How precise are overpass times of sun-synchronous satellites?

Question

sun-synchronous satellites, as their name say, acquire scenes at the same solar time of the day when they pass over the same location. According to this site, sun-synchronicity is achieved by taking advantage of nodal regression and launching a satellite into an orbit where the nodal regression nearly exactly cancels out the daily change in the position of the sun over any point on earth, caused by the earth's orbit around the sun. This turns out to be, depending on the altitude of the satellite, about 95 to 100 degrees inclination.

The local time of descending node (or overpass time) is usually mentioned on the satellite descriptive documents. I would like to know how precise the solar time provided in those descriptive documents actually is and how to improve this precision based on potentially effecting parameters (altitude, latitude, longitude, day of year, age of the satellite). My understanding is that the main difference comes from local solar time vs mean solar time (see equation of time, up to 18 minutes), but I am seeking an order of magnitude of the other possible sources of dicrepancies between the announced overpass time and the actual local solar anywhere in the world.

I have several satellites in mind (the Sentinel's, MODIS, Landsat...), but I am particularly interested in PROBA-V. PROBA-V flies at an altitude of 820 km in a sun-synchronous orbit with a local overpass time at launch of 10:45 h. Because the satellite has no onboard propellant, the overpass times are expected to gradually differ from the at launch value. Examples of drift correction for satellites like Sentinel-2 are also welcome.

Answer 1

I am not a specialist of orbits, but I'll try to answer. Given a theoretical overpass time on a sun synchronous orbit, the exact one is not that easy to determine, as it depends on a lot of factors.

• the theoretical overpass time is valid at the equator, and for the local time under the satellite track when it crosses the equator (which is called the ascending node or descending node depending on the satellite which either acquire images ascending or descending)
• it takes something like 15 minutes to go from Brussels to the equator for instance
• it depends also from the viewing angle from the satellite. If the satellite looks to the west to see your interest point, its overpass is earlier, and if it looks to the east, the overpass time is later
• and the space agencies do not maintain the overpass time exactly, they allow some drift within a window. When the satellite crosses one side of the window, they do a manoeuvre to put it back on the other side and let it drift again

So the only way to predict accurately the overpass time is to use an orbit simulator, using as input the two line elements bulletins that are available at Norad for instance. https://celestrak.org/NORAD/elements/

Much simpler, but less accurate, if your satellite is on a phased orbit, with a repeat cycle of N days, you could also use the acquisition time of the previous acquisition, N days earlier. But I am not sure PROBA-V is on a phased orbit.

Answer 2

Based on the first answer and this post, I tried to put some numbers on the different parameters that affect the local solar time of overpass of a sun-synchronous satellite:

satellite drift

The sun-synchronous orbits need to be adjusted from time to time. For example, every two year in the case of MODIS. In the case of PROBA-V, the drift is not corrected. As can be seen in the PROBA-V product user manual v1.3, uncorrected drift result in a change of overpass time of approximately half an hour over 3.5 year. I guess that this drift is maintained within approximately 10 minutes when corrections are applied.

Mean solar time versus local solar time

Twelve noon local solar time (LST) is defined as when the sun is highest in the sky. Local time (LT) usually varies from LST because of the eccentricity of the Earth's orbit. The local solar time is in the range of +/- 15 minutes compared with the mean solar time. Illustration from Wikipedia below.

Angle of view

sun-synchronicity is achieved at nadir. Due to the large swath of PROBA-V, the observed location have a different local time. Here are some examples derived with the NOAA solar position calculator with the PROBA-V swath of ~2200 km. I (roughly) looked at the footprint at those latitudes.

Equator : +/- 20 minutes

65° North : +/- 40 minutes

With Sentinel-2 and its relatively smaller swath (290 km), the difference would range in +/- 4 minutes at the equator.