Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1999-09-14
2003-02-04
Vo, Don N. (Department: 2631)
Pulse or digital communications
Spread spectrum
Direct sequence
C342S108000, C342S189000
Reexamination Certificate
active
06516021
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the fields of communications and navigation systems. More particularly, the present invention relates to communication systems coupled to inertial navigation systems for improved signal tracking.
BACKGROUND OF THE INVENTION
With the increasing dependency on the Global Positioning System (GPS) for military and civilian navigation systems, it is important that the navigation receiver be able to withstand intentional or unintentional signal interference with robust signal acquisition and tracking. For this reason, inertial navigation systems have been coupled to GPS signal tracking and acquisition system for improved signal tracking and acquisition of the received signal. Inertial navigation systems (INS) have an inertial measurement unit (IMU) for processing inertial measurements. Coupling navigation data with the GPS signal tracking and acquisition system improves signal tracking and acquisition. When the inertial measurements of a navigation processor are used with a GPS signal tracking and acquisition system, the combined system is said to be tightly coupled. The tightly coupled GPS and INS method uses pseudorange and pseudorange rate measurements from the GPS receiver instead of the processed position and velocity measurements. In addition, inertial measurement unit data is usually used to assist the receiver tracking loops and enable more noise filtering than would be possible without tracking loop aiding. The tightly coupled method has been used with the pseudorange and pseudorange rate measurements. The receiver measurements are not the raw receiver correlator samples, but the product of aided or unaided receiver tracking loops and are subject to performance limitations.
A loosely coupled GPS and INS method has an integration Kalman filter used to process GPS position and velocity measurements to aid the inertial navigation system. In a loosely coupled method, no information from the inertial navigation system is used to assist the tracking loops in the GPS receiver. The GPS receiver is unaided when the tracking loops must operate independently of the inertial navigation system. The loosely coupled method is easily jammed and has poor noise filtering.
A tracking loop is a combination of electronic hardware and software algorithms used to track a pseudorandom noise (PRN) code signal and its carrier. A tracking loop that tracks the PRN code is called the Code Loop and one that tracks the carrier is called the Carrier Loop. The Carrier Loop can track the phase or the frequency of the carrier or a combination of both. A tracking loop design is adjusted to each application by designing the closed loop gains and the order of the filter to obtain the desired filtering and dynamic response. A code tracking loop functions by driving the code phase of a replica signal to be aligned with the received signal so as to enable coherent demodulation of the received signal. The tracking loops include a signal generator that generates an estimated replica signal of the received signal using a control signal that advances or retards in time the replica signal relative to the received signal, includes a correlator that multiplies the received signal by the replica signal and passes the multiplied signal result through a low pass filter, includes discriminator generator that generates a discriminator signal having a value related to the difference between the received signal and the replica signal, and includes a controller that filters the discriminating signal into the control signal that is then communicated to the signal generator.
A fundamental function in all GPS receivers built to date is the ability of the receiver to generate a replica signal of the received signal transmitted by a satellite that can be correlated with the received signal being received from the satellite. The replica signal is advanced or retarded until the receiver-generated replica signal correlates with the received one. Traditionally, this process is performed by code and carrier tracking loops that determine an error signal, which is a measure of the range and range rate difference between the generated and received signals. That difference between the received and replica signal is an error signal that is processed by a transfer function for generating input values communicated to a numerically controlled oscillator to advance or retard the generated replica signal. When the replica signal correlates with the received one, the tracking loop is said to be in lock and the feedback error signal is near zero. In this condition, the state of the code and carrier generation process can be sampled to obtain a measure of the pseudorange and pseudorange rate. The pseudorange is the geometric range between the transmit antenna and receive antenna plus a bias due to the user clock error. The pseudorange rate is approximated by the amount of range change for a predetermined amount of time plus a bias due to the drift of the user's oscillator frequency.
The pseudorange measurements are derived from an instantaneous sampling of the state of the code generator at a desired measurement epoch time. The pseudorange rate measurement is obtained by strobing the carrier loop twice over a small period of time and is a measure of a discrete change in pseudorange over a discrete period of time, determined from the carrier phase change. In the limit as the time interval goes to zero, the ratio of the delta pseudorange divided by delta time approaches the instantaneous time rate of change of the pseudorange, which rate of change is the pseudorange rate.
The tracking loops that track the incoming satellite received signal must adjust the phase and the frequency of the generated replica signal for many changing variables, such as user and satellite relative motion and user clock drifts. These tracking loops are traditionally called the code loop and the carrier loop, because the code loop tracks the phase of the pseudorandom noise (PRN) code and the carrier loop tracks the signal carrier frequency and the carrier phase. Although a phase-locked loop is the most common way to track a carrier signal, the GPS signals have a fifty hertz navigation data message superimposed on the code generation process, which can potentially change the phase of the signal by 180o every twenty milliseconds. To avoid a loss of lock when this occurs, a Costas tracking loop is used in place of the conventional phase lock loop. This allows the carrier to be tracked across 50 Hz data bit changes with no loss of lock. Frequency lock loops are also used for the carrier tracking and sometimes are used in combination with phase lock loops to improve robustness.
The design of tracking loop transfer functions is compromised to meet two conflicting requirements. On one hand, it is desired to have a low bandwidth of the tracking loop to filter out as much noise as possible. On the other hand, if the tracking loop is too sluggish because of a low operating bandwidth, the tracking loop cannot track the dynamics of the relative motion often caused by vehicular acceleration. Inertial aiding applies to the use of data from an IMU to assist the tracking loops or the extrapolation of the user's position and velocity. When inertial aiding is used in the context of tracking loops, the relative motion between the satellite's antenna and the user's antenna can be predicted to a certain accuracy based on the IMU measurements and the position and velocity of a satellite evaluated from the ephemeris data. A reduction of the bandwidth of the tracking loop is possible with inertial aiding because the loop need only track the errors in the aiding information as opposed to the absolute motion. A narrower bandwidth has the advantage of filtering out more noise from the loop during tracking.
GPS sets with IMU aiding, including those with communication receivers and navigation processors, must use a large enough tracking loop bandwidth to accommodate the dynamics of the relative vehicular motion and tolerate the associated r
Abbott Anthony S.
Lillo Walter E.
Nguyen Dung X.
Reid Derrick Michael
The Aerospace Corporation
Vo Don N.
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