This product survey provides main features of today's high-resolution (HR) imagery recorded by passive optical and active radar sensors, acquired from orbiting platforms and suited for mapping purposes. The data have been provided by manufacturers as response to our questionnaire submitted to all companies, who control orbiting optical and radar HR satellites and offer the resulting image products on the market place.

By Mathias Lemmens, Senior Editor, GIM International 

Since 21^st June 2010 TerraSAR-X is accompanied by a twin satellite called TanDEM-X enabling single pass interferometry. (DEM in TanDEM is written in capital to highlight that this synthetic aperture radar (SAR) configuration has been put in orbit foremost to generate homogeneous Digital Elevation Models (DEM) with an elevation accuracy of better than 2m covering the entire land surface of the globe.) The radar sensors have four imaging modes: (1) High Resolution SpotLight (HS) mode with Ground Sample Distance (GSD) up to 0.5m (at 300MHz) and possible polarizations: HH, VV, HH/VV; (2) SpotLight (SL) mode with GSD up to 0.75m (at 150MHz) and possible polarizations: HH, VV, HH/VV; (3) StripMap (SM) mode with GSD up to 1.25m (at 150MHz) and possible polarizations: HH, VV, HH/VV, HH/HV, VV/HH; and (4) ScanSAR (SC) mode with GSD up to 8.25m (at 150MHz) and  possible polarizations: HH, VV. Clients with a ground station upgrade for TerraSAR-X reception are offered direct access service.

NASA's Terra/ASTER carriers three sensors covering the range 0.52-11.65 µm of the spectrum: one sensor records three bands in the visible and near-infrared (VNIR) part of the spectrum, another sensor records six bands in the shortwave-infrared (SWIR) part and the third one five bands in the thermal infrared (TIR) part. The images are deliverable through: LP DAAC http://lpdaac.usgs.gov/ and by third party through: GDS http://www.gds.aster.ersdac.or.jp/. The across-track stereo-images are not available commercially.

The spectral bands recorded by RapidEye are: 0.44-0.51µm; 0.52-0.59µm; 0.63-0.69µm; 0.69-0.73µm and 0.76-0.88µm. GeoEye-1 records four spectral bands: blue (0.45-0.51µm); green (0.51-0.58µm); red (0.655-0.69µm) and near IR (0.78-0.92µm). Ikonos records four spectral bands too: blue (0.445-0.516µm); green (0.506-0.595µm); red (0.632-0.698µm); and near IR (0.757-0.853µm).  The off-nadir range of both is unlimited, they can image to limb of earth. In addition to mapping, typical applications of very high resolution satellite imagery, such as captured by GeoEye-1 and Ikonos, include: crisis response and emergency management; law enforcement, facility monitoring, and threat assessment; urban planning & management, exploration & resource development of energy.

The geometric quality of satellite imagery depends largely upon the use of additional information, in particular Ground Control Points (GCP) and DEMs, the latter resulting in ortho-rectified imagery. Most providers deliver products on several levels of accuracy: raw imagery, geo-referenced imagery (using GCP) and ortho-rectified imagery, using DEM. TerraSAR-X / TanDEM-X are provided as Geocoded Ellipsoid Corrected (GEC) imagery without use of GCPs and Enhanced Ellipsoid Corrected (EEC) using GCPs.  The relative accuracy of the latter is 20cm and better while the absolute accuracy is mainly DEM and incidence angle dependent. The accuracy of GeoEye-1 is 5m without using GCPs and 2m with GCPs; for Ikonos these figures read 8m and 2m, respectively, both for positioning in mono and stereo images. For EROS A images the accuracy is 5m for both GCP corrected and ortho-rectified images and for EROs B this is 1m. For RapidEye these numbers read 12m for both GCP corrected and ortho-rectified images. Ortho-rectified images of the ASTER sensors have an accuracy of 20m. Of course, the quality of used GCPs and DEM will affect the final quality of the imagery and derived products.

Remote sensing image processing software is usually sufficiently equipped to process optical and radar imagery. Also some GIS packages are able to process satellite imagery. For typical radar applications and analysis most remote sensing image processing software providers offer radar tools.

Detailed background information on optical and radar satellite imagery are treated in my feature article on HR imagery in GIM's August 2011 issue written in tandem with this product survey.