Lesson 14: Advanced Navigation Systems

Lesson 20: Advanced Navigation Systems AGENDA: NAVSTAR Global Positioning System (GPS) Inertial Navigation Systems (INS) Bottom Contour Navigation Electronic Charts (Raster & Vector) Navigation Sensor System Interface (NAVSSI) Applicable reading: Hobbs pp. 540-555.

NAVSTAR Global Positioning System (GPS) GPS = Global Positioning System all-weather, jam resistant, continuous operation, real-time, passive, worldwide radio navigation system. Provides: Extremely accurate 3D position data Extremely accurate velocity data Precise timing services 3 LOPs provide a Lat. & Long 4 LOPs provide Lat., Long. & Altitude Standard Position Service (SPS) Position: 76 m SEP 40 m CEP Velocity: .5 m/sec Time: 1 milli-second Precise Position Service (PPS) - Military Position: 16 m SEP 12 m CEP (about 40 ft) Velocity: .1 m/sec Time: 100 nano-seconds

The key to GPS accuracy is time The key to GPS accuracy is time. Time determines the user’s position and distance relative to each satellite. The better the satellite’s reported time and position, the more accurate the signal from space. The “average” GPS receiver derives it’s time and position based on four satellite signals. This process is called triangulation. The receiver passively acquires four GPS signals which contain basic time and position data. By subtracting the time the signal was sent from the time the signal was received, a GPS receiver can calculate its distance from that satellite. Thus, the receiver determines a location arc for each satellite and the intersection of these arcs identifies the user’s GPS location. As you can see in the drawing, each color represents an arc from a particular satellite. The intersection of these arcs is centered on the receiver.

NAVSTAR GPS Uses Civilian Uses Marine Navigation Law Enforcement Hydrographic surveying Search and Rescue Collision avoidance Military Uses Rendezvous Close Air Support Mine Warfare Unmanned Aerial Vehicles (UAV/s) WRN-6

WRN-6

Differential GPS (DGPS) A receiving station located at a fixed, known location receives position data from several GPS satellites. The data obtained from GPS is compared to the known location of the station. Any difference between these two is due to GPS error. This difference the applied to the individual GPS receiver and thus, increased positional accuracy is obtained Differential GPS is available to users possessing a specially modified receiver set to access the signal. It can be used in conjunction with SPS or PPS. DGPS significantly improves accuracy because positioning is computed against a pre-surveyed ground station which transmits navigation corrections to users. These ground stations send the DGPS signal locally (Local-Area DGPS) or to a geosynchronous satellite (Wide-Area DGPS) that relays the signal to DGPS users.

NAVSTAR GPS Accuracy Standard Position Service (SPS) Position: 100 m Hor. 150 m Vert. Velocity: .5 m/sec Time: 1 milli- second Precise Position Service (PPS) - Military Position: 16 m Hor. 25 m Vert. Velocity: .1 m/sec Time: 100 nano- seconds

For simplicity, this chart depicts a Local-Area DGPS scenario For simplicity, this chart depicts a Local-Area DGPS scenario. The GPS satellite transmits the normal SPS (black line) and PPS signals (blue line). The red line represents the Local-Area DGPS being transmitted by pre-surveyed ground station. Wide-Area DGPS works the same way except the ground station transmits the DGPS signal to geosynchronous satellite for broader dissemination. Local-Area DGPS can obtain 1 meter accuracy due to close proximity of the ground station where Wide-Area DGPS provides approximately 5 meter accuracy.

Inertial Navigation System (INS) Inertial Navigation: the process of measuring the movements of a vessel based on sensed accelerations in known spatial directions. Gyroscopes Accelerometers Electronic computers

Inertial Navigation Systems (cont) Integrating acceleration gives you velocity Integrating velocity gives you position/distance traveled Need to compare to fix, since output is an EP! Can go up to 30 days w/out update, in theory Typically go no longer than 7-14 days Types SINS ESGN RLGN

Inertial Navigation Systems Current position is inputted Xo Spinning gyro Known mass Accelerometer F=m*a Through Differentiation we get velocity and position (V and Xf) Computer

Bottom Contour Navigation Establishes position by using the geographic features of the ocean floor. An echo sounder (fathometer) is used to produce a trace of the ocean floor beneath the vessel, which can be compared to a bottom contour chart to establish the ship’s position. Can be used as a fix source, but is only accurate when large ocean floor gradients exist

Bottom Contour Navigation 2 Techniques: Line-of-Soundings (page 567 in Hobbs) Contour Advancement (page 568 in Hobbs)

Bottom Contour Navigation Advantages no satelites required subs=> no need to go to Periscope Depth (PD) not vulnerable Disadvantages not very accurate requires a cooperative sea bottom

Electronic Charts Raster Chart Display Systems (RCDS) Electronic Chart Display Systems (ECDS) Not approved for Fleet use, but getting closer

Navigation Sensor System Interface (NAVSSI) Provides/Distributes NAV data (precise position, time, velocity, pitch-roll-yaw) to multiple users. NAVSSI has been successfully installed, interfaced, and tested with WSN-5, WRN-6, EM Log, Tomahawk, Outboard, and NTCS-A. A similar system may be integrated into your ship’s Combat Control System (CCS)

Summary/Review How does GPS work? What is ephemeris and almanac data? Name 3 uses of GPS. How does differential GPS work? How do Inertial Navigation Systems work?

QUESTIONS? Break out charts and tools and pick pt and measure distances……