Session 7 GPS Orientation ONE

Outline of Presentation F GPS, how it works & what you need to check. F Basic receiver functions – what is it telling you? F Selecting a GPS receiver.

How does GPS work? F GPS is a satellite navigation system developed by the US Dept. of Defense (DOD). F GPS provides continuous, all-weather, three-dimensional positioning information (position, velocity and time). F 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 F GPS 10 meters(33 ft) 95% of time F DGPS < 10 meters (33 ft) F Availability % of time F Coverage--world wide

SUMMARY F 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 F The height of the bars in display shows signal quality from each satellite F The number of items show the positions of satellites currently in view to your GPS.

Dilution of Precision F Dilution of Precision (DOP) is a measure of satellite fix geometry quality F Figure at far left has lower (better) DOP F GPS receivers measure and display DOP Good Geometry Poor Geometry

Estimated Position Error “EPE” F 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

With 95 % of the time within 45 meters within 6.3 meters or 148 Feet or 21 Feet 24 hours 5/1/ hours 5/2/2000

DGPS (operated by USCG) enables local corrections for errors

DGPS F Series of coastal stations broadcast corrections to GPS signal. F Former RDF stations used for this purpose. F Accuracy improved to better than 10 meters. Note: Stations on Great Lakes not shown on this illustration

WAAS – Wide Area Augmentation System System of ground-based reference stations and two stationary satellites that broadcast precision differential GPS corrections.

Typical DGPS WAAS Typical GPS with SA OFF) Less than 10 feet of error Be sure that the GPS that you purchase has WAAS capability.

Other things to consider when buying a GPS set.

Intended Use F Imagine your worst case scenario: –Stormy conditions. –Waves beating on the hull. –Boat bouncing back and forth. –Windshield covered with salt spray. –You can’t let go of the helm without losing control of your boat. How are you going to adjust or read your GPS set in these conditions?

 Mount near the helm.  Swivel capability so that it can be used or read by others.  Adjustable for sun glare.  Protected from the weather.  Ability to view satellites. Mounting Capability You can’t just lay the GPS on the dash. Think of your worst case scenario. Think about the magnetic influence that your GPS has on your compass!

Screen Size F Can you read the display at a distance? F Avoid sets with tiny numbers and letters. F Some sets sacrifice readability for more chart area on their screens. F 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 Waterproof means that the device will survive a little dampness. Water Resistant Water Resistant means it won’t last long in a moist, damp marine environment. Submergible 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 F Be sure your set comes with a power cord that will attach to your boat’s 12V system. F Carry spare batteries for backup.

Key and Keyboard Size F Keys too small for your fingers. F Lack of keys. F The argument for pre- planning your trips.

GPS Age and Processor Size F Smaller computing capability. F Slower to calculate. F Less options. F No DGPS or WAAS. The faster a GPS calculates, the more accurate the data that it generates. Most GPS projections are averages of past activity. Should be regenerating position at least once per second.

Chart Plotters F Nice to have but not really necessary. F Colored charts are a definite plus. F Paper charts are easier to read in bright sunlight. It is not a safe practice to be gazing at a chart plotter monitor when you should be focusing on the other boats and obstructions around you.

RayChart 420

Display Clutter F Too much data being presented on a small screen. F Have to adjust your screen to remove some data. F Such as, depth readings, aids to navigation, etc. What you eliminate in the close-up area views, you also lose in the wide-area display views.

Screen Capabilities F How easy is it to switch from one screen to another? F Focus on a few navigation screens for everyday use. F Avoid having to jump to multiple screens for basic navigation data. Minimize your dependency on a GPS screen. Focus on handling your boat. Pre-plot your trip. Use a nautical chart and a trip log. Use your GPS as a quality control check for this process.

Remote Antenna Need F GPS need a clear view of the satellites. F May need a remote antenna on your boat.

Remote

Multiplexing vs Parallel Receivers F Multiplexing receivers often have only one channel to tune into satellites. F Parallel receivers can acquire several satellites at a time. Come in 3 – 5 – 8 and 12 channel versions. –More accurate processing –Quicker start ups. Only buy a parallel GPS receiver.

GPS Sensor Design F NMEA – F NMEA – National Marine Manufacturers Association F Standard sensor capability to RADAR, Chart Plotters, Depth Sounders, etc. F Not all sensors are compatible to other manufacturer’s equipment. F Check to be certain.

Used Sets buy new! The price is so low. buy new! FREE. The best price for a used set is FREE.

GPS Tips F GPS looks like a compass F GPS acts like a compass F Therefore it must be a compass WRONG!!!!!!! WRONG!!!!!!!

Any questions about? AV Task 6-1 Differences between GPS, DGPS and WAAS – page 139.