GPS World, March 2010

Algorithms Methods INNOVATION primary observations will be the carrier phase measurements as code pseudorange observations cannot provide the required accuracy High accuracy GPS positioning needs to address the issue of carrier phase ambiguity The ambiguity can be treated as an unknown in the Kalman filter but it may take several minutes to resolve the ambiguity using GPS alone Using certain ambiguity resolution techniques however the ambiguity can be resolved outside the main filter in the GPS INS high precision carrier phase integration filter Note that if the ambiguity were to be resolved within the filter this would increase the number of states of the filter For the GPS component ionospheric delay should be included for applications that cover a large area Ionospheric delay can be resolved using network based differential techniques but it will affect the ambiguity resolution for single baseline differential positioning if it is not included in the local solution The filter is designed either to use or not to use ionospheric delay which can ensure flexibility to accommodate network based and single baseline differential positioning As mentioned above the measurement model in the tightly coupled model is based on the raw observations For GPS and Locata the observations will be the carrier phase observations The approximate values for the linearization of the GPS and Locata measurement equations are provided by the INS navigation solution The GPS carrier phase ambiguity is solved independently outside the Kalman filter with OTF techniques The GPS differential positioning coefficient matrix remains the same regardless of whether or not a network based differential technique is used For velocity determination the double differenced Doppler observation is used to eliminate the clock error rate as an unknown because it is difficult to model this in the filter The initial carrier phase bias of the Locata is also not included in the filter because it can be resolved instantaneously with dual frequency data in the Locata second generation system The implementation of the filter will be flexible so adjustments can be made to account for actual environmental conditions The filter is designed with an open interface and is modular in structure so that components can be added or removed from the model In open sky areas GPS is sufficient for system positioning so only its observations need to be processed In moderately obstructed environments GPS and Locata observations will be processed In this case the number of GPS observation equations is limited and sometimes will be less than four FIGURE 1 illustrates the flowchart of the triple integration of GPS INS and Locata Field Tests For experimental purposes we used a dual INS based on a navigation grade unit and a tactical grade unit In addition a Locata receiver and a dualfrequency GPS receiver were placed on a vehicle at Locatas Numeralla Test Facility NTF near Canberra Australia This test site features both open sky and obscured environments allowing for testing the systems performance under truly 0050 0045 0040 0035 0030 0025 0020 0015 0010 0005 0 East North Up 0 500 1000 1500 2000 2500 3000 3500 FIGURE 5 The standard deviation of position in the test East North Up Time seconds 0 500 1000 1500 2000 2500 3000 3500 006 005 004 003 002 001 0 Position standard deviation meters Velocity standard deviation meters per second FIGURE 6 The standard deviation of velocity in the test 050 045 040 035 030 025 020 015 010 005 0 Attitude standard deviation degrees 0 500 1000 1500 2000 2500 3000 3500 challenging scenarios The test was repeated by mounting the devices on an autonomous electrical car driven on the UNSW campus FIGURE 7 The standard deviation of attitude in the test In both cases the separation between the rover and the terrestrial transmitters was between a few tens of meters to several kilometers The GPS and Locata data were processed separately for testing the internal consistency as well in a hybrid solution resulting in few centimeter level accuracy per coordinate depending primarily on GPS availability and the geometry between the rover and Locata devices as well as the level of multipath fading Test 1 NTF The first integration test was conducted at the NTF on March 17 2008 The NTF covers an area of approximately three hundred acres 25 kilometers 06 kilometers and is ideally suited to real world system testing over a wide area At the NTF a number of LocataNet configurations are possible through the installation Time seconds Pitch Roll Yaw Time seconds of permanent antenna towers The network configuration used for this experiment is illustrated in FIGURE 2 Before the test a special mounting platform was designed and built The platform shown in FIGURE 3 consists of a two level metal frame The bottom level can accommodate two inertial measurement units while the top level can hold up to four antennas The platform can be easily attached to either the roof of the NTF test vehicle or to the body of UNSWs small electric car described later The devices used in the test include two dual frequency GPS receivers one used as the rover receiver and the other as the base station one navigation grade INS and one Locata rover unit The GPS antenna and the Locata antenna were mounted with the INS together on the top of a truck www gpsworld com March 2010 GPS World 45