APT and LRPT Image Tansmissions

After writing about my adventures receiving images from weather satellites, I wanted to take the opportunity to expand on the two systems that make this possible: Automatic Picture Transmission (APT) and Low-rate Picture Transmission (LRPT). 

Image and video transmissions, like WiFi and cellular signals, are basically just RF (radio) signals similar to what passes through the air to your car radio or walkie talkie. The signal is modulated, oscillated within a metal antenna or electromagnetic fields and propagated in one or many directions via waves in various frequencies. APT and LRPT signals are no different. They send a standard radio signal in VHF (very high frequency) range, then -in layman's terms, "fine-tune" their signal waves by modulating them so different streams of data can be heard on one frequency and decoded by the listening receiver. 

The two types of satellites I listened to, The National Oceanic and Atmospheric Administration (NOAA) satellites from The United States and the Meteor M2-X satellites from The Russian Federation each use basic frequencies in the VHF range, the same as the radio stations broadcasting your local sports guys and "morning zoo" radio hosts. Then, they modulate their signals in different ways to send data, rather than voice and sound. 

NOAA Satellites (APT)

Automatic Picture Transmission is the older and simpler system, and as a novice, I found it easier to connect to NOAA's APT feeds for two reasons. The first is that APT transmission uses only two modulation channels, telemetry and synchronization data, making it easier to receive a steady synchronized feed that generates a recognizable image without pointing my antenna at the approximate point of the satellite in the sky at any given time. The second is that there are three operational NOAA satellites, so there are more opportunities to listen to their APT signals each day. 

When I connect a NOAA satellite's signal with my radio receiver and demodulation software, three things indicate I'm successfully downloading an image. The first is a ticking or beeping sound, which pulses twice-per-section. The second, is that the de-modulator visualization window goes from a cloud of random data points, which is just the back-groud radiation (or radio fuzz) that occurs when no signal is present, to a unified vertical line. The third is that with each half-second pulse a new line of my live image of earth appears.  

The raw image I am left with after a NOAA satellite makes it's pass has two side-by-side images. Typically a visual spectrum image (a normal photograph) and an infrared image, with boarders on each side that indicate one minute of transmission time. 

A raw image from NOAA 19 with both channels

Due to the limitations of APT, NOAA images are always taken in black-and-white or grayscale. Additional software is used to add color, when necessary, once the images is downloaded on earth. 

Image from NOAA 19 with color added

Meteor M2-X Satellites (LRPT)

Low-rate picture transmission uses a newer, faster and more advanced method for modulating transmissions in the 137 Mhz frequencies as APT. It can transmit on 3, rather than 2, channels simultaneously, allowing for more data to be gathered in one pass. For example, a visual image, an infrared image, and another data stream depending on what the satellite's camera or sensor can capture. It uses compressed data packets to transmit more data on all channels in the same time frame as the the older APT system resulting is more detailed high-resolution images.

 

 

Partial Image from Meteor M2-3

Because of the increased data capacity of the LRPT transmissions, downloading images from the Meteor satellites was a bit more difficult. When my antenna would briefly lose the signal sync due to it being a more-conventional style stationary radio antenna or because of other atmospheric factors, the loss in image quality was significant. When my antenna re-gained it's sync, it also took a long time to re-establish the data feed providing the pixels in each image. Additionally, there are only 2 operating Meteor M2-X satellites, M2-4 and M2-3. M2-2 was struck and disabled by a meteor years ago, and M2-3 suffered an antenna malfunction after launch resulting in weaker transmission.  

Still I was able to sync to the M2-3 satellite and download a partial image, which means I should be be able to better connect to both satellite systems with a better antenna specific to receiving satellite signals and/or finding an un-obscured area outside and physically aiming a standard antenna toward the satellites position in the sky throughout it's pass. Two more thought-provoking science experiments to add to my increasing portfolio of hobby stuff! Or as one of my friends says, more opportunities to "tinker".