Weather Satellites Round 2

Back in 2014 I described a weather satellite receiving station that I had built with one of the last remaining RIG RX2 receivers ever sold. They went on sale to clear out the inventory and so I bought the last 5. Most of them went to local schools. I had been a Remote Imaging Group member for quite some time ever since the launch of the RX2 and indeed created a few mods for it along the way. I've been chasing this particular demon since 1997

Back then things were pretty primitive. A wideband FM receiver tuned to 137.5MHzish fed audio into a computer. Some software then listened to that audio and decoded a fax. And that was it.

Modernizing an RX2 to Y2K technology levels

In the picture above we can see my attempt at updating the RX2 to Y2K level technology. I've created a breakout board (under the LCD) to make accessing the various signals coming from the multi-pin header on the top right of the green board easier. I've also added an ATMega328 (of Arduino fame) on its own board to drive the LCD. The original RX2 had a single 7 segment LED display to display its channel number. The "dot" on the display was used to indicate that the receiver was in scanning mode. I wrote a bit of code that looked at the various lines of the LED to see what channel number it was currently on and then transposed that into some LCD display text. It was a total hack but it worked very well! I also converted the RS232 control port to USB.  

Well that was then. Between then and now I've moved house a few times as well as stupidly lost my RX2. But that has given me the opportunity to upgrade my receiving station to more modern technology. Today one would use a USB based Software Defined Radio and a Raspberry Pi or similar computer. 

RTL-SDR USB Sticks

There are 2 of them screwed to the wall in the above picture. Simply feed them with the correct antenna, plug them into a computer (I'm using a Raspberry Pi 3), configure the software and then wait for the satellite to go by.

Speaking of antennas, here's a picture of a very simple to make linear polarized antenna that I made from a broken steel measuring tape. It's simply a few lengths of the tape cut to length (520mm/20.5") and then attached to some coax. I used my 3D printer to create a mounting point for all of the elements. The antenna behaves just like a turnstile type but with slightly less gain.

This antenna is called a V-Dipole. It is a basic dipole antenna but its elements are pushed forward so that it creates a 120 degree V shape. As with any dipole, one side is known as the "driven element" and connected to the inner of the coax while the other side is known as the "counterpoise" and is connected to the coax shield. By placing the driven element on the right hand side of the antenna we can create an antenna that is still linear in design but has some limited emulation of Right Hand Circular Polarization (RHCP). By placing the driven element on the left we can emulate a Left Hand Circular Polarized (LHCP) antenna. Ha! And you thought making antennas was hard!?

Upon closer inspection of the picture to the right it would appear that an antenna element is missing. Not so. The reflector elements below the main antenna must all be connected to the coax shield (there's a wire running up the back of the pole for this) and must entirely encircle the main antenna to create the illusion of a full circle. The reflector is not necessary but does improve the antenna's reception ability by as much as 3dB so its worth doing.

There's about 75 feet of RG8x coax connecting the antenna from the roof of my apartment building to the receiver on the wall in the garage. According to the many online coax loss calculators the coax presents an approximate 3dB loss in signal level. 3dB is roughly half of the signal lost to the coax and significantly diminishes an already weak signal (it has come all the way from outer space!). 

To counteract this loss I found a VHF preamp in my junk box. It is a very old kit from a long defunct but popular electronics kit supplier. This item was designed for use on the 145MHz amateur radio band but its input filter can be tuned over a range of about +/- 25MHz and so I "ghetto tuned" it to the 137MHz weather satellite band using my SDR software looking for a peak in the noise floor.

The picture on the right shows my SDR software tuned to the 137MHz weather satellite band. It's a little hard ot see but there are 3 blue stripes in the bottom right corner of the picture. The aim was to make this blue stripe (known as a "waterfall") brighter. The waterfall descends in real time down the screen. The lower part of is the before where the preamp's input filter was tuned to an unknown frequency. The middle part is where my tuning stuck was inserted into the adjustment coil on the filter. The top part is a little brighter than the "before" waterfall. You get the idea. Fiddling with the coil moves the preamp's reception window up or down in frequency. As we say in ham radio, "tune for maximum smoke". Peaking the filter on a signal is very difficult but increasing the noise floor is somewhat easy with the right tools.

Here's the finished article wrapped in marine heat shrink and installed into the coax just below the antenna on the roof. In theory the preamp delivers an added 20dB of gain and so should more than overcome any coax losses and maybe even add a little improvement to the desired signal.

All the hardware is now built so I guess its time to talk about software. I'm using a Raspberry Pi 3 that I also found my junk box.

Poking about on the Interwebs I came across a project called raspinoaa which is a fully self contained satellite receiver and decoder that uses all the hardware I had in the junk box. It also predicts when the weather satellites are coming and schedules their capture. Finally, it decodes the captured data and creates a web page to display them on. Below is a captured picture from NOAA 18.

I can also capture the Russian Meteor series of satellites These are digital satellites with much better picture quality. They use the LRPT system. However, my antenna is a little weak for this and so I only get partial amounts of data as seen below.

This project was built exclusively from my junk box. I did not buy any parts specifically for it. Obviously at some point in the past I had bought all the components. They were bought for other abandoned projects. The total cost of this build would be about $100. If you want to take a look at my most recent captures and see when the next passes are going over my apartment point your browser at http://wx.ni2o.ampr.org/wxsat  

RadioShack PRO-2055 scanner discriminator modification

I own one of these scanners. It is a bit long in the tooth now but back in its day (early 2000's) it was the top of the line scanner sold at the now mostly defunct RadioShack. So now that its well out of warranty and without much use for it I decided it was time to do the discriminator audio modification.

This modification will take the pre-filtered raw audio and present it via a socket on the back of the radio for later use by sound card or SDR software. Such uses could be for receiving Weather Satellite data around 137.5MHz. Both APT and LRPT are possible due to the bandwidth available. Some 50KHz of audio spectrum (+/- 25KHz) should be usable thereby overcoming issues of doppler shift. 

Its a simple modification requiring only 4 components; 6 inches of RG174 type coax, a 0.002uF (202) capacitor, a 10K resistor (Br/Bl/Or) and a 3.5mm (1/8") mono panel mounted headphone socket.

Remove the radio from its black enclosure sleeve. Remove the top and bottom covers and disconnect the speaker by pulling the connector from the board. On the component side of the board locate TP4. It is located next to a small 8pin surface mount IC. Follow the black coax in the below picture to locate TP4. Solder the inner of the coax to TP4 and the outer/braid to a suitable ground point. I scratched off some of the solder resist to reveal a large copper ground plane and put the braid there. Run the coax through the large speaker hole to the other side of the board.

Take a look on the back of the radio. You will see a few factory made holes including one marked for an M5 screw. As if by magic, the 3.5mm headphone socket will install just nicely into this hole. No drilling required.

With your socket mounted solder the 0.002uF (202) capacitor across the positive and ground pins. Solder one end of the 10K (Br/Bl/Or) resistor to the positive pin. Finally prepare the remaining end of the RG174 coax and connect the inner to the end of the resistor that is now sticking out from the socket. Connect the braid to the ground pin together with the other end of the capacitor.

Check your work for shorts and dry joints. Reassemble the radio making sure to re-attach the speaker connector. Power up the scanner and test that it still works as before.

It [still] Buggers Other Channels - FM Edition

I recently wrote an article about how IBOC (known by its commercial name of "HDRadio") causes interference to other broadcasters. That article was based on the AM/MW band but you'll be interested to hear that it is also the case on the FM band too.

I live in the greater Philadelphia area. A scan of my local FM band reveals some 23 radio stations broadcasting both an FM signal and an IBOC signal.

IBOC in "wrapper" mode

In the screenshot to the left we can see how the digital radio transmission is wrapped around the main analog (FM) carrier.  The IBOC signal (properly known as "In Band On Channel") is seen here in its "wrapper" mode. This is where the data carrier is split into 2 segments and sent out on different frequencies separated by the analog FM signal.

The IBOC system uses a COFDM type transmission method and so the data is in fact on lots of frequencies all at the same time. That there is a gap in the middle of it does not matter. The decoder simply skips over the gap looking for more data carriers. 

The maximum data throughput for this mode is 120kbps. If there were no analogue FM carrier in the middle and the gap was filled with more data the throughput increases dramatically to several Megabits. At that speed we are able to transfer TV images! This is only available on the FM band due the the wider bandwidth of the signals. The AM band is limited to a total of 20kbps.

Similar to our discussion in my previous article, here in the USA radio stations are given a fixed set of 100 KHz wide frequencies spaced out every 300KHz for every radio "market". This allows markets that butt up against each other to coexist interference free. Well, that's the theory at least.

Among my many (too many?) radio receivers is an RTL-SDR. This is a USB stick that is capable of receiving radio signals from about 30MHz up to 2GHz. By computer sampling a received radio signal and then pushing that sample through some software we are able to decode the information it carriers. This is known as Software Defined Radio. 

For this article I connected my RTL-SDR to one of my many (too many?) outdoor antennas and performed a scan of the FM radio band. As noted above I received 23 stations transmitting IBOC data. I used a combination of "GQRX" (Linux) and "RTL-SDR FM-Radio" (Android) as well as the RTL-SDR driver software for the USB device itself. These are all open source applications and are freely available. 

GQRX SDR software showing a few Megahertz of spectrum including some FM/IBOC stations

In the below picture we can see just how offensive IBOC is. We are looking at 3 FM/IBOC stations side by side. As you can see there is no room in between the stations. There should be a gap here. In this gap should be a stations from the adjacent radio markets of NYC, Wilmington, Scranton, and Lancaster.

IBOC shuts out its neighbours.

So, what is this data carrying anyway? Well, obviously its a digital version of the analogue output. But its so much more than that. Many broadcasters operate more than one media stream. Some of them relay their AM broadcasts, others a partner station from somewhere else on the band. Others yet carry special interest media stream such as Russian language or LGBTQ+ tailored music.

WIOQ 102.1 has 3 media streams

Here we see WIOQ on 102.1MHz. You can see that the stations transmit not only the audio stream but often pictures too. Pictures of the artists, album covers. Often head shots of the DJ also.

And that's not all. Radio stations are the traditional source of news, weather and travel information. Every 10 minutes during drive time you can hear them announce the traffic jams on the Schuylkill Expressway. But instead of just hearing about it how would you like to see a map?

Traffic map

Of course, I'm using an application on my computer to display all this information. But most people will be receiving this in their cars, So this map would pop up on the radio's display. Weather maps too. Talk about distracted driving!!

But has this method of media transfer been surpassed by your cellphone? I think so. It is not uncommon for cars today to have a significant display ability in the dashboard that is tied to your phone. For the most part when you you want traffic information you simply turn on your navigation app. You get up-to-the-second traffic and weather data displayed along side your navigation instructions. Has IBOC's time been cut short? Is it a modern dinosaur? Was it ever really useful in the first instance?