09-05-2011, 09:34 AM
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INTRODUCTION
GPS retransmission systems Global Positioning System (GPS) technology is increasingly being applied in many different military applications beyond navigation. Soldiers use GPS to enhance situational awareness on the battle field with systems such as Land Warrior. GPS applications are utilized for precision aerial resupply via the Joint Precision Airdrop System (JPADS) to guide ammunition, medical supplies, or food to units operating on the ground. GPS enabled asset tracking may provide current position and status of high value assets, such as VIPs, nuclear weapons, etc. In training applications, GPS technology may be used to track the participating assets, scoring the exercise and enabling a far more instructive de-brief.
GPS provides accurate target position information to smart weapons deployed from aircraft or ground based platforms, improving accuracy and lethality of these weapons systems. Have successfully been utilized in combat since 2004 to provide live wireless signals to commercial and military GPS receivers inside volumes where a clear view of the sky is unavailable. Successful examples include military free fall (MFF), precision aerial resupply (JPADS), air and ground assault, and ground vehicle patrols (DAGR, Land Warrior). GPS retransmission systems have been successfully employed on a range of platforms, including C-130, C-17, CH-47, MH-60, HMMWV, Stryker, Bradley, and more.
This white paper will discuss how GPS Retransmission can be a very cost effective solution to the problem of GPS denied environments for delivery of GPS guided munitions from aircraft weapons bays, under-wing munitions pylons, or artillery & mortar tubes.
GPS - a Technical Perspective
The GPS system traces its origins to the sixties. In 1960, Aerospace Corporation was founded for the purpose of applying then advanced technology to space and ballistic missile problems. In 1963, the company started work on Project 621, the Global Positioning System, a scheme for replacing strategic aircraft astro-navigation systems with satellite navigation. Whereas astro-navigation systems needed clear sky to track stars, the satellite navigation scheme would use microwaves and a satellite distributed master clock, thereby providing all weather operation and superior accuracy
The Operational GPS Constellation uses 24 satellites, of which 3 are spares, orbiting in precise 12 hour orbits. The orbit geometry is adjusted so that these orbits repeat the same ground track once perday, and at any point on the Earth's surface at any given time the same configuration of satellites should be seen.
The satellites are grouped, nominally in sets of four, into six orbital planes, each of which is inclined at approximately 55 degrees to the polar plane. A user at any point should be able to see between five and eight satellites at any time.
The GPS system provides two navigational services, the military Precise Positioning Service (PPS), and the civilian Standard Positioning Service (SPS). PPS provides nominally 17.8 m horizontal accuracy, 27.7 m vertical accuracy and time accurate to 100 nanoseconds. SPS provides nominally 100 m horizontal accuracy, 156 m vertical accuracy and time accurate to 167 nanoseconds, and is available to civilian users. The degraded accuracy results from the use of Selective Availability. In practice, achieved accuracy can significantly better the nominal figures.
GPS Retransmission System Architectures
GPS retransmission systems, in their simplest form, include at a minimum the following elements:
• Active Antenna (Active meaning the antenna includes an integrated Low Noise Amplifier)
• Interconnecting Coaxial Cable(s)
• Retransmission Amplifier/Signal Conditioner
• Passive Retransmission Antenna (or repeat antenna)
Obstruction
Retransmission
Antenna
LNA
Coaxial
J4 Cable
Retransmission Amplifier/Signal Conditioner
GPS
Active GPS Antenna
Rx.
Figure 1. Simple GPS Retransmission System
In this system, the GPS satellite signals are received by the active (receive) antenna, amplified and conditioned by the retransmission amplifier, and re-broadcast on the GPS frequency(s) by the retransmission antenna. Because the signal delay through the GPS retransmission system is common for each satellite once the signals are received by the exterior antenna,
GPS receivers operating in the retransmitted signal environment will generate a location, not at their actual position inside of the hangar, vehicle, or aircraft, but rather they will calculate the position for the system’s receive antenna that is located outside in view of the LOS signals. This limitation, however, is not critical for the applications described above, as the derived location is close enough to accomplish the intended function.
In the application of a GPS retransmission system repeating a wireless GPS signal to munitions stored in an aircraft’s enclosed weapons bay, the system utilizes the aircraft’s existing active GPS antenna to receive a signal from the satellites. The signal is transferred through an RF splitter to the passive, or repeat antenna, and received by the munitions’ active antenna, thereby providing a live GPS signal to the weapon.
This process creates an efficient manner for installation of munitions onto the aircraft by the weapons technician. Similarly, this system provides a more robust and efficient manner of transferring GPS data to munitions prior to separation. The guided munitions utilize their active antenna to receive a “hot” position signal and do not require a separate MIL-STD-1760 compliant GPS umbilical to provide this positional information then switch over to the antenna post-separation. Most importantly, during separation the weapon seamlessly transitions from repeat GPS signal to organic signals by keeping the ephemeris, within the weapon’s GPS receiver, active.
A transition from the umbilical signal to the munitions antenna is unnecessary, removing the burden from firmware or electromechanical switch within the weapon.
Figure 2. GPS Retransmission System Function in Aircraft Deployed Munitions
Figure 2 graphically depicts the concept of GPS retransmission systems for use with GPS guided munitions deployment
Within the aircraft weapons bay, the repeated GPS signal can be tailored to meet the requirements of different aircraft or weapons configurations. The signal power can be tailored to ranges of a few inches to ranges of 12 ft, depending on the application and requirements. Similarly, one or multiple repeat antennas, or near field antenna couplers, may be utilized in the system design to provide maximum GPS signal coverage for the weapons bay while eliminating the potential for the signal to propagate beyond the intended area of coverage. For example, a GBU-15
Air-to-Surface GPS guided weapon houses the guidance system and GPS antennas in the forward portion of the weapon. Alternatively, the JDAM GPS guided weapon houses the guidance system and GPS antennas at the rear of the system, with one of the GPS antennas facing rearward mounted to the tail section of the weapon, shown in Figure 3.
When loaded aboard an F-35 weapons bay, for example, the GPS retransmission system must be designed in a manner to provide GPS signal coverage for both types of munitions and may require two repeat antennas if both weapons were to be loaded onto the aircraft, simultaneously, for a mission.
This configuration requirement would also hold true with munitions systems loaded onto an aircraft’s wing pylon or underbelly where a clear view of the sky may not always be available. A repeated GPS signal extending 12 to 24 inches from the passive antenna or near field antenna coupler will provide a GPS signal to the munitions to ensure immediate organic positional information upon separation
