Lecture Notes
Updated Monday, 15-Mar-2004 10:14:48 EST

Satellites



Sputnik I -- 60 cm diam. sphere with straight-wire antennas


Explorer I -- 1 m. long and 20 cm in diam., spin stabilized (like a gyroscope), with flexible antennas


A generic military/meteorological/communications satellite -- 1-3 m. on each side, stabilized with internal gyroscopes or external thrusters


Dual-spin stabilized satellite -- 1-3 m. in diameter, up to several meters tall; lower section spins to provide gyroscopic stability, upper section does not spin (or spins very slowly or intermittently) to point antenna and/or other sensors in a specific direction

What does a s/c need? Common subsystems Propulsion Power Thermal control

Communications

Attitude sensing and control (orientation of s/c)






Structure Computers Scientific instruments

Environmental control and life support

For some very nice graphics of early space vehicles, see the Manned Space Flight page created by Dr. Thomas Starchville, Jr. for this course in Spring 1997.

Global Positioning System (GPS)

    The GPS system consists of 24 satellites that broadcast signals containing information about their position and the time that each signal is sent.  A GPS receiver processes signals from at least four satellites to determine the receiver's position to high accuracy (approximately 10 meters).  The basic concept used by the GPS system and receiver is the constancy of the speed of light.  Since each signal broadcast by a GPS satellite travels at the speed of light c = 3 x 108 m/s, the distance that the signal rravels to reach the receiver is  d = cDt, where Dt is the time interval between transmission and reception of the signal.  For example, if the signal is transmitted at 10:36:02.0000453297 (this is read as 10:36AM plus 2.0000453297 s) and received at 10:36:02.0000821946, then Dt = 0.000368649 s.  Therefore the distance between the GPS satellite and the GPS receiver is d = cDt = 11059.5 m.
    For all of this to work, the GPS satellites use ultra-accurate atomic clocks, but a GPS receiver would be too expensive and bulky if it contained an atomic clock.  Instead, the receiver contains a simple quartz-regulated clock, similar to those used in ordinary wristwatches.
    So the GPS signals must be used to correct the clock in the GPS receiver.  The receiver must be able to receive at least four GPS signals simultaneously -- these allow it to calculate the four unknowns -- x, y, and z coordinates of the receiver, and the correct time.  This is a mathematical problem in which four equations are solved simultaneously.  (Note:  if you're not used to thinking of x, y ,z coordinates, consider the position in terms of latitude, longitude, and height above sea level).

Details of the calculation

Copyright © 1998, Robert G. Melton

Updated Monday, 15-Mar-2004 10:14:48 EST