GPS Receviers for Geocaching
GPS receiver is the most important device if you start thinking about geocaching. You can probably get it in your local sports shop in a variety of sizes, colors, covers, with assorted functions and, last but not least, prices. Rest assured, however, that buying it is the only major cost involved in the game.
In order to use the device properly, it is always wise to know the basics of its operations. We will only deal with it on a very simple level; if you're interested in technical details, there is plenty of information available on the Internet. Let us concentrate on the most important issues here.
GPS – Global Positioning System – is technically defined as "a system of satellites, computers, and receivers that is able to determine the latitude and longitude of a receiver on Earth by calculating the time difference for signals from different satellites to reach the receiver." (www.dictionary.com) What you have in your hand now is just a small part of this system.
The modern GPS has been built out of two separate satellite navigation systems created and operated by the US Navy and Air Force. It's full name is NAVSTAR (Navigation System with Timing and Ranging) or NAVSTAR GPS. It's original aim used to be enabling navigation of the US Military Forces everywhere in the world and under various weather conditions. Nowadays it also has many civilian uses such as navigation of civilian boat and aircraft, scientific research and recreation, including hiking and geocaching.
The whole system is based on an arrangement of 24 satellites in orbit of 20,200 kilometers above the Earth. The last satellite was put in orbit in March 1994. The satellites are divided into six orbital planes which guarantees at least four of them are visible at any given time from any place on the planet's surface. The placement and condition of all the satellites is the responsibility of the US Government.
This is how the entire system operates. What you have in your hand is but a small part of it. Let us now see how your new GPS unit uses the network to help you in your treasure hunt.
One of the most important parts of a GPS receiver is a very accurate quartz clock. Once the unit receives data stream from a satellite, the clock is being synchronized and it determines its position on the basis of how long it took for the signal to travel from the particular satellite.
A GPS receiver calculates its position in three dimensions (latitude, longitude and height) by measuring its distance from the closest GPS satellite. It uses at least four measurements, from four different satellites, the last of which is necessary to correct a possible clock mistake. If one of the receiver's coordinates is known (the height for example), only three measurements are needed. The most modern units are able to maintain communication with eight satellites simultaneously and measure its position on the basis of all received GPS signals.
The signals sent by the satellites and received by your GPS unit are in the form of data stream containing such information as the identification of the particular satellite and the precise time the signal was sent. One of the codes used by the system (Course Acquisition), originally intended for civilian use, was deliberately being degraded, hence the location could only be known with very limited accuracy. This used to be done for security reasons. However, the Select Availability (that's how it was called) was removed in May 2000. Since then, civilian users have been able to use the GPS with almost the same accuracy as the Military does.
This is our brief introduction to how the GPS system works. Now, let's move on to explaining some terms useful for operating any model of a receiver.
Bearing – the angle between any given straight line and the prime meridian measured from 0° to 360°.
Distance – a GPS unit calculates the distance between two given points. Most of the receivers can give you the value in chosen units.
Course and Track – horizontal direction in which we're traveling. Basically measured as an angle between the desired track and the prime meridian.
Desired Track – the bearing between the beginning and final point of our track.
Course Made Good – a term used to describe the direction from the last point to our current position.
Speed – apart from calculating its position, a GPS unit is also able to measure its velocity.
Speed Made Good – anticipated velocity toward a given point.
Cross-Track Error – distance between the desired track and any variation from it. The GPS unit constantly monitors the course and displays any anomalies.
Estimated Time en Route – anticipated time needed to reach the next point based on current velocity
Estimated Time of Arrival – anticipated time needed to complete the track. There are also many other values that can be measured, such as time since the beginning of the journey, the completed distance, distances between any given points etc.
Electronic map – even the simplest GPS units are capable of displaying graphically their current position, the closest points on the track and so on. The more modern units have built in more detailed topographical maps.
Even though the modern GPS system is very reliable, it's always advisable to have a map and a compass as well. All the information should be compared with the map to ensure absolute safety in unknown regions.
There's also a number of factors that can degrade a GPS signal making it much less accurate. Ionosphere and troposphere delays – slowing the signal down leads to time calculation errors.
Signal multipath – a delay caused by the signal being reflected from large objects such as sky scrapers or large rocks.
Receiver clock errors – though very accurate, the quartz clock is not as precise as the atomic clocks an board the satellites.
Orbital errors – the inaccuracies of the satellites' reported position.
Number of satellites visible – the more satellites there are within range of your receiver, the better and more reliable your information is.
Satellite geometry – the relative position of the satellites. The ideal situation is when there's a wide angle between the satellites.