KA7OEI - Weather Satellite Page

Weather Satellite Page

Here are the pictures as received from the weather satellites using the receiver described on this page.

A few important comments about the pictures:

Occasionally, the pictures will be out-of-date. Sometimes the computer running the WXtoIMG program (which is being used to decode and process these pictures, control the receiver, overlay maps, and post images to the web site) will have a problem that I might not notice for a day or two before fixing it.
These pictures are received from the 137 MHz APT transmissions of polar-orbiting weather satellites. At the time of writing this, the only operational satellites in this category are the NOAA POES series.
Note that the satellites transmit their strongest signals to the Earth directly below them. When they are nearer the horizon (from the perspective of the ground station) not only are they just farther away, but the signal being received on the ground is that radiated off to the side of the satellite's transmitting antenna and is further-weakened because of that. Because of this:

One will often see "noise bars" near the tops and bottoms of the pictures where the signal was weak because the satellite was at its most distant and at low angles.
Those satellite passes that are far to the east or west (as noted in the information near the top of each image) may also be low on the horizon, with the corresponding weak signals causing some "noise bars" to appear.
During low passes or when the satellite is nearer the horizon, local objects such as nearby trees and houses tend to block some of the signal, causing some "noise bars."

I have configured the program to "listen" for the satellite when it is very low on the horizon - which also means that it will start "listening" even when signals are may be weak. Normally, one would set the program to only listen when the satellite was more-nearly overhead and signals would be stronger and more noise-free. However, doing so would preclude the reception of pictures that were far to the north, south, east or west of the receive location.
Pictures from satellite passes that are very far to the east or west will often show only portions of Canada or Alaska, respectively - sometimes showing only small bits of land.

I have yet to "permanently" mount the receive antenna on my roof: When I do this, the signals should be better, with less noise nearer the horizons.
There may some "wavy lines" that appear in the picture. Because the antenna is fairly low, it is more-susceptible to interference from devices operating from the AC mains. This "buzzing" manifests itself as the "wavy lines" that one sees at times. When the antenna is (finally) placed on the roof, these should greatly diminish. (It seems as though there's something in the garage of the neighbor nearest to the antenna that really makes a racket near this frequency!)
The web page above says that an "APT-06" is used, but I simply built my receiver to use that receiver's codes for frequency control.

About the weather satellite receiver:

Previously, I'd been messing with the WXtoIMG program, variously using my FT-817 or a service monitor (a piece of test equipment with receive capability) to demodulate the weather satellite signals - but neither of these receivers were very satisfactory. These transmissions, from polar-orbiting satellites, include those on frequencies in the 137 MHz area. Using FM and having fairly strong signals, exotic equipment is not required to receive these satellite, but most standard receiving gear (scanners, amateur radio receivers, etc.) isn't particularly well-suited for their reception for a number of reasons:

The bandwidth of the FT-817 was too narrow for the weather satellite signals: Its receiver is only about 15 kHz wide, whereas the weather satellite signal's transmissions are about 30-35 kHz wide. This fact - plus the Doppler shift of the satellite changing the frequency received on the ground - causes a lot of noise and distortion of the signal, considerably degrading its quality!
The service monitor's receiver is about 100 kHz wide. While this was wide enough to accommodate bandwidth of the weather satellite's transmissions, it was really too wide in that not only did receive sensitivity suffer somewhat due to the extra bandwidth, but it was susceptible to interference from the Orbcomm satellite constellation that operates on nearby frequencies.

In November of 2008, I decided that I wanted to put together a 137 MHz APT weather satellite receiver and decided to build it from the ground up using components onhand. A few of the critical design parameters were:

Proper selectivity. For VHF APT weather satellites (those operating at about 137 MHz) the ideal bandwidth is in the 35-45 kHz range. This is large enough to accommodate the modulation on the satellite's carrier plus frequency variation due to Doppler shift as the satellite passes overhead.
"Adequate" sensitivity. The receiver itself was to be reasonably sensitive - in the area of 1 microvolt. This would allow the receiver to be used "barefoot" with an un-amplified antenna connected with a fairly short coaxial cable.
Computer controlled. The WXtoIMG program has provisions to control the receiver to which it is attached. Since there are several weather satellites aloft, each one on its own frequency, the program would steer the receiver to the frequency of the satellite that was to be received.

In reality, it wasn't the desire for a weather satellite receiver that was the main motive for its construction, but rather that I'd always wanted to build a receiver from scratch - plus, I wanted to try out a few circuit ideas, too.

The antenna:

For receiving polar-orbiting weather satellites, one must either have an antenna that tracks the satellite, or use an antenna that is sufficiently non-directional to allow it to receive signals that come from anywhere overhead where the satellite might be. Because the 137 MHz signals from these satellite are quite strong, one need not go through the trouble of assembling an automatically-tracking antenna system.

**Figure 1:**
**Top:** The "Tall and Narrow" Quadrifilar Helix Antenna with the mast-mounted preamplifier used for weather satellite reception.
**Bottom:** A look inside the enclosure containing the GaAsFET preamp (left) and bandapass filter.
**Click on either image for a larger version.**

While there are several types of antennas that may be used, I chose a rather odd-looking antenna, the "Qudrifilar Helix Antenna" (or "QHA".) This antenna is circularly-polarized to match the transmissions of the satellite's antenna: Whereas most antennas radiate their signals in a horizontal or vertical plane, the satellite uses an antenna that imparts a "spin" on the radio waves. This has a definite advantage in satellite work over a "linearly" polarized antenna (e.g. horizontal or vertical) in that as the satellite goes overhead, you would have to make sure that your horizontal or vertical antenna matched however that of the satellite might be oriented: If you are trying to receive a "horizontal" signal on a "vertical" antenna, most of the receive signals will be lost - much as what happens when you try to look at a wristwatch through polarized sunglasses and the dial face darkens! By using circular polarization, rotation of either the transmit or receive antenna becomes irrelevant.

One simple antenna that can be used for circular polarization is the "turnstile" antenna. While a good, simple antenna, it was not used here as it isn't quite as "sensitive" as some of the other options. For satellite reception, I chose to build a "Tall and Narrow" version of the Quadrifilar Helix. By making the antenna "tall and narrow" it exhibits more gain near the horizon (when the satellite is farthest away) than it does at high angles - such as when the satellite is overhead and is the closest with the strongest signals. By making this tradeoff, one can better-receive signals when the satellite is at a low angle and not only be able to receive the signal when the satellite is farther north and south, but also have a better chance of receiving signals when the satellite is farther to the east or west.

This antenna was built on a piece of plastic pipe and using thin, copper refrigeration tubing. While it looks fairly complicated, careful attention to the drawings and dimensions make it easier to duplicate with good results.

For details on construction of various types of weather satellite antennas, go to to the "Quadrifilar Helix Antennas" and another Quadrifilar Helix Antenna page. Follow the links on these pages for more info, and don't forget to try searching on your own, too!

For best results, the antenna should be mounted as high as possible to avoid blockage from ground-based objects such as trees and buildings, as well as to remove it as much as possible from nearby interference sources: A high, clear mounting location can considerably improve signals when the satellites are near the horizon.

The mast-mounted preamplifier and bandpass filter:

In the list of "design parameters" above, the second item - "adequate" sensitivity" - deserves a bit of explanation: For the best receive system sensitivity, it is best to put the preamplifier outside and at the receive antenna to avoid coax losses. Putting a low-noise preamplifier outside also allows one to use inexpensive, small-diameter coaxial cable to connect the receiver to the antenna. It is worth remember, then, that the more amplifier gain one puts in front of the receiver, the more susceptible it would be to overload from strong signals - such as those from passing aircraft, paging systems, or even nearby ham transmitters. By designing the receiver to have "adequate" gain, that meant that it could be used (with "good" but not stellar) performance without the preamp, but it wouldn't be as likely to be overloaded once the preamplifier was added. In addition to the preamplifier, the box mounted outside would also contain a bandpass filter that would reject those signals farther away from the desired 137 MHz weather satellite frequencies, further improving the receiver's overall performance.

Even though the signals from the weather satellites are quite strong when overhead, they can be quite weak when the satellites are near the horizon. This makes sense when you think about it:

When overhead, the satellite is as close as it is going to get, so the signals are stronger.
Most of the satellite's transmitted signal is beamed downwards so that those receiving stations directly below the satellite will get the strongest signal.
When farther away from the ground station below, not only is the satellite farther away, but that ground station is off to the "side" of the satellite's antenna, out of the strongest portion of the beam: These to factors can cause the signal to become quite weak!

Aside from amplifying faint signals, an amplifier also overcomes the losses in the cable that connects the antenna to the receiver that would further degrade an already-weak signal.

For the preamplifier, I constructed a simple GaAsFET amplifier. This amplifier has a fairly low noise figure (in the 0.5-0.9 dB area) and modest (about 18dB) gain - both being sufficient to dig a weak signal out of the noise and boost it enough to overcome moderate cable losses.

A schematic of this amplifier may be found here. Even though this preamplifier is intended for operation in the 2-meter amateur band (144-148 MHz) it is easily retuned to the 137 MHz region. Note that this schematic does not include the bandpass filter or the power-over-coax coupling.

Following the preamplifier is a 2-stage bandpass filter: This filter is placed after the amplifier to minimize losses that would increase the amplifier's noise figure and reduce its sensitivity. Because GaAsFET preamplifiers have quite good dynamic range, only very nearby transmitters (such as my own 2-meter transmission) are likely to overload it. The filters themselves knock out signals on other nearby frequencies (such as passing aircraft) that could overload other stages in the receiver as well as provide a higher amount of "image rejection" to prevent signals 21.4 MHz below that of the desired frequency (which are also in the aircraft band) from getting into the receiver.

To conduct signals from the preamp to the receiver - as well as to provide power for the preamplifier - standard RG-6 TV-type coaxial cable is used with F-type fittings. This cable is ubiquitous, fairly small in diameter, cheap, and exhibits fairly low-loss characteristics at this frequency so it was an obvious choice. Again, to minimize losses the preamplifier assembly is mounted at the antenna in a weatherproof enclosure: Doing so allows a relatively short run of coax from the antenna to the preamplifier to minimize signal degradation.

Receiver front end:

**Figure 2:**
On the workbench: In pieces, but working, the Weather Satellite receiver - before it was put into it's box, of course!
Left to right: Display, CPU board (the small one with the ribbon cable), the main RF/VCO/PLL board, and the IF/Demodulator board - before the audio amplifier was added.
**Click on the image for a larger version.**

The receiver front end is fairly simple - a two-stage bandpass filter with integral JFET preamplifier. For this, a grounded-gate amplifier, based on an MPF102, is used with a tuned circuit on the input and output. This amplifier offers modest performance, providing a gain of 10-12dB with a noise figure of 2-3dB - more than adequate to overcome the majority of the mixer noise. The filtering of this amplifier - while not particularly "tight" - is adequate for the purpose: Remember, there is better (lower-noise) amplification and filtering at the antenna!

The signal from the preamplifier goes to a diode-ring passive mixer - but between the input and ground is a simple, series L/C circuit tuned to the image frequency of the receiver (lower than the receive frequency by twice the IF frequency, or 21.4 MHz below) around 116 MHz. This provides at least 20dB of additional image rejection, providing about 55dB overall of image rejection to the receiver. Again, this figure is "adequate" but not great, remembering that there is more filtering at the antenna that improves the image rejection to well above 75dB.

Mixer:

A standard diode-ring mixer is used. This is a passive device, having excellent dynamic range and requiring only a local oscillator to convert signals. Because it has a fairly high insertion loss (around 7 dB) it is necessary to precede it with the preamplifier described above to achieve good system sensitivity.

Following the mixer is a very simple diplexer to assure that all signals being outputted by the mixer (particularly the "image" that is at about 260 MHz) is properly terminated: Failure to do so can result in undesired mixing products, reducing receiver performance considerably. This is followed by a post-mixer amplifier designed to terminate the mixer at around 50 ohms, which is then followed by a 10.7 MHz ceramic IF filter (the bandwidth being about 200 kHz) followed by another IF amplifier. The use of the ceramic filter at this point is to "pre-filter" the IF somewhat to remove strong, off-frequency signals before applying it to later amplifier stages.

IF Amplifier:

The amplified signal, now at the I.F., from the mixer is further amplified, applied to another wide-bandwidth ceramic filter (about 150 kHz), amplified and filtered (150 kHz) again, and amplified yet again before being passed through a narrow (40 kHz) wide ceramic filter: It is this last filter that sets the ultimate bandwidth of the receiver. While the 10.7 MHz, 40 kHz wide ceramic filter isn't particularly "sharp", it seems to be more than adequate in eliminating adjacent-channel signals - such as those from the Orbcomm satellite network.

Initially, I was worried that having fairly wide-bandwidth filtering in the early IF stages was going to make the receiver susceptible to interference from nearby Orbcomm satellite and Aeronautical traffic - either of which can be within a MHz or two - but "on-air" testing has not revealed any tendency for this to happen.

Demodulator:

The demodulator uses the venerable LM3189 chip. This is a fairly easy-to-use IC and although it is no longer manufactured, it is still readily available on the surplus market: I used it because I happen to have a bunch of them onhand. This chip contains a limiter and quadrature detector and provides both an "AFC" output (Automatic Frequency Control - for determining if the received signal is on-frequency) and an "RSSI" (Received Signal Strength Indicator) output: The former can be used to track Doppler shift, if desired and the latter can be used to provide a signal strength reading.

The demodulated "audio" output of this chip is buffered, filtered slightly, and made available to the computer doing the processing. Also included is an audio amplifier, based on the LM386, that is used to drive a speaker. While an "S-Meter" squelch (one that is based on signal strength rather than signal quality) could have been implemented with the LM3189, I have not done so yet - and since the computer doesn't really care if it "sees" noise all of the time that there's not satellite being received, there doesn't seem to be any real reason to do so. Since I only turn up the volume occasionally to see if things are working properly, the constant "hiss" doesn't bother me!

Local oscillator:

The local oscillator is a simple Colpitts circuit using an MPF102 JFET, operating 10.7 MHz below the receive frequency. The inductor uses some extremely small-diameter (about the size of #18 AWG wire) PTFE "hardline" coax forming a 1/4 wave line. While I could have simply used a standard inductor/capacitor arrangement, I was curious as to how well this 1/4 wave transmission line resonator would work. One advantage of this method is that it is less-sensitive to microphonics - that is, the tendency for mechanical vibration to modulate the oscillator, the result ending up back in the receiver audio. In the worst-case scenario, this can cause feedback with the loudspeaker.

A varactor diode is used to tune the oscillator and a tuning range of about 126.0 to 129.5 MHz is provided, covering all possible satellite frequencies in the 137 MHz range, plus allowing the receiver to tune to 140 MHz: This 140 MHz tuning capability is provided solely to allow the receiver to tune in a harmonic of a 20 MHz reference signal used in the receiver and allow a means of self-calibration in terms of the receiver tuning and the AFC.

Following the oscillator are two emitter-follower buffer amplifiers: One of these goes to yet another stage of amplification which boosts the local oscillator signal to about +7dBm to drive the diode-ring mixer. The other buffered output goes to the synthesizer section, as described below.

The power supply for the VCO and buffer comes from a 78L09 regulator. This device provides a clean, stable source of power for the VCO to prevent changes on the power supply from affecting the frequency.

LO Converter and Master frequency reference:

In order for a frequency synthesizer to work with good accuracy, at least one stable oscillator is needed. In this case, a 100 MHz VCXO (Voltage-Control Crystal Oscillator) is used: This device was rescued from a piece of scrapped equipment and has been found to be stable to better than +-1kHz over a fairly wide temperature range - adequate stability for our purposes. To drive the CPU, a sample of the 100 MHz output is divided by 5 using a 74F191 programmable divider, yielding a 20 MHz signal.

Another 100 MHz output goes to a single-transistor bipolar mixer where it is combined with a sample of the 126-130 MHz buffered output of the local oscillator. The 26-30 MHz output from this mixer is filtered and amplified and then inputted to a 74HC4040 binary counter, where it is divided by 4096 to produce another signal that is in the 6.3-7.3 kHz region.

Phase-locked Loop:

The Phase-Locked Loop (PLL) is used to control the frequency of the VCO. It does this by comparing the frequency of the down-converted LO (the 6.5 kHz-ish signal from the 74HC4040) with a locally-generated audio frequency - also in the 6.5kHz range: This latter frequency is chosen to be the same as that which would be produced by the VCO's down-conversion if it happened to be on the correct frequency. It so-happens that the receive frequency is related to that down-converted frequency in this way:

Down-converted (audio) frequency (in MHz) = ( (Receive Frequency - 10.7 MHz) - 100 MHz) / 4096

The PLL chip, a 4046, compares the phases of the down-converted signal with the locally-generated signal and if the VCO is too-high in frequency, it steers the VCO's frequency downwards: If it is too low, it steers it upwards. With the proper selection of time-constants, the VCO will immediately "snap" onto the desired frequency with good stability.

CPU:

The CPU has several jobs to do:

Display user data (such as frequency) on display.
Take user input - such as that from pushbuttons - to change frequencies, etc.
Digitize data such as signal strength so that it can be displayed
Take input from the computer for remote control
Generate a precise frequency for use by the PLL.

The CPU, a PIC16F88, does all of these functions using its onboard peripherals. For the audio frequency-generation, its onboard PWM hardware is used as a simple D/A converter and DDS (Direct Digital Synthesis) techniques are used to produce a sine wave. The DDS software was written to use a 32 bit accumulator which means that the frequency can be very-precisely generated in steps of 0.000005 Hz: Because the downconverted frequency is related to the local oscillator frequency by a division-by-4096, that means that the local oscillator frequency (at 126-130 MHz) can be controlled to within 0.02 Hz! While this amount of precision is meaningless, it was easy to accomplish in software and provides excellent frequency resolution. Because the CPU is clocked from the 20 MHz signal originally derived from the 100 MHz oscillator, it has a stable, accurate reference for the generation of the local oscillator frequency.

For those readers more familiar with the design of DDS and PLLs, eyebrows might be raised as to the use of such techniques with such a large divisor ratio! Once concern would be the inevitable spurious responses intrinsic to DDS synthesis causing undesired phase modulation of the 6.3-7.3kHz reference signal. With fairly aggressive loop filtering (e.g. fairly long time constants) and the careful selection of the DDS clock frequency, one can generate these audio frequencies with sufficient cleanliness that undesired "reference sidebands" are attenuated adequately. Additionally, relatively simple techniques of using a "dual time constant" in the PLL's loop filter can allow the receiver's local oscillator to be on-frequency in well under a second, even with such aggressive loop filtering.

In addition to generating the reference frequency for the local oscillator, the CPU also drives a 2-line by 16 character LCD as well as reading inputs from pushbuttons (to change frequency) and to digitize some analog input voltages from the AFC circuit (to determine offsets from Doppler shift) as well as the RSSI output (to determine signal strength.) The LCD displays not only the frequency being received, but also signal strength and frequency-offset readings. Finally, the CPU also contains a UART that can receive serial commands from the computer running the weather satellite image decoding program that not only tells the receiver what frequency to select, but also when the computer is processing a signal - a case in which the CPU puts an "RX" on the display to let one know that a satellite pass is in progress!

Note: Additional pictures, block diagrams and, possibly, schematics, will be added soon.

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