GPS Units

 What is GPS?

GPS, which stands for Global Positioning System is a worldwide navigation system that is comprised of 24 orbiting satellites and the ground stations that monitor them. A GPS receiver will use preferably four or more of these satellites as reference points to calculate its relative position on the earth’s surface to an accuracy of metres (centimetres with very advanced systems). GPS works in any weather conditions, anywhere in the world, 24 hours a day and there are no subscription fees or setup charges to use it.

How GPS Works in a Nutshell

GPS satellites orbiting at around 18,000 kilometres high are encircling the earth twice a day in a very precise and known orbit. These satellites transmit 3 different pieces of information, which includes:

• Pseudo-random Code – A unique I.D. code in a high FM frequency which resembles noise signals. This code is used to identify which satellite is currently transmitting and is also used to synchronise the atomic clock on the satellite to the quartz clock on the GPS receiver.


• Almanac Data – This data is periodically transmitted to the GPS receiver. It tells the receiver the approximate locations of the satellites, the status and the current date and time. Using almanac data saves time by letting the receiver skip looking for satellites that are below the horizon. GPS satellites include almanac data in the signals they transmit to GPS receivers. Although variations in satellite orbits can accumulate with time, almanac data doesn't need to be highly accurate to be useful. The data that is collected before your receiver was last switched off may remain usable for weeks or months.


• Ephemeris Data – This positioning data is much more accurate and is monitored and uploaded to the satellites by the US Department of Defence’s ground stations. This in turn comes back down to the GPS receiver and provides an updated position of each GPS satellite and where it should be at any time throughout the day. To accurately calculate your location, ephemeris data is only usable for a limited time (a few hours or less). Up-to-date data is needed to minimize error that results from minor variations in a satellite's orbit.

For the GPS receiver to work, it needs the 3 pieces of information to calculate the positions of at least three satellites (although, four satellites are preferable). The GPS receiver also needs to calculate the distances to each of these satellites. The GPS unit receives the Pseudo-random code from one of the satellites to let the receiver determine the distance to that satellite. The receiver does this by generating the code itself and then shifting the code back until it’s synchronised with the satellite’s code. The delay in this shift determines the length of time the radio signal has taken to reach the receiver. This figure which could be measured to the nanosecond (one billionth of a second) is then used by the receiver to calculate the distance to the satellite.

The reason why it’s preferable that the GPS receiver has four satellites in view is so it can obtain not only the latitude and longitude, but also the altitude. The distance and position of the fourth satellite is also used for obtaining more precise calculations and for error checking in regards to synchronisation. With this information and the data from the Almanac and the Ephemeris, a very highly accurate GPS position can be achieved. Once this position has been determined, the GPS unit can calculate other information, such as speed, bearing, track, trip distance, distance to destination, sunrise and sunset time and more.

 Using a GPS Unit

Electronic GPS receivers are becoming very affordable and the technology is accessible to virtually anyone. Many of these receivers have been designed to be extremely portable, which is perfect for 4WDs and laptops. A GPS unit can be used to mark landmarks, turns and corners on an otherwise non-descript track, allowing you to accurately backtrack if needed. Use it properly and you can confidently venture along unmarked tracks and trails that are not on your maps – relying on your GPS to bring you right back to your start point.

 GPS Types and Applications

There are a many different brands and models of handheld GPS units out there in the market and it can be quite difficult to choose the right one for your intended use. The main types include: basic GPS handheld units and map-based GPS handheld units. There are also GPS receivers for laptops and PDAs that can be used via a GPS Compact Flash card, a GPS mouse or a GPS Bluetooth unit. To learn more about these GPS devices with some practical tips and intended usage applications, please read our article: GPS for Laptops and PDAs.

Before we touch on the different types of GPS units, it’s worth mentioning that it’s highly recommended to carry the traditional magnetic compass and paper maps with your electronic GPS. If the electronic GPS unit fails due to problems like: the batteries failing or the unit gets damaged from water or shock – having a backup means of navigation can be a lifesaver. Another reason is a paper map can give you a larger area to work with than a GPS screen, which is handy for route planning on a grander scale.

Click here to see our range of handheld GPS Units or click on a product below.

Basic GPS Handheld Units

This type of receiver is popular for hiking and bushwalking due to the light weight and compact nature of the basic handheld unit. Many units are designed to be built rugged for outdoor use with some units being waterproof. Most units can display the position in various formats such as latitude and longitude or UTM. Since these receivers have extremely limited mapping ability, they are generally used in conjunction with paper maps with position format grids.

These units can be quite limited in regards to vehicle use, mainly because it’s hard to distinguish what’s going on in the small screen. A navigator in the passenger seat who is working with the GPS unit and a paper map is highly recommended because the driver needs to only focus on driving. Besides being highly portable and generally built rugged, these units are affordable and depending on the quality of the unit, you can expect a battery life of around 12 to 22 hours.

Below is a list of the typical features of a basic GPS receiver:

• Position in the format you choose


• Altitude above sea level (although, altitude will not be as accurate as position)


• Compass heading


• Accurate time in 12 and 24 hour mode


• Satellite status, location and signal strength


• Provide calculations such as the total distance traveled


• Functionalities such as: record path as set of waypoints, navigation of routes and step retracing

Map-based GPS Handheld Units

These units are slightly more expensive than a basic unit due to the added mapping capability. Unfortunately, to have this added capability, more power needs to be consumed. With the extra or larger batteries, the unit is slightly bigger and heavier and the batteries will have a shorter lifespan.

There are benefits with the added mapping feature and this includes the ability to take map images you've previously purchased on CD/DVD and have stored on your computer to the GPS unit so that the maps can be viewed on the screen of the GPS. The biggest limitation with these mapping GPS units is that to date, the units will only accept proprietary map file formats - being exclusive to the manufacturer of the unit. This means that by buying a mapping GPS unit your access to maps is limited by the range available by the manufacturer and does not offer you the mapping flexibility you would otherwise have. These proprietary maps are in vector format, which is an image derived from database values and is not a static image such as a raster map. It’s these values which are used to draw the lines, points, curves and other graphical objects onto the screen. Besides the fact a raster map or a scanned map file is much too big, the main reason they are not used is because there’s no relationship between the map and the dynamic data.

These units are similar to the basic handheld units in regards to vehicle use. Again, the small screens make it hard to distinguish all the various elements in the map, therefore making it only suitable for the passenger. Like basic units, many of these are intended specifically for the outdoors and designed to be robust, shockproof and waterproof. Typically, they will also have more features than a basic unit with some including a barometric altimeter, an electronic compass and calendar.

The main advantages of both types of handheld GPS units are:

• They are great for hiking, bushwalking and other outdoor activities


• Plan your route on a computer or laptop at home and then upload the waypoints and plots to the handheld unit.


• They are extremely portable and usually ruggedly built


• Many hold SD (Secure Digital) cards and Compact Flash (CF) cards for additional memory.

 Accessories

There are a huge amount of accessories available for most makes and models of GPS receivers. These include accessories that will alow easy interfacing to your computer, laptop, palmtop or even between two GPS receivers. You can also get GPS antennas and external antennas that will assist in improving weak signals, allowing you to place your receiver where you want it. Other accessories include: AC and 12v DC chargers, SD memory cards, carry bags, mounting plates and brackets, adapter cables for Bluetooth, data transfer cables and other interface cables and much more.

Click here to see our range of GPS Add-ons or click on a product below.

External Antenna

If you're mounting your GPS unit in the car/boat or in a position without clear access to the sky (e.g. dashboard), then an external antenna gives improved satellite acquisition. External antennas are generally waterproof and you can optionally make a permanent installation on the bull bar or some other external location. Some even have magnetic bases for simply placing on the bonnet.

PC Cable with cigarette lighter plug

These are used for enabling computer interfacing and also allowing you to connect the system to the vehicle battery. There are also

In-car mounting bracket

Most good GPS units have optional swivel brackets that enable you to move the unit around to compensate for glare and for passenger or driver reading.

SD Memory Cards

SD, which stands for Secure Digital are tiny stamp sized cards measuring only 24mm by 32mm and a mere 2.1mm thick. They are commonly used for various kinds of applications such as multimedia storage of photos, video and music files. Its compact design promotes ease of handling and the ability to move or ‘bridge’ between different devices. The cards currently come in 2GB and 4GBs, however, there have been 12 and 16GB cards designed.

There are many GPS units that have SD card slots which can take an SD memory card of your choice. This will provide added memory for your GPS receiver in which case you would load with mapping file data to improve the onscreen mapping of your unit.

 GPS Terminology

GPS:- Global Positioning System. A satellite-based navigation system consisting of a network of orbiting satellites in space. These satellites are consistently moving. One of the biggest advantages is that GPS receivers provide accurate world wide position fixes 24hrs a day in all weather conditions. For many people, simply knowing position is sufficient, but the real power of GPS technology is the ability to direct you TO a destination.

Waypoint:- The term for a co-ordinate identifying a known position. GPS users can enter a Waypoint into their GPS unit and the GPS will tell you the distance and direction to it, time to go until you reach your destination and other functions depending on your unit. Most, but not all GPS units, allow you to store the Waypoint as a name instead of a number, some allow you different colour and shape symbols.

Route:- A route is a series of waypoints that allow you to navigate from one Waypoint to another in that particular order. When you arrive at your first destination, the GPS automatically advises your course and distance to your next destination. The ability to store routes is very helpful. Most GPS units 20 routes and some even have reversible routes, great for when you are ready to come home along the same route.

Geodetic Datum:- If your Waypoint positions are not where you think they should be it is most likely caused by a datum mismatch. Datums are complicated so I shan't go into it here. Putting it simply, a Datum is essentially a measurement system - it describes the size and shape of the earth. Coordinate systems are relative to the datum. Both the GPS receiver and the map in use can be set to use various datum types, however a datum mismatch can result in position errors. Typical map datums for GPS users are the World Geodetic System 1984 (WGS-84), Australian Geodetic Datum 1966 (AGD66), and the Geodetic Datum of Australia 94.

Georeferencing:- Refers to the manner by which locations in digital map images are related to actual earth-surface positions. Digital maps with georeferencing means they are calibrated to enable pointing to any map position with a cursor and obtaining an accurate grid coordinate. Maps with georeferencing are of great benefit to drivers because they can be automatically interfaced to a GPS receiver to set Waypoints, Routes, etc.

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