1. Observations Passive: Natural Signals Photographs Semi-Passive: Artificial Signals GPS receiver Active: Signals Controlled by Observers Radar Echo, Laser Ranging

2. Passive Observations Astrographs: 2-dim. Angles Photographic Plates, CCD Arrays, VTR Transit, Meridian Circle: 1-dim. Angle Interferometry: Precise 1-dim. Angle VLBI, Optical/IR Int., X-ray Ground-based vs Orbital platform

3. Passive Observations (Cont.) Detectors: Arrival Time, Energy Photo-Multiplier Tube, Photo-Diode, CCD, Chamber Clocks: Event Time Eclipse, Occultation, Pulse, Light Curve, Rise/Decay Pattern Doppler: Radial Velocity Spectrograph, Emission/Absorption Lines

4. Semi-Passive Observations Doppler Up/Down Link to Satellites/Space Vehicles Integrated Doppler NNSS, DORIS/PRARE Semi-Passive VLBI ALSEP, RISE Time Difference GPS, GLONASS

5. Active Observations Radar Bombing Mercury, Venus, Mars, Asteroids Range and Range Rate (R&RR) Artificial Satellites, Space Vehicles Radio Transponder Laser Ranging Satellite, Moon