# Difference between revisions of "Group12"

## Executive Summary

This page provides a brief history and introduction to GPS (Global Positioning System) and the science behind it's ability to calculate various parameters on the earths surface.

Contents:

1. Introduction:

GPS Satellite Constellation
• History
• Background
• Data transfer and calculation methods
• Accuracy and time keeping

2. Before Disassemble:

• Components and Features

3. Disassembly Procedure:

• Disassembly steps and descriptions

• Parts and Functions

5. Personal Experience:

• Location
• Procedure
• Data
• Accuracy
• Recommendations

6. Conclusions

## Introduction

The GPS (Global Positioning System) was created on June, 26 1993 with the addition of the 24th satellite launched into space. The GPS system was originally developed by the American Department of Defense (DOD); however, has since been made available to the general public for many uses.

The GPS system is a network of 24 satellites that transmit high-frequency, low-power radio signals to a GPS receiver that allows the receiver to determine information such as its location, elevation, velocity and time. GPS receivers rely on a clear view of the sky for accurate data. They are best used while outdoors and away from tall buildings and heavily wooded areas.

Example of 2D position determination with 3 satellites

The transfer of data occurs between the GPS receiver and at least four other satellites, which each orbit the earth twice a day in a specific orbit while transmitting signals to earth, for accuracy. The location of the GPS device is found by transmission of radio waves sent to four satellites simultaneously at the speed of the light. The small differences in the time it takes for the signals to reach the satellites are used to calculate the distance from the satellite and its three dimensional location. This three dimensional location is found with a set of imaginary spheres of know area located around the center of each satellite. Three spheres are required for two dimensions and four spheres for three dimensions. The common intersection of the three spheres gives the location of the GPS unit. This method of determining the location of the GPS unit based on spheres of known area is called trilateration. Due to the fact that the GPS receivers do not utilize atomic clocks a great deal of inaccuracy of the exact location is existent. Errors in the internal clock of the GPS require that the spheres be adjusted each in an equal amount until the intersection becomes defined. This method often gives inaccuracies of 10 to 20 meters.

The calculated distance from the satellite to the GPS begins with the transmission of a long pseudo-random code sent from the satellite to the GPS. At the same time the GPS also runs the same code sending it to the satellite. The signal sent from the satellite will lag behind the transmitted signal sent from the GPS to the satellite. This lag in time is equivalent to the travel time of the signal and is multiplied by the speed of light to determine the distance the signal has traveled.

The timing of these radio signals from satellite to receiver and vise versa are only accurate if the clocks within the satellite and receiver are synchronized to the nanosecond. Each satellite uses an atomic clock to keep time; however, due to the price of atomic clocks the GPS receiver utilizes a conventional quartz type clock which is less accurate than an atomic clock. The GPS receiver remains synchronized with the satellite by analyzing the incoming signals from four satellites to determine its own inaccuracies. The correct time to use will exist only from the atomic clocks installed within the satellites. Since only one correct time exists the signals will align to a single point in space for the receiver to obtain. The GPS receiver will then set its time to the signals received from the satellites. The GPS receivers will continuously reset the quartz clock time to the atomic time from the satellite as long as the receiver is on.

The resulting distance calculated from the accurate time keeping between the satellites and GPS are used in conjunction with the actual location of the satellites. In order for the GPS receiver to determine the location of the satellites it utilizes an installed almanac which tells the receiver exactly were the satellite is in orbit at any given time. This information is easily obtained because the satellites travel in predictable orbits. The known locations of the 24 satellites are located in six orbital planes at an altitude of 20,200 Km.

## Before Disassembly

Eartmate PN-20 Specification and features

The PN-20 comes with the several components and features that make it an all-in-one device indeed. The PN-20 includes a DeLorme Topo USA 7.0 DVD software with full, updated U.S. topographic and street maps that can be exported as needed to the device. The PN-20 also has on-device highway-level world base map, as well as discs that contain pre-cut map packages of the entire United States that can be transferred onto the SD card for insertion directly into the PN-20. Other components of the PN-20 are 1-GB SD Card & Reader, USB interface cable, baterries, and a comprehensive user's manual with usage scenario tips.

The PN-20 is waterproof standard, has impact-resistant rubberized housing, a bright color screen,measures 2.43″ W x 5.25″ H x 1.5″ D, and weighs 5.12 ounces. It has buttons that provides the user with access to all of the functionality on the device. It features a STMicroelectronics chipset with SiGE RF front-end and DeLorme firmware for outstanding signal acquisition and retention and is said to work equally well under dense foliage or in-vehicle. It also has 75MB of internal flash memory in addition to the preloaded world base map and can provide multiple views of the same GPS location.

Basic Functions of theEarthmate GPS PN-20 Device
Button Function
Power

The POWER button is used to turn the Earthmate on and off, to change the backlight settings, or to reset the device. It is located at the bottom-right corner of the button area.

In/Out

The IN and OUT button allows the user to zoom in or zoom out on a map.

Page

The PAGE button allows the user scroll through all of the enabled “Pages.”

The MENU button allows the user to navigate through a multitude of functionalities available in the PN-20.

Find

The FIND button, labeled with a magnifying glass, allows the user to search for a waypoint, address, coordinate, and etc, based on its name or its proximity to the current map center.

Mark Waypoint

The Mark button, labeled with a push pin, is used to mark a waypoint at one's current GPS location.

The ARROW keypad enables the user to move the map cursor left, right, up, or down, pan the map by moving the cursor on the edge of the map, highlight options in menus or lists, and highlight characters in the keyboard screen.

Enter

The ENTER button is used to select a menu entry or on-screen button/field and to get information about a point on the map.

Quit

The QUIT button is used to cancel any action or reverse the page sequence.

## Disassembly, Assembly, and Component Description

The disassembly process was pretty easy overall. We were able to open the device using a small "phillips" screwdriver. The only minor difficulty was handling the screws, because they are really tiny and can be lost if care is not taken. Also, we needed to pay close attention to the rubber joint when opening or closing the device in order not to put it off track as putting it off track could affect the impermeability negatively and therefore quality of the device. Below is the procedure we went through to take the device apart, explore the different hardware components, and to put it safely back together.

Step Description Picture
1

The back cover was taken off by loosening the screws on the back of the device that hold the cover in place. This process simply required the twisting of the screws by hand. The back cover, which is made of both plastic and rubber, protects the batteries as well as the SD card. The picture shows the batteries' placement. The PN-20 works on two AA batteries or R. CR-V3 Li-Ion approved rechargeable batteries manufactured by Moyoto Industrial Co, Ltd. AA Energizer batteries can last for a maximum of about 14 hours in this device.

.
2

Next, the batteries were taken out and the SD card slot lifted up. The picture shows the 1GB SD card partially inserted into the slot. Pre-cut map packages can be transferred onto the SD card for insertion directly into the PN-20. The the memory size of the SD card enables is to accommodate large files including the Aerial Data Packets and topographic maps.

3 These pictures show the USB attachment and the slot on the back of the PN-20 into which it fits. The USB attachment enables the PN-20 to be hooked up a computer allowing you to create, save, and transfer detailed GPS-accurate information between the unit and the computer. You are able to prepare maps and imagery on the desktop, including waypoints, tracks, and automatically generated routes and then transfer this information to the PN-20 via the USB connection. You are also able, once you’ve been in the field or in-vehicle with the PN-20, to transfer your new and updated waypoints and track logs back to the Topo USA desktop software,edit, and save these files as needed.
4 In this step the plastic back cover was unscrewed using a small screwdriver. A total of ten small screws held the back cover in place. The back cover protects and helps to keep the circuit board and the other internal components in place. A rubber washer that is located in-between the front and back covers ensures that the device is impermeable to water and other fluids that will damage the internal workings of the PN-20.

The different color wires connect the device to the batteries to generate the power required to work the PN-20.

5 Here the back cover containing the antenna receiver and the emergency battery storage is completely separated from the front part containing the circuit board and the display screen. The antenna connects to the receiver and transmitter and allows the GPS receiver to obtain strong data signals from various satellites around the globe. The information received by the receiver is sent to the circuit board which processes the information and displays it on the screen. Information is also transmitted from the SD card to the screen and vice versa via the circuit board.
6

This picture gives a closer look at the circuit board and all the micro chips the are involved in the processing and transmitting of data between the satellites, the screen, the SD card, and the computer the PN-20 may be hooked up to.

7 At this stage the PN-20 is being reassembled. The wires from the power source and those from the antenna receiver were connected back appropriately.
8 Finally, the rubber washer that ensures impermeability was replaced and the two plastic back covers were screwed back in place. The PN-20 worked perfectly after the dissection process.

1. Front End - the GPS L1 signals are received at the antenna and amplified by the Low-Noise-Amplifier. The radio frequency front-end further filters, mixes, and amplifies automatic gain control signal down to the intermediate frequency where it is digitally sampled by a Analog-to-Digital converter.

2. Baseband Processor/CPU - the Analog-to-Digital conversion samples of GPS Course/Acquisition code signals are correlated by the digital signal processor and then formulated to make range measurements to the GPS satellites. The digital signal processor is interfaced with a general-purpose CPU, which handles tracking channels and controls user interfaces.

3. Memory - the processor runs applications stored in memory. The operating system is stored in non-volatile memory such as EE/FLASH/ROM.

4. User Interface - allows user to input data with input demands via buttons or touch screen applications.

5. Connectivity - allows the receiver to connect to the USB port.

6. Power Conversion - converts input power from battery source to run various functional blocks.

## Field Experiment

Our group felt it was important to experiment with the GPS device to determine the practicality of its use in certain situations. We decided to test the device's ability to accurately determine elevations. Earlier in the term, our lab group performed a Differential Leveling experiment to determine the elevation of the first step of the statue outside of the DAC. We attempted to re-do this experiment, using the GPS device to record the elevations of the Benchmark, Turning points, and Statue. The GPS readings were recorded and compared to our lab data to determine the accuracy of the GPS.

When we started we didn't look at the GPS much as we wanted to see how easy it was to use if you just picked it up and tried to start using it. after putting it together and pressing some buttons we had it up and running fairly quickly. After walking around a bit we found it was very easy to get some elevation reading from different pages in the GPS. Also we discovered if you we further into the options it gave an error reading for each spot. The error readings seemed to level out after standing still for a few seconds.

There were a few interesting things to point out with the GPS unit. We started between buildings it took a while to connect to the satellites. Once connected there was a great deal of error. We took three or four readings and then decided to go back and double check what we did. The results changed greatly between the same spots just depending on where we approached from. After we went back and saw there was great error we decided to go up and record the error at each spot with an elevation. Our results below show that while walking in an obviously uphill direction the GPS device kept giving us lower numbers as we were walking downhill. We saw this as a big error in the system.

After doing this small experiment we determined the GPS would be ineffective in determining an accurate reading for a detailed survey area. If you had to be very accurate in a starting location for a building project of some kind this would not be a good method to use. However with the software involved and other capabilities it may be useful as a preliminary survey device to get an overall feel of the site you are working on.

### Experimental Data

Experimental Data Collected
Station Differential Leveling Data GPS Readings
MH-9 202.2 ft 113ft +or- 38ft
TP-1 210.71ft 169.46ft +or- 46ft
TP-2 216.38ft 70ft +or- 42ft
TP-3 216.63 42ft +or- 28ft
Statue 225.29ft 57ft +or- 34ft

## Conclusion

This GPS project tested and taught us a number of things. We learned about one of the "new" technologies in surveying, but we also learned its limitations. To start this project had us work in a Wiki. This was an interesting learning experience for many of us that have not done much with programming. Working within the confines of the Wiki helped with team work and giving all the group members a way to get credit for what they have done.

Once we started working with the GPS we found is was fairly simple to navigate and didn't take a long time to get the basic functions down. We went back after geting the basic uses down and read that there were more functions to use that made it easier to take readings and use the GPS for differnt things. <<<<whoever has the manual should put in some of the functions here to expand this>>>>

In terms of results we did not have a specific assignment to report numbers on. We found that these GPS units are very bad at reporting elevation and hardly work at all if you are surrounded by buildings. These devices work best in open fields so that they can pick up more satellites and get a better idea of location since there in very little to no interference. To get better readings in elevation you just need to pay up and get a better unit.

(Not done yet)

## References

APA Style You must use this format (It's easier than MLA, so don't worry).

### Guide to Writing Wiki Code

The beauty about Wiki is that if you don't know the code, you can steal it from someone's page that does. Feel free to click the "edit" links or tabs to view the code for sections or the pages respectively. Be weary about wrecking havoc on another's page. Each page can be rollbacked to a previous verison and your username is linked to all changes. Although you might think it's cool to go through and insert "MIKE RULES" throughout the page, I'm sure Dr. Lewis would not be pleased.

Here's a few tips on writing with Wiki:

This is a bracket "[" "]"

This is a brace "{" "}"

To create a new page/link within Wiki:

• Double brackets, page name, double brackets
• Typing in a new page name will automatically create a page, which when clicked, you can then edit.
• Whatever name you first type in is the name of the page. You can't change page names, only create new pages. Think before you create a new page.
• Don't worry about slashes or anything, all pages are located in the same directory. If I wanted to create a page called "MAE 277 Template" the code would be ''MAE 277 Template'' Note: Brackets are italicized to prevent creating a new page.

• 1,2,3,4 are level 2 sections
• 1.1, 1.2, 1.3 are level 3 headers

• Section titles are wrapped with two equal signs ==My favorite header==
• Bold headers within a section are wrapped with three equal signs ===My not-so-favorite header===

Asterisks indicate bullets. Be sure to put each asterisk on a new line.

• Here's one
• Here's two *Here's three, but its not on the next line

Bold text:

• Start line with "b" in "<>". Be sure to end the line with "/b" in "<>" if you don't want the whole paragraph to be bold.
• Surround text to be bolded with three " ' " marks on either side. Or highlight the text and click the "B" button on the toolbar.

Italics:

• "i" in "<>". Don't forget to end with "/i" in "<>"
• Highlight the text and click the "I" button in the toolbar (It will put four " ' " on either side).

Media tags are indicated by "Media:", images by "Image:" Broken links in red. Case is not important. Use the toolbar to get examples if you're not sure.

Spacing is werid in wiki. Single return does nothing.

Double return (blank line), breaks the line.

Triple return (two blank lines), puts an extra blank line between lines of text.

"br" in "<>" will break lines. They can also be used to separate section headers.

Finally, use the "Show Preview" button on the bottom of the page to see how it looks before saving. It will allow you to catch and edit your errors without having to edit the page again. Just don't forget to save it when you're really done.

### This is an example table

This is the table title
This starts Row 1 Width values (pixels) in header are used to designate the width of the column for the entire table. Text will wrap but it helps to control the layout. Height of the row is determined by the row's largest content A return and single vertical lines separate columns in rows. A double vertical line is necessary if you don't break up the text for cells.
This starts Row 2 In this part we took the batteries off and lift up the memory card slot. Some html tags can be used, but not many. Notice the align equals center tag at the beginning of the row. It centers the text in the first two columns, but doesn't work for the third column. I don't know why. Adding the tag again to the beginning of the cell in question will center the text.
This starts Row 3 Image tags are in this format:

Double brackets "[["
Image name
| = Vertical Line

The following order is not important, as long as each is separated by a vertical line:

• Horizontal position (left, center, right)
• Thumb (to create clickable thumbnail that links to fullsize image), don't include to make a fullsize
• Size denoted in pixels (if desired)
• You can add a caption if there is a thumbnail

Then close with double brackets "]]"

Broken links show up in red.

This is thumbnail

This is a resized image, not a thumbnail, but notice you can still click on it to get the full size.

This starts Row 4 Notice the repeating code for every row? It's important. A vertical line and a dash indicate the start of a new row. An exclamation point indicates the first column. You can put the entire row onto a single line, but it's easier to read if you break it up. Again, wiki usually ignores new paragraphs. Make sure to end the table correctly (vertical line and closed brace). Not doing so might still display the table, but nothing that comes afterwards.