There are two methods for determine the range from SC to the REC in GPS; codes (
ID: 1825925 • Letter: T
Question
There are two methods for determine the range from SC to the REC in GPS; codes (C/A and P) and waves (L1 and L2). Discuss two elements of GPS signals as they relate to each method. Choose from the list of elements shown below. Elements *choose two to answer the above questions* WELL DETAILED please -determination of time -atmospheric errors -correlation of signal at REC -what is actually measured at the REC -AODC effects on GPS signal -Accuracy also please specify which two elements you are choosing each one should be about one paragraphExplanation / Answer
Selective Availability, or SA, occurred when the DoD intentionally degraded the accuracy of GPS signals by introducing artificial clock and ephemeris errors. When SA was implemented, it was the largest component of GPS error, causing error of up to 100 meters. SA is a component of the Standard Positioning Service (SPS), which was formally implemented on March 25, 1990, and was intended to protect national defense. SA was turned off on May 1, 2000. Table 1. lists the possible sources of GPS error and their general impact on positioning accuracy. Table 1. GPS Error Budget Error source Potential error Typical error Ionosphere 5.0 meters 0.4 meters Troposphere 0.5 meters 0.2 meters Ephemeris data 2.5 meters 0 meters Satellite clock drift 1.5 meters 0 meters Multipath 0.6 meters 0.6 meters Measurement noise 0.3 meters 0.3 meters Total ~ 15 meters ~ 10 meters How to Reduce GPS Error You've probably heard people talk about getting 1 to 5 meter accuracy with a GPS receiver, or even centimeter or millimeter accuracy. Is there a way to cancel out the errors and get better than 15 meter accuracy? The answer is yes, but the level of accuracy depends on the type of equipment you are using. The following discussion describes a technique used to achieve 1 to 5 meter accuracy using mapping (resource) grade receivers. Some mapping grade receivers are even capable of sub-meter accuracy, but the increased accuracy comes at a price. Survey grade receivers are the most accurate, capable of centimeter or even millimeter accuracy, depending on the equipment, but they use more advanced techniques to achieve this level of accuracy and, naturally, are more expensive. Recreational grade receivers usually can receive real-time differential corrections, but they cannot store a file that can be differentially corrected using post-processing methods. Differential Correction Differential correction is a method used to reduce the effects of atmospheric error and other sources of GPS positioning error (differential correction cannot correct for multipath or receiver error; it counteracts only the errors that are common to both reference and roving receivers). It requires, in addition to your "roving" GPS receiver, a GPS receiver on the ground in a known location to act as a static reference point. This type of setup is often called a GPS base station. Since the base station "knows" where it is, it can compute the errors in its position calculations (in reality, it computes timing errors) and apply them to any number of roving receivers in the same general area. This requires that the base and rover receivers "see" the same set of satellites at the same time. The base station, depending upon how it is configured, can correct roving GPS receiver data in one (or both) of two ways. 1) In the first method, called real-time differential correction or real-time differential GPS (DGPS), the base station transmits (usually via radio link) error correction messages to other GPS receivers in the local area. In this case, the positions you read on your GPS receiver while you are out collecting data, are the corrected positions. 2) The second method, called post-processed differential correction, is performed on a computer after the roving receiver data are collected. While you are out in the field collecting data, the positions you read on your roving GPS receiver are uncorrected. It is not until you take your rover files back to the office and process them using differential correction software and data from the base station file, that you get corrected positions. The base station file contains information about the timing errors. This information allows the differential correction software to apply error corrections to the roving receiver file during processing. Since the base and rover receivers have to "see" the same set of satellites at the same time, the base file has to start before the rover file starts, and end after the rover file ends (a base station is normally set up to track all satellites in view, insuring that it will "see" at least the four satellites that the roving receiver is using to compute positions). Post-processed differential correction, then, requires both base and rover receivers that are capable of collecting and storing files. Most recreational grade receivers cannot collect and store files that can be differentially corrected. Differential Correction Sources Several options are available for obtaining differential corrections: 1) use a local base station, 2) use one of the wide-area differential GPS (WADGPS) services. You can set up your own local base station or share a base station with other GPS users in your area. If you are using post-processed differential correction, the base station can usually serve users in an area with about a 2 to 300 mile radius (the further the roving receiver is from the base station, the less accurate the corrections). If you are using real-time differential correction, you must establish radio links, and your coverage area is limited by the strength of the radio transmissions. If you plan to set up your own base station, make sure the manufacturer can supply all the necessary components including base and rover receivers, radios (if using real-time correction) and differential correction software (if using post-processed correction). Many government agencies operate GPS base stations and may provide correction files for post-processed differential correction. If you plan to use files from an operating base station, determine the manufacturer of the base station receiver. If you purchase a roving receiver from the same manufacturer, the base and rover files will be compatible and your rover files can be differentially corrected using software provided by the manufacturer. If your rover is made by a different manufacturer, you will probably have to convert the base files to Receiver Independent Exchange (RINEX) format before they can be used to differentially correct your rover data. Make sure your differential correction software can use a RINEX base file. If not, the rover file has to be converted to RINEX format and then differentially corrected using software provided by the base station manufacturer. In this situation, any attribute data stored in your roving receiver file will be lost because the RINEX format supports conversion of position data only. If you need to use a RINEX conversion, make sure you test it thoroughly before purchasing a receiver. Some companies, such as Omnistar and RACAL provide differential corrections in real-time via their own communication satellite systems. To receive their signals you must purchase a special satellite receiver as well as the subscription service. The signals from satellites are generally available over a widespread area, hence the term wide-area differential GPS (WADGPS). Your GPS receiver must be able to receive the correction data from the satellite receiver and apply those corrections to the data it collects. Some companies offer an integrated GPS/satellite correction receiver so you don't have to purchase a separate GPS receiver. Just be sure the system will allow attribute data collection and can provide any other features you need. The Nationwide Differential GPS (NDGPS) system is another source for differential correction data. NDGPS stations exist around the country, and the system is currently being expanded, with the hope of providing full coverage throughout the continental United States . An additional source of real-time differential correction data is the Wide Area Augmentation System (WAAS) operated by the Federal Aviation Administration (FAA). Many receivers are now WAAS compatible.