Low pass filter is a great device to add to your FPV system, it protects GPS and 2.4 GHz radio signal.
900 MHz transmitters may cause harmonics that aftects 2.4 GHz Radio Control system and some spurious can interfere GPS signals on 1.575GHz (1575MHz),depending on good the built-in filter of the GPS receiver was designed.
The harmonics and spurious of the TXV signal, are strong enough to cause problems, but since they are just a small fraction of the original signal, and they are weak enough to be easily attenuated and filtered out.
It can also be used along with 1.2-1.3G Transmiters (TXV) with some attenuation, however we have a filter specifically for 1.2-1.3GHz TXV HERE.
Low insertion loss
Insertion loss ~0.5dB at 910MHz
Insertion loss ~1.9dB at 1260 MHz
Insertion loss ~30dB at 1575MHz
Insertion loss> 43dB at 2.4GHz
50 Ohm impedance
10W Max Power
-55C to 100C Operating Temperature Range
Most installers are aware that maximizing the distance between the A/V transmitter and the RC receiver is important. But for some reason they ignore such words of wisdom, perhaps because they have a compact installation that makes this difficult to do.
Another way to reduce radiated noise from your transmitter case and wires, is to run your power and video cables multiple times around a toroidal ferrite rings as close as possible to the transmitter.