Satellite Sensors

Principles of Remote Sensing

Multi-spectral Radiometers (sensors)
Electromagnetic Radiation
Solar and Terrestrial Spectra

In our last class we discussed the difference between GOES and POES orbits and reviewed orbital characteristics. We have also viewed (and will continue to view) current weather imagery, which you should now realize comes from GOES 8 or 9, commonly referred to as GOES East and West. We also have combined images which composite GOES East and West, and finally, a composite image based on all the GOES sensors we reviewed is available as a global Mollweide projection. These images are available regularly on the UVA Weather page. The GOES satellites in their geosynchronous orbits 35,790km above the surface rotate with the same angular velocity as that of the earth and appear to remain stationary over one location providing us with a continuous view of the changing meteorological conditions.

How are these images made? What instruments does the GOES platform carry? The following information comes from the NOAASIS web site and provides, by example, an introduction to spectral radiometry.

Multi-spectral Radiometers

The main mission is carried out by the primary payload instruments, the Imager (more details about the Imager below) and the Sounder. The Imager is a multichannel instrument that senses radiant energy and reflected solar energy from the earth's surface and atmosphere. The Sounder provides data for vertical atmospheric temperature and moisture profiles, surface and cloud top temperature, and ozone distribution.

Other instruments on board the spacecraft are the search and rescue transponder, ground-based meteorological platform data collection and relay, and the space environment monitor. The latter consists of a magnetometer, an X-ray sensor, a high energy proton and alpha detector, and an energetic particles sensor, all used for in-situ surveying of the near-earth space environment.

Two advanced GOES satellites are in use: GOES-8, and -9 (Series I-M).

Main Characteristics of GOES I-M (GOES-8,-9...)
---------------------------------------------------------
 Dimensions      :  Main Body:  2.0m x 2.1m x 2.3m
			Deployed Length:  26.9m
 Weight	     :	2104.7 kg
 Attitude Control:  Three-axis body stabilized 
 Design life     :  5-year minimum
---------------------------------------------------------
 Launch Vehicle  : 	ATLAS I 	(GOES 8,9,K)
			ATLAS II	(GOES L, M)
 Launch Site     :	Cape Canaveral, FL
 Launch Date     : 	GOES-8:  April 13, 1994; GOES-9:  May 23, 1995
---------------------------------------------------------
 Orbit Type             : Geosynchronous
Approximate Parameters
       Altitude         : 35,788 km (19,324 nmi.)
       Longitude        : GOES-8: 75 deg W;  GOES-9:  135 deg W;
       Latitude         : GOES-8: +/- 0.5 deg; GOES-9: +/- 0.5 deg
---------------------------------------------------------
Power	|  Solar Array and Batteries (1050 Watts Solar Array 
					  Power Output at end of life)
---------------------------------------------------------
 Sensors : 	Imager
		Sounder
		Space environment Monitor System
          		Magnetometer
           		EPS and HEPAD
           		XRS
           	Search and Rescue (SAR) Transponder
           	Data Collection System (DCS)

For more information about the GOES satellite, see: GOES Project .

Below is a description of the first multi-spectral radiometer we will consider in the class, the 5-channel GOES imager, again, the source of this material is the NOAASIS site.

GOES Imager Instrument

The GOES I-M Imager is a five channel (one visible, four infrared) imaging radiometer designed to sense radiant and solar reflected energy from sampled areas of the earth. By means of a servo driven, two-axis gimbaled mirror scan system in conjunction with a Cassegrain telescope, the Imager's multispectral channels can simultaneously sweep an 8-kilometer (5 statute mile) north-to-south swath along an east-to-west/west-to-east path, at a rate of 20 degrees (optical) east-west per second.
: The Imager consists of electronics, power supply, and sensor modules. The sensor module containing the telescope, scan assembly, and detectors, is mounted on a baseplate external to the spacecraft, together with the shields and louvers for thermal control. The electronics module provides redundant circuitry and performs command, control, and signal processing functions; it also serves as a structure for mounting and interconnecting the electronic boards for proper heat dissipation. The power supply module contains the converters, fuses, and power control for interfacing with the spacecraft electrical power subsystem. The electronics and power supply modules are mounted on the spacecraft internal equipment panel.
Imager Instrument Spectral Bands (micrometers) Characteristics | 1 | 2 | 3 | 4 | 5 | -------------------------------------------------------------------- Wavelength (micrometers) | 0.55| 3.80| 6.50| 10.20 | 11.50 | | to | to | to | to | to | Product | 0.75| 4.00| 7.00| 11.20 | 12.50 | -------------------------------------------------------------------- Clouds | X | X | X | X | X | -------------------------------------------------------------------- Water Vapor | | | X | X | X | -------------------------------------------------------------------- Surface Temp | | O | | X | O | -------------------------------------------------------------------- Winds | X | | X | X | | -------------------------------------------------------------------- Albedo & IR Flux | X | | O | X | O | -------------------------------------------------------------------- Fires & Smoke | X | X | | O | O | -------------------------------------------------------------------- X: Primary Spectral Channel O: Secondary (supplementary) Spectral Channel -------------------------------------------------------------------- Field of View Defining Element: Detector -------------------------------------------------------------------- Optical Field of View : Square -------------------------------------------------------------------- 5-Channel Imaging : Simultaneously -------------------------------------------------------------------- Scan Capability : Full earth/Sector/Area -------------------------------------------------------------------- Channel/Detector Instantaneous Field of View (IFOV) Visible/Silicon : 1 km Shortwave/InSb : 4 km Moisture/HgCdTe : 8 km Longwave 1/HgCdTe : 4 km Longwave 2/HgCdTe : 4 km -------------------------------------------------------------------- Radiometric Calibration : Space and 290K IR internal blackbody -------------------------------------------------------------------- Signal Quantizing (NE'delta'T) : 10 bits all channels S/N : Minimum 3X better than spec -------------------------------------------------------------------- Frequency of Calibration Space : 2.2 sec for full disk; : 9.2 or 36.6 sec for sector/area Infrared : 30 minutes typical -------------------------------------------------------------------- System Absolute Accuracy : IR channel lte 1 K : Visible Channel 5% of : maximum scene irradiance -------------------------------------------------------------------- System Relative Accuracy : IR channel lte 0.1 K -------------------------------------------------------------------- Transmit Frequency : 1676.00 MHz --------------------------------------------------------------------: Additional informaton about the GOES satellites can be found at: GOES Project Office.


A better table of the GOES imager channels, with links to current images is provided here, echoed from the COMET Satellite Meteorology Class (which is linked from EVSC494 links page).
GOES Imager Channel Notation

NOTE: It is recommended that Imager channels
be referred to by wavelength to avoid confusion
with channel number or "aliases."

Central
Wavelength
(µm)* Channel No. Also Known
As . . . Half-Power
Points**
(µm)

0.65 1 VIS
visible
visual 0.55-0.75

3.9 2 IR2
shortwave
shortwave window 3.8-4.0

6.7 3 WV1WV.GIF
water vapor
WV 6.5-7.0

10.7 4 IR4
longwave
IR
window 10.2-11.2

12.0 5 IR5
split window
dirty window
IR12 11.5-12.5

*The symbol µm is an abbreviation for micrometer (one millionth of a meter). It replaces the older unit micron symbolized by µ.
**The wavelengths where the response of the instrument is one half of the peak response. The instrument "sees" most of the radiation between these limits, but only a little radiation outside of them.
Of primary interest to us in EVSC494, at the moment, is to consider how the multi-spectral imaging radiometer works. We will begin with a lecture on one of the basic principles of remote sensing- reviewing electromagnetic radiation and the solar and terrestrial spectra. As stated at the outset of this course, essentially all of the information received by satellites is in the form of electromagnetic radiation. Where does this radiation come from, and how does it get transmitted to the satellite platform? Why were these wavelengths chosen for remote sensing?
Back to syllabus

Central Wavelength (µm)*	Channel No.	Also Known As . . .	Half-Power Points** (µm)
0.65	1	VIS visible visual	0.55-0.75
3.9	2	IR2 shortwave shortwave window	3.8-4.0
6.7	3	WV1WV.GIF water vapor WV	6.5-7.0
10.7	4	IR4 longwave IR window	10.2-11.2
12.0	5	IR5 split window dirty window IR12	11.5-12.5

GOES Imager Channel Notation

NOTE: It is recommended that Imager channels be referred to by wavelength to avoid confusion with channel number or "aliases."

NOTE: It is recommended that Imager channels
be referred to by wavelength to avoid confusion
with channel number or "aliases."