T/G Layout 1

The Advanced Very High Resolution Radiometer (AVHRR) on U.S. polar-orbiting satel-
lites provides the primary imaging system for both High Resolution Picture Transmission
(HRPT) and Automatic Picture Transmission (APT) direct readout.

The AVHRR scans with a mirror rotating at 360 rpm. With each rotation, deep space,
Earth, and a part of the instrument housing are observed. The radiant energy collected
by the mirror is passed through five separate optical sub-assemblies to five spectral win-
dows (detectors). Each of the five detectors is sensitive to radiant energy within specific
spectral regions. All five channels and telemetry data are transmitted at high speed as
digital data for HRPT.

AVHRR data signals are combined, and pre-processed to achieve both bandwidth
reduction and geometric correction before being transmitted as APT.

The analog APT system was designed to produce real time imagery on low-cost
ground stations. The FM signal from the satellites contains a subcarrier, the video
image itself, as a 2400 Hz tone which is amplitude modulated (AM) to correspond to
the observed light and dark areas of an image.

The louder portion of the tone represents lighter portions of the image, low volumes
represent the darkest areas of the image, and intermediate volumes represent shades
of gray (middle tones).

figure 51.

DI R E C T RE A D O U T F R O M NOAA

PO L A R-O R B I T I N G SAT E L L I T E S

HRPT / APT*

wave

length application

1

2

3

4

5

visible cloud delineation, snow & ice monitoring, weather

near sea surface temperature, locate bodies of water, in

infrared combination with channel 1—vegetation assessment

thermal landmark extraction, forest fire monitoring, volcanic activity,

infrared sea surface temperature (nighttime)

thermal sea surface temperature, weather, soil, moisture,

infrared volcanic eruptions

thermal sea surface temperature, weather

infrared

* Any two AVHRR channels can be chosen by ground command for APT transmission.

** In micrometers (μm)

spectral

range**

0.58 to 0.68

0.725 to 1.0

3.55 to 3.93

10.3 to 11.3

11.4 to 12.4