Simultaneous GPS is a technique that can improve location services on a cell phone. It works by enabling a phone to make voice calls and transmit and receive GPS location at the same time. Designing S-GPS poses particular challenges for phone makers, but it adds a layer of functionality that could be particularly useful in certain situations, such as emergency calls.
Older cell phones with GPS functionality traditionally used a system called time-multiplexed GPS. This meant that during a voice call, the handset would switch back and forth between transmitting and receiving the signal for the call itself and transmitting and receiving the signal to GPS satellites. This was a compromise system to make phone design simpler, the downsides being that the GPS positioning was less accurate.
The idea of S-GPS is that the phone should be able to send and receive both call and GPS signals simultaneously. This means the phone can use more data to pinpoint location accurately while still making the voice call. It also makes it easier to automatically provide the phone's location to the other person on the voice call, though this is used only in specific circumstances where the other person needs to know the location.
S-GPS works by using two radios -- one for GPS, one for voice calls -- sharing a single antenna. The phone is set up to automatically filter data by frequency to separate the data from and to each radio. This is an extremely complicated process that phone developers have refined or attempted to refine on several occasions. The key problem is that enforcing a filter to split the two types of data can mean throwing away some GPS data and in turn reducing the accuracy of the location positioning. Another issue is that transmitting and receiving both types of data simultaneously requires more power. Success requires a good balance between power usage and quality of GPS and phone signals.
The main practical benefit of S-GPS is when making an emergency call. Unlike with a landline call, staff receiving an emergency cell phone call can't simply check the registered address of the incoming call. To add to this, a cell phone user could be almost anywhere, decreasing the chances that he'd be able to accurately describe his location. The emergency call center can get a general location by tracking the phone's position relative to local cell towers, but using GPS increases accuracy. S-GPS makes it possible for the phone to automatically determine and pass on its location to the emergency call center's computers without interrupting the voice call.
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