I'm in the process of troubleshooting an odd issue with RAIM failure during GPS approaches. Follow along to see what I find out.
Equipment:
KLN 89B, Dual KX155's and an Ameriking 450 ELT
Issue:
In two different approaches recently (within the past two months) I've had a RAIM failure flag occur right before the final approach fix. One was during an approach to S50 and one during an approach to KTIW. There are a few similarities on these two approaches, the FAF's are ~10 miles apart and the approach frequencies are the same.
What I know so far:
Historically there have been some reports of KX155's causing harmonics on certain frequencies (notably around 121.175) which can cause interference and loss of signal on the GPS. Supposedly this should be part of an IFR GPS installation, and filters can be installed to solve this but I have not yet been able to verify if this is the case with my installation. Likewise there have been reports of incorrect filters (or no filters) on some older Ameriking ELT circuits leading to a harmonic with similar results due to the placement of the ELT antenna (in my case, the ELT antenna is a few inches from the comm 2 antenna).
What I've tested so far:
So far I've tested Comm transmit/GPS status while sitting on the ramp and have the following results:
SV's before: 8
SV's while transmitting Comm 1 on 120.5: 8
SV's while transmitting Comm 2 on 120.5: 8
First test indicates no issue with the comm causing interference. My next test will be to vary the frequency and see if there are any changes, then test in flight near where I've had issues in the past.
Stay tuned!
Equipment:
KLN 89B, Dual KX155's and an Ameriking 450 ELT
Issue:
In two different approaches recently (within the past two months) I've had a RAIM failure flag occur right before the final approach fix. One was during an approach to S50 and one during an approach to KTIW. There are a few similarities on these two approaches, the FAF's are ~10 miles apart and the approach frequencies are the same.
What I know so far:
Historically there have been some reports of KX155's causing harmonics on certain frequencies (notably around 121.175) which can cause interference and loss of signal on the GPS. Supposedly this should be part of an IFR GPS installation, and filters can be installed to solve this but I have not yet been able to verify if this is the case with my installation. Likewise there have been reports of incorrect filters (or no filters) on some older Ameriking ELT circuits leading to a harmonic with similar results due to the placement of the ELT antenna (in my case, the ELT antenna is a few inches from the comm 2 antenna).
What I've tested so far:
So far I've tested Comm transmit/GPS status while sitting on the ramp and have the following results:
SV's before: 8
SV's while transmitting Comm 1 on 120.5: 8
SV's while transmitting Comm 2 on 120.5: 8
First test indicates no issue with the comm causing interference. My next test will be to vary the frequency and see if there are any changes, then test in flight near where I've had issues in the past.
Stay tuned!
