# ORBITAL ELEMENTS

Parameters that specify the position and motion of a body in orbit. The elliptical orbit of a satellite attracted by an exactly central gravitational force is specified by a set of six parameters as follows: Two parameters, the semimajor axis and eccentricity of the ellipse, establish the size and shape of the elliptical orbit. A third parameter, time of perifocal passage, enables determination of the location of the satellite in its orbit at any instant. The three remaining parameters establish the orientation of the orbit in space. These are the inclination of the orbital plane to a reference plane, the right ascension of the ascending node of the satellite, and the argument of pericenter.

## Related Terms

### TWA

Time Weighted Average

1. The time interval between a change in the input signal to a process control system and the response to the signal.
2. The time interval, after a response to one signal or event, during which a system is unable to respond to another. Also known as insensitive time.

### POINCARE SURFACE OF SECTION

A method of displaying the character of a particular trajectory without examining its complete time development, in which the trajectory is sampled periodically, and the rate of change of a quantity under study is plotted against the value of that quantity at the beginning of each period. Also known as surface of section.

### PLANNING HORIZON

In a materials-requirements planning system, the time from the present to some future date for which plans are being generated for acquisition of materials.

### PHASE PORTRAIT

A graph showing the time derivative of a system's position (or some other quantity characterizing the system) as a function of position for various values of initial conditions.

### PHASE PLANE ANALYSIS

A method of analyzing systems in which one plots the time derivative of the system's position (or some other quantity characterizing the system) as a function of position for various values of initial conditions.

### PERS SUNSHINE RECORDER

A type of sunshine recorder in which the time scale is supplied by the motion of the sun.

### PERMANENT BENCHMARK

A readily identifiable, relatively permanent, recoverable benchmark that is intended to maintain its elevation without change over a long period of time with reference to an adopted datum, and is located where disturbing influences are believed to be negligible.

### PEEL-OFF TIME

In seismic prospecting, the time correction applied to observed data to adjust them to a depressed reference datum.

### PAYOUT TIME

A measurement of profitability or liquidity of an investment, being the time required to recover the original investment in depreciable facilities from profit and depreciation; usually, but not always, calculated after income taxes.

### Related questions

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