observation, contains a light source which can be activated electronically for photographic purposes.
Large satellites with no independent source of illumination can be photographed at dusk or dawn when
reflected sunlight illuminates the vehicle. The smaller angular distances between the images of the
satellite and the star background are measured, and these measurements are used to determine the
direction of the satellite from the observation point.
(b) The SECOR system employs a series of transmitting and receiving stations on the ground
which contact a specially equipped satellite as it passes within range. Command signals are transmitted
from the ground to the satellite. When it receives the command signal, the satellite transmits a ranging
signal back to the ground station. The distance to the satellite can thus be assured by comparing the
outgoing signal with the incoming signals at the ground station. Various corrections must be made for
atmospheric effects. Four practice stations are used on the ground. Three of these stations are located
on a known datum, and the fourth at an unknown position (Figure 1-12). A series of simultaneous
measurements are made from each ground station during each pass of the satellite. These
measurements, when converted, establish the position of the satellite and the position of the unknown
station. The unknown station can be located from 100 to 1,500 miles from the known stations.
Although the SECOR system is still in the improvement stage, results of reduced data already indicate
that it will be a first-order geodetic tool.
Figure 1-12. SECOR Ranging of Satellite