NOAA APT Reception Using an Icom + Quadrifilar Helix Antenna Part III

This post is way overdue, but here it is anyway as reviously I have talked about NOAA APT reception here and here.This past June 5th was the Ann Arbor Mini-Maker Faire where this year we demo'd real time APT Reception throughout the day. I had brought my homemade quadrifilar antenna that I made a few years ago, along with my ICOM IC-R7000 receiver, Mini-Circuits ZFL-1000LN low noise preamplifier, computer running WXtoImg which is my favorite APT decoding software, plenty of LMR-400 low loss cable, and an Agilent E3611 power supply to power the preamp.

Setup was ideal with the antenna being mounted outside the building we were in with an almost completely unobstructed view of the sky. The pass list was nice with at least 8 good passes throughout the day. One addition to my setup was that the night before I threw together a serial to CIV Icom interface:

This really made all the difference to receiving APT satellites. With a general purpose receiver like the ICom (still way better than any general purpose scanner) it has less than ideal bandwidth compare to a dedicated APT receiver. With the CIV interface, WXtoImg can tune the Icom as necessary to compensate for doppler shift as the satellite passes by. It will also automatically tune to the correct NOAA APT frequency. Previously I would tune manually by watching signal strength and listening to the familiar tick-tock synchronization sounds for best quality. This course was still not perfect and led to noisy signals. With WXtoImg tuning my receiver for me, there were no issues. I still had some slight noise on the extreme ends of the signal, but it is expected when dealing with the narrower bandwidth. This resulted with near-perfect images from horizon to horizon.

I was able to get some excellent composites from the day:

Along with some good thermal water temperatures:

Here are a few raw images showing both channels (in this case visible and infrared):

These are about the best APT images you can get from a general purpose receiver, which I have been totally happy with. It would be nice to have perfectly pristine images which I have seen others make, you would just need a dedicated special purpose receiver with the necessary frequency bandwidth to receive them.

Quadrifilar Helix Antenna for NOAA APT Reception

Since most of my previous attempts of NOAA APT live weather imagery reception have resulted in rather poor quality images, I finally decided to give it a real try this summer. I started with purchasing a serious receiver, the icom IC-R7000. With continuous coverage of 25Mhz all the way to 2Ghz, it is probably one of the best scanner / receivers I have ever used. It has the Japan built quality of electronics that you just don't find anymore and the feel of a very solid piece of electronics. Along with this I purchased a registered copy of WXtoImg, which is definitely the best APT decoding software available. I have used several of the free APT decoding applications, and while they work... they just don't have the image clarity that WXtoImg offers. WxtoImg also can control tuning of my icom IC-R7000 via rs-232 which is very nice as well.

The final missing piece I need is a good antenna. Here is an example of an APT capture using a dipole antenna with ground plane tuned to 137 Mhz:


The image is excellent just above my location ( indicated by the yellow + sign ) but image quality decreases as noise increases quickly outside of my location. What is needed is an antenna with a much better gain from a high speed moving satellite that doesn't have signal fade caused by the orientation of the propagated wavefront. While many people have had good results using basic dipole whip antennas, discone antennas, turnstile antennas, and more exotic Lindenblads, nothing seems to perform as well as the Quadrifilar Helix Antenna (QHA) .

Looking at the QHA, it is initially somewhat hard to understand it's design. It is a pair of circularly polarized, half turn, half wavelength helical antennas designed for reception of low-earth polar orbiting satellites. Reading into the documentation for the actual NOAA polar orbiting satellites, they actually use this exact same antenna design for APT transmission on the satellites themselves. After some research, I came across an excellent calculator for designing the antenna. I recently built this antenna using measurements from the calculator and ended up with this:


It's not perfect, but it is designed to specifications. I used thick 4mm copper grounding wire instead of small copper pipe that other designers of these antennas use mostly for ease of assembly. Also in the picture is the RF choke balun, which converts the balanced signal from the antenna to an unbalanced coaxial cable. It is simply four turns of RG-8 around the antenna mast as close to the feed point as possible. For extra gain, I'm using a mini-circuits ZFL-1000LN low noise amplifier between the antenna and receiver. Overall antenna parts cost was about $30 and assembly time took about two hours. I plan on testing the antenna this weekend and I'm looking forward to some good satellite imagery.

Listening to satellites

Satellites are cool...

I have always wanted to pull real-time images off of weather satellites. I read articles of people doing this years ago, but never had the necessary equipment necessary to do it. Receiving these images is actually pretty easy... I had some horrible looking images within a few hours of setting up my system. So now that I have the necessary equipment and with it being February along with the fact that I'm getting really sick of winter, it's a good time to finally do this.


First received image: (very noisy since my antenna sucks.)



The United States currently has three transmitting operational polar orbiting weather satellites, NOAA-15, NOAA-17, and NOAA-18. These satellites travel in a sun-synchronous orbit around the earth at an altitude of about 850km. Because they are moving so fast making revolutions of the earth approximately every 90 minutes, each one passes over regions of the US multiple times each day.

Now what's interesting is these satellites are transmitting their imagery of the earth back to us on two separate frequencies. The first is a high resolution digital signal that transmits around 1.7Ghz, but the second is a lower resolution modulated signal that is around 137Mhz! This system is called APT (automatic picture transmission) and is not encrypted. The satellites are constantly transmitting this APT signal at about two lines of imaging per second and if you are able to receive this signal, you can view real-time weather satellite imaging.

Because of the speed of the satellites, you only have about a 12 minute window to capture the satellite imagery as it flies by overhead. This is plenty of time to get some good images.

The equipment:

The only equipment you need to receive these signals is a good scanner/receiver with wide bandwidth, a good antenna for 137Mhz, and a computer/software that can decode APT data.

Most consumer scanners / receivers on the market have two problems. First, they have the 137Mhz spectrum blocked so you can't receive it, and second they don't have a wide enough bandwidth at the receiving frequency to pull in all of the information. The receiver I'm using is a Kenwood RZ-1, which is sufficient for this task.

I have been using two pieces of software, Wxtrac and WxtoImg. Both work and are freeware. WxtoImg is definitely more powerful... but to unlock all of it's features you have to pay for licensing. JTrack is a powerful satellite tracking tool that is available to use free from NASA that I use to track passing satellites.


My results:

The above image is very low quality ( again from an inadequate antenna) but it does show the entire signal received. The left portion of the images in infrared data, while the right is visible data. The bending of the image is Doppler shift caused by the speed of the satellite moving across the sky. Once I receive a stronger signal, the software will compensate and correct this. The map of north america is applied by the software, it uses the known keplers of each satellite to keep track of the satellites exact location.

Here is another pass. I really need an antenna...



Improvements to make:

The antenna! I need a good antenna with a center frequency at 137.000Mhz. There are a lot of good designs out there, the quadrafilar looks to be the most popular, but also somewhat difficult to make. I will have to do more search into the best antenna...

The winter months are also the worst time to be receiving this satellite imagery. The low contrast from the clouds and snow covered ground both being a nice bright white color combined with the low amount of sunlight we receive this time of year makes decoding image data from signals that already have a very high signal to noise ratio extremely difficult.

I'm really hoping to get some good images soon.