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In this section, we present some optical satellites that have taken images that are considered as references today. These satellites were launched between 1984 and 2004. Most of them are inactive today (but not all of them). Such satellites were heavy, and many of them had considerable longevity. At the beginning of the 2000s, new satellites arrived, such as Ikonos and Quickbird-2: these were agile satellites, with submetric resolution and increased dynamic range. For various satellites, several parameters, such as altitude, local time, weight, year of launch, spatial resolution, panchromatic spectral range and swath, are given in Tables 1.6 and 1.7. As we focus on ground motion displacement measurements and DEM extraction, the highest spatial resolution (panchromatic band in general), with its corresponding swath, is given.

Table 1.6. Past mission characteristics and parameters

Satellite	Nation	Altitude (km)	Local time	Period of orbit (min)	Inclination (deg)	Weight (kg)
Landsat 5	USA	705	9:45	99	98.2	2,200
SPOT-1	France	822	10:30	-	-	1,907
SPOT-2	France	822	10:30	-	-	1,907
SPOT-3	France	822	10:30	-	-	1,907
SPOT-4	France	832	10:30	-	-	2,755
Landsat 7	USA	705	10:00	99	98.2	1,973
Terra ASTER	USA–Japan	705	10:30	98.88	98.3	4,864
Ikonos	USA	681	10:30	-	-	817
Quickbird-2	USA	460	10:30	93.5	97.2	1,100
SPOT-5	France	822	10:30	101.4	98.7	3,030
CBERS-2/2B	China–Brazil	778	10:30	100.26	98.5	1,450
Formosat-2	Taiwan	888	9:30	-	99.14	764

Table 1.7. Past mission instrument characteristics

Satellite	Year of launch	End of life	Highest spatial resolution (m)	Panchromatic spectral range (nm)	Number of XS bands	Swath width for highest resolution (km)	Dynamic range (bits)
Landsat 5	1984	2013	30	-	11	185	-
SPOT-1	1986	2003	10	500–730	3	60	8
SPOT-2	1990	2009	10	500–730	3	60	8
SPOT-3	1993	1996	10	500–730	3	60	8
SPOT-4	1998	2013	10	510–730	4	60	8
Landsat 7	1999	-	15	520–900	7	185	8
Terra ASTER	1999	-	15	-	15	60	8
Ikonos	1999	2015	0.82	450–900	4	11.3	11
Quickbird-2	2001	2015	0.65	450–900	4	18	11
SPOT-5	2002	2015	2.5	480–710	4	60	8
CBERS-2/2B	2003,2007	2007,2010	20	510–730	4	113	-
Formosat-2	2004	2016	2	450–900	4	24	8

In the following, some optical satellites are presented to illustrate their interest for surface displacement measurement using archive satellite imagery:

 – Landsat 5: Landsat 5 delivered Earth imaging data for nearly 29 years and set the Guinness World Record for the “longest operating Earth observation satellite”. Its longevity provides a reference for time-series analysis of the Earth.

 – SPOT-1: A commercial high-resolution optical satellite initiated by CNES in the 1970s. Since 1986, the SPOT family has taken more than 10 million high-quality images, which constitute reference data.

 – SPOT-4: The second generation of the SPOT satellites. The panchromatic band is associated with four multispectral (XS) bands at 20 m resolution. The platform is three-axis stabilized, and the viewing direction is selected with a mirror.

 – Landsat 7: This satellite, launched in 1999, is considered a high-accuracy calibrated Earth-observing satellite. Its radiometric measurements are accurate compared to the same measures made on the ground. In October 2008, the USGS made all Landsat 7 data free to the public. It led to a 60-fold increase of data downloads. The satellite is still active in 2020.

 – ASTER: The ASTER instrument on the Terra satellite creates a detailed map of land surface, and scientists have made an elevation model with this data. The first global digital elevation model (GDEM) was released to the public in 2009. It covers the polar regions and complements NASA’s SRTM. The spatial resolution of pixels of 15 m is conducive to the creation of a 30 m resolution DEM. The last version of the ASTER DEM was released in 2019. Since 2016, all satellite images have been available at no charge for all users. The satellite was still active in 2020.

 – Ikonos: Ikonos was a breakthrough among the commercial satellites. The whole platform is used to point towards a geographical region. The whole satellite is agile and, consequently, the satellite weight is reduced. Commercial images with submetric resolution are available. The dynamic range of images is increased.

 – Quickbird-2: Quickbird-2 features enhanced commercial spatial resolution. It is the highest-resolution commercial satellite to date.

 – SPOT-5: Third-generation satellite of the SPOT family operated by Spot Image. It takes 2.5 m panchromatic images and 10 m multispectral ones. Pointing is done with steering mirrors. The HRS instrument is composed of two telescopes pointing around 20° forward and backward. The HRS takes native stereoscopic images with a resolution of 5 m (along track) × 10 m (across track).

 – Formosat-2: This satellite has a daily revisit for event/disaster monitoring, thanks to a body-pointing capability of ±45° in roll and pitch.