High-precision altimetry with satellites working together

In many ways, the orbit of an altimetry satellite is a compromise. But one point that deserves special attention is getting the right balance between spatial and temporal resolution: a satellite that revisits the same spot frequently covers fewer points than a satellite with a longer orbital cycle. One solution is to operate several satellites together.

Topex/Poseidon-ERS and Jason-Envisat are fine examples of how altimetry satellites can operate together. Topex/Poseidon and Jason-1 follow a repeat cycle of ten days designed to monitor ocean variations, so they pass over the same points fairly frequently but their ground tracks are some 315 kilometers apart at the equator-more than the average span of an ocean eddy. On the other hand, ERS-2 and Envisat only revisit the same point on the globe every 35 days but the maximum distance between two tracks at the equator is just 80 kilometers.

Sea level anomaly maps over the Mediterranean on June 11, 2003, made from Jason-1 + ERS-2 (top) and Jason-1+ERS-2+T/P+GFO (bottom). Merging of the four satellite data shows eddies (circles) that are invisible, or just barely visible with two satellites and much better sorted out with four of them. (Credits MFS/CLS).

Buoy trajectory (white line, from 14 to 28 May 2003, from A to B) and merged absolute dynamic topography in the Gulf Stream on 21 May 2003. Left with 'only' two satellites, right with four satellites. The left-hand map corresponds more closely to the eddy revealed by the buoy's path. (Credits CLS).

At least two altimetry satellites are required to map the ocean and monitor its movements precisely, particularly at scales of 100 to 300 kilometres (mesoscale). With four altimetry satellites available (Jason-1, Envisat or ERS-2, Topex/Poseidon and GFO), the resolution of sea surface height measurements is greatly enhanced. At least three satellites are needed to observe eddies and mesoscale phenomenon.

: Altimeter sampling related to applications. Credits CLS.

Further information :

Ssalto/Duacs multimission altimetry products
Image of the month
- September 2006 : Merging of altimetry data: proofs from other measurements.
- September 2004 : Four satellites to watch over the Mediterranean.
Newsletter #9, 2003 : Two altimetric satellites minimum are needed for ocean observation and forecasting, V. Rosmorduc et al.