There have been several recent post about intermittent GPS signal losses. I'm not sure if this is common knowledge, but Garmin has several documents that cover the installation and testing of the antennas, cabeling, and system operation. You can find these on the web.
Document # 190-00848-00 GA 35, GA 36, GA 37 Antenna Instalation Instructions. This covers the mechanical installation, location, and grounding.
The other document(s) are contained within the GPS receiver equipment units, such as the Garmin G5 and GTN 6xx/7xx Instalation Manuals. Of particular note in these documents is the mounting location of the GPS antenna and its proximity to other GPS antenna.
But don't stop reading there. In the section on Antenna Cable Installation and Connectors, there is a NOTE.
"GPS/SBAS antenna cable loss must be between 1.5dB and 6.5dB in order to maintain proper signal rejection of interference signals. If RG-142B or RG-400 is used, it must be between 6.5 and 35 feet long to meet the cable loss requirements. For longer lengths, use low-loss double or triple-shielded, 50 ohm coaxial cable".
Elsewhere in the document it states to keep coax cable routing as short and direct as possible [but maybe this was not intended for the GPS coax].
Finally, there is a VHF com interference test. This is where you monitor the GPS Satalite Status Page and transmit on the documented com frequencys for a period of 35 seconds each. And do this for each com transmitter installed in the aircraft. If any descripencys are noted, it may be possible to install a 1.57542 GHZ notch filter in the offending VHF com coax.
As owners and operators of EXPERIMENTAL aircraft, we sometimes feel that we can install equipment s we see fit. Sometimes we should follow the guidance of the equipment manufacturer.
I hope you found the above info useful.
Document # 190-00848-00 GA 35, GA 36, GA 37 Antenna Instalation Instructions. This covers the mechanical installation, location, and grounding.
The other document(s) are contained within the GPS receiver equipment units, such as the Garmin G5 and GTN 6xx/7xx Instalation Manuals. Of particular note in these documents is the mounting location of the GPS antenna and its proximity to other GPS antenna.
But don't stop reading there. In the section on Antenna Cable Installation and Connectors, there is a NOTE.
"GPS/SBAS antenna cable loss must be between 1.5dB and 6.5dB in order to maintain proper signal rejection of interference signals. If RG-142B or RG-400 is used, it must be between 6.5 and 35 feet long to meet the cable loss requirements. For longer lengths, use low-loss double or triple-shielded, 50 ohm coaxial cable".
Elsewhere in the document it states to keep coax cable routing as short and direct as possible [but maybe this was not intended for the GPS coax].
Finally, there is a VHF com interference test. This is where you monitor the GPS Satalite Status Page and transmit on the documented com frequencys for a period of 35 seconds each. And do this for each com transmitter installed in the aircraft. If any descripencys are noted, it may be possible to install a 1.57542 GHZ notch filter in the offending VHF com coax.
As owners and operators of EXPERIMENTAL aircraft, we sometimes feel that we can install equipment s we see fit. Sometimes we should follow the guidance of the equipment manufacturer.
I hope you found the above info useful.
