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GPS for Day-to-Day Use and Survival, by Mike S.

Reading accounts of people who had evacuated the Gulf Coast during Hurricane Katrina and Hurricane Rita was a sobering experience. Evacuees who took to the interstate highways effectively ended up in giant parking lots. In contrast, those who used the back roads fared much better and were able to evacuate in a timely manner. I live sufficiently inland that hurricanes do not pose a serious threat to me, nor do other foreseeable regional natural disasters such as earthquakes pose a serious risk. However, I live in the middle of a major metropolitan area where man-made disasters and localized natural disasters can and do happen. Similarly, a disaster can impair my ability to even get home. I also know from personal experience that even “normal” weather-related events such as ice storms can turn the major highways into near-parking lots, and knowing the back roads can save precious time.

Global Positioning System (GPS) receivers have come a long way since they were first introduced to consumers in the 1990s. My first GPS receiver, purchased in 1995, had no inherent map capability. It provided position (latitude, longitude, and elevation) information, along with a bearing while traveling. It had the ability to store way-points, and to record tracks for later review or backtracking. Way-points and tracks were displayable on the graphical display of the unit, but it only showed where you had gone or places where you already knew the coordinates. Using it to its full potential required that it be used in conjunction with a high quality map, such as a United States Geological Survey (USGS) topographical map. By the mid-2000s, GPS receivers with mapping capability became available for a reasonable price. Today, GPS receivers with mapping capability are available for under $100.

There are competing systems to the US GPS system. The Russians have their own operational global navigation satellite system (GNSS) called Glonass. The European Union is currently developing their own GNSS, Galileo, expected to be operational in 2013. And, the Chinese are promising to deploy their own GNSS, called Compass, announced to be operational in 2015.

Further, there are regional satellite based position augmentation services that improve the accuracy of GPS. In North America, the Wide Area Augmentation System (WAAS) is operated by the Federal Aviation Administration (FAA). Europe operates the European Geostationary Navigation Overlay Service (EGNOS). Japan has the Multi-functional Satellite Augmentation System (MSAS). Other regional GPS augmentation systems are under development or being deployed.

This discussion is going to focus on hand-held and automotive GPS receivers – receivers I believe would be useful in an emergency situation. Hand-held GPS receivers run on batteries and are intended for outdoor use. Automotive GPS receivers are intended for use in an automobile, and provide turn-by-turn route navigation capability. Some hand-held GPS receivers have route navigation capability. I don’t consider a GPS receiver that relies on a computer or PDA for display of data to be practical for emergency use since there are too many pieces to be forgotten, lost, or damaged in the “fog” of an emergency evacuation.

Many cell phones also have GPS capability, and while GPS-enabled cell phones are useful during normal times, they rely heavily on the cellular network to provide map and routing information, and should not be relied upon during an emergency when the cellular network may be overloaded or compromised.

Automotive GPS receivers

Automotive GPS receivers provide astonishing capability for their price, but they are not truly portable. For example, the TomTom ONE 125 unit has been readily available for around $100. It comes with a fairly detailed pre-loaded map of US streets and highways, and has a built-in lithium-ion battery which will power it for about three hours. It is intended to get its power from a vehicle. The map mode display is nearly as good as that provided by Google Maps which contributes significantly to its usability. More expensive units will provide larger displays, larger maps (e.g. all of North America), and more points-of-interest (POI) in the map database. (Several of the automotive GPS receiver manufacturers have started providing free or reasonably priced map update services for their road map products. This may or may not be important to you depending on how much the streets and roads change in your area of interest.)

Automotive GPS receivers are known for providing turn-by-turn directions from your current location to your destination. Destinations are either selected from the POI database, entered as a street address, or even entered as a latitude-longitude coordinate. Many reviews of automotive GPS units will complain that the unit does not navigate you to the exact address entered, but may be off by a house or two. My automotive GPS receiver misplaces my home address on my street – it appears to assume that addresses are numbered proportionally from 0 to 99 along the block with 50 being the mid-point of the block, and estimates the position along the street from the numerical address. I don’t consider this a significant issue. My automotive GPS receiver is also capable of generating a route to a known latitude-longitude coordinate position, so long as that position is close to a street or road in its map database. However, it won’t generate a route to a previously stored location hundreds of feet from a street or road, such as a location in the middle of a large parking lot.

Handheld GPS receivers

Handheld GPS receivers fall into several broad categories. Bare-bones units whose functionality consists of storing some small number of way points and the ability to direct the user back to one of these waypoints. Basic units whose functionally is not a whole lot dissimilar to those produced in the mid-90s in that they can record tracks and waypoints, and provide coordinate information. And, mapping units that have pre-loaded/built-in maps and usually have the ability to upload additional map information (many caveats here).

Bare-bones: I am only aware of one GPS receiver on the market with this limited feature set. This unit is the Bushnell BackTrack. It has the ability to store three waypoints set by pressing the “Mark” button when you are at a location you want it to return to later. There is no ability to enter waypoint coordinates. It provides a bearing and distance to direct the user back to one of the three previously stored waypoints. The bearing is displayed via one of 16 triangular points spaced around the perimeter of its round display being energized. The three-digit range is displayed in yards (or meters) or miles (or kilometers) depending on the magnitude of the distance to the waypoint. The BackTrack is intended to help a user return to their car in a large parking lot or find their way back to a hotel in a strange city. It may also be useful in helping a day hiker return to his vehicle, or helping a hunter return to a stand. I see little practical utility for a receiver with this limited capability in a SHTF [1] scenario.

Basic: There are many basic GPS receiver models on the market, the most common being the yellow Garmin etrex (not to be confused with the many mapping etrex models). These models generally provide a compass display, velocity displays, position displays (latitude, longitude, and elevation), and can display a map-like plot of your route tracks and way points. They do not contain any type of base map. They have the same basic capabilities of units sold in the 90s, with updated hardware. They must be used in conjunction with high quality maps to be utilized to their full potential.

Mapping: There are many mapping GPS receiver models on the market. They range from units having a limited base map containing major roads, major streets, and larger bodies of water, to units that come pre-loaded with topographical maps for the entire US. Units containing a limited base map generally have memory for uploading additional map data. Some models use a memory card (SD, or micro-SD) to store the uploaded map data, and some models rely on internal memory. The big caveat is that map data can expensive – on the order of $100 for detailed maps of North America – and generally these maps cannot be shared among multiple units.

No matter how new the map, it will contain old and erroneous data. This is a frequent complaint in the product reviews of electronic map products.

Some hand-held mapping GPS receivers have routing capabilities. With the addition of routable maps, the receiver can function as basic automotive GPS receiver. It will beep and display a message to alert the driver/navigator of upcoming turns. At best, a hand-held mapping receiver is a compromise relative to an automotive GPS receiver due to the small screen size and lack of voice prompts. (Do not underestimate the value of voice prompts when traveling in heavy traffic or in a dense urban environment with numerous streets and exits.)

The Garmin user community has developed open source (free) map products using US Government data and other data unencumbered by use restrictions. For US roads, the Ibycus map is very nice, but lacks the metadata utilized by the routing software built into some Garmin GPS receivers. Further, there are open source topographical maps of the US derived from US government data. The Ibycus and topographical maps are available online from GPS File Depot [2].

There is another site (http://garmin.na1400.info/routable.php) that has routable street maps for Garmin GPS units. As of this writing, I have not tried the maps available on that site.

Some mapping units also have the ability to upload satellite images and other image data from the internet. I have not studied those units in any detail.


Supportability relates to the resources required to support the ongoing operation of the GPS receiver. For automotive GPS receivers, this means gasoline to power the vehicles within which they are used. For hand-held GPS receivers, this is largely its battery consumption. In anything other than a short-term emergency situation, I don’t consider an automotive GPS receiver to be sustainable because of the dependence on gasoline supplies for its host automobile.

Currently marketed hand-held GPS receivers have widely varying battery consumption rates. Some are as low as 10 hours on 2 AA cells (many models), while others claim to be as long as 50 hours on 2 AA cells (Lowrance GO and GO2). Most hand-held GPS receivers use AA cells, while a few use AAA cells. Whether disposable alkaline batteries or rechargeable batteries are used, I am interested in units that have longer battery life. Further, I do not consider hand-held GPS receivers with built-in rechargeable batteries or a proprietary battery pack to be supportable since recharging the battery in the field would be impractical.

Mapping GPS receivers, whether hand-held or automotive, are generally dependent on a personal computer (PC) for map installations and updates. Some GPS manufacturers also sell their maps preloaded on memory cards for their GPS units that accept memory cards. Once map data is loaded onto the receiver, it can be utilized without further updates by a PC.

Position Accuracy & Chipset Sensitivity

The typical GPS receiver specifications will state a position accuracy of less than 15 meters (49 feet) RMS 95 percent of the time, or less than 3 meters (10 feet) 95 percent of the time with WAAS. WAAS is a system for North America with two geostationary satellites that transmit GPS correction information to dramatically improve the position accuracy of GPS receivers. (See the Wikipedia entry on WAAS [3] for more information.) Most WAAS capable GPS receivers also support EGNOS and MSAS.

Even when GPS receivers have the same position accuracy specifications, receiver sensitivity and other design parameters make a big difference it the actual position accuracy. Position accuracy is a function of the number of satellites the GPS receiver can receive and the quality of the satellite signals. Three satellites are the minimum required to get a two-dimensional position fix, and four satellites are required to get a three-dimensional position fix. The more satellites that are received, the better the position solution will be. In practice, obstacles like mountains, buildings, or trees are going to attenuate the satellite signals and affect the position solution. But, software and chipset sensitivity also have a big influence on position accuracy.

Most GPS receivers sold now have WAAS capability, but just because the receiver is advertised as being WAAS capable does not mean that the WAAS feature actually functions. In 2007, the FAA moved their WAAS transmissions to new satellites. Magellan GPS receivers had hard coded the WAAS satellite data in the firmware for their hand-held GPS receivers, and many of these receivers did not transition to the new satellites. The firmware for many of their older hand-held receivers (pre-Triton models) can be hacked to update the satellite data and re-enable WAAS. As of this writing, WAAS does not work on the lower-end Triton models, and nobody has yet figured out how to hack the Triton firmware. The Lowrance iFinder GO receivers appear to have a similar firmware problem.

Not all GPS receivers have the same sensitivity. Chipset sensitivity is important. My mid-19s90s vintage GPS receiver has noticeably diminished sensitivity under many trees. In contrast, a modern high sensitivity chipset will pick up most satellites visible above the horizon, even when the signal travels through the brick walls of a typical residence.

So, how important is position accuracy? Well, it all depends on what you want to do with the receiver. If you are trying to return to a camp site, a one-hundred foot position error is probably close enough. If you are trying to find the location of a buried cache, one hundred feet probably isn’t close enough. However, a position error of less than ten feet will probably be close enough to locate the cache.

Using Your GPS Receiver

Start up. When a GPS receiver is first taken out of the box or after it has been stored for several months (a “factory start’), it requires upwards of 15 minutes with a clear view of the sky to download the almanac and ephemeris data necessary to compute an accurate position. (Some GPS receivers come from the factory preloaded with almanac data, and if that almanac data is current the receiver can get a first fix out of the box in seconds.) Older consumer GPS receivers produced in the 1990s that do not have parallel receivers can take far longer (up to several hours) to produce an accurate position result from a factory start.

Subsequent power-ups of the receiver, after having been off for a few minutes (“hot start”) to a few hours (“warm start”) will produce an accurate position result in a few seconds to less than a minute if it has a clear view of the sky. If the receiver is left off overnight or for several days (“cold start”) the receiver should produce an accurate position result in a minute or so if it has a clear view of the sky.

Antennas. Most consumer GPS receivers now have internal antennas. Some are patch antennas and some are “quadrifilar helix” antennas. The patch antenna is normally facing up when the GPS receiver is lying on a flat surface. The quadrifilar helix antenna is normally facing up when the GPS receiver is standing vertically. It is beneficial to know what kind of antenna your receiver has and the orientation of that antenna to achieve optimal results. Some owner’s manuals will tell you what type of antenna the GPS receiver contains or suggest how to hold the receiver for optimal performance.

For example, the Garmin etrex Legend and Legend HCx have patch antennas. In practice, I have had excellent reception having them standing up at about 60 degrees on the dashboard of my vehicle.

Satellite Status Page. Most GPS receivers have a satellite status page that will provide information about the position of the satellites in the sky and the relative signal quality from each individual satellite in the form of a bar graph. Some GPS receivers have a dumbed-down “normal” satellite status page, and an “advanced” page – you want to use the advanced page. When I have seen my estimated position error degrade or I get a “satellite signal lost” message unexpectedly, the satellite status page can be very helpful in determining the source of the problem. No signal from some satellites could suggest that their signal is being blocked by a mountain or a building. Uniformly low signal quality could be the result of the signal being attenuated by tree cover.

Roadway Routing. GPS receivers with routing capability  have preferences that allow you to select the type of route you want it to generate. Typical options are fastest route, shortest route, avoid freeways or highways, walking, or on a bicycle. Some receivers further have options for the type of vehicle (e.g. automobile, bus, truck) you are driving – this option can dramatically change the route generated. Most routing units will automatically recalculate your route to reach your destination if you deviate from the planned route (e.g. you miss a turn), unless you disable this option.

Updates. The major GPS receiver manufacturers occasionally make firmware updates available for those models that can connect to a PC if that PC has internet access. With rare exceptions, it is worthwhile to keep your GPS unit’s software updated to the latest firmware version available from the manufacturer. These updates will correct bugs and may introduce minor enhancements.

Practice, Practice, Practice. Use your GPS receiver. Practice with it. Get to know how it works in different environments, how fast it starts up, how to navigate through its various menus. Figure out now how to mount it in your vehicle – windshield suction mounts work very well.

Paper Map and Compass.  A GPS receiver is not a substitute for a paper map and compass. GPS receivers, especially the mapping variety, are just easier and faster to use. Use your GPS receiver to help refine your map and compass skills. (You can also use your GPS receiver to help verify that your compass reads true by obtaining the coordinates of some prominent feature, and then computing the magnetic bearing from your compass test point to the feature. Yes, even a genuine military lensatic compass can be off by several degrees.)

For information about using GPS receivers with maps (specifically topographical maps), I’d recommend the book GPS Made Easy [4] , by Lawrence and Alex Letham. While the book is directed at hikers and other outdoor enthusiasts, it provides a good discussion, using real-world examples, about navigating with GPS receivers using topographical maps with different coordinate systems. The book is now in its fifth edition. The fifth edition omits a discussion about the use of a map and compass for backup navigation, in the event of GPS receiver failure, found in the previous editions.


I believe GPS receivers have a place in emergency preparations. While probably not useful in a long-term TEOTWAWKI scenario where the GPS constellation will most likely have failed, they certainly have a place in many SHTF scenarios.

I have used GPS receivers from several different manufacturers. For hand-held units, I have a definite preference for Garmin units – they work as advertised. If you get a Garmin handheld unit, I recommend that you go for a “high sensitivity” model that accepts SD or micro-SD cards for map storage, uses AA cells for power, and connects to a PC via a USB cable. The only caveat, and this applies to all manufacturers, is to avoid newly-introduced models. Give the manufacturer some time to work out the bugs.

For automotive GPS receivers, I have had the most experience with the TomTom ONE 125, which is TomTom’s low end model. TomTom’s more advanced models just add features to this basic model. The Garmin automotive GPS receivers are well respected, and I know several people who are happy with their units.

If you can get only one GPS receiver, get a hand-held mapping unit with routing capabilities (e.g. the Garmin etrex Legend HCx), and load a routable map package (e.g. Garmin City Navigator NT) onto it. A handheld GPS receiver can continue to serve you if you are forced to abandon your vehicle, or are otherwise forced to travel on foot. If you can get more than one unit, add an automotive GPS receiver from a major manufacturer.


Below I provide opinions of several currently available mapping GPS receivers that I’ve personally been able to use. My simulated forest canopy is my traditional single-story wood-frame house with asphalt shingles and a brick exterior. GPS receiver performance in my house is similar to that which I have experienced under a tree canopy. Position accuracy is verified by entering the coordinates provided by the GPS receiver into Google Maps with satellite images, and comparing the position plotted by Google with the actual location on the satellite image. Further, position accuracy is only measured after the GPS receiver has had sufficient time to download almanac and ephemeris data from the satellites. All of these GPS receivers perform well outside, including when placed on the dashboard of a moving vehicle.

Garmin etrex Legend: The Legend [5] is a hand-held mapping GPS receiver with a high level base map that contains major streets and highways, larger bodies of water, and cities. The four-level gray-scale display is very readable under most circumstances, and it has a back light for night viewing. It has 8M bytes of memory for storing map data, which will not hold a lot of map data. Battery life is advertised to be 18 hours on two alkaline AA cells. I have not timed the battery life, but I have no reason to believe that the advertised 18-hour run-time is unreasonably optimistic. The GPS receiver chipset is not “high sensitivity” but I can pick up many of the visible satellites under my simulated forest canopy. This receiver also has WAAS capability, which dramatically improves its estimated position error. I have seen estimated position error values as low as 6 feet from this unit. In early 2009, this model was replaced by an upgraded model called the etrex Legend H [6], which utilizes a high sensitivity GPS chipset, has 24M bytes of map memory, and connects to a computer utilizing USB.

Garmin etrex Legend HCx: The Legend HCx [7] is a hand-held mapping GPS receiver with a high level base map. The color display is very readable under most conditions, with an excellent back light for night or low-light conditions. It accepts micro-SD memory cards. Battery life is advertised to be 25 hours on two alkaline AA cells. It utilizes a high sensitivity chipset that picks up virtually all satellites in the sky under my simulated forest canopy. It is WAAS enabled, and can produce position solutions with estimated position errors under ten feet. It connects to a computer utilizing USB. The USB port in the unit can also provide power to the receiver in a vehicle if a cigarette lighter USB power supply is used.

With the purchase of the Garmin City Navigator NT map package ($100) and a 2 GB micro-SD memory card, routable maps can be loaded into the Legend HCx allowing it to function as basic automotive GPS receiver. It will beep and display a message to alert the driver/navigator of upcoming turns.

Lowrance iFinder Go2: The Go2 [8] is a hand-held mapping GPS receiver with a high level base map containing major streets and highways, large bodies of water, and cities. The base map contains many smaller bodies of water not found in the Garmin base map. What makes this unit intriguing is an advertised battery life of 50 hours on two alkaline AA cells. The GPS receiver chipset is not high sensitivity, but it can pick up some satellites under my simulated forest canopy. This receiver also has WAAS capability, but this feature may not be functioning properly since I have not seen estimated position error values below 16 feet. While this unit has 64M bytes of storage, the manufacturer does not support upload of map data into this unit.

Magellan Triton 200: The Triton 200 [9] is a hand-held mapping GPS receiver with a high level base map that contains major highways, larger bodies of water, and cities. After performing a necessary firmware upgrade, a significantly improved base map is loaded in the unit. The color display is difficult to read under many circumstances without the back light being turned on. With the back light turned on, the color display is beautiful. It has 10M bytes of memory for storing map data, which will not hold a lot of map data. Battery life is advertised to be 10 hours on two alkaline AA cells, which seems to be rather optimistic (6 hours is a more realistic estimate). It utilizes the high sensitivity SiRF Star III chipset, which picks up virtually all of the satellites in the sky under my simulated forest canopy. It is WAAS capable, but the WAAS capability may not be functioning (postings on several forums indicate it is disabled) since I have not seen estimated position errors below 13 feet.

The Triton 200 connects to a PC using a proprietary USB cable. However, there are many reviews, substantiated by my personal experience, indicating that many users have difficulty connecting their Triton GPS receivers to their computer. I could not get it to stay connected to my main computer long enough to even start the firmware update. However, it worked flawlessly with my wife’s computer. (Note: My Triton 200 came with the USB cable, but the manufacturer’s web site suggests Triton 200s do not come with the cable.)

TomTom ONE 125: The TomTom ONE 125 [10] is a  basic automotive GPS receiver. It has a nice 3.5 inch color display. It provides voice prompts and warnings, but does not speak street names. It only contains street maps for the US. It has 1G byte of flash memory built into the unit for map and software storage. Memory is not expandable. The auto-route capability makes some surprising choices – choices I would not have made and that are not optimal based on my driving preferences. However, auto-routing will get you to your location. As mentioned above, this receiver is also capable of generating a route to a known latitude-longitude coordinate position, so long as that position is close to a street or road in its map database. Its GPS chipset is incredibly sensitive, capable of picking up virtually all satellites above the horizon under my simulated forest canopy. While I cannot find any information from the manufacturer stating that it is WAAS enabled, its performance and information displayed on the satellite status page lead me to believe it is WAAS enabled. It also has a built-in rechargeable lithium-ion battery that delivers the advertised 3 hour battery life. This GPS receiver is readily available for $100 – a tremendous bargain for the features it provides.

Definitions and Notes

Almanac and ephemeris data – Almanac and ephemeris data are used by the GPS receiver to precisely compute satellite positions, and hence your position. All GPS satellites transmit almanac data providing coarse information about the orbital position of all satellites in the GPS constellation. Each GPS satellite further transmits its own ephemeris data which provides precise position information about its orbit. The almanac data is generally considered to be good for several months, but is updated daily. The ephemeris data is considered good for only about five hours. Almanac and ephemeris data is continuously transmitted. Full download of the almanac data takes 12.5 minutes, after the receiver has locked onto a satellite signal. Each satellite retransmits its ephemeris data every 30 seconds.

Base (or background) map – A base map is the default map built into a mapping GPS receiver. The base map typically contains interstate highways, US and state highways, four-lane city streets, incorporated towns and cities, lakes and rivers, and shoreline information. The detail of the base map varies from receiver to receiver, and can be a differentiating feature between two seemingly similar receivers. GPS receivers are typically regionalized, and will be loaded with base maps for the region (e.g. North America) where the GPS receiver is expected to be sold.

Patch antenna – A compact flat antenna, with a metal “patch” positioned above a ground plane. The greatest sensitivity is perpendicular to the plane of the antenna. The typical patch antenna in a GPS receiver is less than one inch square.

Quadrifilar helix (or “quad helix” or “quadrifilar”) antenna – A cylinder shaped antenna with four spiral elements. The greatest sensitivity is parallel to the axis of the cylinder. Modern quadrifilar helix antennas in consumer GPS receivers can be as small as 10 millimeters (3/8″).

Selective Availability (SA) – A currently disabled feature of GPS designed to deny an enemy use of civilian GPS receivers for precision weapon guidance. SA was designed to intentionally induce errors of up to 100 meters in the unencrypted GPS signals available to civilians. SA was turned off May 1, 2000.

Recent news stories have reported with alarm that the GPS system could fail in 2010. The facts are that GPS Block IIF satellites being built are almost three years behind schedule, and that the probability of maintaining a 24 satellite constellation between 2010 and 2014 falls below 95-percent. The US Air Force’s objective of having a minimum of four satellites visible 95-percent of the time may not be met. As a practical matter, this means that there may be occasions where insufficient satellites are visible to get a 3D position fix [11]. However, there are currently 30 healthy satellites in orbit, and three older satellites that could be reactivated if necessary [12]. The chance of the GPS system failing is infinitesimally small.

This author has no affiliation with any manufacturer, distributor, or retailer of any product mentioned in this article. All brand names and product names used in this article are trade names, service marks, trademarks, or registered trademarks of their respective owners. The opinions expressed in this article are those of the author. And, as always, your mileage may vary, so use this information at your own risk.