How accurately can solar panels be continuously oriented toward the sun on a typical satellite?

Question

I am interested in knowing how accurately a solar panel (or for that matter an RF antenna) can be physically oriented as a satellite in a geocentric orbit travels about the earth. Within 1 degree? Worse? Better? I am interested in both what is typical and what is "best in class".

I am sure that it would be a major challenge to set up a rotation of the satellite about its own center of mass that would perfectly offset the angular rotation about the earth, so there must be some kind of "fine tuning" done as it moves. I am also surprised that any motion of a gimbal does not cause the entire craft to rotate -- as it must by conservation of angular momentum ....

Please enlighten me!

Answer 1

Solar panels don't have to be aimed very accurately. Solar panel efficiency is going to be proportional to the cosine of the error angle, so even if you're 10 degrees off your panels are still 98.5% effective!

High-gain RF antennas do care about pointing accuracy; it's dependent on the wavelength and the size of the antenna -- the larger the dish the narrower the beam. Typically the effective beam width is on the order of a degree.

Here's an antenna pointer that's spec'd for less than a quarter of a degree. I believe the satellite it's mounted on would use reaction wheels or small attitude control thrusters to hold a stable platform while the antenna moved. You'd obviously need comparable accuracy in the control system in order to make use of accurate pointing hardware.

Answer 2

How accurately can solar panels be continuously oriented toward the sun on a typical satellite?

There is no such thing as a typical satellite orbiting the Earth. Satellites range in size from 10 cm cubesats (or even smaller!) to the 100 meter long International Space Station. The orbits range in altitude of a few hundreds of kilometers to beyond geostationary, with inclinations ranging from near 0° to over 90°. Some satellites point at the Sun, others point at space, but most point at the Earth.

One way to look at this question is in terms of the number of rotational degrees of freedom used to control the orientation of the solar arrays.

An Earth-orbiting satellite whose orientation with respect to the Earth is fully constrained needs to have solar arrays with two degrees of freedom to achieve 100% optimal power production. For example, the International Space Station has two alpha joints and eight beta joints that collectively work to keep the ISS close to optimal in terms of Sun pointing.

Those joints represent a good amount of weight and complexity, and also a number of single points of failure. Other satellites make do with less. At the opposite extreme, a number of satellites have fixed solar arrays. This is the only way to go with a satellite whose job is to observe the Sun. The pointing sensitivity of the sensors is much, much greater than the pointing sensitivity of the solar arrays. This is also how most cubesats (and surprisingly, some very expensive satellites) operate. There's a lot to be said for not having to rotate the solar panels. The drive mechanisms that do this are necessarily single points of failure. Getting rid of these mechanisms simplifies operations, simplifies design, and eliminates single points of failure. There's a lot to be said for simplicity, even if this means that the solar cells are almost never producing at 100% of optimal.

In between these two extremes are those satellites whose solar arrays have one rotational degree of freedom. If there is such a thing as a "typical" satellite, this is it. In some cases (e.g., sun-synchronous orbiters), that single degree of freedom is enough to provide optimal power. In others, the little bit of added power offered by a second degree of freedom is more than offset by added weight, added complexity, and reduced reliability. For example, most geostationary satellites have one degree of freedom solar arrays. (A few have fixed arrays or solar cells attached directly to the satellite.) Twice a year, the solar arrays of a geostationary satellite with one degree of rotational freedom will be misaligned from optimal by at least 23.5 degrees. However, the cosine of that angle is 0.917, which means the solar arrays will operate at more than 90% of optimal even under worst conditions. Getting that last 10% isn't worth it.

Answer 3

Three-axis controlled spacecraft can have very fine attitude control. Tens of arc seconds is well within state of the art for even low-cost microsatellites using technology no more advanced than reaction wheels and star trackers.

That level of precision is totally unnecessary to point solar panels, as the cosine losses at even up to 10° are essentially negligible. Even high gain antennas (e.g. 15 dB) have a 3 dB beam width on the order of 10–20 degrees.

An obvious application for high pointing accuracy is imaging. To make the math easy, at an altitude of 1000 km, 0.01 radians (0.57°) of pointing error results in 10 km of error. This could be a significant fraction of your sensor field of view for a payload like a high-resolution imaging telescope, and thus likely unacceptable.

Answer 4

Solar arrays ... as the other answers mentioned, its not worth improving over a few degrees. There is an exception of course, for particularly high power satellites (20kW) it might be worthwhile.

Antennas Its worth adding to the other answers that the idea of "RF autotrack" exists. This basically means the needs to be a feedback loop that adjusts the beam pointing according to how well centred the beam is. In principle there are lots of ways of creating this. Geostationary communications satellites with narrow spot beams have used an on board system, for example as mentionend in the link, that monitors a ground based beacon sited within the antenna pattern.