Updated 26 Dec 2015 10:05

APT Pictures

To receive APT (Automatic Picture Transmission)  pictures from polar orbiting satellites is quite easy. Normally in a 24 hr period four passes from each of the Noaa satellites are available. Presently Noaa 15 is transmitting on 137.50 MHz , Noaa 18 on 137.10MHz and Noaa 19 on 137.91MHz. 

A simple crossed dipole antenna feeding a receiver such as a RX2 will give good results. It is recommended to use a pre-amplifier between the antenna and receiver to increase sensitivity so that low passes can also be received. A good quality 75 ohms coaxial cable should be used from the pre-amplifier to the receiver

 If you have an old VHF TV ant, you could cut it down to make a crossed dipole. (see drawing) The phasing harness and the coaxial going to the pre-amplifier should have an impedance of 50 ohms (RG58). When installed, one of the elements should be oriented roughly east to west.

The RX2 receiver is a kit which is available to members of RIG (Remote Imaging Group) in UK. If you have some basic knowledge of soldering and electronics, it very easy to build. Other receivers are also available but please note scanners are usually not suitable, as the bandwidth is not right. For APT a bandwidth of about 40KHz is required.

To decode the signal from your receiver, you need a program such as WXtoImg. The signal from the receiver is fed to the line in of your sound card.


Weather Station

The weather station I use is a Davis Weather Monitor II.

Davis Monitor 11.jpg (45839 bytes)

The following accessories are connected to it.

  External temperature/Humidity sensor

Hum1.jpg (8553 bytes)

 Rain collector

Rain Guage.jpg (29583 bytes)

Anemometer & Wind vane

Anemometer1.jpg (18340 bytes)

I also use the Weatherlink software from Davis which consists of a data logger and software which allows me to connect the station to a PC to store, view, plot, analyse, export and print weather data collected in my case for Quatre-Bornes.

wbul.jpg (92704 bytes)


Modification RX2 to receive Noaa 18

Noaa 18 is transmitting on 137.10Mhz, therefore the hex file contained in the PIC16C554 (IC6) in your receiver needs to be updated to receive the APT signal. A Pic programmer, a PC and the new hex file are required to do the job. In my case, I elected to remove the PIC16C554 in the receiver and kept it as a spare. I used a PIC16F84 which I had , loaded it with the new hex file.  Use a program such as ChipCat for programming the PIC and also save the new hex file with the extension .hex (i.e. rx2.hex). After installation the receiver will scan up to 7, 7 being 137.10Mhz for Noaa 18 and 6 being 137.91Mhz for Noaa 19 eventually. Note this hex file is for RX2 using TSA6057 synthesiser (IC4).


Repairing your Davis anemometer

It happens that your anemometer starts to indicate wind speed zero or seems to work intermittently. Most of the time the problem is
with the reed switch, the glass is probably cracked and humidity has damaged the contact inside. The switch can be replaced, follow the easy steps below. Maplin UK sells a reed switch which I found can be used to replace the faulty one. The code for the product is CL36P.



Remove the cups from the anemometer to expose the sealant holding the reed switch in place. Carefully pull the sealant with a long nose pliers and unsolder the reed switch from the black and red wires.

Step 2


Take the reed switch and carefully bend the wire on one side as shown without breaking the glass. (Use a long nose pliers to hold the wire before bending). Then solder the reed switch to the black and red wires.

Step 3

Carefully push the wires and the reed switch inside the anemometer assembly and apply some clear silicone sealant to hold it in place.






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