GPS AND THE APPLICATIONS TO AN RV

Why do RVers need GPS? Does GPS make you a better driver?

What is the GPS (Global Positioning System)?

Why is accurate time important?

How do satellites and receivers work?

How is position determined?

What are DGPS, WAAS, & LAAS?

What are LORAN E, GLONAS, and GALILEO ?

What types of mapping systems are available?

How and why do you use the GPS with a mapping system?

Where should the receiver and laptop be mounted in an RV

 

For further information on time.

The GPS system can tell you your position anywhere on or above the Earth to within about 30 feet. Even greater accuracy, usually within less than ten feet, can be obtained with differential corrections. For further information see the military GPS.

For further information on position, navigation, and time and click on Part III, Section 16.

WHY DO RVers NEED GPS?

• An RV driver with "Situational Awareness" is a better driver.
• Know the "view" of the road ahead beyond line of sight day or night.
• Find out where we are relative to a campground, a scenic sight or a friend.
• Determine position to request emergency assistance.
• Determine time to go, at present velocity, to a waypoint.
• Find out how to get there using a mapping system.
• Integrate with a mapping system on a computer.
  
  

WHAT IS THE GPS?

• GPS is a military and civilian Global Positioning System.
• GPS has 3 parts: the space segment, the user segment, and the control segment.
• The space segment consists of 24 to 27 satellites, each in its own orbit about 11,000 nautical miles above the Earth in 12-hour orbits.
• The user segment consists of receivers, which you can take on a hike, mount in your RV or take with you in your tow car, scooter or boat.
• The control segment consists of ground stations (five of them, located around the world) that make sure the satellites are working properly.
• GPS receivers can be integrated with a laptop using mapping information or can have self-contained mapping data.
• GPS can be augmented by WAAS (Wide Area Augmentation System), LAAS (Local Area Augmentation System), and/or DGPS (Differential GPS).
  
  

WHY IS TIME IMPORTANT?

• The speed of light equals about 161,870 nautical miles per second.
• Each of the 24 plus spare GPS satellites are about 11,000 nautical miles above the earth and take about 12 hours to complete their medium earth orbit (MEO).
• Anywhere on earth, at least six satellites should be in view.

WHAT IS THE RELATIONSHIP OF POSITION & TIME?

• A minute of Longitude at the Earth's Equator = a nautical mile =6,080 feet.
• A minute of Longitude converges (shortens) as Latitude increases from Equator.
• A minute of Latitude (north or south) on Earth = a nautical mile =6,080 feet.
• By converting minutes of Longitude to time in seconds,

  360 degrees X 60 minutes = 24 hours X 60 min X 60 seconds.

  21,600 minutes of Longitude = 86,400 seconds. Therefore,

One minute of Longitude = 4 seconds and One foot of Longitude distance = 0.00066 seconds at the Equator or a foot of Longitude distance = 0.00066 seconds X cosine of Latitude. Therefore,

• Accurate TIME is very important for position.
• The signal takes "about" 0.068 seconds to reach a receiver on the ground.
• One millionth of a second error could result in a position error of about 1,000 feet.
  

THEREFORE,

• The accuracy of the very precise atomic clocks in each satellite is about 3 times 109 or 3 billionths of a second.
• This precision timing is important because the receiver must determine exactly how long it takes for signals to travel from each GPS satellite.
• The receiver uses this information to calculate its position.
• Knowing a previous position and the current position and the time difference, the receiver computes velocity.
  

GPS is the first worldwide utility. Without accurate worldwide GPS TIME, the Internet and cellular telephone systems of the world will not work. Accurate time provides synchrosity for telecommunication and encryption systems.

HOW THE GPS SATELLITES WORK

• Radio signals radiate outward from a satellite in the shape of an expanding sphere.
• If we know our exact distance from a satellite in space (about 11,000 miles), we know we are somewhere on the surface of this sphere whose radius equals the distance to the satellite's position in orbit.
• If we know our exact distance from two satellites, we know that we are located somewhere on the circular line where the two spheres intersect.
• If we take a measurement from a third satellite, there are only two possible points where the three spheres intersect and where we could be located. One of these is usually impossible, and the GPS receivers have mathematical methods of eliminating the impossible location especially if it has determined its position on earth previously.
• A forth satellite reduces the ambiguity to only a single solution-our position.
• Each satellite is equipped with two accurate clocks to let it broadcast precise time and its respective position signals relative to the earth.
  

• The standard model of the earth used by GPS is WGS-84.
• Our position, relative to the positions of the "best" four satellites relative to the earth, is computed every 0.1 seconds in Latitude, Longitude and altitude.
• Waypoints of present position can be saved.
• Waypoints of any position can be inserted manually or from mapping systems.
• Maps can be accessed when present position is known.
• Waypoints can be grouped in routes manually or from mapping systems.
• Present positions can be retained as tracks for backtracking.
• Waypoints, routes and tracks can be transmitted to mapping data.
• Present velocity, distance, bearing & time to go between waypoints are computed.
• GPS receivers can be battery operated or remotely powered.
• Some GPS receivers have magnetic compasses and barometric altimeters.
• Some GPS receivers have remote antenna for fixed installation.
  

HOW POSITION IS DETERMINED

• The ground unit receives the satellite signal, which travels at speed of light.
• The signal takes a measurable amount of time to reach the receiver.
• The difference between the time the signal is sent and the time it is received, multiplied by the speed of light, enables the receiver to calculate the distance to the satellite.
• To measure the precise Latitude, Longitude, and altitude, the receiver measures the time for the signals from the four separate satellites to get to the receiver.
  

TYPICAL GPS ERROR BUDGET (meters)

WHAT ARE DGPS, WAAS & LASS?

• DGPS (Differential GPS) uses the signal from a GPS receiver located at a known position to determine the difference or local error. The difference is then transmitted on a different radio signal to other compatible receivers to apply the correction. DGPS signals are available in many places of the world. The US Coast Guard provides many DGPS sites in the US. DGPS receivers are available
  
  
• The Wide Area Augmentation System (WAAS) is an FAA augmentation system that uses Geosynchronous (stationary relative to the spin axis of earth) Earth Orbiting Satellites (GEOS) to broadcast navigation corrections over large segments of the earth. Two WAAS signals provide coverage for CONUS. Other non US GEOS signals are available for augmentation in Europe and Asia. Some GPS receivers are compatible with WAAS.
  
  
• The Local Area Augmentation System (LAAS) is an FAA augmentation system using pseudolites (ground-based satellites) to provide navigation corrections for local areas such as airports. This system is not yet available for use.
  
  
• WAAS and LAAS are designed to provide the necessary redundancy and accuracy for civilian and military safety of flight in the US airspace when/if failures occur in the space or control segments of GPS. There are not enough GPS satellites in view for autonomous safe flight.
  
  
• WAAS and LAAS can also provide augmentation when insufficient "satellite in view" situations occur due to "canyon effect", jamming or other masking of satellite signals.

WHAT ARE LORAN E, GLONAS, AND GALILEO ?

• LORAN C is a mid 1960's ground based system of US, European and Asian transmitters that provide timing and ranging signals for positioning and time transfer. Although not a global navigation system, this system does provide positional and timing information to a large percentage of where people live on the earth. Because of system wide improvements, LORAN C is now being called LORAN E.
  
  
• Hybrid receivers are available to receive signals from GPS and LORAN C/E to improve the accuracy and redundancy of the positional and timing information. LORAN C/E may be a viable backup for accurate time transfer for telecommunication.
  
  
• There are several Global Navigation Satellite Systems (GNSS). Presently, GNSS include the US's GPS, the Russian GLONAS and the European Union's GALILEO System.
  
  
• GLONAS has about 10 satellites operational. Hybrid receivers are available to receive signals from both GPS and GLONAS satellites to improve the accuracy and redundancy of the positional information.
  
  
• GALILEO is in the design stage and may or may not be compatible with GPS.
  
  
  

• Regional, national and worldwide mapping systems.
• Two dimensional (Latitude and Longitude) systems.
• Three dimensional (altitude, Latitude and Longitude) systems.
• Vehicle and boating systems.
• Those that are integrated in GPS receivers.
• Those that are used in laptops/computers.
• Those that can be downloaded into GPS receivers.
• Those that can be integrated into a hybrid system with a GPS receiver.
• Mapping systems where you can make your own map.
  

Make your choice and pay your money for what you think you "need".

HOW AND WHY DO YOU USE THE GPS WITH A MAPPING SYSTEM?

In most cases the display of any mapping information on a handheld GPS is too small to adequately see in a moving RV. A laptop computer is, therefore, the major interface with the RV driver. The laptop provides the display and computation capability needed to combine the positioning data from the GPS with the mapping system data on a CD. A cable from the GPS receiver provides the signals to the laptop.

The display of the laptop provides the "situational awareness" to the driver of the RV. The stress level of the driver is reduced by his knowledge of what the road "looks like" beyond his normal line of sight day or night.

WHERE SHOULD THE RECEIVER AND LAPTOP BE MOUNTED IN AN RV?

• Several GPS receivers have the capability of using remotely installed antenna. If the RV has a non-metal roof, the antenna should be mounted inside the RV. The signal loss through the fiberglass roof is negligible.
• GPS receivers with built in antenna should be mounted in the RV with as much unobstructed vertical clearance. Installation under the TV, stereo or any other electrical devices is not advised.
• Easy access to the GPS receiver by the driver or copilot should be considered.
• The laptop should be in the scan view of the driver as he/she glances from left to right mirror views
• No liability is assumed by the above recommendations
  

Date revised: 4/01/04