Device Feature Considerations for Geocaching

Buying a GPS Device

You can usually find GPS units at electronic, camping and boat supply stores or online. Still cannot decide what device to buy? Ask geocachers what they use in the Groundspeak Forums. In the forums, you can search latest news, post product related questions and even browse for used devices for sale!

• Visit gpsinformation.net for guides and reviews.
• Buy and compare online at Amazon.com and Offroute.com

Device Feature Considerations for Geocaching

• Basemap. This is highly recommended. The additional cost is marginal, and the increase in the equipment’s usefulness is substantial. Driving directions to the area near a cache can be incredibly useful, so consider this feature in a higher end device.
• Channels. Most GPS units today now have 12 parallel channels. Channels help to acquire GPS satellite signals faster and more accurately. If you are looking at older GPS devices, consider models built after 1997. Older, single-channel receivers are much slower and may not be as accurate.
• External antenna jack. With new GPS devices this has become less important since the newer chips are sensitive enough to acquire signal inside a vehicle. However, it is not always possible to obtain good satellite coverage through a front windshield; this is when an external antenna can help. Even backpackers will benefit from the ability to safely store the receiver inside a pack with an antenna attached to backpack shoulder straps. It can also help outdoor use in places, like heavy tree cover, where signals are weak.
• Interface jack. If you are using the receiver with a computer, be sure that it includes an interface cable so you can quickly load maps and waypoints onto your device. Newer GPS units often support USB while older devices generally have serial attachments. If you purchase an older GPS device and have a newer computer, you may need to purchase a USB to serial port adapter as well. If you own a Mac, you should check to ensure that it supports the Mac operating system.
• Memory. This is used to load detailed topographic maps, street-level maps, or additional waypoints into the receiver. Detailed maps can use a great deal of memory on your device. Consider internal storage capacity. Higher end devices also usually accept a memory card for additional storage.
• Power source. It is preferable to have a device that can support external power, like a cigarette lighter power cable. Due to the power requirements the device should support standard batteries (AA or AAA) for easy replacement on the trail. Lithium batteries are recommended for newer GPS models to ensure a longer battery life, and power geocachers should consider rechargeable batteries.
• Rocker keypad or touchscreen. Using a receiver without a rocker keypad is like using a computer without a mouse. Some newer GPS units have touchscreens which helps immensely when navigating using the maps.
• Screen size. For visual ease of operation, use a device with the largest screen that can be realistically carried. Screen size is measured diagonally. Color is great and helps define map features and a backlight is important, although both features burn additional battery power. A screen protector is important in preventing scratches out on the trail.
• Waterproof. Sooner or later your device will get submerged. Get a device that is at least water-resistant, although an IPX7 designation is preferred. An IPX7 designation means the GPS case can withstand accidental immersion in one meter of water for up to 30 minutes. Plastic bags and boxes are also recommended for added protection.
• Geocaching Features. An increasing number of higher end GPS devices now have functionality specifically created for geocaching that can store all the information from Pocket Queries. Some will also store field notes to help you log your caches after a hunt.

(Excerpts from the book, The Complete Idiot’s Guide to Geocaching)

GPS Signal-Why You Should (or Shouldn’t) Care about SiRF GPS Review

I’ve had many readers contact me with questions about SiRF, the SiRFstar chipsets, and especially the SiRFstarIII chipset. As more and more people look at chipsets when making a purchasing decision, more questions arise. This article isn’t meant to specifically address any of the direct questions asked, however it should provide an overview for everyone.

So let’s take a step back look at this from the perspective of a new GPS user since that is what most readers here are. Just like your computer has a central, core processor chip (like an Intel chip or a PowerPC chip, etc) GPS devices also have a core chip inside which receives the GPS signal and passes that information along to the GPS software. (I’m simplifying things a bit for this conversation.)

What many people might not know is that various GPS manufacturers don’t typically use a GPS chipset they have developed themselves. The GPS manufacturer will build the device and construct the software that you see running on the device, however the “heart” of the GPS receiver, the GPS chipset, most often comes from a different manufacturer.

Thus you can have GPS receivers from different companies that use the same underlying GPS chipset. Thus the strength of the reception from GPS receivers using the same chipset should be identical, as should the initial acquisition time. What might differ between the GPS receivers is the shape of the device, weight, screen size, and perhaps most importantly the software you interact with.

Lately a company called SiRF has been getting a lot of attention from GPS enthusiasts, specifically their SiRFstarIII chipset. This chipset is known for having fast acquisition times as well as being able to receive a GPS signal where other GPS chipsets might not have been able to acquire a solid signal, or “fix”.

So should you only consider a GPS receiver that is built using chipsets by SiRF or the SiRFstarIII chipset? No. While we agree the SiRFstarIII chipset is great I wouldn’t avoid a particular GPS receiver if it didn’t have a SiRF chipset. However if you are currently dissatisfied with the reception of your GPS receiver and it has an older chipset, SiRFstarIII might be something you want to consider in your next GPS.

In short, I wouldn’t avoid purchasing a GPS receiver that wasn’t SiRFstarIII equipped, however when I receive new GPS receivers for evaluation if it has a SiRFstarIII chipset I know the signal will be acquired quick and be strong.

via Why You Should (or Shouldn’t) Care about SiRF GPS Review.

GPS Signal – Do Chipset Channels Matter?

When people start comparing different GPS devices, a feature commonly listed among marketing materials is the number of “channels” a GPS has. Inside every GPS device is something known as the “GPS chipset” which is the heart of finding where you are. Different chipsets use a different number of “channels”. So what are these channels, and are they important in determining which GPS to purchase? First, it might be helpful if you have a basic understanding of how GPS works.

t the most basic level, think of your GPS as needing one channel for every satellite it is communicating with. (Note that technical GPS whiz people will know there are exceptions, but for this basic article we’re going to ignore that.) So the more channels your GPS’s chipset has, the more satellites it can communicate with at one time.

So the more channels, the better, right? Well– no, not exactly. At any given time there are at least 24, more more often about 30 satellites that make up the GPS system. They orbit the earth about 12,000 miles above us. The system is designed to provide similar levels of service anywhere in the world. So at any given moment of time you won’t be able to “see” all of the sites at one time. At least half of them will be out of view on the other side of the planet. Others could be out of view due to ground obstructions (mountains, buildings, etc).

Many GPS devices have a satellite information screen that will show you how many satellites your GPS is tracking at any given time. If you view this page and take your GPS device somewhere with an unobstructed sky view, you can see how many satellites your GPS is tracking, and thus how many channels are being used.

Take the picture to the right of the paragraph above and you will see that the GPS was tracking 10 satellites, so essentially 10 channels were being used. It is rare for a GPS to be tracking more than 12 satellites at any given time. So in that sense– having a GPS which has more than 12 channels might seem like a waste… and to some degree it is.

But channels can be used for more than just tracking a satellite “right now”. Sometimes channels can be reserved for “searching” as new satellites are predicted to come into your view. If you watch the satellite info screen over a period of an hour or so you will see that some satellites disappear from view while other satellites appear on the horizon. So while your GPS might be tracking only 8 satellites, it might be using a couple of other channels to search for new satellites it is predicting will come into view.

Why More?

So why have 20, 30, or 60 channel receivers? To some degree it is a marketing gimmick. Back in the “old days” of GPS, a receiver might only have 8 channels. Then came along the 12 channel receivers which improved reception and acquisition times dramatically. Then came along 16 channel receivers which also showed some improvement. So where to go from there? Add more channels.

Unfortunately, now that the number of channels a receiver has has exceeded the amount of satellites you can realistically track at any given time, the benefits of a 20+ channel receiver are muddier. This issue is further compounded by the fact that many of the popular 20 channel chipsets such as the SiRFstarIII, have a 20 channel receiver. The SiRFstarIII is a great chipset– without doubt, but its greatness isn’t just because it has 20 channels. (The reasons are more technical and beyond the scope of this article.)

The Bottom Line

The more channels the better– to a point. You can’t simply say that just because one chipset has 16 channels and another has 20 channels that the 20 channel receiver will have more accurate, or faster performance. So for the most part, unless you are doing some sort of activity that needs the highest precision possible in a consumer level GPS (which is rare) I tell people these days to largely ignore the number of channels a GPS device has. Other factors such as the location of the chipset in the device and the materials the device is made from will have a greater impact on signal performance.

http://www.gpsreview.net/gps-chipset-channels/

More on antenna’s

Making the most out of your GPS depends on getting a good signal from the satellites, which orbit the Earth at an altitude of around 20,000 kilometers (12,000 miles). This signal isn’t a strong one; it’s roughly equivalent to detecting the light from a 25-watt lightbulb from 16,000 kilometers (10,000 miles) away. In other words, without a good signal from the satellites (at least three of them, preferably more), you aren’t going to be able to use GPS at all, or the information it gives you will be erroneous. You can solve this problem by adding an external antenna to your GPS unit. This chapter examines the GPS antenna and how you can maximize a weak signal when you are on the move. The GPS Antenna One part that all GPS receivers have in common is the antenna, whose job is to receive the signal from the satellites and pass that signal on for processing. The signal the antenna picks up is a UHF signal with a frequency of 1575.2 MHz (this is the civilian, unencrypted frequency; the military signal uses 1227.6 MHz). This frequency offers all-weather navigation capability, but is blocked by walls, ceilings, and even trees. GPS receivers usually come with one of the following antenna types attached: Quad-helix Patch Receivers such as the Garmin III, and most Magellan units, make use of the quad-helix style antenna, while units such as the Garmin eTrex use the patch antenna. The debate as to which is best is a complex one, and there are pros and cons to each: Patch antennas are smaller. Patch antennas draw less power. Quad-helix antennas are usually bulky and generally protrude from the unit. Quad-helix antennas are less prone to having the signal from the satellites blocked (or masked, to use the technical term) by the receiver and the person holding the GPS. The surface of the GPS over the patch antenna is prone to catching rain and losing the signal (simply because it is a flat surface). A lot of myth and rumor surrounds antennas. Many people believe that quad-helix antennas offer greater performance under tree cover and are less prone to signal loss in valleys and in urban areas. However, based on personal experience, I’ve not found this to be the case. Usually, such phenomena can be attributed to the particular GPS unit, assuming that your course and speed are unchanged and retaining a false signal lock. A good discussion on antenna sensitivity can be found at www.gpsinformation.net/ main/gpsant.htm. Generally, I don’t really think it matters much which type of antenna you use, but the one difference between the two types of antenna is how you hold them. Quad-Helix Orientation A quad-helix antenna works best when the antenna is placed in the vertical position (as shown in F igure 4-3). This gives the antenna the greatest sky coverage and offers the best signal reception. Some GPS receivers allow you to place the antenna at odd angles, which enables you to get the antenna in the right orientation no matter how the GPS is being held or how it is mounted. However, make sure that the antenna is always vertical and not at odd angles When you are holding the GPS on the move, try to hold it with the antenna as vertical as pos- sible and as far away from the body as comfortably possible to reduce the degree to which your body shields the signal. The human body is very effective at shielding the microwave signal from GPS because your body is largely made up of water, and water is an effective absorber of microwave frequency electro- magnetic radiation. Patch Antenna Orientation How you hold a patch antenna is completely different from how you hold a quad-helix antenna. To get the best out of these antennas, you want to hold them so that they are horizontal to the ground . This is by far the best orientation for patch antennas — the closer they are to being vertical, the less efficient they are. This may be a big part of the reason why some people think that the quad-helix antenna is better than the patch antenna. People have a natural tendency to hold a GPS receiver in a vertical position. Best Performance Summary Before moving on, let’s look at a quick “best performance summary” for built-in antennas for GPS receivers: 1. Hold the antenna as far away from your body (and other human or animal bodies) as possible and away from other electrical devices, as these can all degrade the accuracy of your receiver. 2. If your GPS uses a patch antenna, keep it as close to horizontal as possible. 3. If your antenna is a quad-helix antenna, keep it as close to vertical as possible. 4. Whatever type of antenna you use, keep it as dry as possible — a coating of water droplets will severely affect performance.

External Antennas

One problem with holding your GPS in a nearly vertical or nearly horizontal orientation is that it can quickly become uncomfortable and cause you to start walking odd ly. If you are in a vehicle, then the problem isn’t one of orientation but actually getting the signal to the GPS, as most receivers find it hard to get a lock to the satellites when inside a car or boat. A far better idea is to place the antenna away from the GPS receiver. Some GPS receivers come with either a removable antenna or a socket that enables you to hook up an auxiliary external antenna to the device. The connector is called a BNC connector, and you can get many types of external antennas that connect to this port. F igure 4-7 shows a Lowe external antenna attached to the Garmin GPS III. This antenna has a magnetic base, and draws power from the GPS unit itself. It is a patch antenna and offers better performance than the original antenna. The antenna also has a strong magnet at the base that enables it to be firmly attached to a metal surface (such as the roof of a vehicle). The BNC-style connector isn’t the only style of external antenna connector available. shows the connector on a Garmin 76 GPS receiver. This is called an MCX connector, and it is much smaller than the BNC connector. But they get even smaller! GPS receiver. This is an MMCX connector (or Mini MCX or MicroMate). This connector is much smaller than the MCX connector, but smaller also means that the threads on the connec- tors are more delicate and prone to damage These antennas either replace the existing antenna on the unit or they bypass the built-in antenna after they’re attached. External antennas enable you to place the antenna in a different location than the actual receiver. This means that you (or whoever is holding the unit) will obscure the signal a lot less. It also means you can take the unit into an area with poor signal coverage and put the antenna where reception is better.

Some applications ideally suited to the use of an external antenna include the following: Indoor use: If you want to bring a GPS signal indoors, an external antenna is a great way to do it with little fuss. Car use: If you want to use a GPS in a car, especially a car that has metalized glass installed that blocks microwave radiation, an external antenna might be a possible solution that will enable you to use a GPS. Boat use: The antenna can be attached outside and the GPS kept inside, away from exposure to the elements. Mobile use: Place the external antenna in the top of your backpack while out and about. This can really help to boost the signal! Usually, the higher the antenna, the better the signal.

As with most things, there are disadvantages to using external antennas: Clumsiness: All the additional cables can be a pain, as they can easily get in a tangle. Power: External antennas draw more power from the GPS unit. More power means the batteries don’t last as long. Some external antennas come with their own power source, but remember that this means you have something else to keep charged up while on the move! Antenna loss: If the GPS has a removable antenna, make sure that you don’t lose the original! Cable length: The longer the cable you have attached to the antenna, the greater the sig- nal loss. This isn’t a problem when you are routing the cable over a short distance, but if you are routing the cable over a long distance, then this could be a problem. Keep cables as short as possible; and above all, avoid keeping unnecessary loops of cable in the setup.

Antenna Placement

Where you place an external antenna is just as important as how you hold a GPS receiver. When using a fixed or semi-fixed antenna, you need to carefully consider where you place it in order to get the best coverage. On a car, place the antenna as high as possible. The roof is the best place for it, while the hood and near the flat glass panels are worse. The area around the hood has high electromagnetic interference, while glass and flat metal surfaces cause signal reflections and signal loss Another possible area for placement is inside the front or rear plastic bumper, although bear in mind that small impacts can damage the antenna. The problem with in-car use of GPS is that most people think that because they can get a satellite lock on their GPS when it is in their shirt pocket, in the car, they don’t need to worry about antenna placement. The truth is that poor antenna placement in a car, especially using a GPS signal from inside the vehicle that has traveled through the metal skin of the vehicle, is likely to result in a very inaccurate signal. If your vehicle has tinted metalized windows, this can severly degrade the GPS signal you receive, and an external antenna becomes a must. For trucks and vans, the best place for the antenna is going to be the cab or trailer roof. Both of these offer fantastic views of the sky. Around buildings, keep the antenna away from walls. If possible, either put the antenna high up or at least well away from walls and obstructions. Keep the antenna away from trees In urban areas that have a large number of tall buildings, higher is better because it eliminates multipath errors caused by signal reflection Note one problem with the “higher is better” rule: lightning. A single lightning strike can com- pletely destroy a GPS receiver. This is especially a problem with placing antennas on buildings. If you are going to have a permanent antenna fixed on your building, then prevention is far better than the cure. You are safer if you attach lightning arrestors to the line. There are several manufacturers of lightening arrestors. Here are a few to get you started: Symmetricom: www.symmetricom.com PolyPhaser: www.polyphaser.com Radiall: www.radiall.com LightningMaster: www.lightningmaster.com This solution may seem pricey for something that may never be needed, but if you live in an area where lightning is common, consider how much it will cost you if your antenna is hit and you lose your GPS. In addition, it’s not just the GPS that you can lose — if your GPS happens to be connected to a PC at the time of the strike, you could also lose the PC and possibly even others connected on the same network if you happen to use one.

Other Things to Avoid

Here are a few other things that you should avoid when it comes to using external antennas: Knots and kinks in cables damage the interior of the coax and can cause signal loss. Keep the cables as straight as possible, and if the cable does have to travel around corners, don’t make the loop too tight. Be careful about bringing cables in through windows and door frames, as crushing can severely damage the cable. Another way to damage a cable is to stretch it, so take care to avoid this. If you install the cable in warm weather, make sure you leave additional slack in the cable, as it will contract when the temperature drops. Keep the cable short and reduce the number of connectors: The greater the length of cable and the more connectors and joints you have in it, the more signal loss you will experience.

http://www.gpsreciever.net/antenna.html

GPS Antennas for Consumer GPS Receivers: Which type is best?

GPS Antennas> Patch and Helix types. Which is the best?

Questions are put to us almost daily as to which is the “best” type of GPS antenna. The answer is: There is NO best type of antenna for all GPS applications. The most popular type for consumer model receivers is the Quad Helix style. This style has several advantages and several disadvantages. The most popular alternative is the PATCH antenna. It too has advantages and disadvantages. Other GPS antenna configurations include:

Spiral helices.

Microstrip. (one form is the “patch” antenna.)

Planar rings (aka “choke ring”), and other multipath-resistant designs.

Some people suggest that the Quad Helix antenna is “best” consumer GPS antenna under tree cover. This is not categorically true. There is little difference in performance between patch and quad helix antennas which have the same effective APERTURE. Aperture is directly proportional to antenna size and antenna gain is proportional to antenna aperture. Note that the Garmin G-12xl, G-12map, and eMap (among others) have a patch antenna while the G-76 series and the Magellan Meridian series use quad helix antennas. These models all have similar performance and sensitivity. All GARMIN models except the GPS 12 and eTrex series can be used with an external antenna. Some recent Magellan models have external amplified antenna capability.

The more recent units do have slightly more sensitivity but this is due more to improvements in semiconductor technology than anything else. The trend of improvements in effective sensitivity in newer technology units is likely to continue for the forseeable future and is not necessarily related to antenna type.

Note also that the eTrex models are more compact units than the others listed. As a result, they have slightly less antenna gain (on the order of perhaps 2db

less) than the physically larger models and thus will have slightly less sensitivity which may be important under forest canopy.

Thus, as a rule-of-thumb, physically larger GPS receivers are likely to have better forest canopy receive performance than more compact units regardless of

antenna type.

Now to Specifics.

Many of the earlier Garmin units used a PATCH style antenna similar to the TOKO antenna. Note the antenna patterns and main lobe gain for this model.

An example of a Sarantel Ltd. commercial Quad Helix antenna can be found HERE.

Here is a good article on Quadrifilar Helix Antenna (QFHA) antenna design and construction.

Here is a link to a listing of GPS antennas of various designs which you may be able to construct yourself. (via the Helenav website)

Joe Mehaffey

via GPS Antennas for Consumer GPS Receivers: Which type is best?.

GPS comparison

I have included a link to a spreadsheet that I like to use to compare GPS receivers. You can copy it, or add to it online (which is what I hope people will do).  It contains data for the Garmin GPSMAP 76CSX, and the Lowrance iFinder Expedition C. It also contains link to reviews of these two receivers.

Features

In a previous post on GPS selection I listed some of the things to consider when deciding which GPS to buy. I will elaborate on some of the specific features. For me, the first feature to consider is whether or not I want a mapping GPS. The next step is to see what is available locally, as far as manufactures go. There is no point looking at Delorme units if I can’t buy one. Once that has been decided, it is a matter of which manufacturer has Canadian Topographic maps available, or even Canadian road maps, and what level of detail is included on the basemap, and on purchased map upgrades – who has the best quality maps. I should point out that I will eliminate a manufacturers from my list of potential suppliers if their website sucks. If they can’t make a website clear and concise, chances are that  their firmware will be the same! I also check the website for free downloads and get a sense for how easy it is to access firmware updates, etc. Next I search reviews to try to get a feel for which manufacturer is getting the best reviews overall. Then I look at their GPS models and start comparing features (listed below) and prices.  Then I research the reviews for the models that I short-list.

Features:

• Basic (no mapping): Saves and displays waypoints, locations and points of interest only; no detailed maps
• Includes topographic maps: Comes with preloaded U.S. topographic maps
• Includes freshwater maps: Comes with preloaded U.S. Inland lakes
• Includes saltwater maps: Comes with preloaded BlueChart g2 coastal charts
• Accepts data cards / expandable memory: Add optional detailed maps on memory card
• High-sensitivity receiver (SiRF): Locates your position quickly and maintains signal in heavy cover and deep canyons. DGPS, WAAS, SiRF. DGPS & WAAS seem to be standard on all units.
• Electronic compass: Provides bearing information even while you’re standing still (check accuracy statements)
  
• Barometric altimeter: Tracks changes in pressure to pinpoint your precise altitude (check accuracy statements)
• Geocaching mode: Conveniently view and navigate to caches
• Paperless geocaching: Download caches and details directly to your device from geocaching.com
• Wireless data sharing: Share waypoints, tracks, routes and geocaches wirelessly with similar units
• Color screen: Crisp, sunlight-readable color display
• Touchscreen: Intuitive, waterproof touchscreen
• Floats: self explanatory!
  
• Waterproof: also self explanatory!
  
• Auto Routing: Turn by turn routing on roads
  
• Antenna: Does it have an external antenna connection
  
• Audible Alarms: Proximity, off-course, arrival, etc
  
• PC communication: to upload/download user data
  
• Tables and Calculation: Area calculation, glide ratio, hunt/fish calendar, sun/moon info, tide tables
  
• Trip computer: speed, ETA, time, odometer, tripometer, etc.

Some units include other items such as music players, voice recorders, and cameras – I ignore those features. Then I move on to comparing details like memory, and physical aspects;

• Backlit for night use: screen, buttons
• Number of screens/windows
• Screen size, resolution,
• Thermometer
• Physical dimension and Weight
• Battery life and type, other power options such as 12VDC
• Temperature range for operation
• Accessories

If you are really bored, you can also look at some performance issues;

• Cold and Warm start time
• Acquisition times
• Update rate
• Waypoints/icons (common to all, but quantities vary)
• Tracks (common to all, but quantities vary)
• Routes (common to all, but quantities vary)
• Nearest waypoint lookup
• Altitude included in waypoints
• Backtrack routes (common to all, but quantities vary)
• Map datums (common to all, but quantities vary)
• Position formats (common to all, but quantities vary)

Garmin has a great product selection tool on their website. It allows you to narrow your search by selecting those features that are important to you. You also can compare models side-by-side. They have got all kinds of documentation. Garmin has one of the best websites I’ve seen. Although, getting to some of their extra information seems impossible from the main page. I found the following via a google search. The only thing they could do better is include the details for the competition for side-by-side comparisons. Short of that, you will need to create your own spreadsheet to compare models.

I also download the manuals, software and any product emulators to try and get a feel for how the GPS receiver works; menu’s etc.

Geocaching

Lowrance has an interesting utility

Using a GPS with paper maps

usingagarmingpswithpaperlandmaps_manual1

GPS signal – SiRF

GPS receivers with SiRF are marketed as high sensitivity receivers.

SiRF capitalizes on its architectural innovations, system design and silicon expertise, RF capabilities and state of the art semiconductor manufacturing technology to make GPS capabilities accurate, affordable, power efficient and compact enough to be used in consumer applications. SiRF’s extensive patent and intellectual property portfolio improves on conventional GPS by providing location awareness where other approaches cannot, including under dense foliage, in steep ravines, in “urban canyons” and even in some indoor environments. In the constant tension between lost and found, SiRF’s GPS solutions tip the odds in favor of being found.

GPS signal – WAAS

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

The origins of WAAS

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

How it Works

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

Who benefits from WAAS?

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

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

It just keeps getting better

100 meters: Accuracy of the original GPS system, which was subject to accuracy degradation under the government-imposed Selective Availability (SA) program.

15 meters: Typical GPS position accuracy without SA.

3-5 meters: Typical differential GPS (DGPS) position accuracy.

< 3 meters: Typical WAAS position accuracy.