6.1. Overview of Satellite Systems

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Objectives

The objective of this unit is to introduce some of the principal spaceborne remote sensing systems and their key characteristics. This unit provides an overview of how we get data from satellites and the remaining learning objects in the unit describe important satellite systems which provide widely-used remotely sensed data.

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How do we get Satellite Data?

The operation of the satellite is managed by a control centre (1) which communicates with a signal receiver on the satellite (2). The scanner (3) captures remotely sensed data and this is transmitted to a ground receiving station (4). Data received is subject to pre-processing (5) and onward transmission (6) to a storage centre (7), from which it can be distributed to users (8).

Left: system for receiving data from satellite; Right: Madley Satellite Earth Station (Image source: Geograph.org.uk © Copyright Philip Halling and licensed for reuse under this Creative Commons Licence)

It is therefore apparent that obtaining satellite remotely sensed data requires a complex series of supporting technologies to be in place, including a spacecraft mission to launch a satellite platform carrying an appropriate sensor device into orbit. Many satellites do not carry remote sensing devices but are intended for entirely other purposes such as communications relay or Global Positioning System satellites intended to assist with Earth surface navigation. Remote sensing satellites have often been operated as part of long-running programs intended to ensure that new satellites are launched to replace earlier ones carrying similar sensors and thus to provide continuity of data streams for end users and their applications. There is of course a tension between providing continuity of sensors devices and thus data, and providing more advanced sensors which provide more detailed data which is not directly comparable to that which was previously available.

Data recorded by satellites may be (A) directly transmitted to Earth; (B) recorded on board the satellite or (C) relayed through the Tracking and Data Relay Satellite System (TDRSS). This is a NASA communication relay system which provides links between low earth orbiting satellites and the ground, increasing the proportion of their orbit over which which they can remain in data communication. TDRSS satellites remain in geostationary orbit and relay communications between remote sensing satellites and ground terminals.

(Image source: CCRS)

There are many satellites orbiting the Earth. The figure varies, but it is estimated that there could be over 6,000 satellites of various types orbiting the earth.

	On orbit	Decayed	Total
All	3,375	2,971	6,166
CIS	1,414	1,889	3,303
US	1,078	755	1,833
Japan	122	20	142
PRC	80	47	127
ITSO	64	1	65
GLOB	60	0	60
France	47	9	56
ESA	47	7	54
Germany	36	13	49
India	39	9	48
ORB	41	0	41
UK	27	9	36

(CIS: Commonwealth of Independent States – former USSR; ITSO: International Telecommunications Satellite Organization; GLOB: Globalstar; ORB: ORBCOMM)

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Activity