N14A-T009 TITLE: Precise Positioning with Local Signal Carrier Phase Measurements and Global Positioning System (GPS) Fusion

TECHNOLOGY AREAS: Sensors

ACQUISITION PROGRAM: PMA 213

OBJECTIVE: Develop precise positioning techniques based on a local radio system and fusion algorithms combining Global Positioning System (GPS), which are suitable for shipboard landing systems.

DESCRIPTION: Carrier phase measurements in GPS are well known to enable precision performance at centimeter level. In consideration of GPS possibly degraded or unavailable, however, carrier phase measurements in a local radio system of transmitters and receivers are proposed here for precise positioning within Line Of Sight (LOS) distances. Local transmitters and receivers may be found in legacy military communications e.g. LINK16 and/or commercial pseudo-lite products. Also, they may be newly built in the future for dual purpose of positioning and communications.

In maximally re-utilizing GPS technologies (not necessarily GPS-original), it is important to thoroughly understand differences between the local radio system and GPS. First of all, local signals can be much stronger due to the proximity, and result in higher Signal to Noise Ratio (SNR) for receiver tracking circuitries such as Phase Lock Loops (PLLs), Frequency Lock Loops (FLLs) and Delay Lock Loops (DLLs). Consequently, local receivers can produce better carrier phase measurements even in higher dynamics, see Chapter 12 of Reference 1. The measurement update rate can reach 100 Hertz or higher with less cycle slips. On the other hand, local receivers may experience non-planar such as spherical or paraxial wave fronts due to the proximity and directivity from/to transmitters. The proximity is as close as 100 feet or less for landing aircrafts on the ship. The directivity is likely imposed to avoid multi-paths caused by reflection on aircraft, ship structures and sea surfaces. Thus, it remains as a concern that receiver tracking circuitries may experience less optimal or even ill-functional from non-planar wave fronts, even if they are designed optimal for planar wave fronts. Proposals should include clear understanding of fundamental differences, if any, of the local and GPS receiver tracking performances.

In Differential or Relative GPS (DGPS/RGPS), processing carrier phase measurements can be said to resolve integer ambiguities in a larger context of complementary (Kalman) filtering, which is a particular sensor fusion theory itself. The classical processing method is documented in Reference 2, whereas a new approach is published in Reference 4. In either case, GPS is not a sole source of measurements but the aiding sensor to IMU (Inertial Measurement Unit) in achieving precise positioning. In general, any qualified system other than GPS can be an aiding sensor to IMU. Reference 3 explains an optical system as the aiding sensor to IMU. Here, the proposed local radio system is now intended as the aiding sensor to IMU for precise positioning. Proposers should describe how they will approach precise positioning algorithms in the context of complementary filtering or another sensor fusion framework through theoretical analysis, simulations and/or physical experiments. (GPS and IMU are complementary in that GPS and IMU provide measurements at low and high update rates, respectively. The proposed local radio system and IMU are not complementary in the same sense when the local system produces measurements at the rate as high as IMU.) If possible, proposals should describe how to accomplish precise positioning solely based on the proposed local radio system without IMU.

When properly implemented, the proposed local radio system should have multiple advantages over GPS: less sensitivity to multi-paths, less cycle slips, faster complementary (Kalman) filter convergence and higher receiver dynamics. While, the local radio system would have short-comings. A typical case is Dilution of Precision (DOP) due to a limited geometry as the local radio system is installed shipboard.

As the project becomes mature in later stages, the proposed local radio system should be fused with GPS. The contenders should consider successful reports later on that analyze merits from combining the locality and globalism. It is very desirable to eventually embed the developed technologies inside GPS receivers of the next generation.

PHASE I: Determine feasibility of precise positioning techniques based on carrier phase measurements from a local radio system of transmitters and receivers. Assess maximal reuse of GPS receiver tracking circuitries in the local radio system. Develop and simulate basic complementary (Kalman) filtering with the local radio system and IMU. Clearly present fundamental differences, if any, of the local and GPS receiver tracking performances through theoretical analysis, simulations and/or physical experiments.

PHASE II: Refine the basic complementary filtering, or devise a broader fusion algorithm for the local radio system and IMU. Simulate them to be suitable to real-time aircraft control in shipboard landing environments. In comparison to GPS counterparts, evaluate filtering performances in multi-paths, cycle slips, filter convergence, receiver dynamics, DOP and others. Devise fusion architecture/algorithms with existing RGPS algorithms for shipboard landing systems. Verify and validate improvements from stand-alone algorithms to fusion algorithms for combined GPS and the local radio system that likely include IMUs. Through theoretical analysis, simulations and/or physical experiments, clearly present any advantages or short comings between standalone local radio systems and fused local radio systems with GPS.

PHASE III: Develop dual receivers for GPS and the local radio system. Test and evaluate dual receivers in shipboard landing operations. Develop airborne-to-airborne precise positioning and navigation. Explore applications such as for precision strike and electronic warfare.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: The developed local signal carrier phase measurement techniques will result in LOS navigation applications in general that require high dynamics beyond what GPS can handle. Also, the developed duality with GPS in particular will enable more reliable automated unmanned vehicles such as in open field mining operations and airborne highway traffic surveillances.

REFERENCES:
1. Misra, P., & Enge, P. (2010). Global Positioning System: Signals, Measurements, and Performance (Revised 2nd ed.). Lincoln, MA: Ganga-Jamuna Press.

2. Farrell, J.A., Givargis, T.D., & Barth, M.J. (2000). Real-Time Differential Carrier Phase GPS-Aided INS. IEEE Transactions on Control Systems Technology, 8(4), 709-721. doi: 10.1109/87.852915.

3. Fosbury, A.M., & Crassidis, J.L. (2008). Relative Navigation of Air Vehicles. Journal of Guidance, Control, and Dynamics, 31(4), 824-834. doi: 10.2514/1.33698.

4. Brown, A.K., Nguyen, D., Felker, P., Colby, G., & Allen, F. (2012, March 1). Unmanned Air Systems: Precision Navigation for Critical Operations. Retrieved from http://gpsworld.com/defensenavigationunmann ed-air-systems-12705/.