Communications: directive radio wave systems and devices (e.g. – Directive – Including a satellite
Reexamination Certificate
2000-07-25
2002-04-16
Phan, Dao (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a satellite
C342S357490, C342S357490, C342S003000, C342S357490, C701S213000
Reexamination Certificate
active
06373432
ABSTRACT:
II. BACKGROUND
Conventional satellite-based positioning techniques are based on the use of special navigation signals transmitted from several navigational satellites. In the global positioning system (GPS), for example, a constellation of GPS satellites transmit L
1
and L
2
carrier signals modulated with C/A and P code signals. By measuring these code signals, a user receiver can determine its position to an accuracy of several meters.
To determine the user position with higher accuracy, a differential technique can be used. A reference receiver having a known position also measures the code signals and calculates its position. The reference receiver then calculates a differential correction by comparing its known position with this calculated position, and transmits this correction to the user receiver. Assuming the user receiver is near the reference station, it can use the differential correction data to improve the accuracy of its position estimate down to approximately 1 meter.
Various proposed techniques provide positioning accuracy on the order of 1 cm. In addition to measuring the code signals from the GPS satellites, these techniques use carrier phase measurements of the signals from the GPS navigational satellites. Typically, this carrier phase positioning technique uses differential carrier phase correction data from a reference station in order to improve performance. There is a significant difficulty inherent to this technique, however. When tracking a carrier signal of a navigational satellite transmission, one is able to directly measure the phase of the signal, but one cannot determine by direct measurement how many complete integer cycles have elapsed between the times of signal emission and reception. The measured carrier signal thus has an inherent integer cycle ambiguity which must be resolved in order to use the carrier phase measurements for positioning. Consequently, much research in the art of satellite-based positioning has focused on resolving these cycle ambiguities in carrier phase measurements of GPS satellite signals.
MacDoran and Spitzmesser (U.S. Pat. No. 4,797,677) describe a method for deriving pseudoranges of GPS satellites by successively resolving integers for higher and higher signal frequencies with measurements independent of the integers being resolved. The first measurement resolves the number of C/A code cycles using a Doppler range; these integers provide for independent measurements to resolve the number of P code cycles, and so on for the L
2
and L
1
carriers. This technique, however, assumes exact correlation between satellite and user frequency standards (i.e., the user requires an atomic clock), and provides no means of correcting for atmospheric distortions.
A similar technique, called dual-frequency wide-laning, involves multiplying and filtering the L
2
and L
1
signals from a GPS satellite to form a beat signal of nominal wavelength 86 cm, which is longer than either that of the L
1
signal (19 cm) or the L
2
signal (24 cm). Integer ambiguities are then resolved on this longer wavelength signal. Since the L
2
component is broadcast with encryption modulation, however, this technique requires methods of cross-correlation, squaring, or partially resolving the encryption. These techniques are difficult to implement and how low integrity.
Hatch (U.S. Pat. No. 4,963,889) describes a technique for resolving integer ambiguities using measurements from redundant GPS satellites. Initial carrier-phase data is collected from the minimum number of GPS satellites needed to resolve the relative position between two antennae. From these measurements, a set of all possible integer combinations is derived. Using carrier phase measurements from an additional GPS satellite, the unlikely integer combinations are systematically eliminated. This technique is suited to the context of attitude determination where both receivers use the same frequency standard and the distance between the antennae is fixed. This approach, however, is ill-adapted for positioning over large displacements, where the initial set of satellites is four and the distance between the receivers is not known a priori, the technique is then extremely susceptible to noise, and computationally intensive. Knight (U.S. Pat. No. 5,296,861) details an approach similar to that of Hatch, except that a more efficient technique is derived for eliminating unlikely integer combinations from the feasible set. Knight's technique also assumes that the two receivers are on the same clock standard.
Counselman (U.S. Pat. No. 5,384,574) discloses a technique for GPS positioning that does not resolve integer cycle ambiguity resolution but rather finds the baseline vector between two fixed antennae by searching the space of possible baseline vectors. The antennae track the GPS satellite signals for a period of roughly 30 minutes. The baseline is selected that best accounts for the phase changes observed with the motion of the GPS satellites. This technique, however, assumes that the baseline vector remains constant over the course of all the measurements during the 30 minute interval, and is therefore only suitable for surveying applications. Moreover, it also assumes that the clock offset between user and reference receivers remains constant over the 30 minute measurement interval.
A motion-based method for aircraft attitude determination has been disclosed. This method involves placing antennae on the aircraft wings and tail, as well as a reference antenna on the fuselage. The integer ambiguities between the antennae can be rapidly resolved as the changes in aircraft attitude alter the antenna geometry relative to the GPS satellite locations. This approach, however, is limited to attitude determination and is not suitable for precise absolute positioning of the aircraft itself.
Current state-of-the art kinematic carrier phase GPS navigation systems for absolute positioning have been disclosed. These systems achieve rapid resolution of cycle ambiguities using ground-based navigational pseudo satellites (pseudolites) which transmit either an additional ranging signal (Doppler Marker) or a signal in phase with one of the satellites (Synchrolites). Although this approach rapidly achieves high precision absolute positioning, it provides high precision and integrity only when the user moves near the ground-based pseudolites. In addition, the pseudolites are expensive to maintain.
Therefore, each of the existing techniques for satellite-based navigation suffers from one or more of the following drawbacks: (a) it does not provide centimeter-level accuracy, (b) it does not quickly resolve integer cycle ambiguities, (c) it is not suitable for kinematic applications, (d) it provides only attitude information and does not provide absolute position information, (e) it does not have high integrity, (f) it requires the deployment and maintenance of pseudolities, (g) its performance is limited to users in a small geographical area near pseudolities, or (h) it requires the user receiver and/or the reference receiver to have an expensive highly stable oscillator.
III. SUMMARY OF THE INVENTION
In view of the above, it is an object of the present invention to provide a system and method for centimeter-level kinematic positioning with rapid acquisition times and high integrity. In addition, it is an object of the invention to provide such a method that does not depend on additional signals transmitted from nearby navigational pseudolite transmitter, and does not require a highly stable oscillator such as an atomic clock. It is further an object of the invention to provide a navigation system requiring only carrier phase information. These together with other objects and advantages will become apparent in the following description.
In order to obtain high-integrity estimation of integer cycle ambiguities, carrier-phase measurements must be made for a time interval long enough that the displacement vectors between the user and the signal sources undergo substantial geometric change. Surprisingly, the pres
Cohen Clark E.
Lawrence David G.
Parkinson Bradford W.
Rabinowitz Matthew
Fish & Richardson P.C.
Phan Dao
The Board of Trustees of the Leland Stanford Junior University
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