In other words, it's LightSquared's word against Garmin's.
Elsewhere in the world, the European Union is proposing that old analog TV frequencies around 800MHz be used for European 4G networks now that everybody has moved to digital TV, which uses a different and smaller frequency range. However, cable TV services in Europe still pipe data down coaxial lines at these old, analog frequencies. Using a European LTE phone in a room near a cable TV box has been proven to disrupt the image and disrupt data services. (Click here (PDF) for a study undertaken by a U.K. government agency proving the effect.)
The underlying problem is that as efficient digital services replace their bandwidth-hogging analog equivalents in the radio spectrum, national governments are keen to sell off the suddenly unoccupied radio space. However, as Garmin's tests show, such services don't always sit comfortably alongside each other.
In an ideal world we'd simply start all over again and assign frequencies to digital services in a sensible manner, but the current systems of cell phone networks, Wi-Fi, and GPS all have arisen organically over many years, during which they had to coexist peacefully with neighbouring analog services.
Other 4G, LTE services are safe. For example, Sprint's WiMAX service operates at 2500MHz, and Verizon LTE at 700MHz. Neither are known to cause issues.
However, it's becoming clear that a competitive marketplace combined with limited radio spectrum opportunities means that service providers will have to tread very carefully as they roll out faster phone services in the coming years.
Keir Thomas is an IT journalist who's been making his opinion known about computing since the last century. More recently he has written several best-selling books. You can learn more about him at http://keirthomas.com, and his Twitter feed is @keirthomas.