Double Cross Antenna for the reception of NOAA weather satellite signals

We see the weather images on various news channels. they show the cloud cover, contours etc. The main objective of our project is to obtain such weather images from satellites. Capturing signals from weather satellites doesn't require any patent rights. It is a free service and perhaps most of us dont know about it.

The flow chart of the entire process is as follows

• Satellite takes images of earth and modulates the image with a carrier
• Then it transmits the weather fax using an antenna (generally a Quadrifilar Helix Antenna)
• At the receiving end we need an antenna to capture the signal
• A receiver which demodulates, amplifies, removes noise etc
• A computer with an inbuilt sound card
• Weather fax to image converter software

The first step is to identify the current weather satellites in orbit and then choosing the ones which radiate the signals in your location. We preferred the NOAA (National Oceanic and Atmospheric Administration) satellites. These satellite belong to the United States of America. The main reason we chose them is because there are many satellites of NOAA, so even if you fail/miss to capture signal from one satellite that day, there is always another one the same day. NOAA satellites are low earth orbiting satellites i.e they orbit at a distance of 800 to 900 km from the earth. Also they are polar orbiting satellites i.e they orbit from north to south or vice versa. These satellites transmits signals with a power ranging from 5-10 Watts.

NOAA satellites work in the VHF region i.e the Very High Frequency region (30MHz to 300MHz). there are 4 NOAA satellites which pass over hyderabad. they are
         
                   Satellite                    Transmitting Frequency (carrier frequency)

                   NOAA 15                    137.6200MHz
                   NOAA 17                    137.5000MHz
                   NOAA 18                    137.9125MHz
                   NOAA 19                    137.1000MHz


NOAA APT (Automatic Picture Transmission) transmissions are analog in nature. The signals are frequency modulated (FM) with the carriers as mentioned above. The modulating signal is 2.4KHz and the frequency deviation is 17KHz. So the minimum bandwidth required to acquire a faithful image is 40KHz (according to Carsons Rule). the modulation index is approximately 7, so we are operating in wide-band FM.

SATELLITE TRACKING

For satellite tracking it is better to use a software rather than searching websites because many of the sites are not updated or providing incorrect information. There are many softwares which do this work and the one we preferred is WXtoImg. It is a free downloadable software. the software takes input of your location and gives us the satellite pass list over your location. However the primary use of this software is to process the weather fax signal and give the corresponding image.


 ANTENNA

We need an antenna to capture the circularly polarized waves. Currently we are working on the double cross antenna. After completing the work we will update this column.

RECEIVER

We need an FM receiver to demodulate and enhance the signal. As the bandwidth required is 40KHz we need a receiver whose bandwidth is atleast 40KHz. We can use a receiver with less bandwidth but the disadvantage is that you wont get a very good image. So we need a wide bandwidth receiver. We borrowed an FM receiver from NIAR (National Institute of Amateur Radio) Hyderabad. the receiver bandwidth was 12KHz. We dint get the image as expected but it was a good enough image for a beginner.

COMPUTER

All modern computers have a built in sound card. If your computer doesn't have one you can buy it and use it externally.



DESCRIPTION 

Satellite takes images of the earth and converts the image into a fax signal. It then modulates with its respective carrier siganl. transmission is done using a Quadrifilar Helix Antenna. At the receiving end we can use a Quadrifilar Helix Antenna, Double Cross Antenna, Yagi-Uda antenna, etc. We can use any antenna provided it is built and tuned to that particular frequency. The signal from the antenna is fed to the FM receiver using coaxial cables like RG-58, RG-174, RG-213. etc. Demodulation, amplification and noise reduction are done in this receiver. the output of the receiver is a sound signal which when heard sounds like the "tick-tock" of a clock. This signal is from the receiver is given to the microphone input plug of your computer by using an adapter. WXtoImg software must be installed on your computer. this software automatically records the "tick-tock" sound in .wav format and gives the picture instantly. This sotware also provides various image view types and many other image enhancement options.



 













                 

 THE A-TEAM

 

From Top to Bottom Pics: Abhinav, Kushal, Pradeep

We are students of ECE working on this project.