NOAA Weather Satellites

This is a another side of radio/space that I really enjoy. I started decoding Noaa weather satellites images back in the 1980's and 90's but was put aside for a while and came back to it in June 2017.


More about NOAA


Polar orbiting weather satellites circle the Earth at a typical altitude of 850 km (530 miles) in a north to south (or vice versa) path, passing over the poles in their continuous flight. Polar satellites are in sun-synchronous orbits, which means they are able to observe any place on Earth and will view every location twice each day with the same general lighting conditions due to the near-constant local solar time. Polar orbiting weather satellites offer a much better resolution than their geostationary counterparts due their closeness to the Earth. The United States has the NOAA series of polar orbiting meteorological satellites, presently NOAA 19 and NOAA 18 are primary spacecraft, NOAA 15 as secondary spacecraft, NOAA 16 and NOAA 17 NOAA have been decommissioned.



Doppler Shift


We must also talk a little about doppler shift. The doppler shift effect is identical with the familiar change in pitch of a police car siren as it approaches then recedes from you - initially high pitched, becoming lower as the satellite firstly approaches the listener, flies overhead, then recedes again, this variation is added or subtracted to the signal thus widening its bandwidth.

The doppler shift effect is quite pronounced in Low Orbit satellites. It affects both up-link and down link signals. The magnitude of doppler frequency shift is proportional to the closeness of the earth station with the ground track of the satellite. Compensating doppler shift requires frequency tracking in narrow band receivers. So that means we really have to continually adjust the frequency. I personally have not done this and still get good results.


Getting Started


This is not going to be an in-depth tutorial, just a quick outline to give you the general idea. It's not that difficult at all.



The Antenna


The signals from the NOAA weather satellites are transmitted with right-hand circular polarisation. There are two common solutions to this problem, the crossed dipole (or turnstile) and tuned specifically for the 137 MHz band. The antenna should be positioned with a clear aspect to all horizons. If roof-mounted, make certain that no metal objects are likely to come between it and the satellites. Ideally, it should be the topmost structure on any antenna mast. If you cannot roof-mount your antenna, good results are still possible so long as there are no obstructions nearby. Good results have been reported with the antenna secured to a post just a metre above ground level. Buildings, trees and heavy foliage coming between the satellite and the antenna will all degrade the signal you receive.


Test Image



Now you've seen the antenna, don't let that put you off from your first attempts, I wasn't. All I use at the moment is a vertical dipole cut for the 137 Mhz band, but I will build a cross dipole at some point.



The Receiver



Each NOAA weather satellite transmits an Automatic Picture Transmission (APT) signal, which contains a live weather image of your area. To receive signals from the NOAA weather satellites you'll require a receiver capable of receiving the 137 Mhz band. The satellites transmit their datastream using frequency modulation (FM). Although you can listen to their 'tick-tock' signature using a fairly inexpensive scanner and whip antenna such a system will not enable the reception of good images. The reason for this apparent anomaly is due to the bandwidth of the signal. Most scanning receivers have available FM bandwidths of either 150kHz, for FM broadcasts, or about 5kHz for amateur or mobile service transmissions. Weather satellite signals require a bandwidth of 40kHz - 50kHz to accommodate the signal itself. There is a Weather Satellite receiver available here WeSaCom As for me, I'll stick to my Uniden Bearcat UBC-9000XLT for now.




My attempts at receiving images



As I said I use a Uniden Bearcat UBC-9000XLT scanner and I've also used my Alinco DR-150E 2M rig. Although the images obtained by this method aren't as good as those by using a dedicated satellite band receiver. The image on the left is not that bad, and is one of the many images I have received in June 2017. The images I had years ago were much better but long lost. You can see that this image has some noise on it ( the thin speckled lines ) that's partly due to the antenna, but it is better than nothing.





You'll need a satellite tracking program and the program to decode your APT images from the NOAA weather satellites. For tracking I use Orbitron you can download from here Orbitron - Satellite Tracking System When you've installed Orbitron you'll need WXtoImg get this from here WXtoImg weather satellite signal to image decoder So, it's that easy? Well yes and no, you'll have to play with some settings on the WXtoImg and setup your location etc in Orbitron, so a little reading will be required to get it all working. You can contact me for any help if you need it, but don't forget this is only a quick introduction.



They say a picture is worth a thousand words... so see the diagram on the left (don't feel insulted this is for beginners). You simply connect a 137-138MHz FM communications receiver, scanner, or weather satellite receiver to the soundcard of your computer, if you have a "line out" socket on the receiver use that. I'm just about to try SDR, I'll let you know how I get on.


The Noaa satellite frequencies you'll need are:


NOAA 19 – 137.1000 MHz.


NOAA 18 – 137.9125 MHz.


NOAA 15 – 137.6200 MHz.


I hope you have found this interesting and it will help you to get started. There is a section on the forum for NOAA discussions so please join in.