Communications: directive radio wave systems and devices (e.g. – Directive – Including a satellite
Reexamination Certificate
2001-08-24
2003-10-14
Blum, Theodore M. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a satellite
Reexamination Certificate
active
06633256
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to surveying methods and apparatuses for measuring coordinates of a target point. The methods and apparatuses are based on the signals of satellite radio navigation systems. The invention is especially efficient under strong multipath conditions.
BACKGROUND OF THE INVENTION
Satellite navigation systems include the global positioning system (GPS) and the global orbiting navigation system (GLONASS) and are used to solve a wide variety of tasks that related to determining object position, object velocity, and precise time. Land surveying is an important application of receivers based on satellite navigation systems. Such receivers have a lot of advantages compared to the conventional devices for land surveying. In comparison to conventional surveying devices, satellite-based surveying systems are more responsive, can operate in nearly all types of weather and at all times of the day, and can be used in areas which do not have line-of-sight conditions.
Any measurement procedure is characterized by its efficiency (productivity). In the case of surveying, it is the number of point position measurements that can be made per unit of time within a predetermined accuracy. To improve efficiency, we should reduce the time duration of a single measurement. However at this, it is necessary to simultaneously increase measurement accuracy, because a reduction in the time duration of the single measurement can result in a deterioration in accuracy if special precautions are not used.
Many survey applications require sub-centimeter positioning accuracy, i.e., accuracy to within several millimeters. To achieve this, the receiver, which is often called the “rover”, operates in phase differential mode with a base station that has a position known with high accuracy. The coordinate difference between a rover and a base station, which is called the “base vector,” can be determined in this mode. For this, we use the satellite carrier phase difference between the base and rover. It can be calculated by processing data sets from the base and rover. Data sets of measurements from the base station are called differential corrections. The rover is placed on a point whose coordinates need to be ascertained, i.e., a target point, while the base station is placed on a point with precisely known coordinates, i.e., a land mark. The receiver antennas are mounted, for instance, on respective tripods.
Knowing the coordinates of the base vector and the base station, it is possible to compute the rover's coordinates by summing the base-station and base vector coordinates together. For computing, one needs to know both the land-mark position relative to the phase center of the base antenna and the target point position relative to the phase center of the rover's antenna, since a satellite navigation system can determine a base vector only between the phase centers of the antennas.
To simplify the transformation of the phase center position into target point position (and vice versa, landmark position into phase center position), the phase center of the antenna is usually situated vertically above the landmark or target point using a plumb bob, level vial, or other instruments. In this case, for the transformation we need to know just the difference in height between the antenna's phase center and the corresponding landmark or target point.
However, such a procedure of vertical alignment is time-consuming, and is an acceptable burden only for the base station, not the rover. The base station is usually set up to operate for a long time, while the rover is usually fixed for a short time. For instance, such a procedure as real time kinematic (RTK) surveying usually requires that the minimum possible amount of time be used to set up the rover on target point in order to improve measurement efficiency. In practice, almost all of the time needed for an RTK surveying measurement is spent on this set-up time. As usual, in such cases one employs a range pole with a bubble level vial. The antenna is mounted on top of the range pole, and the bottom pole tip is placed on target point.
We should note that using such an instrument does not provide sub-centimeter accuracy because of the possible trembling of the operator's hand. To reduce this trembling, one can use a bipod which has two extra legs to achieve a stable pole position, but this results in an undesirable increase in the set-up time.
An alternative way is to provide the range pole with a tilt sensor and magnetic sensor (compass) that determines direction of the tilt in the horizontal plane. When processing this sensor data, it is possible to ascertain the direction and amount of the pole's tilt, and to then transform the position of the phase center in the target point (U.S. Pat. No. 5,512,905). However, due to their inherent errors, sensors do not allow the accuracy of transformation to better than 1 to 2 cm when using a two-meter length range pole. Moreover such a device is relatively expensive and complicated. The main source of errors for the tilt sensor is temperature drift of measurements, and for the magnetic sensor, it is both neighboring iron objects and local magnetic anomalies.
There is a possibility of doing without any of the above sensors. It is possible if we process a set of measurements obtained when swinging the pole while keeping contact of the pole tip with the target point (U.S. Pat. No. 5,929,807). As the pole length is constant, all of the measured points will be placed on a sphere with a radius equal to the pole's length. The set of measurements can be processed with the least squares technique (LST) to determine the position of the sphere's center, which will be the position of the target point. This approach provides high accuracy in the height of the target point. But at this, the accuracy of the plane position will be poor (not better than 3 to 4 cm) because of the limited swing angle sector of the pole (tilt angle not greater than 20 degrees). This limitation is connected with the shape of an antenna radiation pattern, because if the tilt is greater than 20 degrees, the signal power at the antenna output is too weak to reliably track satellites having low elevation angles. The limitation is also related to inconvenience for the operator to swing the pole with a greater angle since it makes him bend. Note the angle sector of 90 degrees is needed to reach better accuracy, that is, the antenna should be swept through the range of a semi-sphere. However, this is impossible. So, both considered alternative approaches for the transformation of the phase center position into the position of the target point cannot provide sub-centimeter accuracy.
Another source of coordinate errors are multipath errors arising from the reception of signal replicas along with the line-of-sight signal from the satellite. These replicas are reflected from neighboring objects and have parameters different from line-of-sight signal parameters. The total signal received by the antenna and measured by the receiver will be a combination of the parameters of the line-of-sight signal and the parameters of the multipath signals. Thus, the parameters of the total signal will be different from the parameters of the line-of-sight signal, and there will be a resulting multipath error. This error can be about 1 to 3 cm, depending upon operation conditions. In addition, multipath signals can result in anomaly errors having values much greater than the ones given above.
Under differential mode at short baselines, when baseline length is less than 10 km, the multipath error in the antenna's phase center position becomes prevailing. In reality, using differential mode enables one to eliminate almost totally the majority of position error sources which are related to the satellites (selective availability, ionosphere delay, instability of the satellite clock, inaccuracy of ephemeris information). Error elimination is achieved by their inter-compensation at subtraction, since they are present
Ashjaee Javad
Zhdanov Alexey Vladislavovich
Blum Theodore M.
Sheppard Mullin Richter & Hampton LLP
Topcon GPS LLC
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