What are Keplerian elements or Keps for satellites?

 If you look at satellite information page at N2YO.COM, there are a couple of lines named as Two Line Element Set (TLE) near the bottom of the page. These are also known as Keplerian elements or Keps in short. They can be used in satellite tracking softwares like Gpredict. Once TLE is added to the software, it gives information on possible passes in your region as well as Doppler correction needed during the pass. After the line number in each row, there is the NORAD ID of the satellite, which is 25544 for International Space Station.  NORAD stands for North American Aerospace Defense Command. Orbital positions of satellite can be calculated from TLE using mathematical models which predict the effect of perturbations caused by Earth's shape, atmospheric drag, radiation and gravitation effects from other heavenly bodies like the Sun and Moon.

The mathematical model for near Earth objects with an orbital period of less than 225 minutes is known as Simplified General Perturbations (SGP) model. For those with orbital periods greater than 225 minutes, which corresponds to an altitude of 5,877.5 km in case of circular orbit, Simplified Deep Space Perturbations (SDP) models apply. Of the five simplified perturbations models SGP, SGP4, SDP4, SGP8 and SDP8, SGP4 is the most commonly used. As there can be errors in SGP4 calculation model which can grow at the rate of 1-3 km per day, data is updated frequently by NASA and NORAD.

Of the eight Keplerian elements used in the calculations, atmospheric drag, often called as just drag, is optional. Other elements are Epoch, Orbital inclination, right ascension of ascending node (RAAN), argument of perigee, eccentricity, mean motion and mean anomaly. In the Keplerian model, satellites orbit in an ellipse of constant shape and orientation with Earth at one of its focus. Keplerian elements define the ellipse and orients it about the Earth and places the satellite on the ellipse at a particular time.

For most amateur satellite, weekly updates of TLE may be enough. An important exception is the International Space Station for two reasons. One because of its low orbit, aerodynamic drag is quite significant. Secondly, its orbit is periodically raised by onboard thruster. This is out of question for most of the tiny amateur radio CubeSats with no thrusters onboard! Still, TLEs are updated daily in the first hour of each UTC day as per AMSAT.

Epoch is a number representing the time at which the TLE was taken. In the current format of TLE, it comes after the International Designator of the satellite. Last two digits of the year is followed by day number and fractional portion of the day. Orbital inclination is the angle between the orbital plane and the equatorial plane of the Earth. Orbits with inclination near zero degrees are called equatorial orbits. LAPAN A2 (IO 86) has a near equatorial orbit. Orbits with inclination near 90 degrees are called polar orbits. Other elements are a bit too much for an average radio amateur like me to grasp, except the eccentricity of the orbit!