Learn About GPS Technology Here!
 

* All coordinates posted on this page and any other page within my site, use the “NAD83/WGS84” Datum no matter what grid system was used (Lat. & Long., UTM, MGRS, etc.).
 

What is GPS?
        GPS stands for Global Positioning System. The original name of this system was called NAVSTAR, which is the acronym for: NAVigation Satellite Timing & Ranging. GPS is a worldwide navigation system (developed by the U.S. D.o.D originally for military purposes) that consists roughly of 28 satellites (4 extra, in case one fails) that orbit the earth, along with their corresponding receivers on land. The satellites orbit the earths about 12,500 miles above the earth surface making two complete orbits every 24 hours in a certain spherical pattern at about 7,000 mph!
        Each GPS satellite is powered by solar energy with battery backup for events like solar eclipses. The first satellite was launched in 1978 and the complete 24-satellite constellation was not complete until 1994. GPS was opened to the pubic for use in the 1980's, but Selective Availiblity or SA made it extremely inaccurate for civilian use. SA was intentional data error enforced by the U.S. Gov't for security purposes. SA was later removed in May of 2000.
        The GPS satellites continuously transmit data through digital radio signals that contain information about satellite location & time. With this information the earth bound receiver (GPS Unit) can calculate various data such as: location, speed, altitude, etc.
        Three satellites are needed to calculate a 2D fix/location (latitude & longitude) called trilateration, four satellites are needed to include altitude and is known as a 3D fix. Five or more satellites locked in by the receiver helps to increase accuracy. Many GPS units are capable of receiving data from 12 satellites at once. Remember without a GPS Unit the GPS navigation system is useless to you.
        Parallel multi-channel design of GPS receivers have allowed them to be very accurate. Additional strengths added to GPS units are stronger processors, electronic downloadable maps & the new SiRF chip. These vital additions have allowed GPS receivers to complete the intrinsic calculations very quickly.

Three Vital Components Needed To Make GPS Operable

1. Space
2. Ground control
3. Users

GPS Satellites Information (24 Active, 4 extra for 28)

What Will Interfere With GPS Accuracy

What is Differential GPS
            Improves GPSr accuracy to within 1 in. (2 mm). DGPS uses the difference between two measurements to improve GPS position and was put into operation by the U.S. Coast Guard. There are currently 60+ land based sites in the U.S. and it's provinces. DGPS is highly accurate for navigational purposes, but would be too costly, heavy and bulky for the consumer looking to use this technology out in the wilderness. Why? in order to use DGPS you must have an additional antenna and a DGPS unit, WAAS is more convenient to the outdoor enthusiast (WAAS is simply a form of DGPS).
            DGPS does come in handy for other uses like: dam monitoring, the precise accurately of this system is an excellent way to control water levels and fill rates below the failure point of the dam for flood control.

Two Methods Used By DGPS To Increase Position Calculations

What is WAAS?
        An acronym for Wide Area Augmentation System. This is another U.S. developed navigational system (primary purpose was/is for civil aviation) that further enhances GPS accuracy by correcting minutely flawed data caused by atmospheric disturbance, timing and satellite orbit error.
        WAAS is a form of DGPS that is very similar in the sense that it uses corrected data to further improve GPS accuracy, but does this in a different way. In WAAS, land-based stations (located at various points all across the U.S.) calculate any GPS satellite error at any time. The corrected data is up-linked from the land-based stations to the four visible WAAS satellites (#35, 38, 47 & 48) located along the equator at opposite ends of the United States. The WAAS satellites then beam the corrected data all across the continental U.S. & Alaska (soon Hawaii & Canada) where WAAS capable/enabled units can use the corrected data to further enhance the receivers true location. Garmin units display WAAS corrected data with a "D" within the satellite bar. In September of 2002, WAAS testing confirmed accuracy performances to 1-2 meters horizontally and 2-3 meters vertically throughout most of the continental U.S. and parts of Alaska.
        For some users that have an obstructed view of the horizon due to mountains, trees, buildings, etc. it is not necessary to enable WAAS on your GPS unit, because the unit can not see the satellites so therefore, it can not communicate with the satellite to correct the data. But with today's technological changes, this may not be the case in the near future.
             WAAS is a form of DGPS that is very similar in the sense that it uses corrected data to further improve GPS accuracy, but does this in a different way. In WAAS, land-based stations (located at various points all across the U.S.) calculate any GPS satellite error at any time. The corrected data is up-linked from the land-based stations to two WAAS satellites (#35 & #XX) located along the equator at opposite ends of the United States. The WAAS satellites then beam the corrected data all across the continental U.S. & Alaska (soon Hawaii & Canada) where WAAS capable/enabled units can use the corrected data to further enhance the receivers true location. In September of 2002, WAAS testing confirmed accuracy performances to 1-2 meters horizontally and 2-3 meters vertically throughout most of the continental U.S. and parts of Alaska.

Did Other Countries Create Their Own Global Positional Systems?
YES.
European System: EGNOS (Euro Geostationary Navigation Overlay Service)
Russian System: GLONASS (Global’naya Navigatsionnaya Sputnkovaya Sistema)
Japanese System: MSAS (Multi-Functional Satellite Augmentation System)

Will My Receiver Be Capable of Using Other Countries Systems?
From my knowledge, NO. I suggest further research to confirm this.

Global Grid Systems

Latitude & Longitude
        A spherical coordinate system (global grid system) where the lines of latitude and longitude form an imaginary grid system over the globe. This system is used for navigational purposes on land, sea & air. You probably learned about this grid system in school.

Anything to the right (east) of the Prime Meridian increases in number and is followed by an E for east.
Anything to the left (west) of the Prime Meridian increases in number and is followed by a W for west.

Basic Latitude & Longitude Measurements

Latitude & Longitude Coordinate Formats

*The coordinates above are for the Ganaga Falls (100’+) located in Ricketts Glenn State Park off Rt.118.

Lat. & Long. Coordinate Example in "National Geographic TOPO!" Software

          Ex1.  41°26'38" N     76°40'43" W     in Degrees, Minutes & Seconds
          Ex2.  41°26.626' N   76°40.713' W    in Degrees & Decimal Minutes
          Ex3.  41.44377° N    76.67855° W    in Decimal Degrees

The above coordinates are for "High Knob Vista," which is absolutely amazing and gigantic. This vista looks out over 7 counties in PA. I would recommend anyone to see this vista and on the way up stop and check out Dry Run waterfalls off the road.

Coordinate Example When Using Garmin's "GPSMap 60CSx"

          Ex1.  41°25'49.1" N     76°40'14.0" W   in Degrees, Minutes & Seconds
          Ex2.  41°25.826' N      76°40.234' W     in Degrees & Decimal Minutes
          Ex3.  41.43043° N       76.67056° W     in Decimal Degrees

The above coordinates are for “Dry Run Waterfalls Parking Area” located off Dry Run Road, which is accessible off Rt.87. The Dry Run Waterfalls are a very pretty waterfall that is at least 20' high.

Universal Transverse Mercator (UTM)
        A global grid system that splits the earth into 60 zones that run east to west, each zone of which is 6° wide for two reasons reduced distortion when converted from a spherical globe to a flat paper map & the ease of use on a paper map with northing & easting values being measured in meters. This system starts at the zone meridian (180° aka the International Dateline, when referencing to the latitude & longitude system) and uses letters of the alphabet along with easting & northing values.
        UTM maps do not cover areas above 84° N. latitude & 80° S. latitude. The remaining degrees above & below these lines are drawn using Universal Polar Stereographic Grid (UPS). This grid system will be talked about at a later time.
        Definition #2: UTM stands for Universal Transverse Mercator. This system is popular for use on land. Unlike lat. & long., which projects imaginary grid lines on the globe. UTM projects section of the globe on a flat surface, like a map. Again there are 60 sections or zones in the UTM system that cover 6° each. This type of grid system starts with section 1 at 180° - 174°, continuing west to zone 60. Two other elements describe this system in greater detail; they are easting and northing values. Easting & northing values measure how far in meters, within the zone the reference point is.

UTM Coordinate Example in "National Geographic TOPO!" Software

          Ex. 18T 295718mE  4621044mN

          18T represents the zone            295718mE represents the Easting Value & 4621044mN represents the Northing Value

The above example is coordinates in UTM format for "Owassee Rapids" on the Pine Creek. This rapids is a Class II rapid on a 6 scale system. Notice how NG TOPO combines the zone letter & splits the easting & northing values. Also note how the numbers are all the same size.

UTM Coordinate Example When Using Garmin's "GPSMap 60CSx"

          Ex. 18 T 363327 4591410

The above example is coordinates in UTM format for “Split Rock” on the Loyalsock Trail. This landmark is a cool rock that split over time creating crevasses between the rock. Notice how the GPS splits the zone, the zone letter, easting value & northing value. But does not tell you which is which. So you must have some knowledge of UTM to interpret this coordinate.

How Do I Measure or Calculate Coordinate Distance Using UTM?

            It’s quite simple actually. If you remember each UTM grid line is divided by 1000 meters, with this in mind, it’s real simple to figure out a waypoint coordinates.           

            Ex. Someone has told you that there is a really cool vista near Blackwell off the MST known as the “Materhorn.” They show you its general location on the map, but neither of you know its coordinates and you would like to plot it on your map & in your GPS. To figure this out, take your UTM legend that shows the map scale and measure from one of the main UTM easting lines, either ³02^000m.E. or ³03^000m.E.. If you use ³02^000m.E., then you add those meters to the last three 0’s. If you use ³03^000m.E., then you subtract those meters to the last three 0’s and vice versa for the northing lines.

            Easting line:     ³02^000m.E. + 129 (measured meters) = ³02^129m.E. (Easting value)
        Northing line:    ⁴⁶02^000m.E. + 763 (measured meters) = ⁴⁶02^763m.E. (Northing value)
           Coordinates: ³02^129m.E. ⁴⁶02^763m.E.

UTM Grid Zone Chart

Military Grid Reference System (MGRS)
        The MGRS is the global grid system designed by the U.S. Military. This system is very similar to the UTM grid system, except that the majority of the numbers (first two) are replaced with letters & the letters run horizontally instead of vertically.

Ex. UTM:        12 S   ⁰⁵01^788m.E.          ³⁶90^619m.N.
                 MGRS:      12 S WB 01788,            90619
                 GPS:          12 SWB 0178890619 

The MGRS system divides the globe into 8o sections instead of 6o sections like the UTM grid system. So therefore, there are 19 sections, each covering 8o that run horizontally, except for the northern most section which covers 10o. The letters start from the southern most hemisphere using the letters C, D, E, F, G, H, J, K, L, M, (all in southern hemisphere) N, P, Q, R, S, T, U, V, W, X (all in the northern hemisphere). The southern most section of the MGRS system is assigned letter C & the northern most grid section is letter X. This grid system is not popular among outdoor enthusiast, because USGS maps do not display this grid on their maps.

Universal Polar Stereographic Grid (UPS)
            A special uniform grid system developed for use in the Arctic & Antarctic regions of the world. The UTM grid system can be used for these regions also, but the zones would be very narrow and confusing to read. UPS does share some similarities of the UTM system in that both have eastings and northings and each is 1000 meters (.62 miles) in size.

Ordnance Survey Great Britain (OSGB)
            This is the national grid system for Great Britain and Great Britain only. This grid system is separated into square sections each 100km in size (62.1 miles). Each grid is assigned a double letter that is 7 grids wide by 13 grids high. This grid system is also similar to the UTM system in that: it has easting and northings, its unit of measure is in metric, the last three numbers a grid coordinate represent meters and its easy figure out distance between two coordinates within the same grid.

 

Maidenhead Grid
            This grid system was developed and operated by amateur radio operators. It divides the world into rectangular grids each 20^o of longitude and 10^o of latitude for a total of 18 grids wide (A-R) running east to west and 15 grids high (D-R) running south to north. Each grid is identified by two letters and are further subdivided by 2^o by 1^o and labeled with two numbers (00-99). This grid system subdivides one more time; each sub-area is 5’ of longitude by 2.5’ of latitude and is labeled with the letters AA-XX.

            EX.      EM18BX 

FYI: Grid letters above and to the right of the letter “J” are located in the northern hemisphere and in the eastern hemisphere.

How Do I Know Which Datum & Format My Map or GPS Is In?
            To figure out how your map has been setup look over the legend and look to the edges of the map. The USGS Topo! software tells you the datum used in the upper right & lower right-hand corners. It can also generate maps in four formats (3 lat. & long. & UTM). The format markings are along the outer edges of the map & would contain the coordinates. 

            The USGS maps you would purchase at outfitters are 7.5’ topographic maps. You will find (9) 2.5’ blocks on each map. 7.5’ is the distance it covers in both directions using the latitude & longitude grid system. It will also contain the UTM grid system with markings every 1000 meters. 

            Ex. Map Name: Cammel, PA  

Northern Corners: 41^o30’00” N.  77^o30’00” W.               41o30’00” N.   77o22’30” W.                                    
                                                                               ⁴⁵97 N.

                                                               ⁴⁵84 N.     ²91 E.        ³01 E.
Southern Corners: 41^o22’30” N.  77^o30’00” W.               41^o22’30” N.   77^o22’30” W.
                                                                                                                                                       
                                                Midpoint of Map:  41^o26’15” N.  77^o26’15”

Cammel Quad in:   

UTM by 1000m intervals    &   Latitude & Longitude    (Large Files! 3.5MB)
 

 
Latitude & Longitude

UTM

Working with and Converting Degrees, Minutes, Seconds & Mils
 

Degrees, Minutes, Seconds & Mils

Navigation begins with a point and everything around that point. So the circle is a very important part of navigation. The circle in navigation is subdivided into degrees, minutes, seconds and mils.

All of our navigation systems, formats and datums are based on this circle design.

The most common subdivision is the degree (^o). There are 360^o in a circle; 0 is the start, a 1/4th of a circle is 90^o, 180^o to a ½, 270^o to 3/4th and so on.

When working with degrees, in the case of adding & subtracting the values, it starts at 0^o - 359^o.

The degree’s subdivision is the minute, expressed by a tick mark (‘). There are 60’ to one degree (^o).

The minute’s subdivision is the second, expressed by two tick marks (“). There are 60” to one minute (‘)

When working with minutes & seconds, in the case of adding & subtracting the values, it starts at 0 – 59.

Various Examples Listed Below:

Working with Minutes & Seconds:

37” + 21” = 59” (almost one minute)

45” – 15” = 30” (half a minute)

46” + 33” = 73” = 1’ 13” (one minute & 13 seconds)

1’ 07” + 49” = 1” 56” (one minute & 56 seconds)

5’ 45” – 2’ 15” = 3’ 30” (three minutes & 30 seconds)

Working with Degrees, Minutes & Seconds:

270^o 15’ 45” + 10^o 7’ 23” = 280^o 23’ 08”

165^o 23’ 17” – 42^o 12’ 48” = 123^o 11’ 31”

Converting Decimal Degrees to Degrees & Decimal Minutes

76.257^o = 76^o (.257 x 60’) = 15.42’ (remember 60 degrees to 1 degree)

Answer: 76^o 15.42’

320.926o = 320^o (.926 x 60’) = 55.56’

Answer: 320^o 55.56’
 

* If you want to convert these values the opposite way, just divide 60, instead of multiplying

Converting Decimal Degrees to Degrees, Minutes & Seconds

Using the same examples above we can further break down the decimal minutes to seconds.

76.257^o = 76^o (.257 x 60’) = 15.42’ (remember 60 seconds to 1 minute)

= 76^o 15.42’ (.42 x 60) = 25.2” exactly or rounded to 25”

Answer: 76^o 15’ 25”

320.926^o = 320^o (.926 x 60’) = 55.56’

= 320^o 55.56’ (.56 x 60) = 33.6” exactly or rounded to 34”

Answer: 320^o 55’ 34”

* If you want to convert these values the opposite way, just divide 60, instead of multiplying

How to Find the Opposite Direction

Simply add or subtract 180^o to your bearing.

74^o 32’ 45” = 74^o (+ 180^o) = 254^o

Answer: 254^o 32’ 45”

315^o 45’ 30” = 315^o (- 180^o) = 135^o

Answer: 135^o 45’ 30”

What is a Mil & How Does It Relate to Navigation?

A circle can also be subdivided into mils, instead of degrees, minutes & seconds. The cool thing about mils is there is no subdivision of a mil, so there are no silly conversions! A full circle is made up of 6,400 mils and/or 1 mils is one sixty-four hundredths (1/6400) of a circle.

Mils are best related when you know how to convert the two from one to another. Examples below:

1 mil = 360^o / 6400 = .05625^o

1^o = 6400 mils / 360 = 17.778 mils

To Convert Degrees to Mils, Simply Multiply the Number of Degrees by 17.778.

68^o = 68^o x 17.778 =

Answer: 1208.904 exactly or 1209 mils

* TO convert degrees, minutes & seconds, be sure to convert it to decimal degrees, then multiply by 17.778

To Convert Mils to Degrees, Simply Multiply the Number of Mils by .05625.

4200 mils = 4200 x .05625 =

Answer: 236.25^o

A Few Equivalent Examples:

North (N) = 0^o = 0 mils

East (E) = 90^o = 1,600 mils

South (S) = 180^o = 3,200 mils

West (W) = 270^o = 4,800 mils

Map Scale Conversions from USGS Scale to Feet & Miles
* Each inch of map represents the numbers below according to the map scale they were printed in

General USGS Map Scale Conversions

National Geographic TOPO! Software Scale Conversions

Reading Distance on a Paper Map
            Yes, you can (to a certain extent) do this by knowing the distance between o (degrees), ‘ (minutes), “ (seconds) or UTM’s meters, but the west way is:

Additional Information on Declination & What It Means
            You must know the declination of the area you are in, for example: declination around this area is roughly 11-12^o W. Since we are right of the Magnetic North line we add the declination difference to our compass to get to a certain point on a paper map. (Remember East is Least, West is Best!) 

Determining Your Declination – Expressed as either East or West Declination according to your present location of the magnetic North Pole.

East is Least, West is Best – Which means you subtract east declinations and add west declinations to your calculations depending on which side of the magnetic pole line you stand on. The examples below show how this works.

* Most all maps will tell you the declination difference somewhere on the legend, the USGS Topo! software maps declination is located in the lower right corner.

Things You May Want To Take On The Trail

* Always check to maps & GPS to make sure that both are using the same grid system and datum setup. If they are incorrect your calculations could be off by a mile.

Useful Terms

2D Operating Mode – In GPS terms, it is a two-dimensional fix on a location that only contains a horizontal fix without elevation. A minimum of 3 satellites is required for this operation mode.

3D Operating Mode – In GPS terms, it is a three-dimensional fix on a location that includes both a horizontal fix and elevation. A minimum of 4 satellites is required for this operating mode.

Almanac Data – Information transmitted by the GPS Satellite telling the receiver its current health & orbital status.

Altimeter – An instrument used to determine elevation.

Altitude/Elevation Profile/ing - Is a cross-section of the trail, route or drawn route that displays the changes in elevation at every point on the trip. This is an invaluable feature provided by topographical software that displays to you how easy or strenuous the hike may be.

Atomic Clock – A very precise clock. Each GPS satellite contains many of these clocks. They are extremely accurate, because of the rubidium & cesium components (one-second error per one million years).

Azimuth – Aka bearing, is the compass position between your present position and your destination. It is mostly expressed in degrees (^o), but sometimes can be displayed in mils.

Barometer – An instrument used to determine the pressure in the air. It is calculated by either.

Coarse/Acquisition Code (C/A Code) – The standard code transmitted by GPS Satellite. The information contained within this code is for civilian use and is accurate to within 10 feet. The best reception I ever had in combination with WAAS was 6 feet!

Compass – A navigational instrument used to find direction. 

Coordinates – These are numbers and letters that precisely label any and all positions on earth. Every position on earth has a unique coordinate. The displayed coordinate of a position is determines by the grid & datum used.

Coordinated Universal Time (UTC) – This system of time replaced the Greenwich Mean Time (GMT) in 1986 and is now the world standard. This time compensates for the leap year and any changes in the earths rotation.

Data Formats – There are generally three data formats, each format is best suited to a particular grid system.

1. Metric: The European & world unit of measurement, Ex. Kilometer, meters
                Grid System Preference: UTM, UPS, MGRS & OSGB
2. Nautical: Used on the open waters by sailors & others, this is used in reference to marine charts, Ex. Nautical mile, knots
                Grid System Preference: Marine Charts, Latitude & Longitude
3. Statue: The U.S. units of measure, such as: feet, Fahrenheit, miles per hour.
                Grid System Preference: Latitude & Longitude

Declination – Aka Magnetic Declination is the difference between true north & magnetic north in mils or degrees. 

Differential GPS (DGPS) – An extension of the GPS system used by WAAS. This system consists of land-based radio beacons located a various locations across the country (east coast, middle & west coast) that transmits corrected GPS position data. DGPS improves GPSr accuracy to within 10 meters, I have experienced an improvement to within 10 feet! DGPS does have the potential to increase positional accuracy to within 1 in or 2 mm! But not without added equipment & substantial costs; not to mention we civilians do not have access to the P-Code.

Easting – Similar to a northing, except is refers to the distance east or west of a fixed reference point, which is the International Dateline. Again you will see this value used in the UTM, MGRS and others.

Elevation – The distance above or below the mean sea level.

Elevation Profile – shows the elevation gain or loss along a trail from a horizontal aspect. This information is very informative when it comes to the amount of time and miles you will hike when planning out a hike or backpacking trip. 

Emphemeris Data – Data that is valid for several hours and contains current satellite position and timing information. This data is transmitted by the GPS satellite as part of the satellite data message.

GPS – A satellite system used to navigate. This system enables anyone on Earth who owns a GPS receiver to know where they are at any time, no matter what the weather.

GPSr – This is short for the GPS receiver. The GPSr communicates with the satellites to calculate the fixed or moving location of the individual or object that is equipped with the unit.

Ionosphere –  A region of the earth’s atmosphere 50–250 miles wide (50–400 km) and affects GPS radio waves due to the incoming solar radiation.

Local Area Augmentation System (LAAS) – A real-time DGPS correction system with a 20-30 mile radius developed by the U.S. Federal Aviation Administration for landing aircraft at small to medium-sized airports. The LAAS system is extremely accurate to within 1 meter on the horizontal and vertical plane.

L-Band – The radio frequency that carries the L1 & L2 frequency that consists of the all GPS satellite data and operates within the range of 390-1550 Mhz.

Magnetic North – Points to the magnetic North Pole, which is located in northern Canada. All compasses point to the magnetic North Pole, not true north. Most papers are laid out in true (grid) north. (You must know the declination from your location to correctly navigate to your destination.)

Map Datum – All Maps are drawn with respect to a reference point (specific location). This reference point is known as a datum, so therefore datums are used as a basic of calculating and measuring. Since maps generally only cover a portion of the earth in a larger scale, datum's have been created to pinpoint a certain location. The transition from a global view to a paper map view is where a datum comes in. Similar to the grid system, just on a more precise scale, 100’s of map datum's been created by the U.S., and other countries for navigational purposes. The U.S. generally uses the World Geodetic System 1984 (WGS84) or North American Datum 1927 (NAD27).

Map Scale – Maps are scaled out according to a certain measurement. Most maps, especially USGS maps are scaled to 1:500,000, 1:100,000 & 1:24,000. The most popular and most detailed map scale is 1:24,000, which means that for every one inch on the map, it represents 24,000 inches on the ground. Generally the 1:24,000 covers less area on a map (but is more detailed) than the 1:100,000 and so on.

Navigational Message – Information within the GPS satellite signal that contains velocity, time, position.

Navigational Screens – Visual screens or pages that display statistical data about your adventure. Some data on these pages can be changed and some can not.

1. Map Page: displays the map of the area you are in, along with variable data fields.

2. Compass Page: displays the compass bearing, along with variable data fields.

3. Trip Computer Page: displays a variety of variable statistical & navigational data used on the expedition.

4. Satellite Page: displays the satellite locations in the sky, how many your GPSr has locked and the accuracy of the receiver.

5. Menu Page: displays icons where the user can change preferences and advanced features of the unit.

6. Altimeter Page: displays altimeter readings or profile & barometric pressure plots, plus more.

Navigational Statistics – Calculated data according to its title. Many GPS receivers are capable of recording or displaying a wide variety of statistical data that you may find useful on your adventure.

NAVSTAR – The original name of the GPS Satellite System. It stands for NAVigation Satellite Timing & Ranging.

Northing – This refers to the distance north or south of a fixed reference point such as the equator. You will see this value used in the UTM system and some others like the Military Grid Reference System (MGRS). 

Precision Code (P Code) – Code typically used by the U.S. military that increases GPS accuracy to within a foot. This code is encrypted and reset weekly to prevent unauthorized use. Also known as the Y-Code.

Pseudo-Random Code – Helps to separate the GPS Signal sent by the satellite and mirrored by the receiver from background noise.

Quadrifilar Helix Antenna – A type of GPS antenna (used in the GPSMAP 60 Series) and others that is more sensitive then a patch antenna and is capable of searching for satellites on a 360o 3D plain. This type of antenna contains four spiraling elements that are contained within a plastic case for protection and durability.

Selective Availability – Intentional random error distributed by the U.S. Government that can be added to GPS signal to degrade accuracy. SA is currently disabled.

Triangulation – Uses the laws of trigonometry to determine the location of an unknown point. An example of its use is in GPSr’s to determine location along with the GPS Satellites.

True North – Is simply the direction to the north pole (very similar to grid north, both are used interchangeably), but the north pole is not magnetic north. The difference between the two is called declination.

The United States Department of Defense (DoD) – Maintains and controls the GPS system along with a variety of other duties.

Units of Measurement Based off Miles In Decimal Format

The charts below depict decimal miles and its equivalents in yards, feet, meters & kilometer.

Basic Conversions