US 5,296,861 A · Method and apparatus for maximum likelihood estimation direct integer search in differential carrier phase attitude determination systems

Abstract

A method for comparing the relative phase of carrier signals received from GPS satellites to determine the roll, pitch and azimuth attitude of ships, aircraft, land vehicles, or survey instruments, accomplishes a maximum likelihood estimation (MLE) optimum solution over the full range of integers and vehicle attitudes. The problem is formulated as an MLE optimization, where vehicle attitude and integers are regarded as parameters to be adjusted to maximize probability of first-difference carrier phase measurements that are actually generated by hardware. Formulation results in weighted-fit error W as the objective criterion to minimize. A Kalman filter is introduced, having same objective criterion. Minimizing computation in Kalman filter leads to a decision tree for the integers. Two ways are shown to prune decision tree. The first is to exclude impossible combinations, such as those that produce an antenna upside down. The second is to generate a lower bound for W at each branch of the tree. A running sum is kept at each stage moving down the tree. When that sum exceeds a reasonableness bound or the current best W found elsewhere in the search, it is guaranteed that all subsequent integer combinations further down the current branch will produce a larger W and the remainder of the current branch can be cut off, speeding up the search.

An apparatus for determining a direction or heading in a reception environment that includes radio reflections, reception noise, antenna phase center motion, temperature-sensitive drifts and detected signal phase corruption, comprising:

a pair of GPS antennas separated by a known first physical distance that exceeds one wavelength of a radio carrier signal emitted by a GPS satellite for simultaneously receiving radio carrier signals emitted by radio transmitter in at least four corresponding GPS satellite each having a known position;
a pair of first and second phase coherent GPS correlators connected to the pair of GPS antennas and having respective first and second correlator outputs derived from said GPS satellite radio carrier signal as independently received by each of the pair of GPS antennas wherein said first and second correlator outputs include a phase difference representative of a phase difference that exists in the pair of GPS antennas for said GPS satellite radio carrier signal;
first computer means connected to the pair of first and second phase coherent GPS correlators for determining a first plurality of possible headings to a first of said GPS satellites based on a single first phase difference between said first and second correlator outputs for a first radio carrier signal from said first GPS satellite, wherein said first plurality of possible headings includes more than one possible heading due to whole cycle carrier phase ambiguities;
second computer means connected to the pair of first and second phase coherent GPS correlators for determining a second plurality of possible headings to a second of said GPS satellites based on a single second phase difference between said first and second correlator outputs for a second radio carrier signal from said second GPS satellite, wherein said second plurality of possible headings includes more than one possible heading due to whole cycle carrier phase ambiguities;
third computer means connected to the pair of first and second phase coherent GPS correlators for determining a third plurality of possible headings to a third of said GPS satellites based on a single third phase difference between said first and second correlator outputs for a third radio carrier signal from said third GPS satellite, wherein said third plurality of possible headings includes more than one possible heading due to whole cycle carrier phase ambiguities;
fourth computer means connected to the pair of first and second phase coherent GPS correlators for determining a fourth plurality of possible headings to a fourth of said GPS satellites based on a single fourth phase difference between said first and second correlator outputs for a fourth radio carrier signal from said fourth GPS satellite, wherein said fourth plurality of possible headings includes more than one possible heading due to whole cycle carrier phase ambiguities;
and
simultaneous heading solution computer means connected to the first through fourth computer means for matching said first through said fourth pluralities of possible headings and for identifying at least one geometry for the pair of antennas that satisfies at least three of said first through fourth plurality of possible headings and eliminating any additional geometries for the pair of antennas that satisfies at least three of said first through fourth plurality of possible headings that have a less perfect fit to said respective geometries before identifying any geometry for the pair of antennas that satisfies four of said first through fourth plurality of possible headings.

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Method and apparatus for maximum likelihood estimation direct integer search in differential carrier phase attitude determination systems

Life of this patent

Method and apparatus for maximum likelihood estimation direct integer search in differential carrier phase attitude determination systems

Life of this patent

Abstract

Claims7