GPS Systems

GPS is now a part of everyday life, the ability to know exactly where you are on the planet was the dream of every navigator for millennia, now this is available at the press of a button. GPS is widely used in a variety of technologies such as mobile phones apps, cars navigation for both mapping and incident reporting, wildlife tracking which has yielded some interesting results for migratory birds and mammals and preventing crime by tracking desirable objects. This tech has only been available to the public since the year 2000 and has since become the most popular method of accurately establishing a location within metres. 

Introduction

A short history of GPS Systems

The first publicly available Global Navigation Satellite Systems (GNSS) was the USA’s Navstar GPS satellite constellation. This was a satellite-based radio navigation system owned by the United States government established in 1978 for the USA military and made public in the year 2000 by the Clinton administration. America’s newest GPS system is now just one of many that provide geolocation and time information to a GPS receiver anywhere on Earth where there is an unobstructed line of sight to four or more GPS satellites. Other GPS systems include Russia’s GLONASS, the European Union’s Galileo, and China’s Beidou Navigation Satellite System which offer varying degrees of accuracy, the Galileo system being the most accurate at less than 1 metre for public use and up to 20 cm accuracy for paying customers in 2021.

NAVSTAR Global Positioning System satellite
Artist’s concept of a NAVSTAR Global Positioning System satellite, a space-based radio navigation network.

How does a GPS work?

GPS systems use sets of dedicated satellites, called constellations, these are not stationary but are circling the earth so ‘rise and set’ twice a day, the satellites constantly send out signals, the GPS receiver listens for these signals they don’t transmit anything back to satellites. To determine the location of the GPS satellites two types of data are required by the GPS receiver: the almanac and the ephemeris. The almanac contains information about the status of the satellites and approximate orbital information allowing the receiver to see which satellites should be visible. After establishing what satellites should be available for you to get a fix, your GPS receiver requires additional data transmitted by each satellite, called the ephemeris, this data gives very precise information about the orbit and location of each individual satellite. The GPS receiver uses the ephemeris data to calculate the location of a satellite within a couple of metres and then by using the information that was transmitted to the GPS, your position can be calculated by triangulation using the delta in time signal transmitted and when it was received plus the location of the satellites. The ephemeris is updated every 2 hours and is usually valid for 4 hours, so If your GPS receiver has been off for a while, it may take up to several minutes to receive the ephemeris data from each satellite, before it can get a fix, this is known as a cold start. Obstacles such as mountains and buildings block the relatively weak GPS signals, this will ‘lengthen’ and therefore distort the time to receive the data or even give a false location. On a Mobile phone, there is an additional A-GPS mode which uses the cell towers to calculate the initial position of the user very quickly but with less accuracy, unlike pure GPS this may send information back to a server where that might be helpful to process position. Once the receiver calculates its distance from four or more GPS satellites, it can figure out where you are to approximately a 7.8-metre accuracy and depending on the system in use the accuracy might increase. Using GPS for locating a point of the earth is a key component for Geocachers in their quest to find caches, hence the saying follow the arrow.

Photo: ©GSA, ©European GNSS Agency
One of the Galileo Satellite Constellation

Tips and tricks
1 Having obstructions between the GPS and satellites causes issues where 3 – 4 satellites cannot be seen simultaneously, this is most often seen in cities and forests where accuracy quickly degrades.

2 Multi-path or Signal reflection occurs when the GPS signal is reflected off buildings or other objects, this can delay the time-clock signal sent out by satellite and cause a miscalculation again resulting in degradation of accuracy.

3 In the Northern Hemisphere Face the internal Antenna toward open Southern, SW, SE, in the Southern Hemisphere revers this, most of the satellites are clustered around the equator, this will make getting a fix and maintaining on easier.

4 Low batteries invariably cause issues with GPS systems, make sure that your batteries aren’t on their last legs.

5 When you switch on GPS after moving more than 25 miles or replacing batteries keep it in one position in the open air to allow the ephemeris data to update, it will get a fix far quicker than moving with it.

6 Keep firmware up to date, if there are any bugs in your GPS program an update will fix the issues. If there are more up-to-date base maps in GPS standalone system, an update will repair that, too. 

7 GPS systems are not infallible so use common sense and your Mk 1 eyeball to check what you are being told by the system matches reality, learn to read a map and research where you are going. Most Geocachers will have stories of reaching a river or cliff face and being just meters from a cache that could not be reached without retracing their steps and trying another path. 


Favourite Locations

Some areas have it all, great countryside and some of the best geocaches. If you have a favourite location and would like it to be included here, please contact us.

The Moseley’s Recommend Kentisbeare

If you are ever in Devon, you must visit a little village called Kentisbeare! This area is filled with fantastic geocaches placed by a geocacher called heartradio. 

We travelled to Devon for the Mega in 2017, but decided against camping on site because we dragged along a then 6 month old Moseley_Bach with us, so we stayed in a static caravan near Kentisbeare, and accidentally stumbled across these caches nearby. 

In this village and the surrounding area you can find traditional caches, multi caches, a devious mystery cache geo-[he]art, earth caches, letterbox caches and wherigos! And within these caches, there are Church Micros, Village Halls, Little Bridges, Village sign and a Fine Pair!

We spent days exploring the numerous paths and finding the varied containers and some near impossible caches. 

We had to message heartradio a few times for some help, and he always obliged. He passed us one day as we searched for a particular difficult cache and stopped to say hello!

We didn’t manage to complete his caches, and will hopefully return to the area one day to find more! 

Kentisbeare, definitely worth a geocaching visit. 

Kentisbeare Geocaches

Contributed by Lon Moseley, Welshgirl11

Blue Switch Day 2021

On May 2, 2000 the U.S. government “flipped the switch” so high-accuracy GPS was available to everyone. For reasons unknown, geocachers know this as Blue Switch Day. However, there really wasn’t a blue switch!

Blue Switch Day

Global Positioning System (GPS) when first developed was only available to the US military. Later civilians were allowed access with Selective Availability (SA) but the GPS signal accuracy was greatly reduced.

May 2nd 2000, on the instructions of President Bill Clinton, the US government turned off its Selective Availability. By “flipping the switch” everyone had access to high accuracy GPS signal. Twenty-four satellites around the globe processed their new orders, and instantly the accuracy of GPS technology improved tenfold. Tens of thousands of GPS receivers around the world had an instant upgrade. Even though there was not a blue switch, for some unknown reason, geocachers refer to this day as Blue Switch Day.

With the availability of accurate GPS signal the world changed forever with the introduction of new technologies that today we take for granted. Gone are the days of the road atlas!

May 3rd 2000 saw the first geocache hidden by Dave Ulmer in Oregon and the beginning of the game that we know today.

2017 Souvenir

In celebration of this Blue Switch Day geocaching first introduced a souvenir in 2017. To obtain your Blue Switch Day 2021 souvenir simply find either a geocache, an Adventure Lab or attend an event anytime between May 2nd to December 31st.