VIEWS: 4 PAGES: 49 POSTED ON: 9/16/2011
Session 7 GPS Orientation ONE Outline of Presentation GPS, how it works & what you need to check. Basic receiver functions – what is it telling you? Selecting a GPS receiver. How does GPS work? GPS is a satellite navigation system developed by the US Dept. of Defense (DOD). GPS provides continuous, all-weather, three-dimensional positioning information (position, velocity and time). GPS is used by aviators, mariners, surveyors, truck and auto operators, and hikers. 5 ft wide 17.5 ft long 1,860 lbs Delta II Confused yet? How GPS works in a few logical steps The basis of GPS operation is „triangulation” from the overhead satellites in view. In order to triangulate, your GPS receiver measures the distance from a satellite using the time of travel of the radio signals from the satellite. In order to measure time of travel, a GPS set needs very accurate time which is achieved with some electronic tricks. Along with the distance, the satellite must know exactly where it is positioned in space at all times. High orbits and careful monitoring are required. All delays that the GPS signal experiences as it travels through the atmosphere must be corrected. ANIMATION Specifications Accuracy GPS 10 meters(33 ft) 95% of time DGPS < 10 meters (33 ft) Availability--99.85% of time Coverage--world wide SUMMARY GPS positions are not error free – User and/or Satellite clock bias. – Satellite signal can be blocked. – Atmospheric interference is possible. – Poor satellite geometry is possible. You can monitor these errors with your GPS receiver. Signal Quality and Accuracy The height of the bars in display shows signal quality from each satellite The number of items show the positions of satellites currently in view to your GPS. Dilution of Precision Dilution of Precision (DOP) is a measure of Good satellite fix Geometry geometry quality Figure at far left has lower (better) DOP Poor Geometry GPS receivers measure and display DOP Estimated Position Error “EPE” Many GPS units display EPE – It uses satellite geometry plus other calculations to improve accuracy of the estimated position error – EPE is expressed in feet or meters – user choice CEP = Circular Error Probable SEP = Spherical Error Probable SA – Selective Availability Error 24 hours 24 hours 5/1/2000 5/2/2000 With 95 % of the time within 45 meters within 6.3 meters or 148 Feet or 21 Feet DGPS (operated by USCG) enables local corrections for errors DGPS Series of coastal stations broadcast corrections to GPS signal. Former RDF stations used for this purpose. Accuracy improved to Note: Stations on Great better than 10 Lakes not shown on this meters. illustration WAAS – Wide Area Augmentation System System of ground-based reference stations and two stationary satellites that broadcast precision differential GPS corrections. Typical GPS with SA OFF) Be sure that the GPS that you purchase has WAAS Less than 10 WAAS capability. feet of error Typical DGPS Other things to consider when buying a GPS set. Intended Use How are you going Imagine your worst case scenario: to adjust – Stormy conditions. or read – Waves beating on the hull. – your GPS set in Boat bouncing back and forth. – Windshield covered with salt spray. – You can‟t conditions? these let go of the helm without losing control of your boat. Mounting Capability Think about the Mount near the helm. You can‟t just lay the GPS Swivel capability so that it can be magnetic influence on the dash. used or read by others. Adjustable for sun glare. that your GPS has your weather. Think offrom theworst case Protected on your compass! scenario. Ability to view satellites. Screen Size Can you read the display at a distance? Avoid sets with tiny numbers and letters. Some sets sacrifice readability for more chart area on their screens. If you can‟t clearly read the screen at an arm‟s length, you won‟t be happy with the set in stormy conditions. RayNav 300 Waterproof versus Submergible Waterproof means that the device will survive a little dampness. Water Resistant means it won‟t last long in a moist, damp marine environment. Submergible means that the device will survive a dunking in the ocean. Display Lighting Buy a back-lighted display so you can view it at night. Color displays are usually easier to read than black and white. Viewable in bright sunlight. Glare resistant screen. 12v Attachment Be sure your set comes with a power cord that will attach to your boat‟s 12V system. Carry spare batteries for backup. Key and Keyboard Size Keys too small for your fingers. Lack of keys. The argument for pre- planning your trips. GPS Age and Processor Size The faster a GPS calculates, the Smaller computing more accurate the data that it capability. generates. Slower to calculate. Most GPS projections are averages of past activity. Less options. Should be regenerating position at No DGPS or WAAS. least once per second. Chart Plotters It is not a safe practice to Nice to have but not really be gazing at a chart plotter necessary. Colored charts are a definite monitor when you should plus. be focusing on the other Paper charts are easier to read in boats and obstructions bright sunlight. around you. RayChart 420 Display Clutter Too much data being presented on a What you eliminate small screen. Have to adjust your screen area in the close-up to remove some data. views, you also lose Such as, depth readings, aids to in the wide-area navigation, etc. display views. Screen Capabilities How easy is it to switch on a GPS Minimize your dependencyfrom one screen. to another? screen Focus handling your boat. Focus onon a few navigation screens for everyday use. Pre-plot your trip. Avoid having to jump a multiple Use a nautical chart and totrip log. screens for basic navigation data. Use your GPS as a quality control check for this process. Remote Antenna Need GPS need a clear view of the satellites. May need a remote antenna on your boat. Remote Multiplexing vs Parallel Receivers Only buy a Multiplexing receivers often have only one channel to tune into satellites. parallel GPS Parallel receivers can acquire several satellites at a time. Come in 3 – 5 – 8 receiver. and 12 channel versions. – More accurate processing – Quicker start ups. GPS Sensor Design NMEA – National Marine Manufacturers Association Standard sensor capability to RADAR, Chart Plotters, Depth Sounders, etc. Not all sensors are compatible to other manufacturer‟s equipment. Check to be certain. Used Sets The price is so low. buy new! The best price for a used set is FREE. GPS Tips GPS looks like a compass GPS acts like a compass Therefore it must be a compass WRONG!!!!!!! Any questions about? AV Task 6-1 Differences between GPS, DGPS and WAAS – page 139.
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