Revolution in GPS: Advanced Spinning-Vehicle Navigation

GPS World, Sept, 2004 by James H. Doty

GPS-aided weapons such as the Joint Direct Attack Munition (JDAM) family of bombs have revolutionized warfare, dramatically improving accuracy and cost effectiveness, while significantly reducing collateral damage. Augmenting artillery rounds with GPS aiding can furnish similar improvements. Unfortunately, the rapid rotation on launch of many shells and rockets complicates GPS-aided guidance. This rotation can cause amplitude and phase modulation of the GPS signal, reducing navigation performance and effectiveness of traditional anti-jam systems. Further complicating matters, traditional attitude determination techniques, utilizing inertial sensors, tend to develop significant errors at high roll rates.

To address the special navigational needs of rotating vehicles, my colleagues and I have developed a suite of GPS technologies known as Advanced Spinning-Vehicle Navigation (ASVN). These technologies enable high-performance, interference-robust GPS navigation as well as roll-angle determination on both rapidly spinning and slowly rotating vehicles. Potential applications include artillery shells, rockets, missiles, space vehicles, and unmanned aerial vehicles (UAVs).

Bottom-Line Drivers. ASVN provides an innovative, low-cost, compact, and robust solution to both rotation angle determination and the navigation of rapidly rotating vehicles. ASVN can:

* Improve jamming immunity by using interference to aid navigation and guidance;

* Increase range by delaying deployment or reducing the required authority of control fins and actuators;

* Reduce cost, size, and weight by eliminating or decreasing the requirements of the inertial sensing system and simplifying antenna design; and

* Enhance accuracy by enabling early GPS acquisition for improved estimation of vehicle trajectory and inertial measurement unit errors.

ASVN Technology Suite

ASVN uses the amplitude and phase modulation of the signals received by a GPS antenna on a spinning vehicle to track both vehicle rotation and improve GPS jamming and interference immunity. Analyses and laboratory and field hardware simulations illustrate the viability of ASVN technologies to reduce costs and improve the performance and capabilities of a range of spinning and rotating applications. Four core solutions comprise ASVN technology:

GPS Roll Angle Determination. Using the GPS signal modulation with rotation to measure vehicle roll angle;

Interference-Aided Navigation. Leveraging a jamming signal or interference source to aid vehicle navigation;

Temporal Beam Forming. Improving jamming resistance with only a single rotating antenna element; and

Coriolis Pitch- and Yaw-Rate Sensing. Providing inertial aiding (including rotation rates) without the use of gyroscopes.

GPS Roll-Angle Determination

In rapidly rotating vehicles (such as artillery shells and missiles), high roll rates complicate attitude determination using conventional inertial sensing techniques. Gyroscope scale factor inaccuracies trigger small roll angle errors with every rotation. At high spin rates, these errors rapidly accumulate into large errors in the vehicle attitude estimate. Added sensors such as magnetic detectors can provide an absolute roll reference, but the performance of these systems depends on the geometry of the flight trajectory with respect to the Earth's magnetic field and the magnetic properties of the vehicle.

ASVN GPS roll-angle determination (GRAD) uses the GPS signal as a reference for roll determination. The diverse geometry of the GPS constellation provides a very trajectory-independent roll reference. Because a GPS receiver typically is present in guided munitions for position determination, no additional sensors are required. The GPS signal is very weak and buried in the background noise, rendering traditional radio frequency (RF) direction-finding techniques ineffective for GRAD. However, rotation demodulators controlled by a roll-angle estimate and GPS correlators help observe the signal.

Figure 1 shows a block diagram of the GRAD receiver with rotation demodulators preceding the GPS correlators. Figure 2 shows the amplitude and phase modulation measured for an example antenna, as well as the rotation demodulator amplitude and its phase correction centered on the rotation modulation. When the estimated roll angle aligns with the actual vehicle roll angle, as shown in the figure, the rotation demodulator corrects the phase modulation and passes the largest portion of the GPS signal amplitude on to the carrier demodulator.

Two more signal-processing channels provide feedback for rotation tracking. The roll-angle control for one channel is advanced ahead of the centered roll-angle estimate and the other is retarded behind it.

When the roll estimate is properly aligned with the true roll angle of the vehicle, the correlation magnitude from the centered channel will be maximized, and the advanced and retarded channels will be reduced in amplitude and approximately equal. The rotation-angle-tracking servo loop can accurately maintain alignment of the centered channel with the actual roll angle of the vehicle by controlling the angle estimate to keep the advanced and retarded magnitudes equal.