and a fixed position antenna will provide good results. The two most frequently recommended antennas are the crossed dipole and the quadrifilar helix antenna (QHA or QFH). Displaying the images Years ago, to receive APT images, a specialized decoder was required in addition to the receiver to display or print images, much like HF WEFAX (serving the maritime community). Often both receiver and decoder were combined into one unit. Nowadays, with the advent of personal computers, all that is required is dedicated software such as WXtoIMG (many of which offer "free" versions ) and a sound card. The sound card acquires and digitizes the slow scan video (in the audible range) coming from the speaker, phones, or line-out of the receiver, and then the software will process the various visible and infrared channels of the AVHRR sensor. Most software will automatically save every image and publish processed image onto the website of choice, putting up a new image on every pass of an APT satellite. Enhanced images Since each channel of the AVHRR sensor is sensitive to only one wavelength of light, each of the two images is luminance only, also known as grayscale. However, different materials tend to emit or reflect with a consistent relative intensity. This has enabled the development of software that can apply a color palette to the images which simulates visible light coloring. If the decoding software knows exactly where the satellite was, it can also overlay outlines and boundaries to help in utilizing the resulting images. History Developed by the National Earth Satellite Service Tested on TIROS-8, launched December 21, 1963 Nimbus 1, launched August 28, 1964, was the first application satellite First NOAA polar-orbiting vehicle to use it was TIROS-N, launched on October 13, 1978, and it has flown on all NOAA polar-orbiting vehicles since then. Also flown on the Soviet METEOR, Sich, Resurs and Okean weather satellites. Current status NOAA satellites transmitting APT NOAA-18 NOAA-15 NOAA-19 Soviet / Russian satellites transmitting APT None. Future With improvements in electronics, analog transmission systems have given way to digital transmissions systems. NOAA-19, called NOAA-N' prior to its launch on 6 February 2009, is the last satellite to carry an APT system. The MetOp program, a collaboration between NOAA and EUMETSAT, has switched to Low Rate Picture Transmission (LRPT) for its new polar-orbit satellites. See also EUMETSAT Radiofax Weather satellites References NOAA's KLM User's Guide: Section