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 stick 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  

Satellite Exploits

 This is a reprint of a few pages from my old g7ltt.com website originally posted in 2002 ...

Firstly lets look at my station. I own the ubiquitous Yaesu FT-847 transceiver. This radio is very commonly found in the stations of many a satellite enthusiast. I have beam antennas for 2mtrs, 70cms and an MMDS down-converter with dish for 13cms (2.4GHz) all of these antennas are mounted on a cheap azimuth and elevation rotator arrangement by Satelectronics.

I have owned this radio for about a year with the intention of getting active on the satellite bands and was finally able to install my antenna system onto my new house in the early summer of 2002. All was not well. I bought one of the first Sat-El rotor systems that were made and it came with a few surprises. Firstly, the software was severely lacking. So much so that I took to developing my own program so that I could make my satellite prediction software control the antennas. QA was also a serious issue. One of the 2 rotators wouldn't work. After several trips to the roof and almost 6 weeks in waiting I finally got a replacement rotor. Having proved that the new one worked I took it apart to compare it with the broken one. The broken one was missing a very large capacitor! It never worked so how could it have passed the manufacturers QA process?

Aligning the antennas was another pain. The antennas must be accurately mounted onto the rotator so that when the rotators think they are looking at the Pole Star the antennas are too. In this way whenever the software controlling the rotators wants to look at a given point in the sky the antennas will correctly look there. This was not an easy task. It took me almost 6 months to complete this stage of the alignment. I couldn't seem to get the rotators to line up with anything.

A few nights ago I was watching "The Dish" on video. Its the story of the Parkes Radio Telescope in Australia. This was the dish that brought the TV pictures of Neil Armstrong's Apollo 11 moon walk back to the world. The story told in the movie is apparently true and it chronicles the goings on during the 2 weeks around the moon walk. Parkes was the only dish in that part of the world capable of receiving the pictures as the moon was only visible from Australia at the time. Well Parkes computer had a problem just a few hours before the big event and they lost contact with the space craft. After an hour of bluffing NASA about land line problems they finally found the space craft by randomly pointing the dish at the moon and waving it around a bit. This gave me an idea!!

So picture the scene. Its 10:30PM. Its dark. Its DAMN cold. The moon is out. Mark is on the roof trying to work out why his antennas are not pointing at the moon when he tells them too. I figured that I could align the antennas with the moon as my point of reference rather than the Pole Star. I can see the moon. I don't know what the Pole Star looks like. I start by guestimating the amount of error between my antennas and the moon's location. I guess 20 degrees. I go down to the basement where I control them from and fiddle with the settings. I go back up onto the roof. I reckon I made it worse. I go back down to the basement. And so on for about half an hour.

I'm getting cold and annoyed. It's getting late and the neighbors dog keeps barking at me up on the roof. I retire to the bedroom to spend some time with my wife before she forgets what I look like. On the way I fall over my laptop computer. A light goes on! I have a wireless network card in my laptop and some remote control software on my computer in the basement. Quick as a flash I'm back on the roof with my laptop. Now I can see what I'm doing!! Within about 5 minutes I was able to align the antennas so that they pointed at the moon. As the antennas are only about 6 feet above the roof of my house I was able to look from the back of them up to the moon and see that they were perfectly lined up. Back down the ladder and off to bed.

Sunday morning and I'm up early and down in the shack. I run up Nova (satellite tracking program) on my computer and find that I'm just in time for a PacSat pass. I instruct my antennas to point to it. A quick check of  Amsat's Operational Satellites page tells me what frequency I need to listen on. Sure enough, there's a whole bunch of data being transmitted. Result! My antennas are looking at a satellite. I don't have anything set up do receive the data so I just listen to it. Not quiet 10 minutes of data was heard here at KC2ENI QTH.

Next up was a crack at AO-40. This is what I started building my satellite system for. I have an MMDS down converter and a 3 foot BBQ grill dish (more about this here) which allows me to listen to the 2401MHz band. This is where all the activity is now as AO-40 has had some sort of unexplained accident which killed off all its other facilities. Sure enough when the satellite came around later that day I was hearing some hams sending each other SSTV pictures. I tried to decode the pictures but I wasn't able to get a good enough signal from the satellite. I was able to decode the telemetry beacon with some success as you can see from the screen capture above.

You can see I was only getting about 40% of the data packets from the telemetry beacon. I'm not sure quite why that is but I suspect that it has something to do with interference from all the 2.4GHz (2401MHz) gizmos we have around the house like our wireless network and some CCTV cameras.

On the next pass I'll turn all the 2.4GHz stuff off and see of it makes an improvement on the telemetry. If so it looks like I'll be spending a lot of time hard wiring the network devices to the main switch here in my shack.

Outernet Receiving System

Have you seen this? It's a free filecasting (one way file transmission) service that is delivered by satellite. "Satellite, you say? Sounds expensive." Not at all. And the skill level required to access this media source is also quite low.

So what's this then? It's an L-band patch antenna for an Outernet receiving system. It is currently sitting just outside of my living room window looking up at the Inmarsat satellite at 98 degrees west.  

As you can see from this picture, the Outernet receiving system is little more than a USB based software defined radio. In my case it's plugged into a C.H.I.P. computer (similar to a RPi) which then makes the received files available via it's WiFi interface.

I bought the development/DIY kit from Outernet for $79. Mine did not come with the USB battery as I already had one of those.

Aligning the system was trivial. One of the advantages of the L-band (1.5GHz) is that as long as you have a clear sky you can "see" the satellite. You do not need to be aligned with the satellite as you would with a Ku (TV) band satellite. That said, if you can align the antenna you will increase the reliability of the data downlink. As you can see from the picture above, I got quite a good downlink from my window.

So far all I've received is a few Wikipedia pages and some weather maps etc. I also received the latest AMSAT keplarian elements too.

The system runs at 2400bd and so is very slow. But considering that its free I can live with it. The ultimate goal of Outernet is to launch 3 Cubesats which will circle the earth and have a faster downlink.