Information about satellite navigation

GPS global positioning system, this thing is made by Americans. It is mainly divided into three parts, the control station on the ground, the satellite flying in the sky and the receiver in our hands.

Simple nagging

Let's talk about equipment first. Of course, the big one was prepared for us by the United States.

On the ground, there is a main control station, of course, in the United States, in Colorado. Three ground antennas and five monitoring stations are distributed all over the world. It mainly collects data, calculates navigation information, diagnoses system status and dispatches satellites.

There are 27 satellites in the sky, 20200 kilometers from the ground. Of the 27 satellites, 24 are in operation and 3 are on standby. These satellites have been updated for three generations and five models. Satellites emit two kinds of signals: L 1 and L2. L 1: 1575.42 MHz, L2: 1227.60 MHz. The clock on the satellite uses cesium atomic clock or rubidium atomic clock. I plan to use hydrogen maser in the future, which is more accurate than my watch.

In your hand, it's a receiver. Large and small, all kinds, there are pockets, backpacks, cars, ships, airborne and so on. Common mobile phones receive L 1 signals, as well as dual-frequency receivers for accurate positioning.

2. About the GPS receiver

GPS is now generally 12 channel, which can receive 12 satellites at the same time. Early models, such as GARMIN 45C, were 8 channels. When the GPS receiver receives signals from three satellites, it can output 2D data, only longitude and latitude, but no altitude. If it receives more than four satellites, it will output 3D data, which can provide altitude. However, because the earth itself is not a standard circle, there are some errors in the height data. At present, some GPS receivers have built-in barometers, such as SUMMIT and VISTA of etrex. These machines should synthesize the final height according to the height data obtained from two channels, which should be more accurate.

When the GPS receiver is turned on for the first time or turned off for the last time at a starting distance of more than 800KM, it needs to be relocated on the receiver because the ephemeris stored in the receiver cannot match.

The GPS receiver should be used in the open and visible sky, so it can't be used in the house. The accuracy of hand-held GPS is generally around 10 meter, which means that a road can tell whether to go left or right. The accuracy mainly depends on the reception of satellite signals and the distribution of satellites that can receive signals in the sky. If several satellites are scattered, the positioning accuracy provided by GPS receiver will be higher.

If you have a notebook, if you sometimes have to drive to other places. Then this is the cheapest and practical GPS solution. /auction/0/item _ detail-0db 2-2 E8 F9 B9 f 1c 10b 969 a 837 db 222 1653d 92 . jhtml

3. Positioning accuracy

When it comes to positioning accuracy, you have to say SA and AS.

What is SA, AS? Don't worry, you have to learn from the beginning, or you won't understand.

There are two kinds of GPS signals: C/A code and P code.

The error of C/A code is 29.3m to 2.93m. General receivers use C/A code to calculate the position. In the mid-1990s, the United States added SA (Selective Availability) to the signal for its own safety, which increased the error of the receiver to about 100 meters. SA was cancelled on May 2, 2000, so our current GPS accuracy should be within 20 meters.

The error of P code is 2.93m to 0.293m, which is one tenth of that of C/A code. However, code P can only be used by the US military. AS (anti-deception) is an interference signal added to the P code.

In short, America is also very tired. Many satellites were launched for military positioning. Then I didn't think it was worth it. I wanted to make some money, so I developed a civil signal. The accuracy can't be too high, but if the accuracy is low, everyone will swear. Because GPS is in the hands of the United States, although it can be used for free, it is not practical in other countries. Two days ago, the United States processed the GPS signals in this area, and the positioning accuracy became lower.

Russia has its own satellite positioning system, global navigation satellite system. Europe should also develop its own positioning system NAVSAT. China also has its own satellite positioning system called Beidou, which is a binary star system. It can only be located in the country and nearby areas, and it is only used by the military at present.

GPS application knowledge ii

What I said today is boring, but it is very useful. You can talk to others with it.

1.GPS settings

As mentioned last time, if it is a new machine, obtain GPS for positioning. There are also some settings, such as coordinate system, map datum, reference orientation, metric/English system, data interface format and so on.

Coordinate system: Latitude /LON and UTM are usually used. LAT/LON means latitude and longitude, and UTM will leave him here alone.

Map datum: WGS84 is generally used.

Reference bearing: that is, where the north is. Where is the north? In fact, there are two North, Magnetic North and True North (CB and ZBY for short).

The north pointed by the compass is magnetic north, and the north pointed by the Big Dipper is true north. The angle of difference between the two is different in different regions, and the north on the map is the true north.

Metric/English: choose by yourself. I will use the metric system.

Data interface format: this needs to be discussed in detail. GPS can output real-time positioning data for other devices, which involves data exchange protocol. At present, almost all GPS receivers follow the standards and specifications stipulated by the National Institute of Oceanography, which stipulates the communication standards between all marine electronic instruments, including the format of data transmission and the communication protocol of data transmission. There are three kinds of NMEA protocols: 0 180, 0 182 and 0 183. 0 183 can be considered as a superset of the first two, and it has been widely used now. There are several versions of 0 183, namely V 1.5 V2. 1. So if you want to connect your GPS receiver with the general GPS navigation program in your notebook, such as OZIEXPLORER and my GPS receiver, you should choose the protocol above NEMA V2.0 The communication speed stipulated by NMEA is 4800 b/s, and now some receivers can provide higher speed, but to be honest, it's useless, and 4800 is enough.

GARMIN, for example, has its own mapsource software. In order to prevent other brands of GPS from using this software, it designed a private GARMIN protocol. Only GARMIN's machine can output this data, and MAPSOURCE can only receive GARMIN protocol, so MAPSOURCE can only be used by GARMIN's machine and will be knocked down! ! !

2. Representation of Latitude and Longitude

Let's talk about data representation. Generally, the data obtained by GPS is latitude and longitude. Latitude and longitude can be expressed in many ways.

1.)ddd.ddddd, the fractional part of the degree. Degree (5 digits)

2.)ddd.mm.mmm, the fractional part of the degree. One minute. Minutes (3 digits)

3.)ddd.mm.ss, degree. Minutes. second

Not all GPS have these displays. My GPS3 15 can only choose the second and third.

How far was it? If you ask, it's amateurish.

In the latitude /LON coordinate system, the latitude is evenly distributed, from the south pole to the north pole *** 180 latitude. The diameter of the earth is 12756KM, the circumference is 12756*PI, and the latitude is12756 × pi/360 =11.133 km (to be clear,

Longitude is not like this. Only when the latitude is zero, that is, on the equator, the distance between longitudes is111.319 km. With the increase of latitude, the longitude is getting closer and closer, and finally the north and south poles are met. Think about it? So the unit distance of longitude is closely related to the latitude that determines longitude. The simple formula is:

Longitude 1 length =11.413cosφ, at latitude φ. (This formula is not accurate either. You can be a fool. )

Question: The longitude of Beijing is 1 19 degrees, and the latitude is 40 degrees. What are the units of latitude and longitude?

Answer: unit latitude11.133km unit longitude11.413× cos40 = 85.347km.

The purpose of this talk is to understand the representation of latitude and longitude easily.

1.)ddd.ddddd, in Beijing, the last decimal place of latitude increased by 1. How much did you actually go? About 1. 1M

The last decimal place of longitude is increased by 1. How much did you walk? About 0.85 meters

2.)ddd.mm.mmm, in Beijing, the last decimal place of latitude increased by 1. How much did you actually go? About1.85m.

The last decimal place of longitude is increased by 1. How much did you walk? About1.42m.

3.)ddd.mm.ss, in Beijing, the latitude increases by 1 every second. How much did you walk? About 30.9 meters

Longitude increases by 1 per second. How much did you actually walk? About 23.7 meters

What I said today is not an accurate formula, and it is generally no problem to estimate the approximate figures.

New progress of GPS navigation technology

The global positioning system (GPS) navigation satellites in the United States are gradually modernizing. Starting from the navigation equipment of the US Air Force, GPS has gradually developed into an important dual-use technology in Cheng Junmin. The precise positioning and timing information of GPS has become an important resource for Zhong Junmin's use, scientific research and commercial activities all over the world.

Development of GPS satellites and improvement of signals Since the launch of 1978, the types of GPS navigation satellites have developed from the first, second and second A batches to the second R batch. There are 40 satellites in Ⅰ, Ⅱ and Ⅱ A batches, which are made by Rockwell Company, and 20 satellites in Ⅱ R batches are made by Lockheed Martin Company. Boeing acquired Rockwell's aerospace and defense business on 1996, and is currently manufacturing 33 more advanced II F batches of satellites. The United States is still considering the development of a new generation of global positioning system satellite (GPS-III) using spot beams.

Since 1994 was put into full operation, the improvement of GPS has been in progress. This is because civil users require GPS to have better anti-jamming and interference performance, higher security and integrity; The military requires satellites to launch new military signals with greater power and separation from civilian signals; For "smart" weapons with GPS navigation, it is more important to speed up the signal acquisition.

The biggest improvement of civil GPS navigation accuracy so far occurred on May 2, 2000, when the United States stopped deliberately reducing the performance of civil signals (called selective availability, that is, S/A). When working in S/A, the 99% time accuracy of civil users is only 100 meter. However, after cutting off the S/A, the navigation accuracy is improved, and 95% of the position data can fall within a circle with a radius of 6.3 meters.

GPS satellites send two kinds of codes: coarse acquisition code (C/A code) and fine acquisition code (P code). The former is for civilian use, and the latter is limited to users approved by the US military and its allies and the US government. These codes are spread spectrum modulated and transmitted at two different frequencies: L 1 band transmits C/A and P codes at 1575.42 MHz; The L2 band only transmits the P code of 1227.6 MHz.

The most remarkable improvement of GPS satellite navigation capability will begin with the launch of the first batch of Ⅱ R-M (improved Ⅱ R) satellites of Lockheed Martin in 2003. Ⅱ R-M satellite will transmit enhanced L 1 civil signal and new L2 civil signal and military code (M code). Further improvement will begin in 2005 when the Boeing Ⅱ F satellite is launched. In addition to the enhanced L 1, L2 civil signal and M code, the IIF satellite will add a third civil signal (L5) at 1 176.45 MHz. Before the introduction of Ⅱ F, M code will transition from development type to working type. As it will take some time to launch the navigation satellite constellation, it will take 18 L2 civil signal and M-code satellites to reach full operational capability in orbit in 2007. The constellation of the third civil signal (L5) consisting of 18 satellites is not expected to be launched until 20 1 1.

After that, the US military will get a new signal with enhanced anti-jamming capability-M code. It can send more power without interfering with civilian receivers. M code also gives the military a new ability to interfere with the enemy's use of signals, but its details are confidential.

L2 civil signal, the second civil signal, called L2C, enables civil users to compensate the uncertainty error of atmospheric transmission, thus improving the civil navigation accuracy to 3 ~10m. The US military and its allies have always had this capability, because they can receive the P codes in L 1 and L2 from the beginning.

The design limitation of L2 is that it must be compatible with the new M code. In order to avoid any damage to the military L2 P(Y) receiver, the new civil L2 should have the same power and spectrum shape as the existing C/A code. Here, the Y code in parentheses is the encryption type of the P code. In fact, the civil L2 signal will be 2.3 dB lower than the existing L 1 C/A signal. In order to capture very weak signals quickly, modern multi-correlator technology will overcome the problem of low power.

The signals transmitted by GPS satellites must be modernized while maintaining backward compatibility. The combined civil signal and military signal must be placed in the existing frequency band and have sufficient isolation to prevent mutual interference. The United States decided to put the C/A code signal between the L 1 band and the new L2 band for civil use, while retaining the Y code signal.

M code will use split spectrum modulation, and most of the power will be placed near the edge of the allocated frequency band. The anti-jamming ability mainly comes from the powerful transmitting power of the receiver that does not interfere with C/A code or Y code.

The security design of M-code signal is based on the next generation cryptography and new key structure. In order to further distinguish between military and civilian codes, satellites will provide separate RF links and antenna apertures for M codes. When the satellite can work, each satellite can transmit two different M-code signals on each carrier frequency. Even if the same satellite transmits at the same carrier frequency, the signal will be different in carrier, spreading code, data information and so on.

M code will be modulated by binary offset carrier (BOC) signal, with subcarrier frequency of 10.23 MHz and spreading rate of 5. 1 15 million spreading bits per second, so it is called BOC (10.23,5.1/kloc). Since BOC (10/0,5) modulation is separated from Y and C/A code signals, it can be transmitted with greater power without degrading the performance of Y or C/A code receivers. Boc (10/0,5) is insensitive to the interference of C/A code signals, and it is difficult to distinguish it from the binary sequence structure used for spread spectrum modulation.

L5 will be located in 960 ~ 12 15 MHz band, which has been widely used by DME/TACAN navigation station and military data link (Link 16), but it will only interfere with aircraft flying at high altitude in Central Europe and the United States. The United States plans to reallocate DME frequency within L5 9 MHz, so that L5signal can be well received at all altitudes in the United States.

Some new anti-jamming technologies

Because the navigation signals transmitted by GPS satellites are weak and transmitted at a fixed frequency, military GPS receivers are vulnerable to enemy interference.

The Defense Advanced Research Projects Agency (DARPA) is developing a new anti-jamming method, which uses unmanned aerial vehicles over the battlefield to create a pseudo-GPS constellation, so that its signal power exceeds the enemy's jamming signal.

The so-called pseudolite is to install the GPS navigation signal transmitter on the plane or on the ground to replace the GPS satellite for navigation. The research of DARPA using UAV as pseudolite is called GPX pseudolite concept, which aims to make our troops have accurate navigation ability in the disturbed battlefield environment. This method is to broadcast high-power signals by four pseudosatellites on the flying UAV, thus creating an artificial GPS constellation over the battlefield area. Four "Hunter" drones can cover 300 square kilometers of war zones.

As long as the software of the existing GPS receiver is changed, the signal transmitted by pseudolite can be used. When navigating with the actual GPS constellation, the receiver needs to know the positions of satellites, that is, ephemeris, so the challenge of the concept of pseudolite is to tell the receiver the positions of four mobile pseudolites with available low data rate information. Therefore, the key task of DARPA and Collins designers is to send pseudolite ephemeris in the available 50 bits/second information. The stability of the drone is quite good, and it will not maneuver like a fighter; But any action will make the position a little uncertain. Therefore, compared with navigation using satellite constellation, the total positioning error will increase by about 20%. DAPRA has been used to test a single pseudosatellite on a business jet at an altitude of 7500 meters and a hunter drone at an altitude of about 3000 meters. The navigation accuracy has dropped from 2.7 meters when using real satellites to 4.3 meters.

Of course, pseudolites are not necessarily airborne, and a hybrid scheme of ground and airborne transmitters can also be adopted. The disadvantage of setting some pseudolites on the ground is that the coverage is reduced, but the navigation accuracy is improved. In order to overcome the interference, the pseudolite can transmit a signal of 100 watt, which makes the signal strength at the ground receiver increase by 45 decibels compared with the signal strength from the satellite.

Northrop Grumman is developing an improved GPS receiver, which can provide 30 ~ 40 decibels of anti-interference ability. This anti-jamming method called "anti-jamming autonomous integrity monitoring extrapolation" will be realized through the full coupling of inertial navigation and GPS receiver at the carrier phase level. Fully coupled filter will reduce the bandwidth of GPS tracking loop, thus reducing the chance of interference signals entering GPS receiver.

The G-STAR high anti-jamming GPS receiver jointly developed by Collins Company and Lockheed Martin Company for JASSM air-to-ground missile adopts the method of zero adjustment and beam control. The receiver weighs 1 1.3 kg, and uses a space-time adapter. When the adapter detects a threat, it will adjust its signal to zero, and increase the gain in the direction of the navigation signal transmitted by the satellite.

This anti-jamming technology is realized in a digital way, so it is called a digital beamformer. It is more accurate than the conventional analog zeroing method, and the beam of the receiver can be adjusted to the available navigation satellites. In digital signal processing, noise can be eliminated by dynamically moving the zero position, gain can be increased, and beam can be controlled by 6 yuan antenna array.

Civil GPS receivers also have anti-interference problems, but users of civil GPS receivers are more concerned about unintentional interference. Unintentional jamming is basically broadband, which is different from jammer concentrating power on GPS frequency. The digital signal processing method closely related to software is ideal in dealing with broadband interference.

American Electric Radiation (ERI) Company pointed out that the conventional anti-jamming method is to use a zeroing antenna composed of phased array antennas, which not only increases the weight, but also costs a lot. However, the anti-jamming technology implemented on the receiver usually has limited anti-jamming ability or is specially designed to deal with some kind of interference.

This company has developed an interference suppression device (ISU), which can effectively deal with all known types of interference. It does not need expensive and bulky antennas, and can be added to new and existing GPS receivers in a low-cost and efficient way, which is suitable for both military and civilian use.

This kind of interference suppression equipment includes patch antenna and electronic equipment, which can be inserted into any GPS receiver antenna interface to suppress broadband noise and narrowband interference. It improves the anti-wideband noise ability of GPS receiver by 20 decibels and the anti-narrowband interference ability by 35 decibels.

Application of GPS in aircraft landing

U.S. Navy test pilots flew the F/A- 18 aircraft and made the first automatic landing on the aircraft carrier Roosevelt using GPS system. It is said that the performance of this system is equivalent to or exceeds the current automatic landing system.

The landing system being developed by the US Navy is a naval model of Raytheon Joint Precision Approach and Landing System (JPALS), which has been improved on the basis of JPALS. According to the contract of the US Air Force, Raytheon is developing JPALS system for all military aircraft. The system will use local differential GPS correction to provide Class I and Class II instrument approaches for fixed-wing aircraft and rotorcraft at land airports.

The shipborne GPS(SRGPS) system led by the US Navy will replace the shipborne TACAN system. It will add a one-way low probability of interception (LPI) data link to JPALS, providing the aircraft with a position within 370 nautical miles.

Within the radius of 92.5km, the two-way LPI data communication will enable the aircraft carrier to track as many as 100 aircraft by adopting position reports similar to those used in the modernization plan of civil aviation air traffic control (ATC).

With the installation of SRGPS, aircraft carriers and other ships will be able to contact the aircraft more invisibly, without using TACAN system and primary or secondary radar signals, and minimize voice communication. Compared with the update rate of TACAN 15 Hz, the LPI link will work at a very low data rate (0.2 Hz).

The development of FAA's GPS Wide Area Augmentation System (WAAS) has been delayed due to recurring problems. The system is made by Raytheon Company, and attempts to transmit integrity warning information, differential correction and other data to GPS users through geosynchronous communication satellites over the equator, so as to improve the navigation accuracy of GPS and meet the requirements of Class I approach.

WAAS originally planned to start a 60-day trial in 1999 12, and then put it into use at the end of 2000. However, these tests were cancelled in June 2000 at 5438+ 10 due to signal interruption and high false alarm rate. However, WAAS shows that its accuracy can reach 3 meters, far better than the 7.6 meters required for testing, so its development work is still going on. It is expected that WAAS which has passed the safety certification will be put into operation in early 2003.

The delay in the use date of WAAS may also have an impact on the subsequent local augmentation system (LAAS), which will provide the airport with accurate GPS instrument approach capability and the ability to track taxiing aircraft on the ground. LAAS is planned to be put into use in 46 first-class airports and 1 14 second/third-class airports in the United States in 2002. A Boeing 727-200 cargo plane of FedEx took the lead in using the satellite landing system (SLS) with LAAS capability for accurate approach in commercial operation.

Miniaturization of GPS and its application in shell guidance

With the decrease of cost and volume of GPS/ inertial guidance system, even some shells will adopt GPS/ inertial guidance now. IEC has developed a miniature GPS receiver for projectile guidance, which is installed on the top of the 127mm projectile propelled by rockets of the US Navy and Army. This kind of GPS receiver can withstand the overload exceeding 12500g when the shell is launched, and can intercept the GPS signal quickly. The receiver adopts fast interception/direct Y code processing, which can intercept signals in less than 6 seconds and track up to 8 satellites. In order to suppress the interference signal, it is designed to cooperate closely with the inertial measurement device and adopt some narrow-band tracking loop technologies. The inertial sensor in its guidance system adopts silicon micro-electromechanical system (MEMS) technology, so it is small in size and low in cost. In order to alleviate the problem of unstable phase of GPS clock oscillator in long-term storage, an advanced correlator is used to search the uncertainty caused by clock oscillator through time domain search and data transformation of GPS signal, so as to capture Y code directly.