This plugin can be used to demodulate APT (Automatic Picture Transmission) signals transmitted by NOAA POES weather satellites. These images are at a 4km/pixel resolution in either the visible, near-IR, mid-IR or thermal-IR bands.
The received images can be sent to the Map feature to be projected on to the 3D globe (this requires the Satellite Tracker feature to be running and start the decode):
The status of the NOAA POES satellites is available at: https://www.ospo.noaa.gov/Operations/POES/status.html The instrument used to generate the APT images is the AVHRR (Advanced Very High Resolution Radiometer).
<h3>1: Frequency shift from center frequency of reception</h3>
Use the wheels to adjust the frequency shift in Hz from the center frequency of reception. Left click on a digit sets the cursor position at this digit. Right click on a digit sets all digits on the right to zero. This effectively floors value at the digit position. Wheels are moved with the mousewheel while pointing at the wheel or by selecting the wheel with the left mouse click and using the keyboard arrows. Pressing shift simultaneously moves digit by 5 and pressing control moves it by 2.
<h3>2: Channel power</h3>
Average total power in dB relative to a +/- 1.0 amplitude signal received in the pass band.
<h3>3: Level meter in dB</h3>
- top bar (green): average value
- bottom bar (blue green): instantaneous peak value
- tip vertical bar (bright green): peak hold value
<h3>4: RF Bandwidth</h3>
This specifies the bandwidth of a LPF that is applied to the input signal to limit the RF bandwidth. APT signals are nominally 34kHz wide, however, this defaults to 40kHz to allow for some Doppler shift.
<h3>5: Frequency deviation</h3>
Adjusts the expected frequency deviation in 0.1 kHz steps from 10 to 25 kHz. The typical value for APT is 17 kHz.
For images displayed on the Map, these dials set the greyscale levels below which a pixel will be fully transparent and above which will be fully opaque. In between these values, pixels will be partially transparent. The opacity threshold should be greater or equal to the transparency threshold.
These settings can be used to remove land and sea from the APT image, which are typically at lower greyscale levels than cloud, allowing the 3D map surface to be visible, underneath the cloud.
Starts or stops decoding. A maximum of 3000 scanlines can be decoded, after which, the Reset Decoder (10) button needs to be pressed, to start a new image.
- Whether the APT demodulator can be controlled by the Satellite Tracker feature. When checked, the image decoder will be enabled and reset on AOS and the satellite pass direction will be used to control image rotation. The decoder will be stopped on LOS.
- Which satellites the APT demodulator will respond to AOS and LOS indications from the Satellite Tracker. This can be used to simulataneously decode images from multiple satellites, by having multiple instances of the APT Demodulator and setting a unique satellite name for each demodulator.
- After how many scanlines image processing is applied and updates sent to the map. Lower values require more CPU power.
- The number of pixels per degree longitude and latitude (i.e. resolution) for the equidistant cylindrical projected images.
- Satellite time and yaw offsets that can be used to more precisely align the image projected on the map.
- A list of colour palettes. Colour palettes are 256x256 image files that are used to enhance various features within an image. Some examples are here: https://github.com/Xerbo/aptdec/tree/master/palettes
Temperature maps require enough scanlines to have been received in order to determine calibration data. Until then, the image will appear blank.
The temperature range of the image is from -100C to 60C. Moving the cursor over image will display the temperature under the cursor at the bottom of the legend.
When checked, noise is cropped from the top and bottom of the image. This is noise that is typically the result of the satellite being at a low elevation.
