architecture most commonly used in Down-converter mixer design, as shown in (Figure 4), this design does not require other more components, because it uses a bi-orthogonal structure, it has a very high Quadrature phase accuracy.

The phase lock loop is mainly composed of VCO and Prescaler. In the design of VCO, what we care most about is phase noise. The most commonly used designs are the so-called "Ring Oscillator" and "LC-tank Oscillator". The principle is to use Bondwire as a High Q inductor to make an LC-tank Oscillator. 

"Figure 5 Common CMOS architecture of Down converter"

At first glance, it seems that the design of RF IC and GPS receiver is not dependent, but at present each RF IC design has its own frequency plan, that is, there are different IF frequencies and base frequencies. The frequency and reference frequency cannot be shared.

Most baseband signal processing ICs are made of CMOS. In addition to de-spread correlators, common signal processing chips also include UART, Memory and other logic circuits. We take DRAM as an example to illustrate the design of the memory. (Figure 5) is a basic DRAM block diagram. The peripheral circuits are used to select locations and read and write data. In a cell's memory (Cell) (Figure 6), Cs stores charge to indicate the state, and n-Channel MOSFET acts as an Access switch.

"Figure 6 Basic DRAM Block Diagram"
GPS market status

With the generalization of global positioning systems in daily life, laptops, personal electronic assistants, mobile phones, and all of them are integrated with GPS. In addition to the market trend (the added value of mobile communications), the reason why GPS has to be integrated with mobile communications, the Federal Communications Commission (FCC) E911 regulations are also one of the driving forces.

Market opportunity

E911 stipulates that between October 2001 and June 2002, more than 50% of mobile phones must have a location positioning function, and the accuracy must be within 125 meters. Then, by the end of 2005, more than 95% of mobile phones must have a positioning function, and the accuracy is further required to be within 15 meters.

Under such regulations, all major mobile phone manufacturers seek cooperation from GPS manufacturers. Of course, the positioning solution is not only the global positioning system, we will also explain other technologies.

In addition to FCC regulations, NTT DoCoMo, Japan’s largest telecommunications company, also launched a value-added service for Location Agent. Although this is not mainly used for emergency rescue applications, the business opportunities created by its value-added services cannot be ignored. Therefore, this It is also a pie that GPS manufacturers are eager to share.

In addition, there are similar FCC E911 emergency rescue regulations in Europe. These are the driving forces for the integration of GPS and mobile communications. There are many applications derived from mobile communication and location information, which can simply provide information about the surrounding area, such as gas stations, parking lots, and restaurants. For complex ones, location data can also be used to charge fees. The most obvious example is Singapore’s entry fees.

GPS manufacturer profile

"Figure Seven Memory of a Unit"

At present, the manufacturers that provide GPS can be roughly divided into four categories. The first category is those that only provide GPS chips and then support customers to develop end products (end products), such as Cyfer, Phillip, IBM, A&D, ST, etc. ; The second category is manufacturers that only provide GPS modules, such as Motorola, Rockwell and the customers of the first category of suppliers.

As for the third category, it refers to manufacturers that provide general commercial terminal products. Such terminal products include car navigators, marine/aircraft/personal navigators, which are characterized by convenient navigation software and friendly human-machine interfaces (such as LCD, monitor, Keypad), even plus 2-way transceiver (referred to here in general, GSM CDPD Trunk Radio WCDMA, etc.), such manufacturers such as Garmin, Trimble, Magellen Taiwan's Changtian, Dingtian, etc. The fourth type of manufacturers refers to the production of high-precision GPS receivers, that is, those who have obtained P code authorization, such as Leica. Of course, there are also a few vendors that provide IP integration solutions such as SiRF and Parthus.

At present, it is particularly focused on consumer electronic products, and has cooperated with major mobile communication manufacturers, and only SiRF and SnapTrack can provide Location Based Service.

With the increasing application of consumer electronics products, many electronic products are actively planning to integrate GPS navigation functions, the most common ones are electronic map companies, car navigation systems, mobile phones, PDAs, and Smart Phones. Once the GPS navigation function is paired with appropriate electronic maps and well-planned navigation software, GPS will become an essential daily necessities for everyone in the future.

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