There are four altimetry satellites currently in service:

• One satellite - Jason-1 - with a relatively short repeat cycle (10 days), able to observe the same spot on the ocean frequently but with relatively widely-spaced ground tracks (315 kilometres at the equator), in the same orbit as its predecessor, Topex/Poseidon (1992-2005).
• One satellite - Envisat - with a longer repeat cycle (35 days) but tighter ground track spacing (90 kilometres at the equator), and another one, ERS-2, on the same track with a small time-lag (but no longer any onboard recorder).
• An 'intermediate' satellite - GFO - with a repeat cycle of 17 days and 160 kilometres between ground tracks at the equator, in the same orbit as its predecessors Seasat (1978) and Geosat (1985-1990).