was adjusted by varying the radio signal strength so that traffic lights could be activated only nearby (if desired), or at greater distances. The downside to these preemption systems (which also performed collision avoidance) was that they would display the direction of impending collisions, but not be able to effectively (or accurately) calculate the distance to collision by any method other than RF signal strength, which was only a rough estimate at best. Global Positioning System With the advent of widespread Global Positioning System (GPS) applications came the introduction of a GPS-based traffic preemption system, that could also do collision avoidance. Recently some GPS preemption systems have found a way to overcome the nagging problem that "blinds" many GPS systems: how to prevent the system from being "blinded" by the loss of a GPS signal. In dense cities with tall buildings, GPS receivers may have difficulty obtaining the four required GPS satellite signals, required for trilateration to determine location. If the vehicle systems are not designed with a backup "IMU" (Inertial Measurement Unit), lack of GPS availability may adversely affect the system's performance. Extremely heavy cloud cover or severe weather can also adversely impact the ability of the GPS receiver from obtaining the four required satellites. Some systems offer an Optical Compatible GPS system with features that also include an Inertial measurement unit (IMU). The price point of some compare to Optical Systems. Therefore, cities that do not have preemption can get a GPS based system for the price of Optical Systems (typically 1/3 the price of many GPS systems). Additionally, cities that have existing optical systems can start upgrading to a GPS-based system while maintaining compatibility with their existing Optical vehicle emitters. These systems also come with Collision Avoidance. GPS systems typically convey their information in one of three ways - via 900 MHz FHSS, via 2.4 GHz FHSS, or by way of cellular modem. Each of these methodologies has a different set of advantages/disadvantages. 900 MHz FHSS appears to be the best option, because it is capable of the greatest range (often over for a 1 watt transceiver). 2.4 GHz is able to communicate more data, but is typically considered more "directional" or "line-of-sight". It only has a maximum range of about . This can often preclude the system from preempting soon enough to ensure a clear intersection upon arrival. Cellular overcomes the "distance" issue, but can be