GPS/IMU Navigation and Simulation for Higher-Than-GPS Orbits

NAVSYS Corporation has modified its software GPS receiver (SGR) and built an engineering development unit of the High-gain Advanced GPS Receiver (HAGR) optimized for GPS tracking in an orbital environment. Enhancements to the SGR include the ability to track both direct GPS signals, as well as weaker GPS sidelobes. This requires digital beam steering and beam nulling to increase gain towards the weaker signals, while partially nulling direct lobe signals to achieve signal strength balancing. The space based SGR approach also makes use of inertial aiding data to allow continuous navigation through high dynamic mission phases such as launch, or orbit transfer. Goddard Space Flight Center ́s GEONS navigation software has also been integrated with the SGR to allow for real time onboard navigation. To test the engineering development unit, significant enhancements were made to the NAVSYS Advanced GPS Hybrid Simulator (AGHS) to allow simulation in an orbital environment. The AGHS is driven by Matlab and can receive trajectory information in real time from Satellite Tool Kit or other commercial-off-the-shelf or government-off-the-shelf profile generation software. The simulator also contains a Digital Storage Receiver (DSR) capable of recording live GPS data from complicated RF environments (such as urban canyon or indoor GPS) for later playback with an RF Modulator, or that can be played back directly into a digital GPS receiver. The capabilities offered by the AGHS substantially enhance the utility of GPS simulation. Additionally, a capability to simulate inertial measurement unit (IMU) data simultaneously with the GPS and from the same input trajectory has been added. The simulator is useful in testing and developing next generation receiver and navigation which will utilize coupled GPS/inertial navigation scenarios. The AGHS is capable of simulating 12 GPS satellites in 8 RF channels. The RF channels of the AGHS are fully coherent, allowing for GPS wavefront simulation. IMU measurement data coherent in time with the GPS data is generated at a rate of up to 400 Hz. Simulation and tracking results will be shown for various orbital scenarios. Orbital platform simulation and tracking results will be shown for highly eccentric orbits, in which GPS tracking of side lobes of satellites on the other side of the Earth are achieved. INTRODUCTION NAVSYS has developed the design for a flexible, highperformance Space-based Software GPS Receiver (SSGR) and is currently building an Engineering Development Unit (EDU) to demonstrate the next generation capabilities of the SSGR for space applications. The SSGR will provide an integrated precision navigation and attitude determination solution for space applications including Low Earth Orbit (LEO), Highly Eccentric Orbit (HEO), and Geostationary Earth Orbit (GEO) missions. The ability to track low power GPS satellites will extend the use of GPS for precision navigation and timing, particularly for high altitude space missions (above the GPS satellite constellation). The SSGR will be suitable for supporting multiple space missions including GPS metric tracking during launch, orbit determination during transfer to geostationary orbits, and high accuracy navigation, attitude control, and timing. The flexibility of the SSGR design will allow it to be reprogrammed for use in launch and orbit entry, station keeping, and autonomous orbit estimation applications. One of the biggest challenges of designing a space-based GPS receiver is testing, since the dynamics involved are radically different from anything achievable on the ground. Commercially available GPS simulators that can simulate the space environment are very expensive, generally have high learning curves, and are limited in capability. The multi-element AGHS designed by NAVSYS addresses many of these concerns and will, therefore, be used to test performance of the SSGR design. AGHS generates simulated digital signal sets using profiles generated by NAVSYS’ MATLAB Signal Simulation capability and can be used to generate digital representations for the GPS signals under the various scenarios for playback either into an RFM or directly into the GPS receiver. The NAVSYS MATLAB Toolbox has been augmented with many new tools to allow easy simulation of various space-based mission profiles. The added simulation scenarios cover the different phases of a space mission as mentioned above. New toolbox features include functions to determine the visibility and expected signal strength of GPS signals that will be received in each scenario and the ability to drive AGHS under each of these scenarios. Orbit and attitude information is easily entered into the tool via a text-based file, which can be generated with Satellite Tool Kit or other similar tools. MISSION PHASES OF INTEREST The SSGR design has been optimized to address the various complications of space-based GPS usage. The receiver must have the capability to maintain lock through dynamic maneuvers; both during launch and orbit transitions. GPS visibility must be maintained even for spinning satellites and when the satellite is in higher orbit than GPS orbit. The use of beam steering and the addition inertial data to aid the GPS tracking and recovery during outages gives the receiver the ability to maintain lock in such situations. The Digital Beam Steering (DBS) capability utilized in the SSGR allows for the construction of a composite GPS signal from multiple non-coplanar antenna elements placed around the spacecraft. The beam steering/null forming functionality also allows for tracking of weak GPS signals (such as GPS sidelobes) from higher than GPS orbit. Table 1 summarizes the enhancements that will be required for the SSGR and the benefits that will be gained in each mission phase. Table 1 Benefits Gained in Various Mission Phases Mission/ Capability Launch & Orbit Entry Station-keeping Formation-Flying Recovery & Landing 3-D Beamsteering Maintains SV visibility at all attitudes Provides gain towards GPS SVs Provides gain towards GPS SVs Maintains SV visibility at all attitudes Inertial-aiding High dynamic aided tracking for data continuity and navigation through SV outages N/A N/A High dynamic aided tracking for data continuity and navigation through SV outages Precision GPS Navigation Provides high accuracy code/carrier observations for Wide Area Differential GPS (WADGPS) solution Provides high accuracy code/carrier observations for WADGPS solution Provides high accuracy code/carrier observations for Kinematic GPS (KGPS) solution Provides high accuracy code/ carrier observations for WADGPS and KGPS solution Attitude Determination N/A Provides interferometric attitude data from array Provides interferometric attitude data from array N/A HIGH-GAIN ADVANCED GPS RECEIVER NAVSYS’ HAGR is a software reprogrammable, digital beam steering GPS receiver. The HAGR components are illustrated in Figure 1. With the HAGR digital beam steering implementation, each RF input from an antenna element is converted to a digital signal using a Digital Front-End (DFE). The HAGR can be configured to operate with up to sixteen antenna elements (L1 and L2), with the antenna elements installed in any user-specified antenna array pattern. Each DFE board in the HAGR can convert signals from eight antenna elements. The digital signals from the DFE modules are then provided to the HAGR digital signal processing cards. The HAGR can be configured to track up to twelve satellites providing L1 C/A and L1/L2 P(Y) observations when operating in the keyed mode. The digital signal processing is performed in firmware downloaded from the host computer. Since the digital spatial processing is unique for each satellite channel, the weights are optimized for the particular satellites being tracked. The digital architecture allows the weights to be computed in the HAGR software, then downloaded and applied pre-correlation to create a digital adaptive antenna pattern to optimize the signal tracking performance. DFE Module DFE Module