(c) Because of the high altitudes of satellites, it is possible to triangulate across the ocean and

to determine relative positions between continents using the appropriate tracking method. In addition to

obtaining the geodetic position, knowledge of the exact period of the satellite's orbit gives a measure of

the earth's flattening. To be useful for extensive geodetic work, a satellite must be placed in a very

stable orbit so that its spatial position at any moment can be accurately predicted.

differences in elevation of points on the earth's surface is known as vertical control or geodetic leveling.

The highly accurate leveling instrument used is aligned perpendicular to the geoid, and the line of levels

follow the geoid's curvature. The purpose of leveling is to determine the elevation above MSL of any

number of vertical control points from which the elevation of any other point in a survey can be

computed.

a. The determination of the acceleration of gravity over the earth's surface provides a method of

determining the shape of the earth. In using the earth's gravity field to determine the earth's shape, the

acceleration of gravity is measured at or near the earth's surface.

b. The theory behind gravitation acceleration depends directly on Newton's law of universal

gravitation. The story of Newton and the falling apple has led toward the law that governs the universe.

Newton reasoned that the force that pulled the apple down was the same force that holds the moon in its

orbit around the earth. He also reasoned that the force diminished as the distance from the earth

increased. From this reasoning he arrived at his inverse square law which states that the force is

inversely proportional to the square of the distance from the center of the earth. When the distance is

doubled, the force of gravity decreases by the square of two or four. Newton also deduced that the

intensity of the force of gravity depended on the mass of an object. The force of mutual attraction

exerted by two bodies was directly proportional to the masses of the pair--the larger the mass, the

stronger the attraction. From these deductions Newton arrived at the following law of universal

gravitation: every particle of matter in the universe attracts every other particle with a force directly

proportional to the product of the masses and inversely proportional to the square of the distance

between them. In mathematical form this law can be expressed in the following equation:

F is the attractive or gravitational force between two bodies of masses (m1m2), with m1 and m2

expressing grams; r representing the distance between them; and G showing the constant of gravitation.

To put Newton's equation to use, it was necessary for scientists to find the exact value of G from

laboratory experiments. Newton presented only a rough estimate of its value from astronomical

observations.

c. The universal gravitational constant (or G) is so named because its value is the same everywhere

in the universe. An English scientist named Henry Cavendish completed the first accurate laboratory

measurement of G in 1798. His device consisted