Simon Collings - Polar Weather Satellites

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Polar Satellites

Sample composite NOAA satellite image recevied in the
UK on an AOR AR5000 using a crossed dipole and
WXSAT software running on a PC

The satellites

Quite a number of polar orbiting satellites are available. The National Atmospheric and Oceanographic Administration (NOAA) operate several popular satellites, but there are also several Russian satellites providing a good service. The transmission frequencies are: NOAA 137.500, 137.620; Meteor 137.850, 137.300; Resurs 137.850, 137.300; Sich 137.400; and Okean 137.400. All satellites use radio signals transmitted to ground using right hand circular polarisation, which means that a circularly polarised antenna (such as a crossed dipole) is needed on the ground to receive the signal properly.

Since the polar satellites are orbiting continuously from pole to pole, the Earth will slowly rotate underneath them. In this way over the course of many orbits the whole surface of the Earth can be covered. The NOAA satellites each have a 15-minute pass (as seen from the ground) and produces an image with a width of about 3000 km (see example above). The resolution is about 4 km. Passes made by NOAA different satellites overlap and you get four good passes per satellite per day, so about 12 images per day in total (NOAA 14 very early morning & afternoon and NOAAs 12 & 15 early morning & evening).

The modulation

The radio signal from both geosynchronous and polar orbiting satellites is transmitted using frequency modulation but the underlying signal is an amplitude modulated 2400 Hz tone. Polar satellites use automatic picture transmission (APT) whereas geosynchronous satellites use WEFAX. Both these methods are very similar and it is usually possible to demodulate and decode both schemes with the same equipment. The only significant difference is that with polar satellites, there is no start or end to the image as transmission is continuous (the start of each each line is designated with a brief tone burst).

NOAA satellites also offer a digital high resolution picture transmission (HRPT) service which transmits on 1707 MHz and requires a motorised 90cm dish to track the satellite as it passes overhead. Equipment for this is understandably expensive, but the results are apparently very good with a 1.1 km resolution in five spectral bands.

NOAA APT

APT format provides two channels of signal to be transmitted together. Each NOAA satellite actually has five advanced very high resolution radiometers channels, but only two are selected for transmission on APT (the full resolution data is only sent using the HRPT service). The five radiometer sensors operate as follows:

0.58 ... 0.68 µm visible light
0.725 ... 1.1 µm red end of visible light and near infra red
3.55 ... 3.93 µm mid infrared
10.3 ... 11.3 µm far infrared
11.5 ... 12.5 µm far infrared

Sensors 1 & 2 are sensitive to the heat profile of the SUN, so can pick up a reflection of the SUN in the ocean (see Mediterranian Sea to the South of France in example above). Sensors 4 & 5 are sensitive to the (black body) radiated heat from the Earth iteslf and are useful for observing cloud structures especially at night. Sensor 3 lies between these two ranges and can be used to detect ground heat sources such as large forest fires.

Here is a 5 second exerpt of a NOAA polar satellite signal received on an AOR AR5000 and crossed dipole antenna from my location in the West of England. The signal's characteristic clip-clop with accompanying ringing sound can clearly be heard.

noaa.au (28 kb NEXT/SUN audio .au file)

Geosynchronous satellite WEFAX

WEFAX uses a 3-second 300 Hz start tone and a 5-second phasing signal using white-to-black transitions (12.5 ms black/237.5 ms white) before sending the image iteslf. The end of the image is indicated with a 5-second stop tone at 450 Hz.

Weather Satellites | Polar Satellites | Geosynchronous Satellites