GPS Info

What is GPS?
The Global Positioning System (GPS) is a satellite-based navigation system made up of a network of 24 satellites placed into orbit by the U.S. Department of Defense. GPS was originally intended for military applications, but in the 1980s, the government made the system available for civilian use. GPS works in any weather conditions, anywhere in the world, 24 hours a day. There are no subscription fees or setup charges to use GPS.

How it works
GPS satellites circle the earth twice a day in a very precise orbit and transmit signal information to earth. GPS receivers take this information and use triangulation to calculate the user's exact location.

Essentially, the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference tells the GPS receiver how far away the satellite is. Now, with distance measurements from a few more satellites, the receiver can determine the user's position and display it on the unit's electronic map.

A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D position (latitude and longitude) and track movement. With four or more satellites in view, the receiver can determine the user's 3D position (latitude, longitude and altitude). Once the user's position has been determined, the GPS unit can calculate other information, such as speed, bearing, track, trip distance, distance to destination, sunrise and sunset time and more.

The GPS satellite system
The 24 satellites that make up the GPS space segment are orbiting the earth about 12,000 miles above us. They are constantly moving, making two complete orbits in less than 24 hours. These satellites are travelling at speeds of roughly 7,000 miles an hour.

GPS satellites are powered by solar energy. They have backup batteries onboard to keep them running in the event of a solar eclipse, when there's no solar power. Small rocket boosters on each satellite keep them flying in the correct path.

Here are some other interesting facts about the GPS satellites (also called NAVSTAR, the official U.S. Department of Defense name for GPS):
  
   The first GPS satellite was launched in 1978.
  
    A full constellation of 24 satellites was achieved in 1994. 
 
   Each satellite is built to last about 10 years. Replacements are constantly being built and launched into orbit.

   A GPS satellite weighs approximately 2,000 pounds and is about 17 feet across with the solar panels extended.

   Transmitter power is only 50 watts or less.

Sources of GPS signal errors
Factors that can degrade the GPS signal and thus affect accuracy include the following:

Ionosphere and troposphere delays — The satellite signal slows as it passes through the atmosphere. The GPS system uses a built-in model that calculates an average amount of delay to partially correct for this type of error.

Signal multipath — This occurs when the GPS signal is reflected off objects such as tall buildings or large rock surfaces before it reaches the receiver. This increases the travel time of the signal, thereby causing errors.

Receiver clock errors — A receiver's built-in clock is not as accurate as the atomic clocks onboard the GPS satellites. Therefore, it may have very slight timing errors.

Orbital errors — Also known as ephemeris errors, these are inaccuracies of the satellite's reported location.

Number of satellites visible — The more satellites a GPS receiver can "see," the better the accuracy. Buildings, terrain, electronic interference, or sometimes even dense foliage can block signal reception, causing position errors or possibly no position reading at all. GPS units typically will not work indoors, underwater or underground.

Satellite geometry/shading — This refers to the relative position of the satellites at any given time. Ideal satellite geometry exists when the satellites are located at wide angles relative to each other. Poor geometry results when the satellites are located in a line or in a tight grouping.

Intentional degradation of the satellite signal — Selective Availability (SA) is an intentional degradation of the signal once imposed by the U.S. Department of Defense. SA was intended to prevent military adversaries from using the highly accurate GPS signals. The government turned off SA in May 2000, which significantly improved the accuracy of civilian GPS receivers.

Source: www.garmin.com

You've heard the term WAAS and maybe even know it stands for Wide Area Augmentation System. Okay, so what the heck is it? Basically, it's a system of satellites and ground stations that provide GPS signal corrections, giving you even better position accuracy. How much better? Try an average of up to five times better. A WAAS-capable receiver can give you a position accuracy of better than three meters 95 percent of the time. And you don't have to purchase additional receiving equipment or pay service fees to utilize WAAS.

The origins of WAAS 
The Federal Aviation Administration (FAA) and the Department of Transportation (DOT) are developing the WAAS program for use in precision flight approaches. Currently, GPS alone does not meet the FAA's navigation requirements for accuracy, integrity, and availability. WAAS corrects for GPS signal errors caused by ionospheric disturbances, timing, and satellite orbit errors, and it provides vital integrity information regarding the health of each GPS satellite.

How it Works
WAAS consists of approximately 25 ground reference stations positioned across the United States that monitor GPS satellite data. Two master stations, located on either coast, collect data from the reference stations and create a GPS correction message. This correction accounts for GPS satellite orbit and clock drift plus signal delays caused by the atmosphere and ionosphere. The corrected differential message is then broadcast through one of two geostationary satellites, or satellites with a fixed position over the equator. The information is compatible with the basic GPS signal structure, which means any WAAS-enabled GPS receiver can read the signal.

Who benefits from WAAS?
Currently, WAAS satellite coverage is only available in North America. There are no ground reference stations in South America, so even though GPS users there can receive WAAS, the signal has not been corrected and thus would not improve the accuracy of their unit. For some users in the U.S., the position of the satellites over the equator makes it difficult to receive the signals when trees or mountains obstruct the view of the horizon. WAAS signal reception is ideal for open land and marine applications. WAAS provides extended coverage both inland and offshore compared to the land-based DGPS (differential GPS) system. Another benefit of WAAS is that it does not require additional receiving equipment, while DGPS does.


Other governments are developing similar satellite-based differential systems. In Asia, it's the Japanese Multi-Functional Satellite Augmentation System (MSAS), while Europe has the Euro Geostationary Navigation Overlay Service (EGNOS). Eventually, GPS users around the world will have access to precise position data using these and other compatible systems.

It just keeps getting better

What is e-dif?
e-Dif is a differential technology that offers an alternative to subscription-based differential sources like OmniStar.  Once your Satloc SLXg or SLXg3 receiver is programmed to utilize e-Dif, there are no more fees or subscriptions.  Its use is completely free.

e-Dif has two requirements in order to be effective:
1. you must have 4 GPS satellites in view for the duration of e-Dif use
2. you must have an e-Dif enabled SLXg or SLXg3 receiver with an authorization code

e-Dif works by using GPS data gathered from a minimum of four satellites.  This data is gathered over a period of 5 to 8 minutes (the longer the data gathering period, the more accuracy you can achieve), either by sitting still on the ground or in straight and level flight.  This data is then modeled inside the SLXg or SLXg3 receiver which can then apply corrections based on your location.  Once the e-Dif data is gathered, it will continually adjust to match changing conditions reflected in GPS signals that you receive.

The initial accuracy of the e-Dif differential is very high, however it will degrade over time at a rate of approximately 1 to 2 meters per hour.  

e-Dif is accurate to between 1 and 4 meters.  OmniStar is accurate to less than one meter.  So, while e-Dif is ideal for some sprayers, where accuracy is of the utmost importance, users should still consider using OmniStar or WAAS as their primary source of differential.

Satloc's new SLXg3 receiver is capable of using two differential sources.  For instance, a user can switch between OmniStar or e-Dif on the fly with new software from Satloc. 

Any Satloc guidance product is capable of using e-Dif, provided it is being run on an SLXg or SLXg3 receiver. 

If you want to know if your receiver is capable of running e-Dif, contact us at GPS Yorkton.
For more information on e-Dif, please contact GPS Yorkton.

Basic Maintenance Tips for Your GPS System

With the spray season winding down, and the pace of life returning to normal, its time to consider how to maintain and store your precision guidance system over the winter months.

The GPS components in your guidance system (the lightbar, receiver, CPU, antenna, etc.) are highly susceptible to damage from freezing.  It is imperative that you remove these systems from your spray equipment or airplane, and store them inside a heated environment.

Data cards too, are very prone to damage from cold temperatures, and heat as well.  Freezing data cards may cause them to lose the data stored on them, leaving you with no operating software or data logs.

Dust and moisture are also detrimental to your precision guidance system.  When removing your system for storage, its a good time to check over the components to ensure they are sealed tightly and that they are clean and free of dust and moisture.  If the gaskets need replacing, make sure to send your unit to an authorized Level II center to perform the repairs to avoid voiding your warranty.

The antenna should be cleaned as well, with a non-abrasive pad and a little soap and water.  Be careful to avoid getting water inside the antenna.  Clean antenna housings will improve your GPS signal reception.

Now is also a good time to take the logs off of your data cards and store them on a more permanent basis, such as writing them to CD's for later reference.  CD's provide great storage because they are reliable, easy to archive, and easy to search for data retrieval. 

For more information on storing your GPS equipment, click here.