Is the change in range in one direction or is it a variation during a 360 turn? Can you connect the change to special weather conditions at your home (strong rain and wind, long periods of fog/mist )?
At a frequency of 1090MHz the atmospheric influence (lift etc.) is very very low. Sometimes I'm able to receive high altitude airframes at a range were they are one or two degrees beneath the horizon (line of sight). On days with very strong rain I have a reduction of hits on the edge of my range. Further the maximum range is slightly lower than on days with clear blue sky. This is caused by the attenuation due to high amount of water in the amosphere. On days with high temperature I've the same effect as on days with heavy rain, but this is related to the increase of noise of my amplifier.
Jumps in one direction of about 150 and 240nm can be caused by the following(list may not be complete):
- defective part in the reception chain, such as loose connector, corroded connector, temporary water contamination of connector, defect in antenna, colder solder of antenna jack on pcb of Comstation.
- difference in the flight level: as line of sight is needed, the following is valid: the higher the aircraft flies, the longer you can see it, the higher the reception range.
- for long distance range of high altitude airframes a high obstacle that is several miles away can reduce the range.
- corrupt data packet received. It is possible, that a received packed is altered (collision of two or more signals at the same time) in a way, that a false position is generated while the error correction/cecksum (Similar CRC) stays valid.
- airframes that sending out false position data.
- glitches in the data processing of the Comstation. Extremely rare and close to almost impossible to fix, but they can happen. I have mine up and running for almost 6000hours. Until now I haven't seen one.
- On days with high air traffic I have noticed, that I sometimes get a slight increase in the count of airframes that I'm able to receive beneath the line of sight horizon (of obstacles). These airframes haven't performed any position jumps and the whole flight path until reception ended was plausible. These range extention can be caused by reflection of the signal on the metallic hull of other airframes. Typical indications of this effect are, that these range "lifts" are never at exactly the same position, that they never happen at same time of day/week, that they are not predictable and that they have an increase on days with lot of other airframes in the sky. This one is rare, difficult to detect and you need an awful long time in front of your monitor to see one happen.
Ingo
