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Executive Summary

This site explores the notion of Geocaching using the Delorme Earthmate GPS PN-20 and how various set coordinates may be compared to other coordinates at the same location. Three locations were pinpointed by three group members by recording the coordinates at each location. These locations may be seen in the table below. The other two group members were to find these locations and determine the coordinates at the location. We were then able to compare these coordinates with the set coordinates. By doing this, the accuracy and precision of our GPS system was tested. It turns out that the accuracy and precision was tremendous and only off by a few numbers. Therefore, it may be stated that our GPS system may aid in finding a hidden place a unknown destination. The remaining portion of this site looks at the background of GPS systems,how the GPS navigational system works, GPS uses, personal experience, and other types of GPS systems.

History of the GPS system

General notes about the product (it's condition, how it works, how many parts, types of materials, etc)

A GPS (Global Positioning System) is a device that when intercepted by satellites, provides useful information to the user in finding destination/locations. The GPS was created by the American Department of Defense and runs on 30 satellite systems orbiting Earth. Signals are transmitted by the satellites to the GPS providing information such as the following: Latitude, Longitude, land details (roads, buildings, grass, water, etc), current dates and times, sunrise/sunset times, when and where the best places to fish are, etc. These satellite transmitted signals show whether the GPS is on the ground, in the air at high or low altitudes, or on a body of water. GPS systems were mainly used for military use only in determining where opponents were and to aid in dropping bombs on specific locations. Over time, GPS systems continued to grow through technology and were used in aviation as a guide to aid in flying altitudes and ashore locations. GPS systems may also be used in engineering surveying work to plot specific land points to guide the site contractors in constructing the designed site. The GPS provides the engineer with information such as; setting up property lines, how much excavation needs to be performed, etc. In 1973, the decision was made to generate a device that would aid in locating various objects or destinations by the transmission of various satellite signals that would be launched into space. It wasn’t until 1980 when the first Block I satellite was launched into space. However, the Block I satellite only carried sensors to detect atomic explosions. Over a time span of a approximately 30 years or more, satellites were launched into space with more and improved technology which developed a more sophisticated GPS system. A few types of satellites that have been launched into space include the following: Block I, Block II, Block IIA, Block IIR, and Block IIR-M. Of these satellites, the Block IIR-M satellite is the most recent satellite with updated information for both civil and military signals.

How the GPS Navigational System Works

GPS, or global positioning system, is a system of satellites, computers, and receivers that can determine the latitude, longitude, and altitude of a receiver located on Earth through the use of a mathematical principle known as trilateration. Trilateration is the computation of rectangular coordinates using only distance measurements and trigonometry. There are two types of trilateration, two-dimensional and three-dimensional. The old satellite navigational system prior to the creation of GPS, called Transit, used 2-D trilateration while GPS navigation systems use 3-D trilateration.

In order for 2-D trilateration to work, the locations of three or more reference points are needed as well as the measured distances between the desired point and each reference point. By knowing at least three reference points and the distances from each point to the desired point, the location of the point can be determined very accurately. This can be easily understood by the use of circles. If two distances from a known location were known, a circle around each known location could be drawn using the measured distance as the radius. Both circles would intersect at two points, meaning there are two possible positions for the desired point. If there was a third known location, another circle could be drawn using its measured distance away form the desired point as a radius as well. With three circles, there is only one point of intersection shared by all three making that point the desired location.

3-D trilateration functions under the same principles of 2-D trilateration except that it is working with a three-dimensional axis instead of a two-dimensional plane. So instead of working with circles, 3-D trilateration works with spheres. In the GPS navigational system, the system of satellites acts as the known reference points while the receiver acts as the desired point of location. The purpose of the GPS receiver is to locate at least four or more of these satellites, calculate the distance it is away from each one, and using this data determine its location through the use of trilateration.

As long as the GPS receiver is able to lock on to at least three satellites, two possible positions are given. However, one of those positions is not on the surface of the earth, and therefore can be disregarded. The more satellites the receiver can lock on to, the higher the accuracy of the calculated coordinates determined by the GPS system. For example, if the receiver were to lock on to three satellites, three distances would be known and therefore their spheres would be able to drawn, one for each satellite. However, if there were errors in the data, maybe due to signal interference by obstructions such as buildings, the three spheres would still intersect at one point. This would cause some error in the determination of the coordinates of the desired point since false data was being used. However, if a fourth satellite were to be able to connect to the receiver, accuracy would increase and error would be very small because four spheres would not intersect at one point if there were some form of error in the data. Having four or more satellites, and hence reference points, would help to reduce and estimate error.

So how does a GPS receiver calculate its distance from a satellite that it has locked on to? It does this through the use of radio signals; by calculating the time difference for signals from a satellite to reach the GPS receiver, the distance away from the satellite can be accurately measured. Each satellite in the system has an atomic clock, while the receivers have a quartz clock. The satellite sends a pseudo-random code to the receiver, which it then compares to its internal quartz clock. The delay in the radio signals is then determined by the receiver, and with this information the distance is calculated. Quartz clocks are not nearly as accurate as atomic clocks, but since the GPS receiver relies on four or more satellites, the error is negligible.

GPS Uses

A GPS can have a wide variety of both military and civilian uses. GPS can be used in the military to help soldiers find objectives in unfamiliar territory, or to coordinate movements of troops and supplies. It is also extremely useful in the guidance of ballistic missiles and to point out targets for air strikes and artillery attacks. One very important feature is that a GPS allows command to pinpoint the location of soldiers caught behind enemy lines, and makes search and rescue missions much easier to complete successfully. This particular use, search and rescue, can be quite useful to civilians as well. For example if a hiker is carrying a GPS, rescue teams will be able to pinpoint his location, and bring him to safety much faster than if he was simply lost. This becomes especially important when the members of the lost party are injured, and are in need of medical assistance. In an emergency situation, the extra time granted by knowing the location of a person in need of rescue can mean the difference between life and death.

GPS also allows for much easier navigation in any vehicle, whether it travels by sea land or air. GPS navigation systems are becoming more and more common in new vehicles, and are making travel to unfamiliar places much easier for many people across America. In addition to travel by car, a GPS can also make navigation easier for fishermen, speedboats, even large shipping vessels, as well as any type of aircraft.


For our project we decided to explore the more useless uses of the GPS. Caches have been placed all over the world just waiting for other people to find them and place new ones in other places. These caches are hidden and only their latitudes and longitudes are given. Even with the best civilian GPS units the readings are only accurate up to 10 feet, this is why geocaching can be so difficult. Even if you arrive at the lat and long that the has been given to you, you must now find the cache that |you are looking for. This is what makes this game so exciting. We decided to try and test the accuracy of our own GPS by going around campus and creating our own caches for our teammates to find.

We set our caches as waypoints in the GPS. Three members went around campus and set the waypoints and two members went around and tried to find the waypoints. I was on the team that went around finding these waypoints. The precision of the unit was incredible. It gave the degree and minute up to the fourth decimal place. However, finding these locations turned out to be harder than we thought. As we walked the GPS lagged behind, the distance to the cache jumped and swiveled while standing still, but still we moved on. We were not able to find the exact lat and long of the waypoints, but we were able to get close enough to the points to find the caches.

The caches laid out for us to find were not actually items but certain places. We went around twice to see how close we could get to the actual readings.

Our Caches
Set By Original Corrdinate 1st Reading 2nd Reading Cache
Tim N-39° 57.2831’

W-75° 11.2471’

N-39° 57.2843’

W-75° 11.2468’|

N-39° 57.2828’

W-75° 11.2460’

North East corner of Disque Hall
Marc N-39° 57.2250’

W-75° 11.3280’

N-39° 57.2241’

W-75° 11.2911’|

N-39° 57.2257’

W-75° 11.3361’

Out front of Café near bikes
Jon N-39° 57.2648’

W-75° 11.2911’

N-39° 57.2648’

W-75° 11.2911’|

N-39° 57.2663’

W-75° 11.2892’

On top of the new amphitheater

As you can see the accuracy and precision of our unit was incredible, but we were never able to get exactly on top of the same spot as our teammates. I believe that if we had a military GPS unit we would be able to tell which pavement block our teammate stood on.

-Ben Cohen

Varieties of GPS devices

(Here we have several types of devices including general information)

Top Cover of Delorme Earthmate® GPS PN-20
Basic compents that came in the box
  • Handheld Devices
    • Earthmate® GPS PN-20 (This is the GPS device we analyzed and used for geocaching.)
      • Whats included in package:
        • 8-pin-to-Mini-USB Connector
          • Required to transfer data to and from the GPS receiver and the computer.
        • USB-to-Mini-USB Cable
          • To keep the GPS receiver charged when on the go. Plugs into ash tray 12-volt DC.
        • A/C Adapter
          • Used to keep GPS receiver energized while not on the go. The GPS receiver constantly uses battery even when the system is turned off to keep accurate time.Using the A/C adapter will keep the GPS device from drawing power from the battery.
        • Lithium-ion Battery and Charger
          • The Earthmate® GPS PN-20 is capable of using both lithium-ion reusable and alkaline disposable batteries. Rechargeable batteries will keep costly battery purchases to a minimum.
        • LCD Protective Film Kit
          • This kit contains three protective films and one microfiber cloth to keep the screen lint and scratch free.
        • 12-volt Adapter for battery charger. If on the go, this adapter can be plugged into the ash tray allowing extra portability.
        • Topo USA 7.0 mapping software
        • $100 data download certificate*
          • Good to download detailed maps uploadable to the Earthmate® GPS PN-20. Maps include USGS quad maps, NOAA nautical charts, and satellite imagery.
        • USB data transfer cable
        • Getting Started Guide
        • Two AA batteries
Common inexpensive RAZR
    • Common Cellular Telephone such as the Motorola RAZR V3m
      • Package includes:
        • Standard Li-Ion Battery, Travel Charger, Headset Adapter and User's Guide
        • Ultra-thin design for easy transport
      • Low initial cost (price of phone), hi service fee ($2.99 per day or $9.99 per month)
      • Capabilities include:
        • Mapping points of interest such as banks, gas stations, food.
        • Gives land speed and altitude.
        • Uses both GPS signals and cellular telephone tower triangulation.
        • Can search by coordinates in certain models.
      • Drawbacks:
        • Phone is insensitive to direction that its pointing, but relies on the vectors of travel.

Personal experiences

Jon's Personal Experience:

After using the Delorme Earth mate GPS PN-20, I found many versifying features that aids in finding a destination. My first time using this GPS system was becoming acquainted with the use of our hand held GPS and all of its components. Not realizing the amount of accessories that came along with this GPS, I decided to tear apart every single package found with in the packaged box. This GPS came along with a USB cable, two double AA batteries, user manual, a charger system, and a copy of the USA map. The GPS itself is made of rubberized material for ease of grip and a waterproof shield, that if I were to drop this GPS in a river and remove it, it would probably still work. However, due to a limited time period, I was not able to prove my point. Although, I was able to make use of the GPS as a navigation guide to aid me in finding the building my Music Theory II class was in. There is a zoom in and zoom out feature that shows you various details of land locations and what is around you. When you zoom in, more land detail surrounded by you is shown whereas; when you zoom out a general overview of other states are shown. When I first used this feature, I zoomed all the way out not only to realize that I was somewhere in Maryland. After realizing that I was zoomed out, I was actually still right here in Philadelphia. Since the maps were not loaded on to the GPS system at the time of use, it was not apparent of what type of land implications surrounded me. What I like about this GPS system, was that it guided it me with in a reasonable range of finding my Music Theory II class. It appeared to bring me to the Drexel book store, where I had made my entrance through the double doors, around a few corners, and up the stairs to the second floor of Macalister (room 219) where I sat my ass down and waited for approximately 5 minutes for my professor to show up. The GPS provided useful information at the time of my arrival to the bookstore. This information included: the time, date, bearing, and heading. Other useful information this GPS system provides includes; time and elevations of high and low tides, distance to stop, speed, average speed, elevations, sunrise/sunset times, moonrise/moonset times, and many more. It also provides information of the best times to fish and where to fish. So, for those fishermen who crave to each fish every day, this would be a handy dandy device to use to aid in finding their fish. Overall, GPS systems are versatile in many aspects in aiding one to find a location that he/she is not familiar with. There are many types of GPS systems that may be used in various situations such as; in dash units for cars, portable add on-units for cars, laptop systems, handheld systems, and many others. Therefore, the next time you are at circuit city and are staring down that price for that one and only expensive GPS system you want, don’t deny it “buy it.”

Steve’s personal experience:

I felt that the Earthmate GPS was somewhat of a disappointment. It seemed that it could be quite useful as a tool to help find your way around but I ran into a few difficulties. It was a bit difficult to use when searching for the caches that the other group members had set. The readings of direction and distance to the point were not consistent. For example, at one point while I was standing still, the GPS said the cache was 6 feet in front of me, and then 15, and 25. Another time, the device said the point was 200 feet to my left, when in reality, the point I was looking for was about 30 feet in front of me. Also, I set up a waypoint and then went around looking for a few caches, and when I returned to the exact same position, it told me that I was 40 feet away from where the original point was located. This lack of accuracy makes it a bit difficult to use in the city. There were some features that I did like though. One thing that wasn’t particularly useful, but pretty cool, was that the GPS has the ability to tell you how fast you are traveling. I feel that if I had the chance to download topographic maps onto the device and go out hiking, it could be an extremely useful tool.

Tim's Experience:

(Some basic pointers when geocaching with the Earthmate GPS PN-20)

  • To gain the most from your experience of using GPS it is important to read the manual thoroughly. If time is limited, read the first half of the manual (most important features), and read the rest of the manual on an as need basis.
  • It is important to learn how to find waypoints using a given set of coordinates. For practice, it became necessary to zoom out onto the map to a minimum of four mile visibility range. This helps avoid the tendency to cheat.
  • The GPS device is sensitive to the direction it’s pointing. It needs to be in motion to have a sense of direction.
  • When searching for a geocache waypoint near a wall, it was observed that the GPS coordinates for that specific point changed steadily. It is likely that satellite signal was bouncing off of the wall creating a false sense of location.
  • The GPS device gets the best satellite reception under clear weather conditions. The GPS can operate in both day and night. Geocaching waypoints are more easily spotted when visibility is high. Poor conditions include night and moderate to heavy precipitation.

Preparation before Geocache Survey

  • Wear appropriate clothing for weather conditions.
    • In cold weather, dress warmly. Since the GPS device needs to be in the open for optimal satellite reception, be sure to be sure to keep your hands warm by wearing gloves.
    • In the summertime, be sure to wear sunscreen.
  • Before tracking a long distance geocache waypoint, practice on a small site. Be sure to practice in good weather, optimally with clear skies and no precipitation. Searching for a waypoint as a beginner can be a very time consuming process. If not prepared for spending a long time on your feet, geocaching could be a bad experience.
  • It is a good idea to practice finding coordinates with a partner. If occupied with tracking waypoints in the city, it is likely that you are unaware of your surroundings including buildings, streets, and most importantly other people.

Marc's Personal Experience:

I found the Earthmate GPS PN-20 to be an interesting device to use. I had it for at least a day or so, while setting way points for my group-mates to find. I did not have the manual, so it was a hands-on experience for me from the get go. From the time I had with using the Earthmate GPS, it had many tools in aiding a person to find their specific location. The Earthmate GPS had a wide variety of options to view the data and coordinates of the current location, which were divided into a few pages. One page showed how many satellites that the Earthmate GPS was able to lock onto, and for me it ranged from three to six satellites depending on where I was standing when walking around Drexel University. Whenever I was surrounded by buildings, the buildings seemed to interfere with the reception of the receiver and the number of satellites connected would decrease. When I entered buildings, the receiver lost all connections with any satellites. Since I was using this GPS receiver in a city area (Philadelphia), the coordinates were not as accurate as it should have been if there were no obstructions causing interference with signals coming from the satellite. This page not only displayed the number of satellites that the receiver was connected to, but also the coordinates of the current location and a measurement of how off these coordinates were. At one time, I believed it was plus or minus fifty-six feet, which shows how inaccurate the receiver is when used in areas where the signal can be obstructed. The coordinates that were calculated would also contain fluctuations making this receiver even more inaccurate. It was also windy that day, which could have led to another factor for the inaccurate readings. Another page displayed the path that you were walking, way points in the vicinity, and the speed at which you were traveling. These were the only two pages that I took interest upon since they were the only two I needed to set way- points for. Overall, the manual could have been useful to some extent because the Earthmate GPS PN-20 is capable of aerial photos as well as USGS topographic maps. Being able to use these functions of the Earthmate would have probably been more intriguing since these functions relate to a class that I am taking called "surveying."