Introduction
Weather information is absolutely critical for any marine passage especially for blue water sailors. In previous posts I looked at receiving weather information from the NOAA GOES16 geostationary satellite (Ref.1), from Marine HF Weather Fax (Ref.2) and GRIB Files using OpenPlotter (Ref.3). In this post we will examine reception of weather information from the NOAA POES Polar Operational Environmental Satellites 15/18/19 (Ref.4). Unlike the geostationary GOES16, POES satellites are low earth orbiting over the poles.
NOAA POES Low Earth Orbiting Satellites
| NOAA POES Satellite | APT Freq MHz | HRTP Freq MHz |
| NOAA-15 | 137.6200 | 1702.5 |
| NOAA-18 | 137.9125 | 1707.0 |
| NOAA-19 | 137.1000 | 1698.0 |
NOAA-15 was launched in 1998, NOAA-18 in 2005 and NOAA-19 in 2009. They were designed for environmental monitoring and fly in low earth orbits approx. 850Km above the Earth’s surface with a period of 102 minutes or 14 orbits per day. They contain a host of various sensors, but for weather, the APT or Automatic Picture Transmission and HRPT High Resolution Picture Transmission is of interest. APT operates at 137MHz, just above Air Band Voice and HRPT operates in the microwave S band at 1700MHz (Fig.1) .
Satellite Tracking
Since GOES16 is geostationary, the microwave antenna was pointed at a constant azimuth and elevation which did not change (Ref.1). In the case of NOAA POES satellites, they have low earth orbits that constantly change. This means that the satellites are only visible in a fixed time window of about 12 minutes. Figure 2 shows the orbit of NOAA15 (Ref.5). The red circle shows where it was and the yellow circles show future positions. Since the period is about 102minutes, the Earth rotates (102/60)*15=25.5deg in this time, so the next pass of yellow dots is located this amount to the west of the current track.
In order to receive NOAA15/18/19 planning is required. Figure 3 shows a 10 day prediction window for NOAA15 (Ref.6). This depends on your location which needs to be an accurate Lat/Long. If you are in the open water, then you have full 360view without obstructions. In a city environment, this can be tricky. You need to know if the orbit is South to North or vice versa and the range of azimuths. In my home QTH, I have a view of approx. 0deg to 175deg with large building obstructions which is not ideal. So for instance, the Mar1st orbit at 07:49 goes from 17deg to 105deg might work at my location. Generally the satellites will be visible during two time windows every day
SDRangel Configuration
Figure 4 shows the SDRangel configuration for receiving NOAA15/18/19 satellites. The Source Device Set R0 is an RTL-SDR. The Channels module is the APT Demodulator (Ref.7) and the Features module is the Satellite Tracker (Ref.8). The Satellite tracker is key and has many options. The first thing to do is set up your QTH Lat/Long in the Preferences. The next thing is to update the Satellite Data, an internet connection is required to do this step. Then you enter the Satellites NOAA15/18/19. Finally setup the Satellite Radio Control.
The way I did this was to set R0 at a nominal frequency of 137.5MHz and save this as a Preset. The Satellite Tracker automatically monitors what satellites are available for your location and upon AOS Acquisition of the Satellite, it turns on the R0 to the required frequency and adjusts for Doppler shift. It also turns on the APT Demodulator. You need to set this up for the three satellites (Fig.5). So when you start SDRangel, call the NOAA Preset and only start the Satellite Tracker, it will control the other two modules.
Satellite Reception
| PD Km | Lfs Free Space Loss dB | Rx Level dBm | SNR dB |
| 850 | 133.8 | -96.8 | 23.2 |
| 2000 | 141.2 | -104.2 | 15.8 |
| 3000 | 144.7 | -107.7 | 12.3 |
| PD = Propagation Distance | 32.44+20log(F_MHz) +20log(PD_Km) | Tx = +37dBm |
In order to receive NOAA15/18/19 signals, adequate SNR is required to get a good picture. In my urban location, the receive noise power in 50KHz at 137.5MHz is -120dBm (Fig.6). NOAA documents indicate that the transmit ERP is 5W or +37dBm. So the receive SNR will vary depending on the propagation distance. Figure 7 shows the SNR for various propagation distances from directly overhead to 3000Km.
My initial test was my marine antenna mounted on my balcony. The ambient noise was too high to receive a signal. Mounting the antenna inside looking east outside the balcony window I received a weak signal as shown in Figure 8. Having the proper antenna is key to getting good reception. NOAA15/18/19 use RHCP Right Hand Circular Polarization. Various web sites have excellent antenna designs for helical and various dipole antennas. The Vee Dipole is the simplest that I tried as shown in Figures 9/10. An adjustable Vee dipole can be purchased on Amazon for approx. $25 or it can be easily constructed (Ref.9). In order to reduce QRM/QRN I added a SAW BPF/LNA module to filter out the intermod and increase the SNR. Figure 12 shows the signal capture for March 1st for the 07:49 orbit. I need to experiment with the Vee Dipole and the BPF Filter/LNA in an open location to see what unobstructed reception is like. So this is a start, more to come!
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References
#1. – “RTL-SDR for Satellite Weather on GOES16”
https://jeremyclark.ca/wp/telecom/rtl-sdr-for-satellite-weather-on-goes16-image-capture/
#2. – “OpenPlotter – Marine HF Weather Fax”
https://jeremyclark.ca/wp/telecom/openplotter-marine-hf-weather-fax/
#3. – “OpenPlotter GRIB Files”
https://jeremyclark.ca/wp/telecom/openplotter-grib-files/
#4. – “NOAA POES Satellite Page”
https://www.ospo.noaa.gov/Operations/POES
#5. – “NOAA15 Track AMSAT”
https://www.amsat.org/track/satloc.php?lang=en&satellite=NOAA-15
#6. – “Live Real Time Satellite Tracking”
https://www.n2yo.com
#7. – “SDRangel APT Demodulator”
https://github.com/f4exb/sdrangel/blob/master/plugins/channelrx/demodapt/readme.md
#8. – “SDRangel Satellite Tracker”
https://github.com/f4exb/sdrangel/blob/master/plugins/feature/satellitetracker/readme.md
#9. – “Simple NOAA/Meteor Satellite Weather Antenna”
https://www.rtl-sdr.com/simple-noaameteor-weather-satellite-antenna-137-mhz-v-dipole/
