I know I read that somewhere, too.
But not understanding what the cause was, I did a bit of searching.
Consider this answer:
• Ellipticity of the Earth causes gravity wells.
Stable points are 75E and 105W.
The Himalayas and the Rockies.
Unstable points are 165E and 5W.
Marshalls and Portugal.
BUT, how fast it'll shoot through there, overshoot, and fall back, et al.... I have nothing to offer.
I'm not an orbital mechanic... but my Dad did play one on TV ...
I don't understand these points either, but I was really having a hard time believing that a sat can just stop at one of these points. If so, you'd think that a sat at one of those points wouldn't need much in the way of station keeping, and you'd think that there would be a lot of dead sats ending up at these points, and I don't really see any evidence of either. On the other hand, there DOES seem to be something special about these locations.
As an experiment, I used a well respected tracking program with current keps to predict a few satellites over a LONG period of time. I did this being aware that tracking programs like this are less and less accurate the further you get from the epoch time when the keps were produced, plus the fack that there are all sorts of forces on these satellites not considered by these tracking programs. However I think this experiment will give some illustration of just how these points might affect the orbits of sats.
Anyway, the sats I experimented with were G15, AMC-15 and several old dead sats. Basically, the reason for the choice was to look at what the program says will happen to G15, what the program says would happen to a sat at 105 if it were left to itself, and also I wanted to look at several very old sats that have been dead for years, looking to see what has happened to them, and what the programs predict will continue to happen.
First of all, with respect to G15, the program predicts that over a 10 year period (I know), that G15 will go through several cycles of drifting to about 75W then back to 133W, then back to 75.6W, etc, etc, etc, and a cycle takes about 2 and a half years. Notice that the center of this oscillation is (133-75.6)/2+75.6=104.3 , which is pretty close to 105. Ie it seems to be a pendulum type thing drifting back and forth with 104.3 at the center.
I was next curious what the program would say about AMC-15, which is already fairly stable at 105. The results for AMC15 show drifting from 104.6 to 105.1, ie VERY STABLE at that position. (Which I must admit surprised me very much.)
I then wanted to look at some very old sats that were initially located both near to 105 and far away from 105 when they were released, however I don't have any information about just where the various sats WERE when they were finally abandoned, so this part was mainily an exercise in looking at random sats, and seeing what their orbits look like now, years after they were released.
One sat I looked at was Westar-4, since it used to be located not too far from 105, at 99. Problem is, however, that I don't know if it was moved to some other location after leaving 99. But anyway, W4 is NOT oscillating around 105 or any other point. It is drifting endlessly, circling the earth about once every 6 months. THis suggests that depending on where the drift starts, and/or with how much initial drift, the sat may just continue to drift forever.
I also tried GSTAR-2, which used to be located right at 105, and it too, is continually circling the earth a couple times each year. W4 and R2 also have inclinations of 13 and 12 degrees too BTW.
I also tried Westar-5, which once was located at 122, and it too, is circling the earth.
I will keep trying to look at old sats, hoping to find one that has stabilized into an oscillating orbit around 105W or 75E, however I really get the impression that perhaps these gravity valleys are more theoretical in nature, and other forces, such as solar wind, effects due to lunar or solar gravity, effects due to increasing inclination and or changes in the diameter of the orbit are enough to overcome the tendency to stay at these theoretical stable points.
If anyone is aware of sats that were definately abandoned in the vicinity of 105 (perhaps one of the Anik sats), and/or were intentionally placed in an orbit intended to locate the sats in one of those stable points, I'd be interested in running the simulation on them.
Again, the tracking programs are NOT accurate over long periods of time due to forces they don't take into account, however the current keps ARE an indication of what the orbit looks like NOW.
Anyway, this is an interesting topic.
