# Do GPS satellites use the earth's magnetic field in any way?

There's a lot of speculation about how changes or flips in the Earth's magnetic field would affect us, our machines, and sea turtles.

I'm curious about the GPS satellites themselves. In one way they are basically an atomic clock and transmitter. But it's silly to break it down to that level. They aren't over 2000kg without significant brains.

That leads me to wonder what sensors modern GPS (block III) satellites use. Do they even have a magnetometer? Most cell phones do, so it seems reasonable they might. Do they have a star tracker? What is used for standard stationkeeping, and what else might they have in this vein?

I'm not talking about "second order effects", like "the atmosphere will be affected, so the signals might get attenuated", or changes in solar radiation, etc.

• Satellites in general might use Magnetorquers en.wikipedia.org/wiki/Magnetorquer , which can provide some attitude control and cheap momentum dumping, see smallsatops.com/index.php/2018/09/21/… However, according to the links, this works best on LEO so GPS might not have them.. (they are higher, in MEO). Wikipedia link for GPS says they have " an electromagnetic pulse (EMP) sensor (W-sensor), that form a major portion of the United States Nuclear Detonation Detection System..." – BlueCoder Mar 8 at 6:55
• These are just two ideas to check, a link to the specs of block III GPS would be better :) – BlueCoder Mar 8 at 6:55
• Many GPS enabled devices use magnetometers to determine the direction the user is facing, however this wouldn't cause problems for the satellites, only the end users. – Dragongeek Mar 8 at 12:27
• The "atomic clock and transmitter" part itself requires lots of brains in order to get the incredibly high precision and accuracy we're used to from GPS satellites - for instance, the timing required is so tight that the satellites have to compensate both for even the tiniest shifts in their orbits (caused, for instance, by the Earth's gravity being weaker in places like Canada and India) and for several different relativistic effects (from both special and general relativity) on the fly. – Sean May 10 at 3:43
• @BlueCoder: That would only be the case for U.S. GPS satellites, though, not for GPS satellites in general. Non-U.S. GPS satellites (GLONASS, Baidu, Galileo, etc.) wouldn't have those specific sensors, although they might have their own national equivalent. – Sean May 10 at 3:44

tl;d: At only a handful of milliGauss, the Earth's field out there is so weak that it probably wouldn't be very useful. It's doubtful that the critical GPS satellites would depend on it.

What's the Earth's field like out where the GPS satellites are?

First of all, where are they? The approximate period of a satellite with semimajor axis $$a$$ is given by (from here)

$$T = 2 \pi \sqrt{a^3 / GM_E},$$

where Earth's standard gravitational parameter is about 3.986E+14 m^3/s^2. Flip that around and you get

$$a = \left(T^2 \frac{GM}{4 \pi^2} \right)^{1/3}.$$

Put in a period of a half-sidereal day (about 43082 seconds) and you get a distance from the center of the Earth of about 26,560 kilometers give or take, or about 4.2 Earth radii.

A dipole field drops as $$1/r^3$$. For example

$$\mathbf{B} = B_0 \frac{3(\mathbf{\hat{p}} \cdot \mathbf{\hat{r}}) \mathbf{\hat{r}} - \mathbf{\hat{p}}}{r^3}$$

where $$\mathbf{\hat{p}}$$ is the dipole vector of the field and $$\mathbf{\hat{r}}$$ is the vector from the dipole to the field point. Here $$B_0$$ is about 3.12E-5 Tesla, or about 0.312 gauss.

If we ignore that Earth's field is tipped by about 11.5 degrees, and put in two points at one and 4.2 earth radii, we get about 0.31 gauss and 0.0042 gauss, which is only about 1.3% as strong as the field near the Earth's equator.

At the poles the field is double that, but the ratio is the same.

This is an incredibly weak field, there's not much you can do with a handful of milliGauss, and GPS is so critical that they'd never depend on something so weak.

That doesn't mean that they don't have some backup systems available in an emergency, they might exist. But I don't think they will depend on Earth's field for torque.

• thanks! It looks like the GPSII had some form of magnetorquer but I don't know if they actually used it. I didn't see anything in the modern (GPSIII) spec. I actually emailed a contact at Lockheed to see if they can point me at a decent spec for the A2100 bus and the GPS part itself. – tedder42 Mar 8 at 18:28
• @tedder42 that's really interesting; it will be great to hear more about it. If you have some helpful references I can ask a new question about it. – uhoh Mar 8 at 23:41