signal,
explaining why a three foot dish is required for reliable reception.
HRPT is High Resolution Picture Transmission. These satellites give a
resolution of 1.1Km, while APT only gives a 4 Km resolution. All of the NOAA
Polar Orbiting Satellites and two Chinese Polar Orbiting Satellites contain a
sensor called the Advanced Very High Resolution Radiometer (AVHRR). October 13,
1978 saw the launch of TIROS-N and the first AVHRR. The AVHRR is completely
digital as is the 1700MHz transmitted signal,
explaining why a three foot dish is required for reliable reception.
In the case of the NOAA Satellites, five individual data channels are transmitted. Two are visible, and three are infrared. Each channel has a specific use, but can be combined with other channels to form stunning false color images. The Chinese satellites (CHRPT) have ten data channels and when combined produce amazing true color images. I'm not going to explain any more about CHRPT because the NOAA satellites are much more reliable.
As I mentioned the AVHRR outputs to five data channels, here is what each channel is used for. There are actually six channels, but only five are transmitted at any given time.
| Channel | Purpose |
| 1 | Visible - Gives good cloud detail |
| 2 | Visible - Gives good land detail |
| 3A* | Near Infrared - used for daytime cloud mapping and ice discrimination. Satellites prior to NOAA 15 suffer from striping on this channel when it was simply Channel 3. |
| 3B** | Infrared - Very accurate temperature readout. |
| 4 | Infrared - Day / Night cloud mapping |
| 5 | Infrared - Similar to channel four, but shows slightly better cloud detail. |
* Transmitted
during daylight hours on satellites after NOAA-14
** Transmitted during the night on satellites after NOAA-14
ve2dx December 24th 2003