• Perifocal Coordinate System: fundamental plane is the satellite orbit; origin is at the center of the Earth; P points towards perigee (近地點); Q is $90^{\degree}$ from P in the direction of satellite motion.
• Satellite Coordinate System RSW: R points out from the Earth; S points to the velocity vector and perpendicular to R.
  • Radial replacements are parallel to R.
  • Along-track replacements are parallel to S.
  • Cross-track replacements are normal to the plane.
• Satellite Coordinate System NTW: defined by normal and tangential vectors.
• Apparent Solar Time (視太陽日): interval between successive transits. It’s a poor choice because observed length varies throughout the year.
• Mean Solar Time (平太陽日): now derived from the Earth orientation.
  • Mean solar time at Greenwich is defined as universal time.
  • Difference between apparent and mean solar time is defined as the equation of time.
• Coordinated Universal Time (UTC, Zulu time): derived from an ensemble of atomic clocks.
• GPS Time: starting from January 6, 1980. It counts the week and seconds. It needs to reset in around every 20 years.
• Julian Date: interval of time measured in days from the epoch January 1, 4713 B.C., 12:00. The Julian Date provides a continuous, simple, and concise method of preserving year-month-day-hour-minute-second in one variable.