《GPS定位原理与应用》习题集与部分参考答案
Errors caused by the interference of a signal that has reached the receiver antenna by two or more different paths. This is usually caused by one path being bounced or reflected. The impact on a pseudo-range measurement may be up to a few metres. In the case of carrier phase, this is of the order of a few centimetres.
57、Relative Positioning
The determination of relative positions between two or more receivers which are simultaneously tracking the same GPS signals. One receiver is generally referred to as the Reference or Base Station, whose coordinates are known in the satellite datum. The second receiver may be stationary or moving. However its coordinates are determined relative to the Base Station. In carrier phase-based positioning this results from the determination of the baseline vector, which when added to the Base Stations coordinates generates the User's coordinates. In pseudo-range-based GPS positioning, the coordinates are derived from the User receiver's observations after they have had the differential corrections applied (either in the real-time or post-processed mode).
58、RINEX
Receiver INdependent EXchange format. A set of standard definitions and formats to promote the free exchange of GPS data and facilities the use of data from any GPS receiver with any post-processing software package. The format includes definitions for three fundamental GPS observables: time, phase, and range.
59、Static Positioning
Location determination when the receiver's antenna is presumed to be stationary on the earth. In the case of pseudo-range-based techniques this allows the use of various averaging techniques to improve the accuracy. Static Positioning is usually associated with GPS Surveying techniques, where the two GPS receivers are static for some observation period which may range from minutes to hours (and even in the case of GPS geodesy, several days).
60、Tropospheric Delay
The tropospheric delay depends on the temperature, pressure, and humidity along the signal path through the troposphere. Signals from satellites at low elevation angles travel a longer path through the troposphere than those at higher elevation angles. Therefore, the tropospheric delay is minimized at the userís zenith and maximized near the horizon. Tropospheric delay results in values of about 2.3m at zenith (satellite directly overhead), about 9.3m for a 15° 安徽理工大学测绘工程教研室 余学祥 吕伟才 编
.２１.
《GPS定位原理与应用》习题集与部分参考答案
-elevation angle, and about 20?28m for a 5°-elevation angle.
Tropospheric delay may be broken into two components, dry and wet.The dry component represents about 90% of the delay and can be predicted to a high degree of accuracy using mathematical models. The wet component of the tropospheric delay depends on the water vapor along the GPS signal path. Unlike the dry component, the wet component is not easy to predict. Several mathematical models use surface meteorological measurements (atmospheric pressure, temperature, and partial water vapor pressure) to compute the wet component. Unfortunately, however, the wet component is weakly correlated with surface meteorological data, which limits its prediction accuracy. It was found that using default meteorological data (1,010 mb for atmospheric pressure, 20°C for temperature, and 50% for relative humidity) gives satisfactory results in most cases.
61、Satellite geometry measures
The various types of errors and biases discussed earlier directly affect the accuracy of the computed GPS position. Proper modeling of those errors and biases and/or appropriate combinations of the GPS observables will improve the positioning accuracy. However, these are not the only factors that affect the resulting GPS accuracy. The satellite geometry, which represents the geometric locations of the GPS satellites as seen by the receiver(s),plays a very important role in the total positioning accuracy [5]. The better the satellite geometry strength, the better the obtained positioning accuracy.As such, the overall positioning accuracy of GPS is measured by the combined effect of the unmodeled measurement errors and the effect of the satellite geometry.
In practice, various DOP forms are used, depending on the userís need. For example, for the general GPS positioning purposes, a user may be interested in examining the effect of the satellite geometry on the quality of the resulting three-dimensional (3-D) position (latitude, longitude, and height). This could be done by examining the value of the position dilution of precision (PDOP). In other words, PDOP represents the contribution of the satellite geometry to the 3-D positioning accuracy. PDOP can be broken into two components: horizontal dilution of precision (HDOP) and vertical dilution of precision (VDOP). The former represents the satellite geometry effect on the horizontal component of the positioning accuracy, while the latter represents the satellite geometry effect on the vertical component of the positioning accuracy. Because a GPS user can track only those satellites above the horizon, VDOP will always be larger 安徽理工大学测绘工程教研室 余学祥 吕伟才 编
.２２.
《GPS定位原理与应用》习题集与部分参考答案
than HDOP. As a result, the GPS height solution is expected to be less precise than the horizontal solution. The VDOP value could be improved by supplementing GPS with other sensors, for example, the pseudolites (see Chapter 9 for details). Other commonly used DOP forms include the time dilution of precision (TDOP) and the geometric dilution of precision (GDOP). GDOP represents the combined effect of the PDOP and the TDOP.
To ensure high-precision GPS positioning, it is recommended that a suitable observation time be selected to obtain the highest possible accuracy.A PDOP of five or less is usually recommended. In fact, the actual PDOP value is usually much less than five, with a typical average value in the neighborhood of two. Most GPS software packages have the ability to predict the satellite geometry based on the userís approximate location and the approximate satellite locations obtained from a recent almanac file for the GPS constellation. The almanac file is obtained as part of the navigation message, and can be downloaded free of charge over the Internet (e.g.,from the U.S. Coast Guard Navigation Center).
62、Fast (rapid) static
Fast, or rapid, static surveying is a carrier-phase?based relative positioning technique similar to static GPS surveying. That is, it employs two or more receivers simultaneously tracking the same satellites. However, with rapid static surveying, only the base receiver remains stationary over the known point during the entire observation session. The rover receiver remains stationary over the unknown point for a short period of time only, and then moves to another point whose coordinates are sought. Similar to the static GPS surveying, the base receiver can support any number of rovers.
63、Stop-and-go GPS surveying
Stop-and-go surveying is another carrier-phase-based relative positioning technique. It also employs two or more GPS receivers simultaneously tracking the same satellites: a base receiver that remains stationary over the known point and one or more rover receivers. The rover receiver travels between the unknown points, and makes a brief stop at each point to collect the GPS data. The data is usually collected at a 1- to 2-second recording rate for a period of about 30 seconds per each stop.Similar to the previous methods, the base receiver can support any number of rovers. Thismethod is suitable when the survey involves a large number of unknown points located in the vicinity (i.e., within up to 10～15 km) of a known point.
安徽理工大学测绘工程教研室 余学祥 吕伟才 编
.２３.
《GPS定位原理与应用》习题集与部分参考答案
64、RTK GPS
RTK surveying is a carrier phase?based relative positioning technique that, like the previous methods, employs two (or more) receivers simultaneously tracking the same satellites. This method is suitable when: (1) the survey involves a large number of unknown points located in the vicinity (i.e., within up to about 10～15 km) of a known point；(2) the coordinates of the unknown points are required in real time；and (3) the line of sight, the propagation path, is relatively unobstructed. Because of its ease of use as well as its capability to determine the coordinates in real time, this method is the preferred method by many users.
In this method, the base receiver remains stationary over the known point and is attached to a radio transmitter. The rover receiver is normally carried in a backpack and is attached to a radio receiver. Similar to the conventional kinematic GPSmethod, a data rate as high as 1 Hz (one sample per second) is required. The base receiver measurements and coordinates are transmitted to the rover receiver through the communication (radio) link [7, 8]. The built-in software in a rover receiver combines and processes the GPS measurements collected at both the base and the rover receivers to obtain the rover oordinates.
65、Real-time differential GPS
Real-time differential GPS (DGPS) is a code-based relative positioning technique that employs two or more receivers simultaneously tracking the same satellites (Figure 5.7). It is used when a real-timemeter-level accuracy is enough. The method is based on the fact that the GPS errors in themeasured pseudoranges are essentially the same at both the base and the rover,as long as the baseline length is within a few hundred kilometers.
66、Ambiguity-Resolution Techniques
The ambiguity parameters are initially determined as part of the leastsquares, or Kalman filtering, solution. Unfortunately, however, neither method can directly determine the integer numbers of the ambiguity parameters. What can be obtained are the real-valued numbers along with their uncertainty parameters (so-called covariance matrix) only. These real-valued numbers are in fact difficult to separate from the baseline solution . As such, since we know in advance that the ambiguity parameters are integer numbers, it becomes clear that further analysis is required.
安徽理工大学测绘工程教研室 余学祥 吕伟才 编
.２４.
《GPS定位原理与应用》习题集与部分参考答案
安徽理工大学测绘工程教研室 余学祥 吕伟才 编
.２５.