Most people are aware that every day weather satellites pass overhead to get a glimpse of the nation's weather patterns. Many people, especially those outside the ham radio community, are unaware that the signals these NOAA weather satellites transmit are readily accessible with a minimum amount of equipment. These satellites use a technology known as APT, or Automatic Picture Transmission. NOAA-19 is perhaps the easiest APT satellite to receive because it provides the best, strongest signal for visual satellite imagery. Because of this, we'll focus on NOAA-19 for this post.
All you really need to receive the satellite's signal is a radio receiver like an old police scanner (found at thrift stores) or a simple 2m ham radio handitalkie (like the Baofeng UV5R). An external antenna is usually better, but not a requirement for casual reception of the image. Other than the radio, the only other pieces are a computer with sound input and an audio cable (to get the audio out from the radio to the input on the computer).
If you would like to get the best images possible from every pass of the satellite, use an outdoor antenna connected to your radio. Discone antennas for scanners work well, as will any 2m amateur radio antenna. These antennas do suffer from "fades" where the gain of the antenna is weakest. To minimize these anomalies, eggbeater antennas or the very common quadrifilar helical antenna are used by serious hobbyists and weather professionals.
Once you have the required hardware, download the free WxToImg software which is available for Windows, Mac and Linux. There are other features and enhancements to the software if you upgrade, but it's still not a requirement.
Once the program installs, the first time you start the program you will be prompted to enter your Latitude and Longitude. This information is vital to synchronize the timing of the satellite pass. To locate your longitude and latitude, you can use a web tool like iTouchMap, Google Maps, a weather site or any other means. A help menu will appear showing the calibration steps which I'll cover later.
With the software installed, there are a few settings to change to prepare for our first APT satellite pass. Below is a listing of the various settings I've found that work best for the NOAA 19 satellite.
- Under Options check the following:
- Show all
- Crop telemetry
- Under contrast select: Linear (Constant)
- Under Illumination Compensation select: Full
- Under Gamma, select: 1.6
- Under Sharpen, select: 0.6
- Check Normal if you just want to capture a pass
- Check Mercator if you want to build a composite of several passes (paid version only)
- Active APT Satellites:
- uncheck box next to each satellite except NOAA 19.
- uncheck box Update this table when updating Keplers.
With these settings in place, you can now download the latest Keplerian Elements within WxToImg. The "Keps", as they're known, are mathematical values that are used in formulas to calculate the exact position of a satellite in relation to a given point on a map. Keps are sometimes referred to as TLEs, which is short for Two Line Elements, because of the two lines of values that represent a satellite's orbit. With this information, any satellite's relative position can be determined by simply providing the latitude, longitude, altitude of the observer and the keps for the satellite. You already gave the software your latitude and longitude, now provide the latest keps and the computer will have all the data it needs to predict the next pass.
To update the Keps for WxToImg, click on File then choose Update Keplers
Doing this action will download the latest elements from CelesTrak which hosts large lists of all the latest elements for many, many satellites as published daily by NORAD.
Now that the software is configured, there is really only one last piece of the puzzle and that involves setting the audio level. WxToImg requires the volume of the incoming signal to be within a particular range. Too low and the computer can't hear all the tones. Too loud and the computer can't discern between the various tones being sent by the satellite. Thankfully, to aid in this balancing act of getting the audio set "just right", WxToImg provide an audio level indicator in the bottom right corner of the screen. This feature is only active when the Record process is activated. Under File, click on Record, then at the bottom of the screen you will see "Manual Test". Click the Manual Test button to tell the software to begin listening. You can then adjust your microphone or line-in audio level. In the right corner you will see a yellow, green or red colored bar with the text vol: 50.2 or similar. Adjust your volume so the audio level color is green (this will be somewhere in the 45 - 80 range, give or take).
Once the audio level is set during the Manual Test, click on File > Stop. You may then clear the screen by clicking File > Clear. Your audio is now setup and ready to record the next pass.
To learn when the next pass of NOAA 19 will be for your location, click on File > Satellite Pass List. The date of the pass will be shown along with the number of passes per day (usually 2) for a given satellite. (Because we selected NOAA 19 as the only active satellite, our pass list will only show NOAA 19 passes.) Included in the pass list is the direction of the pass (N = northbound, S = southbound) along with the Maxium Elevation, shown as MEL. MEL will show how many degrees from the horizon, East or West, the pass will reach its maximum perceived altitude. The duration of the satellite's pass (anywhere from 11-12 minutes) and the time of the start of the pass (both UTC and local time). The table will also show you the operating frequency the radio should be tuned to. (In the case of NOAA 19, it shows 137.100 MHz.)
Of course, none of the process will work without a radio tuned to the satellite's downlink (transmitting) frequency. As noted previously, the NOAA 19 satellite transmits at 137.100 MHz, so it is vitally important that your radio be capable of tuning this frequency. It is also important that the radio selected for this project be capable of "Wide FM" and not the "Narrow FM" used for amateur and commercial radio services. APT Satellite signals are 34kHz wide, which is wider than the 6kHz and 15kHz of Narrow FM. Wide FM is most commonly used for FM Broadcast stations which are very wide at 230kHz. Ideally, the radio chosen will have the ability to filter somewhere in between these extremes - wider than Narrow but narrower than Wide - however, few radios offer adjustable filtering. Because of this, for practical purposes, overly wide is better than not wide enough. (FYI: receivers like the Hamtronics R139 offer 34kHz width IF specifically for receiving APT signals.)
Once an adequate radio is selected and tuned to the proper frequency, it is not necessary to set the squelch on the radio. Because the audio will be going directly from the radio's headphone port into the computer, leaving the squelch fully open is perfectly acceptable. The computer will not record the audio (thus ignoring all signals) until the satellite is within range. It's better to leave the squelch fully open to begin with so the radio receives even the faint signal of the satellite while it is near the horizon. Once the receiving technique has been mastered, it is possible to fine tune the radio's squelch but, again, this is not necessary as the computer will reject all unexpected radio signals until the satellite is within range.
With the radio tuned and ready to go, all that remains is to connect the audio output of the radio to the computer's input via an audio cable (typically a ⅛"-to-⅛" stereo cable). It may be necessary to further adjust the radio's and computer's audio levels during a live pass to ensure a good copy. (to get the volume level in the green) However, once the audio is set it should seldom if ever need further refinement.
The final piece of the puzzle is to set the recording options. Click File > Record to open the Recording menu. Check the box next to Create image(s) then click Auto Record. Doing so will send WxToImg into a "monitoring mode" where it will await the next satellite pass (displayed in the bottom left corner of the application window).
When the time arrives, it may be a good idea to physically monitor the screen as the image is downloaded to ensure a good copy. The big thing to check as the pass begins are the audio levels, especially on your first pass.
Do not expect to get a pristine image your first pass, this is merely a "tuning" pass to ensure everything is in working order.
After the pass completes you may need to adjust the Slant Correction or Move Image Overlay which are both available under the Image drop-down menu. You can later configure other features to automatically upload your images to an FTP server for archiving and alter the Image Settings from within the Recording menu.
Good luck with your recordings!
update: For much more in-depth information, check out NOAA's User Guide for Building Receive Stations or one of the countless other APT Satellite guides online.