A Beginner’s Guide to APT Satellite Image Reception
The National Oceanic
Atmospheric Administration (NOAA) operates several polar orbiting, low earth
orbit weather satellites. The satellite data is used to create forecasts for the
public, television, radio, and weather advisory services. Satellite information
is also shared with various Federal agencies, such as the Departments of
Agriculture, Interior, Defense, and Transportation; with other countries, such
as
NOAA's operational
weather satellite system is composed of two types of satellites: geostationary
operational environmental satellites (GOES) for short-range warning and
"now-casting" and polar-orbiting satellites for longer-term
forecasting. Both types of satellite are necessary for providing a complete
global weather monitoring system.
Data from all the
satellite sensors is transmitted to the ground via a broadcast called the High
Resolution Picture Transmission (HRPT). A second data transmission consists of
only image data from two of the AVHRR channels, called Automatic Picture Transmission (APT). For users who want to
establish their own direct readout receiving station, low resolution imagery
data in the APT service can be received with inexpensive equipment, while the
highest resolution data transmitted in the HRPT service utilizes a more complex
receiver.
The polar orbiters are
able to monitor the entire Earth, tracking atmospheric variables and providing
atmospheric data and cloud images. They track weather conditions that eventually
affect the weather and climate of the
Currently, NOAA is
operating five polar orbiters. A new series of polar orbiters, with improved
sensors, began with the launch of NOAA-15 in May 1998 and NOAA-16 on September
21, 2000. The newest, NOAA-17, was launched June 24, 2002. NOAA-12,
NOAA-14 and NOAA-15 all continue transmitting data as stand-by satellites. NOAA-16
and NOAA-17 are classified as the "operational" satellites.
The table below shows the
frequency that the satellite transmits it’s APT on and
also the type of orbit.
|
Frequency MHz |
Orbit Type |
|
|
NOAA-16 |
137.620 FM |
LEO, Sun-Sync. |
|
NOAA-15 |
137.500 FM |
LEO, Sun-Sync. |
|
NOAA-12 |
137.500 FM |
LEO, Sun-Sync. |
|
NOAA-14 |
137.620 FM |
LEO, Sun-Sync |
|
METEOR 2-21 |
137.400 FM |
LEO, Circular |
|
METEOR 3-5 |
137.300 FM |
LEO, Circular |
|
OKEAN O |
137.400 FM |
LEO, Circular |
|
RESURS 01-N4 |
137.850 FM |
LEO, Circular |
|
SICH-1 |
137.400 FM |
LEO, Circular |
APT Transmission Format
The transmission
includes a series of synchronization pulses, minute markers, and telemetry
information.
The
synchronization information, transmitted at the start of each video channel,
allows the receiving software to align its sampling with the baud rate of the
signal, which can vary slightly over time. The minute markers are four lines of
alternating black then white lines which repeat every 60 seconds (120 lines).
The
telemetry section is composed of sixteen blocks, each 8 lines long, which are
used as reference values to decode the image channels. The first eight blocks,
called "wedges," begin at 1/8th max intensity and successively
increase by 1/8th to full intensity in the eighth wedge, with the ninth being
zero intensity. Blocks ten through fifteen each encode a calibration value for
the sensor. The sixteenth block identifies which sensor channel was used for
the preceding image channel by matching the intensity of one of the wedges one
through six. Video channel A typically matches either wedge two or three,
channel B matches wedge four.
The first
fourteen blocks should be identical for both channels. The sixteen telemetry
blocks repeat every 128 lines, and these 128 lines are referred to as a frame.
