Some thoughts on the earlier issue of Lightsquared's proposed terrestrial transmitters on the GPS frequency bands:
The not-so-obvious casualties in this situation are the myriad users of the GPS system as a precise global clock. There are a lot of critical bits of infrastructure that use ubiquitous GPS receivers on a computer's serial port for millisecond timing. Server farms use it, Telecommunications, seismographs ...
Among the hidden users of GPS clock signal: some our country's major urban public safety radio systems. EMS, fire, police, and ambulance radios and mobile data terminals in hundreds of cities, counties, and port facilities, along with a number of statewide and federal systems. Trunked radio systems are on a different frequency band from GPS, but they use the GPS clock signal to synchronize dozens of "simulcast" transmitters at different locations on radio towers, high rises, and mountain tops in order to give better coverage of the service area. Without this split-second synchronization from the GPS clock the system's coverage and intelligibility can be greatly impacted, central dispatching of widespread units becomes slow and chaotic. Not what you want when you're calling for an ambulance or a cop. This alone should be reason enough to stop Lightsquared in their tracks. Never mind any concerns about positioning!
Many of these trunked systems are already facing problems sharing their own frequency band with adjacent cell phone frequencies, which causes crackly interference at times. The "rebanding" of these public safety radio systems is expensive and slow -- many rebanding programs have had a schedule slippage of several years already. And the rebanding will not solve the GPS clock interference problems mentioned above.
There are other ways to synchronize these clocks. Use the internet. Use a dedicated phone line. Use a different radio frequency to send the clock signal. Some of them don't work very well after an earthquake, or a flood, or in a power outage. The GPS clock signal is used everywhere because it works better and for cheaper than anything else.
Implementation of Lightsquared's plan to roll out 400+ powerful terrestrial transmitters in key urban areas would basically require wholesale replacement of all currently-deployed GPS devices, whether your recreational handheld, the navigation in your car, precise GPS equipment used in construction and land surveying, or the vast amount of public safety infrastructure that keeps society safe. To replace every one of the GPS receivers used solely for its clock signal in the public safety infrastructure would cost a fortune.
What happens if you roll in powerful GPS jammer in, say, downtown Seattle, something that overloads the front end of any GPS device within a 5 mile radius?
Cabbies get lost? Soccer moms can't find the library? Surveyors have to break out the total stations? Minor irritation, no problem, you just need an upgrade ... Nope. Here's what happens: Utter chaos, and for no apparent reason. Fire, medics, police, and ambulance dispatches are garbled or delayed. For weeks, as workarounds are worked on and equity fights equity in court.
Maybe maybe every last one of these radio installations has a dedicated hardline to WWV at Ft. Collins. But I doubt it. Never heard of it. Costs extra. We just stick up a GPS antenna and get a clock that way and it works great.
So I had this thought and I emailed a few friends about it, and now I'm sharing it here, because the thought disturbs me almost enough to pick up the phone and call whoever is running the local radio systems and ask, have they heard of this Lightsquared thing, and what happens to those radio systems when someone jams the GPS clock signal. And I think maybe all of us surveyors should make that phone call.
