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This section presents optical satellites that are still operational and whose data may be used for DEM extraction and ground motion displacement measurement. Their spatial resolution is below 15 m. The list is not exhaustive. The key parameters of the missions are summarized in Tables 1.8 and 1.9, and the main characteristics of the instrument are listed in Tables 1.10 and 1.11. Some figures are different depending on the web sources and may differ from the real ones in some cases. Swath width and dynamic ranges are given for the highest resolution (mainly panchromatic bands).

During the last 15 years, the main drivers were increases in the spatial resolution and the revisit time. This is conducive to the conception of constellations of agile satellites (simpler, if possible, and smaller). It is possible to observe several evolution patterns for optical satellite missions:

 – agility is often the reference: it simplifies the instrument and reduces use of steering mirrors;

 – operators reduce the satellite altitude in order to find a compromise between smaller aperture and better spatial resolution. Examples: Cartosat-2C and Cartosat-3A;

 – a large aperture is chosen to optimize the resolution. Examples: WorldView-3 and Gaofen-8;

 – satellite constellations are increasingly proposed: this decreases the weight of the satellite, the payload and platform complexity and the satellite unitary cost, while increasing the number of satellites (constellation) and improving the revisit time. Constellations of high-resolution satellites are used to retrieve change detection information, geo-statistics, etc. Examples: SkySat, SuperView and RapidEye;

 – usually, a focal plane is composed of several time delay integration (TDI) image sensors, mounted in staggered configuration. CCD sensors are used where ultralow noise is preferred. However, CMOS detectors with low power, high frame rate and low cost are also used. The use of matrix detectors (CCD or CMOS) adds video mode functions into the satellites. Examples: Zhuhai and BlackSky constellations.

In the following, some missions with a specific interest for DEM computation or displacement measurement are presented:

 – Asnaro: The overall aim of this project is to develop a new generation of mini-satellite buses with high-performance characteristics. It is based on open-architecture techniques and manufacturing methodologies to reduce the cost and development period.

 – Carbonite-2: This satellite is a technology demonstration of low-cost video-from-orbit with commercial off-the-shelf (COTS) technologies. At only 100 kg, it delivers 1 m ground sample distance (GSD) images. The swath width is very limited.

Table 1.8. Mission characteristics and parameters (1/2)

Satellite	Nation	No. of satellites	Altitude (km)	Local time	Period of orbit (min)	Inclination (deg)	Weight (kg)
Alos	Japan	1	692	10:30	-	98.2	4,000
Alsat-1B	Algeria	1	700	10:30	98.5	98	103
Alsat-2A/2B	Algeria	2	637	10:30	-	-	130
Asnaro	Japan	1	504	11:00	-	97.42	495
BKA (Belka-2)	Belarus	1	505	-	-	97.49	473
BlackSky constellation	USA	20	500	10:30	-	53	56
Carbonite-2	UK	1	505	10:30	94.6	97.5	100
Cartosat-1 (IRS-P5)	India	1	618	10:15	-	97.87	680
Cartosat-2A/2B	India	2	635	9:32	75	90	690
Cartosat-2C–2F	India	4	505	9:30	94.72	97.4	727
Cartosat-3/3A/3B	India	1	505	9:30	-	97.5	1,625
CBERS-4/4A	China–Brazil	2	779	10:30	100.32	98.54	1,980
CESAT-1	Japan	1	500	-	-	97.45	50
Deimos-1	Spain	1	686	10:30	-	-	91
Deimos-2	Spain	1	600	10:30	-	97.9	310
Dove PlanetScope	USA	55	420	9:30–11:30	-	51.6	-
Dove PlanetScope 2 (PS2)	USA	100–150	475	9:30–11:30	-	98	-
DubaiSat-1	UAE	1	685	10:30	98.2	98.13	200
DubaiSat-2	UAE	1	600	10:30	-	97.1	300
DubaiSat-3 (KhalifaSat)	UAE	1	613	10:30	-	-	330
Eros B	Israel	1	520	14:00	94.79	97.45	350
Eros C	Israel	1	510	-	-	-	400
Gaofen-1	China	1	635	10:30	-	98.36	1,080
Gaofen-2	China	1	608	10:30	98.02	-	2,100
Gaofen-6	China	1	634	11:00	-	98.05	1,080
Gaofen-7	China	1	502	-	-	-	2,400
GeoEye-1	USA	1	770	10:30	98	-	1,955
Jilin-1/7/8	China	3	658	10:30	-	-	450
Kanopus-V-IK	Russia	1	510	10:30	-	-	473
Kanopus-V1–V6	Russia	5	510	10:30	-	-	450
KazeoSat 1	Kazakhstan	1	759	10:15	100	98.54	830
KazeoSat 2	Kazakhstan	1	630	10:30	-	-	185
Kompsat-2	South Korea	1	685	10:50	-	-	800
Kompsat-3	South Korea	1	685	10:50	-	98.14	980
Kompsat-3A	South Korea	1	528	10:50	-	97.5	1,100

Table 1.9. Mission characteristics and parameters (2/2)

Satellite	Nation	No. of satellites	Altitude (km)	Local time	Period of orbit (min)	Inclination (deg)	Weight (kg)
Landsat 8	USA	1	705	10:00	98.8	98.22	2,780
Nemo-HD	Slovenia–Canada	1	500	-	-	-	70
Pléiades-1A	France	1	695	10:30	-	-	980
Pléiades-1B	France	1	695	10:30	-	-	980
Prisma	Italy	1	614	10:30	99	98.19	830
RapideEye	Germany– USA	5	630	11:00	-	-	156
ResourceSat-2 Resurs-P	India	1	817	10:20	-	-	1,206
1, 2, 3/GEOTON Resurs-P	Russia	3	473	10:30	-	-	6,570
1, 2,	Russia	3	473	10:30	-	-	6,570
3/SHMSA-VR Sentinel-2-A/B	Europe (ESA)	2	786	10:30	-	98.5	1,200
SkySat-1	USA	1	450	13:00	-	-	110
SkySat-2-21	USA	6	450	10:30–13:00	-	-	110
SPOT-6	France	1	694	10:30	98.8	98.2	712
SPOT-7	France–Azerbaijan	1	694	10:30	98.8	98.2	712
SuperView-1–4	China	4	530	10:30	97	-	560
TeLEOS-1	Singapore	1	550	-	-	15	400
TH-1	China	3	500	13:30	-	97.35	1,000
TripleSat/DMC3/SSTL-S1-2-3	UK–China	3	651	10:30	97.7	-	350
TripleSat/DMC3/SSTL-S4	UK–China	1	636	10:30	97.7	-	458
UK-DMC2	UK	1	665	10:00	97.9	97.8	120
Vision-01	UK	1	583	10:30	96.2	97.5	-
VRSS-1	Venezuela	1	635	10:30	97.48	97.7	880
VRSS-2	Venezuela	1	645	10:30	97.6	98.05	1,000
WorldView-1	USA	1	496	13:30	94.6	97.5	2,500
WorldView-2	USA	1	770	10:30	100	98.58	2,800
WorldView-3	USA	1	617	10:30	97	98	2,800
WorldView-4	USA	1	617	10:30	97	97.97	2,485
Zhuhai-1 OHS-2A-2D/3A-3D	China	8	500	10:15	-	97.3	50
Zhuhai-1 OVS-1A/1B/2	China	3	500	10:15	-	97.3	50–90
Ziyuan-3/ZY-3 01-03	China	3	506	10:30	94.7	97.4	2,630

Table 1.10. Instrument characteristics (1/2)

Satellite	Year of launch	End of life	Highest spatial resolution (m)	Panchromatic spectral range (nm)	No. of XS bands	Swath width for highest resolution (km)	Dynamic range (bits)
Alos	2006	2011	2.5	520–770	4	70	-
Alsat-1B	2016	-	12	-	3	-	-
Alsat-2A/2B	2010, 2016	-	2.5	-	4	17.5	-
Asnaro	2014	-	0.5	-	-	10	12
BKA (Belka-2)	2012	-	2.1	-	4	23	-
BlackSky constellation 2020	2018 (2), 2019 (2), (4), 2021	-	0.8–1	-	-	-	-
Carbonite-2	2018	-	1	-	4	5(x5)	-
Cartosat-1 (IRS-P5)	2005	-	2.5	500–850	0	30	-
Cartosat-2A/2B	2008, 2010	-	1/0.8	500–850	0	9.8	-
Cartosat-2C–2F	2016–2018	-	0.65	500–850	4	10	-
Cartosat-3/3A/3B	2019	-	0.28	500–850	4	17	-
CBERS-4/4A	2014, 2019	-	5	510–730	16	60	8
CESAT-1	2017	-	1	-	-	6x4	-
Deimos-1	2009	-	22	-	3	600	-
Deimos-2	2014	-	0.75	450–900	4	12/24	10
Dove PlanetScope	2018	-	2.7–3.2	-	4	24x8	12
Dove PlanetScope 2 (PS2)	2020	-	3.7–4.9	-	4/8	24x16,32x19.6	12
DubaiSat-1	2009	2017	2.5	420–720	4	20	8
DubaiSat-2	2013	-	1	420–890	4	12/24	-
DubaiSat-3 (KhalifaSat)	2017	-	0.70	420–890	4	16/12	-
Eros B	2006	-	0.7	500–900	0	7	10
Eros C	2019	-	0.38	450–900	4	11.5	12
Gaofen-1	2013	-	2	450–900	8	70	-
Gaofen-2	2014	-	0.8	450–890	4	45	-
Gaofen-6	2018	-	2	450–900	-	90	-
Gaofen-7	2019	-	<1 m	-	-	-	-
GeoEye-1	2008	-	0.46	450–800	4	15.2	-
Jilin-1/7/8	2015	-	0.72–0.8	450–900	4	48	-
Kanopus-V-IK	2018	-	2.5	540-860	6	23	12
Kanopus-V1–V6	2012–2018	-	2.1	540–860	4	23	12
KazeoSat 1	2014	-	1	450–750	4	20	12
KazeoSat 2	2014	-	6.5	-	5	77	12
Kompsat-2	2006	-	1	500–900	4	15	10
Kompsat-3	2012	-	0.72	450–900	4	16	14
Kompsat-3A	2015	-	0.55	450–900	5	12	14

Table 1.11. Instrument characteristics (2/2)

Satellite	Year of launch	End of life	Highest spatial resolution (m)	Panchromatic spectral range (nm)	No. of XS bands	Swath width for highest resolution (km)	Dynamic range (bits)
Landsat 8	2013	-	15	500–680	10	185	12
Nemo-HD	2020	-	2.8	400–900	4	10	-
Pléiades-1A	2011	-	0.7/0.5	457–848	4	20	12
Pléiades-1B	2012	-	0.5	457–848	4	20	12
Prisma	2019	-	5	400–700	66 + 171	30	12
RapideEye	2008	2020	6.5/5	-	5	77	12
ResourceSat-2	2016	-	5.8	-	11	23/70	-
Resurs-P 1, 2, 3/GEOTON	2013, 2014 2016	-	1	580–800	5	38	-
Resurs-P 1, 2, 3/SHMSA-VR	2013, 2014 2016	-	12	430–700	6	97	-
Sentinel-2-A/B	2015, 2017	-	10	-	13	290	-
SkySat-1	2013	-	0.8	450–900	4	2/8	-
SkySat-2-21	2014	-	0.65–0.8	450–900	4	2/8	-
SPOT-6	2012	-	1.5	455–745	4	60	12
SPOT-7	2014	-	1.5	455–745	4	60	12
SuperView-1–4	2016, 2018	-	0.5	450–890	4	12	11
TeLEOS-1	2015	-	1	-	0	12	10
TH-1	2010, 2012, 2015	2018, 2020	2–5	510–690	4	60	-
TripleSat/DMC3/SSTL-S1-2-3	2015	-	1	450–650	3	23.8	10
TripleSat/DMC3/SSTL-S4	2018	-	1	450–650	3	23.8	10
UK-DMC2	2009	-	22	-	-	600	-
Vision-01	2018	-	0.9	450–650	4	20.8	10
VRSS-1	2012	-	2.5	450–900	4 + 4	57	10
VRSS-2	2017	-	1	-	4 + 5/6	-	-
WorldView-1	2007	-	0.46	448–895	0	17.6	11
WorldView-2	2009	-	0.46	464–801	4 + 4	16.4	11
WorldView-3	2014	-	0.31	450–800	8 + 8 + 12	13.1	11
WorldView-4 Zhuhai-1	2016	2019	0.31	450–800	4	13.1	11
Zhuhai-1 OHS-2A-2D/3A-3D	2018, 2019	-	10	-	-	150	-
Zhuhai-1 OVS-1A/1B/2	2017, 2018	-	0.9–2.0	-	-	22.5	-
Ziyuan-3/ZY-3 01-03	2012, 2016, 2020	-	2.5	500–800	4	51	10

 – Pléiades: Pléiades represents the new generation of French satellites with submetric resolution. The main characteristics of Pléiades are the high image quality, the high agility of the satellites, high image geolocation, the ability to provide images with a short delay (exemplified with the International Charter “Space and Major Disasters”) and the ability to take stereo or tri-stereo images on a single track.

 – RapidEye: The first constellation of Earth observation satellites, with five identical satellites launched simultaneously with five spectral bands and 5 m resolution. The aim of the constellation was to provide Earth observation imagery for decision-making and geospatial services. In April 2020, Planet Labs decided to end the operation of the RapidEye constellation.

 – Sentinel-2: An Earth observation mission from the Copernicus Program to systematically acquire imagery at a high resolution (10–60 m). It was developed and is being operated by the ESA. Its main characteristics are high swath (290 km), systematic acquisition of land surfaces and a large revisit (five days with two satellites) and a large number of multispectral bands with high SNR. However, the main breakthrough is the free and open data policy. After many years of acquisition of decametric images by several Earth observation satellites, Sentinel-2 has boosted the general use of satellite images at 10 m resolution. The easy access to satellite images opens the door to many new applications: agriculture (crop monitoring), land cover change, detailed vegetation monitoring, ice extent mapping, flood mapping, etc.

 – SPOT-6: SPOT-6 is the proposition by Airbus Defence and Space to ensure the continuity of SPOT-4 and SPOT-5 satellites. The swath is similar, with a resolution of 1.5 m. It is thus able to cover large areas for agriculture, deforestation and environmental monitoring, as well as for industries, coastal surveillance and defense.

 – WorldView-1: This satellite offers a high capacity and high resolution (0.46 m). It is based only on panchromatic acquisitions. WorldView-1 has an excellent geolocation capability and rapid targeting of in-track stereo images.

 – WorldView-2: This very-high-resolution satellite adds multispectral bands to the panchromatic one for enhanced spectral analysis. WorldView-2 has a higher agility and works at a higher altitude. Consequently, the revisit time of any place on Earth is reduced to 1.1 days. Despite its higher altitude, the spatial resolution remains the same as WorldView-1. WorldView-2 has the capacity for direct tasking: some customers can load imaging profiles directly to the satellite and it delivers the data directly to their own ground stations.

 – WorldView-3: While WorldView-3 is similar to WorldView-2 in terms of conception, several multispectral instruments are added: it contains eight multispectral bands (visible to near-infrared), eight short-wave infrared (SWIR) bands and 12 CAVIS bands (at 30 m resolution). It is the first super-spectral high-resolution commercial satellite.

 – WorldView-4: In 2019, WorldView-4 suffered a failure in one of its control moment gyroscopes. It is therefore considered no longer usable.

 – Dove PlanetScope: PlanetScope is one of the satellite constellations operated by the company Planet. It has almost daily coverage worldwide with more than a hundred nanosatellites. Each PlanetScope is a CubeSat 3U with dimensions 10 cm × 10 cm × 30 cm, producing images at 3.7 m resolution at nadir, with four spectral bands (red, green, blue and near-infrared).