Previous guided missiles had to be delivered within a certain range of the target and then released. The extent of the horizon was a major limitation to the early guided missiles. Most early guided missiles did have preset flight paths that were input into the onboard flight control systems, yet weather conditions, air currents, and other physical phenomena could blow the missile off course if not prevent the missile from completing its objective at all. With time technology improved and so did defensive capabilities.
Many of the early guidance systems used in missiles where based on gyroscope models. Many of these models used magnets in their gyroscope to increase the sensitivity of the navigational array. In modern day warfare, the inertial measurements of the missile are still controlled by a gyroscope in one form or another, but the method by which the missile approaches the target bears a technological edge. On the battlefield of today, guided missiles are guided to or acquire their targets via a radar signal, wire, laser, or most recently GPS.
Many machines used in battle, such as tanks, planes, etc. and targets, such as buildings, hangers, launch pads, etc. have a specific signature when a radar wave is reflected off of it. Guided missiles that use radar signatures to acquire their targets are programmed with the specific signature to home in on. Once the missile is launched, it then uses its onboard navigational array to home in on the preprogrammed radar signature.
Most radar guided missiles are very successful in acquiring their targets, however, these missiles need a source to pump out radar signals so that they can acquire their target. The major problem with these missiles in today's battlefield is that the countermeasures used against these missiles work on the same principles that these missiles operate under. The countermeasures home in on the radar signal source and destroy the antenna array, which essential shuts down the radar source, and hence the radar guided missiles cannot acquire their targets.
Wire guided missiles do not see the target. Once the missile is launched, the missile proceeds in a linear direction from the launch vehicle. Miles of small, fine wire are wound in the tail section of the missile and unwind as the missile travels to the target. Along this wire, the gunner sends navigational signals directing the missile to the target. If for some reason the wire breaks, the missile will never acquire the target. Wire guided missiles carry no instrument array that would allow them to acquire a target.
One strong downside to wire guided missiles is the fact that the vehicle from which the missile is fired must stay out in the open to guide the missile to its target. This leaves the launch vehicle vulnerable to attack, which on the battlefield one wants to avoid at all costs.
In modern day weaponry the buzzword is fire and forget. In other words, the gunner sites the target through the instrument array onboard the missile. Once the target is acquired, the gunner makes his way into a clearing, fires the missile, and then immediately returns back into his cover. Under this principle many modern day laser weapons were designed.
Laser guided missiles use a laser of a certain frequency bandwidth to acquire their target. The gunner sights the target using a laser, this is called painting the target. When the missile is launched it uses its onboard instrumentation to look for the heat signature created by the laser on the target. Once the missile locates the heat signature, the target is acquired, and the missile will home in on the target even if the target is moving.
The early models of laser guided weapons required the target to be painted at all times for missile to acquire the target. If the gunner stopped painting the target before the missile detonated, there was a possibility that the missile would lose acquisition and go astray. Here again we see that the gunner is in a vulnerable position until the missile detonates. The new age laser weapons overcome this problem with the fire and forget principle.
In the new age laser guided missiles, the gunner sights the target through the instrumentation onboard the missile. Once the target is acquired, the gunner simply launches the missile and the onboard computer on the missile takes care of the rest.
Despite the much publicized success of laser guided missiles during the Persian Gulf War, laser guided weapons are no good in the rain or in weather conditions where there is sufficient cloud cover. For ground troops, the laser used by the missile to acquire the target is diffracted by rain; each raindrop acts as a prism deflecting the laser light. Planes that use laser guided weapons sight the target through the instrumentation onboard the missile. If there is sufficient cloud cover, the droplets of water in the air will act as prisms diffracting the laser light. The laser light operates on a certain bandwidth. If the cloud cover is too dense, the laser may become completely diffracted and the missile will not acquire the target.
To overcome the shortcomings of laser guided missiles presented in unsuitable atmospheric conditions and radar guided missiles entered GPS as a method of navigating the missile to the target. GPS is unaffected by weather conditions and is designed to be resilient to jamming affects.
In many communication systems, the main means to avoid jamming is to take the small bandwidth of the information waveform and modulate it with a large-bandwidth waveform. This process is known as spread spectrum and is used in GPS signals. The information waveform of the GPS has a bandwidth of 100 Hz, whereas the transmitted signal has a bandwidth of 20 MHz. 
Potential target areas are mapped out ahead of time, identifying potential targets by their location (height above sea level, latitude, and longitude) with respect to the GPS satellite system. When an attack raid is planned, the GPS guided missile is programmed with the target's GPS coordinates and the topology of the region surrounding the target. When the missile is then launched, it reads its current GPS position and plots a course to the target. Additional software onboard the missile enable the missile to follow the terrain in its approach to the target. This makes it difficult to spot the missile on radar or to intercept the missile by any other means.
Although GPS guided missiles have not been used in combat yet, their success during their testing phase has ranked them next in line as America's heavy hitter. The method of adapting guided missiles has been so successful that officials in the military are considering adapting GPS to other systems as well. One such application that has received much attention lately is the application of GPS as a solution to G Lock. G - Lock occurs when a pilot pulls too many G's, his brain does not receive sufficient blood flow and the pilot blacks out. Once the pilot blacks out, no one is flying the plane and in most cases of G Lock, the plane crashes. To solve this problem, researchers have developed a ground avoidance system. This system uses the GPS satellite system to gain a navigational bearing of the plane, with this information it takes over control of flying the plane if it perceives that the plane is going to collide with the ground.
In the near distant future, the battlefield will actually exist inside a virtual world. There are projects in the works now that if successful will convert manned machines into autonomous drones piloted by humans in a virtual battlefield. In these projects, military vehicles, such as tanks, are being outfitted with GPS transmitters and other remote control devices so that the vehicle can be piloted by a human without the human being physically present. The GPS transmitter located on the vehicle gives the physical bearing of the vehicle on the battlefield. This information along with sensory input from the area surrounding the vehicle is sent back to base where a computer interface receives this information and interprets it into a virtual environment where the human pilots the vehicle.
In summary, GPS guided weapons are not affected by harsh weather conditions or restricted by a wire, nor do they leave the gunner vulnerable for attack. GPS guided weapons, with their technological advances over previous, are the superior weapon of choice in modern day warfare. As technology continues to advance, the face of warfare will continue to change. In time there will be a superior system to replace GPS guided weapons.
1) GPS Theory and Practice. B. Hofmann-Wellenhof, H. Lichtenegger, and J. Collins. Springer-Verlag Wien. New York. 1997. Pg[1-17, 76].