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.
|fig 1. Sea level anomaly maps over the Mediterranean from 11 June 2003, made from Jason-1 + ERS-2 (top) and Jason-1+ERS-2+T/P+GFO (bottom). Merging the data from the four satellites shows eddies (circles) that are invisible, or barely visible with two satellites and much better resolved with four.
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 kilometres apart at the equator – wider 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 kilometres. Other combinations are possible, but 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, especially in the Mediterranean.
|fig 2. 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.
For hydrology applications, more satellites primarily mean that a larger number of lakes and rivers can be observed.
|fig 3. Number of lakes seen by ERS (or Envisat) (215 lakes of more than 100km2) and Topex/Poseidon (43 lakes)
Ice and land
– Report of the High-Resolution Ocean Topography Science Working Group Meeting, Dudley B. Chelton, Ed., March 2001.
– Ducet, N., P.-Y. Le Traon, and G. Reverdin, Global high resolution mapping of ocean circulation from Topex/Poseidon and ERS-1 and -2, J. Geophys. Res., 105 (C8), 19477-19498, 2000.
– Pascual, A., Y. Faugère, G. Larnicol, P-Y Le Traon, Improved description of the ocean mesoscale variability by combining four satellite altimeters. Geophys. Res. Lett., 33, L02611, doi:10.1029/2005GL024633, 2006.