First, gravitational time dilation slows the on board tick rate deep in the gravity well. As you get out of the well your on board tick rate increases relative to the surface due to decreasing strength of the gravitational field.
Second, motional time dilation slows the on board tick rate down as your velocity increases. This effect always slows the on board tick rate down and can never increase it relative to the surface. However, the magnitude of the affect decreases. As you move to higher orbit your orbital velocity decreases and so does the motional time dilation.
There are a few useful reference points which help to think about tick rate on board various satellites relative to the tick rate on the surface.
1. On the hypothetical orbit at altitude zero gravitational time dilation relative to the surface has no effect and tick rate slows down solely due to motional time dilation. On board tick rate is lower than for a stationary observer on the surface.
2. On the geostationary orbit motional time dilation has no affect (since relative velocity is zero) and tick rate increases solely due to reduced gravitational time dilation (we're further out the gravity well). On board tick rate here is higher than for a stationary observer on the surface.
3. Somewhere between these two extremes is an orbit where the two effects balance out and the on board tick rate is the same as the tick rate on the surface. This occurs roughly one half of Earth's radius (~3186km) up above the surface.
ISS is merely ~423km above the surface and so it's the tick-rate-slowing motional time dilation that matters and their clocks run slower from our point of view.
GPS is over 20,000km above the surface and it's the tick-rate-increasing effect of the reduced gravitational time dilation that matters so onboard tick rate is higher from out point of view.
There are two effects here.
First, gravitational time dilation slows the on board tick rate deep in the gravity well. As you get out of the well your on board tick rate increases relative to the surface due to decreasing strength of the gravitational field.
Second, motional time dilation slows the on board tick rate down as your velocity increases. This effect always slows the on board tick rate down and can never increase it relative to the surface. However, the magnitude of the affect decreases. As you move to higher orbit your orbital velocity decreases and so does the motional time dilation.
There are a few useful reference points which help to think about tick rate on board various satellites relative to the tick rate on the surface.
1. On the hypothetical orbit at altitude zero gravitational time dilation relative to the surface has no effect and tick rate slows down solely due to motional time dilation. On board tick rate is lower than for a stationary observer on the surface.
2. On the geostationary orbit motional time dilation has no affect (since relative velocity is zero) and tick rate increases solely due to reduced gravitational time dilation (we're further out the gravity well). On board tick rate here is higher than for a stationary observer on the surface.
3. Somewhere between these two extremes is an orbit where the two effects balance out and the on board tick rate is the same as the tick rate on the surface. This occurs roughly one half of Earth's radius (~3186km) up above the surface.
ISS is merely ~423km above the surface and so it's the tick-rate-slowing motional time dilation that matters and their clocks run slower from our point of view.
GPS is over 20,000km above the surface and it's the tick-rate-increasing effect of the reduced gravitational time dilation that matters so onboard tick rate is higher from out point of view.