Block II/IIA. The ranging error with the improved clocks is almost halved. These ranging accuracy improvements allow a proportional improvement in positioning and time outputs from a GPS receiver.

Further improvements in stability and ruggedness of the atomic clocks flown on GPS satellites could do the following:

• Improve the ranging accuracy, reducing collateral damage, enabling more accurate weapons delivery, and subsequently reducing the number of missions required to destroy a target set.

• Allow longer GPS operations in the event of a ground upload outage.

• Enhance clock lifetime, lowering the clock failure rate and reducing the cost of sustaining the GPS satellite system.

Naval operations are unique in their distributed nature, so four-dimensional coordination of weapons systems is of particular importance to the Navy. For example, missile defense systems typically use radar to locate incoming missiles and predict their flight paths. This four-dimensional location must then be sent to an interceptor that is using the same four-dimensional coordinate system. It is clear that the four-dimensional coordination requirements to achieve success in such a system are formidable. Through GPS, PTTI plays a central role in four-dimensional coordination, helping to define the spatial coordinate frame and coordinating the time dimension. As warning times decrease, the demands on the system and PTTI will become greater, placing greater demands on four-dimensional coordination. There are similar examples in antisubmarine warfare and the attack of fleeting targets.

Advances in PTTI such as more accurate time dissemination and synchronization would provide better overall time coordination between distributed assets. For the warfighter, this could mean

• More accurate targeting, particularly for time-of-arrival location systems,

• Faster reaction times, and

• Increased probability of kill, with subsequent reduction in sorties or rounds fired.

If enemy jamming is present, direct acquisition of GPS signal is necessary. Direct acquisition requires a search for the start of the code signal. If the user has accurate knowledge of time from his local clock, the time required for this search can be reduced. This can be critical in battle situations— for example, in a guided weapon that may have less than a minute to lock up, as in the current weapon system design for a joint direct-attack munition (JDAM). To maintain 100-microsecond acquisition time uncertainty over 12 hours requires a clock with stability of 2 × 10⁻⁹. Current receiver clocks, with typical stability of 1 × 10⁻⁶, drift this far in 100 seconds. While other solutions to this problem are possible, clocks better in all aspects would allow trade-offs when the system is being designed.

In other situations—for example, under jungle canopies or in urban canyons and mountainous terrain or in case of satellite failure—fewer than the normal four GPS satellites are in view. If the user has accurate time from his local clock and independent knowledge of altitude, a position fix can be obtained with only two satellites. Ranging accuracy goes approximately as (drift rate × time since update). If the last time update was 2 hours earlier and the equivalent ranging accuracy requirement was 200 meters, drift rates of less than about 10⁻¹⁰ would be required in a low-power, rugged, lightweight device. A solution to the two-satellite problem would be particularly useful to Marines and Army personnel.