GPS gloabal Positioning System, this thing is the American people. There are three main components, the ground control station, the satellite flying in the sky, and the receiver in our hands.
Simple chatter
First of all, let's talk about the equipment, of course, the big one is the old U.S. ready for us,
Ground, there is a main control station, of course, in the old U.S. territory, in Colorado. Three ground antennas, five monitoring stations, distributed around the world. Mainly collecting data, calculating navigational information, diagnosing system status, scheduling satellites, that sort of chore.
In the sky, there are 27 satellites, 20,200 kilometers from the ground. 24 of the 27 satellites are operational, and three are on standby. These satellites have been updated for three generations and five models. The satellites transmit two kinds of signals: L1 and L2.L1:1575.42MHZ, L2:1227.60MHZ.The clocks on the satellites use cesium or rubidium atomic clocks, and there are plans to use hydrogen atomic clocks in the future, which are more accurate than my watch.
In the hand, there are the receivers. Large and small, in a thousand different forms, there are pocket-sized, backpack, vehicle-mounted, ship-mounted, airborne and what not. Generally common handheld to receive L1 signals, there are dual-frequency receivers, do precision positioning with.
2. On the GPS receiver
GPS is now generally 12-channel, can receive 12 satellites at the same time. Early models, such as the GARMIN 45C is 8 channels. GPS receivers receive signals from 3 satellites can output 2D (that is, 2-dimensional) data, only latitude and longitude, no altitude, if you receive more than 4 satellites, the output of 3D data, you can provide altitude. But because of the earth's own problems, not too standard circle, so the altitude data has some errors. Now some GPS receivers have built-in barometers, such as etrex's SUMMIT and VISTA, and these machines synthesize the final altitude based on the altitude data received from the two channels, which should be more accurate.
The first time a GPS receiver is turned on, or turned on at a distance of more than 800KM from the last location where it was turned off, because the ephemeris stored in the receiver is out of alignment, so it has to be repositioned on the receiver.
GPS receivers should be used under open, visible skies, so they can't be used inside the house. The accuracy of a handheld GPS is generally an error of about 10 meters, which means that a road can be seen to go left or right. Accuracy is mainly dependent on the satellite signal reception, and can receive the signal of the satellite distribution in the sky, if several satellites are distributed more dispersed, GPS receivers provide positioning accuracy will be higher.
If you have a laptop, if you have to drive to the field sometimes. Then this is the cheapest and most practical GPS solution.
3. Positioning Accuracy
When it comes to positioning accuracy, it's time to talk about SA and AS.
What is SA and AS? Don't worry, this has to start from the beginning, otherwise you can't understand.
There are two types of GPS signals: C/A code and P code.
The error of C/A code is 29.3m to 2.93 meters. General receivers utilize the C/A code to calculate positioning. The U.S. added SA (Selective Availability) to the signal for its own safety in the mid-1990s, which increased the receiver's error to about 100 meters. On May 2, 2000, SA was removed, so our current GPS should be accurate to within 20 meters.
P-code has an error of 2.93 meters to 0.293 meters, which is one-tenth of C/A code. But P-code can only be used by the U.S. military, AS (Anti-Spoofing), a jamming signal added to P-code.
In short, the old U.S. is also quite tired. Sent a bunch of satellites for military positioning. Then think it's not worth it, want to make some money, so the development of the signal to the civilian population, the accuracy can not be too high, but the accuracy of the low everyone and cursed. Because GPS in the hands of the old U.S., although free to use, but other countries use is not solid, two days ago to fight Afghans is, the United States on the region's GPS signal to do the processing, positioning accuracy becomes lower.
Russia has its own satellite positioning system, the Global Navigation Satellite System (GLObal NAvigation Satellite System). Europe is also developing its own positioning system, NAVSAT. china also has its own satellite positioning, called Beidou, which is a two-star system that can only locate its own country and nearby areas, and is currently only used by the military.
GPS Application Knowledge 2
Today's talk is rather boring, but useful ah, you can take it and others God Kan.
1. GPS settings
GPS get your hands on, if it is a new machine to locate, the last time has been mentioned. In addition, there are a number of settings, commonly used coordinate system, map datum, reference bearing, metric/imperial, data interface format and what not.
Coordinate system: commonly used are LAT/LON and UTM. LAT/LON is latitude/longitude representation, UTM is ignored here.
Map datum: commonly used WGS84.
Reference bearing: that is, where north is. Where is North? There are actually two norths, magnetic north and true north ya know (CB and ZBY for short).
The north that the compass points to is magnetic north, and the north that the Big Dipper points to is true north. The two are at different angles in different areas, and the north on the map is true north.
Metric/Imperial: take your pick, I use metric.
Data Interface Format: It's important to talk in detail about how GPS can output real-time positioning data for other devices to use, which involves a data exchange protocol. Almost all GPS receivers now follow the standard specifications specified by the National Marine Electronics Association (NMEA), which establishes a standard for communication between all marine electronic instruments, including the format of the transmitted data and the protocol for transmitting the data.NMEA protocols are 0180, 0182, and 0183, 0182 and 0183 three kinds, 0183 can be regarded as the superset of the first two kinds, now being widely used, 0183 has several versions, V1.5 V2.1. So, if everyone's GPS receiver if you want to link up with the laptop in the common GPS navigation program, such as OZIEXPLORER and my GPSRECEIVER, you should choose NEMA V2.0 or higher, NMEA specifies a communication speed of 4800 b/s. Nowadays, some receivers can offer higher speeds, but to be honest, it's not very useful, 4800 is enough.
Like GARMIN, they have a mapsource software, in order to not let other brands of GPS use the software, they designed a private GARMIN protocol, only the GARMIN machine can output this kind of data, and the MAPSOURCE can only receive the GARMIN protocol, so the MAPSOURCE can only be used by the GARMIN machine, and the MAPSOURCE can only be used by the GARMIN machine, and the MAPSOURCE can only be used by the GARMIN machine. GARMIN's machine to use, down down down down!!!!
2. Latitude and Longitude Representation
Tell us more about data representation. Generally the data obtained from GPS is latitude and longitude. There are several ways to represent latitude and longitude.
1.) ddd.ddddd, degrees . Decimal decimal part of degree (5 digits)
2.) ddd.mm.mmm, degree . min . Decimal decimal part of minutes (3 digits)
3.) ddd.mm.ss, degrees . min . Seconds
Not all GPSs have these displays, my GPS 315 only has the second and third options
How far is one degree? It would be amateurish to ask that.
In the LAT/LON coordinate system, latitude is evenly distributed, from the South Pole to the North Pole a **** 180 degrees of latitude. The diameter of the earth is 12756 KM, the circumference is 12756*PI, and one degree of latitude is 12756 x PI /360 = 111.133 KM (just to be clear, it's not precise).
Longitude is not so, only when the latitude is zero, that is, at the equator, the distance between a degree of longitude is 111.319 KM, the meridian with the increase in latitude, the distance is getting closer and closer, and finally intersected in the north and south poles. People think about it, right. So the unit distance of longitude and determine the latitude where the longitude is closely related, the simple formula is:
Longitude 1 ° length = 111.413cos φ, in latitude φ. (This formula is not precise, masking people can)
Do the problem: Beijing's longitude 119 degrees, latitude 40 degrees. What is the unit longitude and what is the unit latitude?
Answer: Unit latitude 111.133KM Unit longitude 111.413 x COS 40 = 85.347KM
The purpose of telling these is to make it easy to understand the representation of latitude and longitude.
1.)ddd.ddddd, in Beijing, the last decimal of latitude increments by 1. How much have you actually traveled? About 1.1M
Longitude last decimal increment by 1, how much did you actually walk? About 0.85M
2.) ddd.mm.mmm, in Beijing, how much did you actually travel by incrementing the last decimal of latitude by 1? About 1.85M
Longitude last decimal increment by 1, how much did you actually walk? About 1.42M
3.) ddd.mm.ss, in Beijing, latitude seconds increment by 1, how much did you actually walk? About 30.9M
Longitude increments by 1 second, how much did you actually walk? About 23.7M
Today's statement is not a precise formula, the general estimate of the approximate number of no problem.
New advances in GPS navigation technology
The U.S. Global Positioning System (GPS) navigation satellites are being progressively modernized.GPS began as a navigational aid for the U.S. Air Force, and has evolved into an important technology for both civilian and military use.GPS's precise location and timing information has become an important resource for a variety of civilian and military, scientific research, and commercial activities around the world
The development of GPS satellites and the improvement of the signal GPS navigation satellites since its launch in 1978, its type has been developed by the first Ⅰ, Ⅱ and Ⅱ A batch to Ⅱ R batch. Batches I, II and IIA satellites***, of which there are 40, are manufactured by Rockwell, while 20 IIR batch satellites are manufactured by Lockheed Martin. Boeing, which acquired Rockwell's aerospace and defense business in 1996, is currently building 33 more advanced Batch IIF satellites. The U.S. is also considering the development of a new generation of GPS satellites (GPS-III) using spot beams.
Improvements to GPS have been ongoing since it became fully operational in 1994. This is because civil users require GPS to have better anti-jamming and interference performance, higher security and integrity; the military requires satellites to transmit higher power and new military signals that are separate from civilian signals; and for "smart" weapons that use GPS for navigation, it is more important to speed up signal capture.
The biggest improvement in civilian GPS navigation accuracy to date occurred on May 2, 2000, when the U.S. stopped the practice of intentionally degrading the performance of civilian signals (known as Selective Availability, or S/A). When S/A was working, civilian users had only 100-meter accuracy 99% of the time. But when S/A is cut off, navigation accuracy rises, and 95 percent of position data can fall within a circle with a radius of 6.3 meters.
GPS satellites send two types of codes: coarse capture codes (C/A codes) and fine codes (P codes). The former is for civilian use, while the latter is restricted for use by the U.S. military and its allies, as well as by users authorized by the U.S. government. The codes are modulated in a spread-spectrum manner to transmit on two different frequencies: the L1 band transmits the C/A and P codes at 1575.42 MHz; while the L2 band transmits the P codes only at 1227.6 MHz.
The most significant improvements in GPS satellite navigation capabilities will begin with the launch of Lockheed Martin's first IIR-M (modified IIR) satellites in 2003. The IIR-M satellites will transmit an enhanced L1 civil signal, along with a new L2 civil signal and military code (M-code). Further improvements will begin in 2005 with the launch of the Boeing IIF batch of satellites, which will add a third civilian signal (L5) at 1176.45 MHz, in addition to transmitting the enhanced L1 and L2 civilian signals and M-code. Prior to the launch of IIF, the M-code will be transitioned from developmental to operational. Since it takes time to launch a constellation of navigation satellites, full operational capability in orbit will not be realized until 2007 when 18 L2 civil signal and M code satellites are launched. 18 satellites comprising the third civil signal (L5) constellation are not expected to be launched until 2011.
After that, the U.S. military will get a new signal with improved immunity to jamming, M-code. It can send more power without interfering with civilian receivers.M-code also gives the military a new ability to jam enemy use of the signal, but its details are classified.
The L2 civilian signal, the second civilian signal known as L2C, allowed civilian users to also compensate for atmospheric transmission indeterminacy errors, thus increasing civilian navigation accuracy to 3 to 10 meters. And the U.S. military and its allies have always had this capability because of their ability to receive the P-codes in L1 and L2 to begin with.
The design constraint on L2 was that it had to be compatible with the new M-code. To avoid any damage to military L2 P(Y) receivers, the new civilian L2 should have the same power and spectral shape as existing C/A codes. Here, the Y-code in parentheses is a crypto-type of the P-code. In practice, the civil L2 signal will be 2.3 dB lower than the existing L1 C/A signal. The lower power will be overcome by modern multi-correlator technology to capture very weak signals quickly.
The signals emitted by GPS satellites must be modernized while maintaining backward compatibility. The combined civil and military signals must be placed in existing frequency bands with sufficient isolation to prevent interference with each other. The U.S. decided to place the C/A-code signals in the middle of the L1 band and the new L2 band for civilian use, while retaining the Y-code signals.
The M-code would use a split-spectrum modulation that placed most of its power close to the edge of the band to which it was assigned. The immunity to interference would come primarily from not interfering with the strong transmit power of the C/A-code or Y-code receivers.
The design of secrecy for M-code signals is based on next-generation cryptography and new key structures. To further separate military and civilian codes, satellites will have separate RF links and antenna apertures for M-code. When the satellites are operational, each satellite may transmit two different M-code signals on each carrier frequency. Even if transmitted by the same satellite on the same carrier frequency, the signals will be different in terms of carrier, spreading code, and data information.
The modulation of the M-code will use a binary bias-carrier (BOC) signal with a subcarrier frequency of 10.23 MHz and a spreading rate of 5.115 million diffusion bits per second, so it will be called BOC (10.23, 5.115) modulation, or BOC (10, 5) for short. Because the BOC (10,5) modulation is separate from the Y and C/A code signals, it can be transmitted at greater power without degrading the performance of the Y or C/A code receivers.The BOC (10,5) is insensitive to interference directed at the C/A code signals and is indistinguishable from the structure of the binary sequences used to extend the modulation.
L5 will be in the 960-1215 MHz band, which is already heavily utilized by ground-based rangefinder/tacan (DME/TACAN) navigational stations and military datalinks (Link 16), but which will only interfere with aircraft flying at high altitudes in central Europe and the United States. The U.S. plans to reallocate DME frequencies within ±9 MHz of L5 so that L5 signals are well received at all altitudes in the United States.
Some new anti-jamming techniques
Because GPS satellites emit weak navigation signals and transmit them at a fixed frequency, military GPS receivers are susceptible to enemy interference.
The U.S. Defense Advance Research Projects Agency (DARPA) is developing a new anti-jamming method that employs drones over the battlefield to create pseudo-GPS constellations whose signal power exceeds that of enemy jamming signals.
The so-called pseudo-satellites are GPS navigation signal transmitters mounted on aircraft or on the ground that stand in for GPS satellites for navigation.DARPA's research on using drones as pseudo-satellites, called the GPX pseudo-satellite concept, is designed to give its own forces precise navigation capabilities in a jammed battlefield environment. This is accomplished by broadcasting high-power signals from four pseudo-satellites on board a drone in flight, creating an artificial GPS constellation over the battlefield area. four Hunter drones can cover a 300-kilometer area of the battlefield.
The pseudo-satellite signals could be used with a few software changes to existing GPS receivers. When navigating with the actual GPS constellation, the receiver begins to need to know the satellite positions, i.e., the ephemeris, so the pseudo-satellite concept faces the challenge of telling the receiver the positions of the four moving pseudo-satellites using available low-data-rate information. Therefore, the key task for the DARPA and Collins designers was to send the pseudo-satellite ephemeris in the available 50-bit/second information. UAVs are fairly stable and do not maneuver like fighter jets; however, any movement makes the position somewhat uncertain. As a result, the total positioning error will increase by about 20 percent compared with navigation using a constellation of satellites.DAPRA has tested a single pseudo-satellite on a business jet at 7,500 meters and a Hunter UAV at about 3,000 meters, and the navigation accuracy has dropped from 2.7 meters with real satellites to 4.3 meters.
Of course, the pseudo-satellites don't have to be all airborne, but can be a mix of ground and airborne transmitters. The disadvantage of locating some pseudo-satellites on the ground is that it reduces coverage but improves navigation accuracy. To overcome interference, pseudo-satellites can emit a 100-watt signal that increases the signal strength at the ground receiver by 45 decibels over the signal strength from the satellite.
Northrop Grumman is developing GPS receivers that provide 30 to 40 decibels of improved anti-jamming. This anti-jamming approach, called "anti-jamming autonomous integrity monitoring extrapolation," will be realized by fully coupling the inertial navigation and GPS receivers at the carrier phase level. The full-coupling filter will reduce the bandwidth of the GPS tracking loop, thereby reducing the chance of an interfering signal entering the GPS receiver.
The G-STAR high anti-jamming GPS receiver, jointly developed by Collins and Lockheed Martin for the JASSM air-surface missile, uses zeroing and beam manipulation. The receiver, which weighs 11.3 kilograms, employs a space-time adapter, which detects a threat, zeroes its signal and adds gain in the direction of the satellite transmitting the navigation signal.
This anti-jamming technique, which is implemented digitally and is therefore called a digital beamformer, is more precise than the conventional analog zeroing method, while adjusting the receiver's beam toward available navigation satellites. Digital signal processing allows for dynamic shifting of the zero to cancel out noise, increase gain, and manipulate the beam through a 6-element antenna array.
Civilian GPS receivers are also resistant to interference, but civilian GPS receiver users are more concerned with unintentional interference. Unintentional interference is essentially a wide-band type, as opposed to a jammer that concentrates its power on GPS frequencies. Digital signal processing methods, which are closely related to software, are ideal for dealing with wide-band interference.
The U.S. company Electro-Radiation (ERI) points out that conventional anti-jamming methods use phased-array antennas consisting of zero manipulation antennas, which not only increase weight and cost, but also anti-jamming technology implemented on the receiver usually has only a limited rejection of jamming or anti-jamming capabilities designed specifically to deal with a certain kind of interference.
The company has developed an interference suppression unit (ISU) that is effective against all known types of interference, eliminates the need for expensive and bulky antennas, and can be retrofitted to new and existing GPS receivers in a low-cost, efficient manner that is suitable for both military and civilian use.
The interference suppression unit, which includes a patch antenna and electronics that can be plugged into the antenna connector of any GPS receiver, is used to suppress broadband noise and narrowband interference. It adds 20 dB of wideband noise immunity and 35 dB of narrowband interference immunity to GPS receivers.
GPS in aircraft landing
U.S. Navy test pilots have piloted F/A-18 aircraft on the aircraft carrier USS Roosevelt using the GPS system to do the first automatic landings. The performance of this system is said to equal or exceed that of current automatic landing systems.
The landing system being developed by the U.S. Navy is the naval version of Raytheon's Joint Precision Approach and Landing System (JPALS), which is based on a modified version of JPALS. Raytheon is developing the JPALS system under contract to the U.S. Air Force for aircraft of all services, and the system will use local differential GPS corrections to provide Class I and Class II instrument approaches to land airports for both fixed-wing and rotary-wing aircraft.
The U.S. Navy-led Shipboard GPS (SRGPS) system will replace the shipboard Tacom system. It will add a one-way Low Probability of Intercept (LPI) datalink to JPALS to provide ship's position to aircraft within 370 nautical miles.
And within a 92.5-kilometer radius, two-way LPI data communications using position reporting similar to the Automatic Dependent Surveillance-Broadcast (ADS-B) used in the Civil Air Traffic Control (ATC) modernization program will allow carriers to track up to 100 aircraft.
With SRGPS, carriers and other ships will be able to communicate more stealthily with aircraft without having to use the Tacom system and primary or secondary radar signals, and minimize voice communications. The LPI link will operate at a very low data rate (0.2 Hz) compared to Tacom's 15 Hz update rate.
Development of the FAA's GPS Wide Area Augmentation System (WAAS) has been delayed by repeated problems. The system, built by Raytheon, attempts to improve GPS navigation accuracy for Class I approaches by using geosynchronous communications satellites over the equator to transmit integrity warning messages, as well as other data such as differential corrections, to GPS users.
The original plan for WAAS was to begin 60 days of tests in December 1999 and then go into service later in 2000. However, these trials were withdrawn in January 2000 due to signal outages and a high false alarm rate. However, WAAS has shown that it can achieve an accuracy of 3 meters, far better than the 7.6 meters required by the trials, and thus its development continues. It is estimated that a WAAS with certified safety will be operational by early 2003.
The delay in WAAS's operational date may also have an impact on the subsequent Local Area Augmentation System (LAAS), which will provide airports with sophisticated GPS instrument approach capability, as well as the ability to track aircraft taxiing on the ground.LAAS is scheduled to go into service in 2002 at 46 Class I and 114 Class II/III airports in the U.S. The system is expected to be operational in the U.S. by the end of the year. A Federal Express Boeing 727-200 freighter was the first to use the LAAS-capable Satellite Landing System (SLS) for precision approaches in commercial operations.
Microminiaturization of GPS and its application in artillery shell guidance
With the reduced cost and size of GPS/inertial guidance systems, even some artillery shells will now use GPS/inertial guidance. The U.S. Interstat Electronics Corporation (IEC) has developed a micro-small GPS receiver for artillery shell guidance, mounted on the tip of the U.S. Navy's and Army's rocket-boosted 127-millimeter artillery shells. The GPS receiver can withstand overloads of 12,500g or more when the shell is fired and can rapidly intercept GPS signals. Using fast intercept/direct Y-code processing, this receiver intercepts signals in less than six seconds and will track up to eight satellites. To suppress interfering signals, it is designed to work in tight coupling with an inertial measurement unit and uses some sort of narrow-band tracking loop technology. The inertial sensors in its guidance system utilize silicon microelectromechanical systems (MEMS) technology, resulting in small size and low cost. To mitigate the phase instability of the GPS clock oscillator in long-term storage, an advanced correlator is used to search the GPS signal in the time domain as well as a data transform, which is used to search for the uncertainties generated by the clock oscillator so that the Y-code can be captured directly.