NOAA Weather Project Overview Receiving Satellite Imaging ......NOAA Weather Satellite Imaging...
Transcript of NOAA Weather Project Overview Receiving Satellite Imaging ......NOAA Weather Satellite Imaging...
NOAA Weather Satellite Imaging
Jayhawk Engineering Design Laboratory
Project OverviewThe goal of this project was to build a weatherstation capable of receiving and processingweather imaging data from the NationalOceanic and Atmospheric Administration’sPolar Operational Environmental Satellites.This project is a unique development in theweather satellite imaging field because thesignal processing was accomplished by writingopen source code that will allow future weatherhobbyists to better understand the translationof audio to image.
Weather Satellite Imaging ResourcesGEOS POES Program (http://goespoes.gsfc.nasa.gov/poes/index.html)NOAA KLM User’s Guide (http://www2.ncdc.noaa.gov/docs/klm/html/c4/sec4-2.ht)Nova Satellite Tracking Software (http://www.nlsa.com)WXtoImg Software (http://www.wxtoimg.com)WXSat Software (http://www.hffax.de)QFH Antenna Design (http://perso.wanadoo.es/dimoni/ant_qha.htm)Entire Project Findings at http://www.markroland.com/APT
EECS 502 Senior Design Team, 2006
Bill DowerEric MbakaMark RolandVijo Varghese
ReceivingIn order to receive the signals sent from thesatellites an antenna with a resonant frequencyof 137 MHz must be attached to the receiver.One antenna design that is particularly suitedto this application because it can receive signalsfrom any position in the sky without a trackingmechanism is the quadrifilar helix antenna.This antenna is composed of two loops inquadrature phase and can be constructed usingcommon components from a hardware storelike copper tubing, PVC pipe, and coaxialcable. A preamp was placed at the base of theantenna in order to boost the signal strengthbefore being sent through a long length ofcoaxial cable to the control station.The control station was
located indoors beneaththe antenna. At thecontrol station, thecoaxial line coming fromt h e a n t e n n a w a sconnected to a receiverdesigned especially forthe NOAA weathersatellite bands. The receiver used here was the Hamtronics R139Weather Fax Receiver. This receiver has 5 different frequenciesthat are tuned to receive signals from NOAA 15, 17, and 18.The receiver performs FM demodulation and provides an outputto a speaker and a mono input jack that may be plugged into asound card. Free software called “Nova” was used to track thesatelllites so that it would be known when a satellite was overheadand recordings could be made.
Signal ProcessingThe audio that comes out of the receiver is in the AutomaticPicture Transmission (APT) format. This signal is constructedas the satellite flies overhead and scans the strip of Earth directlybelow. The satellite then turns what it sees into an amplitudemodulated signal. It takes the satellite half of a second to scanone strip, and the satellite transmits these strips continuously.Each strip contains two images; one image is composed fromthe visible spectrum and the other is composed from the infraredspectrum. In addition to the picture, each frame contains
synchronization pulses, telemetry data and minute markers. Theseextra data markers can be used for map overlays and thermalimaging. Signals can be received as long as the satellite is abovethe horizon and within line of sight at the receiving station.Reception time can last up to 15 minutes and provide an imagecovering from Cuba to Canada and more than half the width ofthe United States.
There is a variety of ways to decode the APT signal. Free softwareexists for this purpose, however the programs offer no insight intothe decoding process. This weather station uses a customprogrammed decoding process that has been implemented in theMATLAB programming environment.
To begin the decoding process the signal is imported into MATLABas a .wav file recorded at a 11.025 kHz sampling rate. Next, theenvelope is extracted from the AM audio signal; this new signalis a vector that contains the information in its most raw form. Thissignal is cross-correlated with a generated waveform that matchesthe APT Sync A specifications. The result of the correlation is avector with maximum values located at the beginning of each APTframe and is used to turn the data vector into a data matrix. Afternormalizing the data matrix to 255, MATLAB functions are usedto display the matrix as an 8-bit greyscale image.
At this point a few more steps are included to optimize theimage. For several reasons like the position of the sun and cloudcover, the image may not have adequate contrast. Our programprovides an opportunity for the user to adjust the contrast. Finally,since it is possible for satellites to travel south-to-north the imagemay come out “upside down”, so one last prompt is given sothat the user may flip the image.
SummaryThe goal of this project was to build a weather station capableof receiving and decoding satellite images from the NOAA POEsatellites without using closed-source software. Severalchallenges were overcome during this project including antennaconstruction, a mechanically failing pre-amp, demodulating thethe AM signal, and seperating the vector data into a coherentmatrix. By comparing the images generated from a closed-sourcesoftware program and this project’s MATLAB process, it canbe concluded that this project successfully met its goals.
Mark Roland, Eric Mbaka, Vijo Varghese, Bill Dower with the Quadrifilar HelixAntenna atop Eaton Hall.
This graph shows that the there is a dip in reflected power at 137MHz andthat the antenna was successfully built to receive the desired signals.
Screenshot from Nova satellite tracking software
ABOVE: Audio waveform from one frame of an APT signal lastingexactly half of a second
LEFT: The technical specifications for the APT Video Line Format asprovided in the NOAA KLM user’s maunaul
These are decoded APT images that clearly display the eastern portion of the United States. The left image was decodeded using WXtoImgSoftware. The right image was decoded using a custom process in MATLAB. As you can see they are almost perfectly identical.