GPS Background

The GPS system that we use each day in our car satnav & handheld GPS units is run by the US Air Force and has been operational since 1995. It uses 24 space satellites each orbiting the Earth every 12 hours, and the system has global coverage. Each point on Earth can generally see four or more satellite.

The satellites orbit at about 24,000 km above the Earth and their ‘messages’ are sent by radio signals sent on the UHF band, and are ‘line of sight’, meaning that the signals will not pass through solid objects such as a tree canopy, cliffs, or buildings, but will travel through clouds.

GPS is the US system. The Russians have GLONASS, Europeans are developing Galileo, and the Chinese and Indians are planning their own systems, although only GPS and GLONASS are operational. The collective name for such navigation systems is GNSS, but for now, any reference to GPS on this site is interchangeable to mean any satellite based location provisioning system.

GPS signals can also be augmented by additional signals, either ground or satellite based. In Europe, the system is called EGNOS, and in the US it’s called WAAS. You don’t really need to know this, but if you see the acronyms in specification sheets for GPS units, you’ll know what they mean.

How the GPS System Works

There is lots of technical information available elsewhere on the web if you’re that way inclined, but there are some key pieces of information that you need to know so that your receiver works when you need it to.

To get a two dimensional fix, aka a grid reference or position on a map, you need to have at least three satellites available. This is important information to know, as you need to have a clear view of the sky with as broad an horizon as possible. So, you may be able to look up and see the sky, but the satellites may be located closer to the horizon than you can see. Position yourself appropriately.

The positional fix from satellites should be accurate to within a few metres 95% of the time. Always bear in mind that the position shown on your GPS receiver could be within that 5% that is less accurate. If your life depends on the GPS position, wait a few minutes, maybe even switch it off and on, and see if the position is still the same. To get a positional and height fix, you need to have at least four satellites in view, all with good signal strength. Height measures are suppose to be approx within 20m 95% of the time

GPS units also have limitations, which you will have noticed if you powered one up and it took an age to acquire a signal and even longer to provide a position. There are reasons for this.

Ephemeris and Almanac Data

GPS satellites transmit two types of data, being ephemeris and almanac data. Almanac data describes to the GPS receiver the approximate position of the orbiting satellites, and such information is updated regularly by the USAF. Receivers need that kick-start to know where in the sky to look for satellites, which is in part based on your geographical position at the time. For this reason, if you’ve not used your receiver for a few days, or have travelled approx 600 km, the initial satellite acquisition will be what’s known as a ‘cold start’. Almanac data is valid for a maximum of 60 days, and most be downloaded if required. On older units, that download could take up to an hour.

Ephemeris data consists of two types of data; time and position. Basically, the satellites send a message every few seconds, which consists of the time that the message was sent, and position of the satellite at the time sent. From this information, the GPS receiver can calculate its distance from the satellite by subtracting the time sent from time received, as the speed travelled by the signals is a constant speed. Combine that calculation messages from at least two other satellites, and the receiver calculates its position by triangulation.

Ephemeris data is updated about every 2 hours and remains valid about 4 hours, so if you have your GPS receiver switched off for more than that time, it will typically take a little longer to get a satellite fix. This is referred to as a warm start, and may take a couple of minutes to get a positional fix.

If you travelled 600 miles or more, or your GPS unit no longer has an accurate time, a new almanac will be downloaded so that your unit knows where to look in the sky for satellites, hence taking longer to obtain a positional fix. This is a cold start. You can sometimes kick-start a cold start by manually entering a grid reference to the unit.

By contrast, if your GPS unit is switched off for only a few minutes, or not at all, then the positional fix will be quick, taking seconds. This is a hot start.

Map Datums

GPS receivers calculate co-ordinates in latitude &longitude, but the software of the unit can convert such co-ordinates into something more useful for land based navigation. In the UK the norm is to use the British National Grid, but other conversion systems exist, call datums.


Most modern GPS units now come equipped with in-built mapping, meaning that an accurate location can be shown on a map on screen too, and not just represented by a grid reference.

Mobile Phone Based GPS Apps

On the face of it, some smart phone apps would render stand alone GPS receivers obsolete. However, there are a number of issues you need to be aware of.

Always Carry a Map

Whether you carry a dedicated GPS receiver or GPS app for your phone, you should always carry a detailed map for the area in which you’ll be walking.

Smartphones GPS

Most smartphones have inbuilt GPS chips and benefit from warm to hot GPS acquisition by downloading the GPS almanac & ephemeris data over their data connection rather than relying on a slow satellite download speed that stand-alone GPS units have to endure. When you use a smartphone location type app in the city, its position acquisition time (or Time to First Fix – TTFF) is likely to be extremely quick as its almanac & ephemeris data is updated quickly and when required. This operation requires a mobile phone signal, with data connection. If you try to use your phone’s GPS functionality when away from a mobile signal, and not having used the GPS of the phone for some time, it will take longer to TTFF and use more power in the process, as the phone has to obtain data directly from weak signalled satellites rather than the mobile network.

Battery Life

One of the limiting factors of using a mobile phone as a navigation device on the hill is the limited battery life and inability with some phones (iPhones for example) to change a battery part way through the day. Heavy use of the GPS functionality of your smartphone will drain the phone’s battery more quickly, potentially leaving you without a means to communicate in case of emergency. If you can change the battery, carry a spare, or an emergency charger if a battery swap is not possible. Just be aware of additional battery drain, and experiment so you know how much extra drain is likely with your chosen app and with your phone.

The recommendation is that your phone’s primary task on the hill communication in case of an emergency, and GPS use is kept to a minimum.

A further reason for reduced battery life of mobiles when in the hills is that they try hard to retain a mobile signal, and in doing this in areas of poor signal they use more power in the process. The recommendation to maintain higher battery levels is to switch off the phone’s mobile signal as soon as the signal becomes low. Do this by putting the phone in airplane mode, or similar. A limiting feature of iPhones is that once in airplane mode, the location based services (aka GPS) become unavailable. Android phones do not have that issue.

GPS Apps

Smartphone apps can be classified into three categories in the context of the outdoors and navigation:

  • map only
  • maps & navigation
  • position only

Map Only Apps


This category of apps area great for urban situations and navigating from street to street in a location with which you are now familiar.

Google Maps falls into this category. From a rural perspective, maps are scant on detail and the app requires a mobile phone data connection to operate, thus if no mobile signal, there are no maps. It is possible to download map tiles to your phone for use when away from coverage, but again, the detail of information is not sufficient to navigate on a hill. These types of apps do not allow the user to show coordinates, which is of course a limiting feature.

This type of app is not recommended for use in the hills as maps are of insufficient detail to be used for navigation, phone signal likely to be patchy at best and inability to provide a grid reference.

Do not rely on apps that require a mobile signal to function. You should ALWAYS take a map onto the hill. ALWAYS.



Maps & Navigation

Some of these apps are more like well featured stand-alone GPS receivers, as you can connect them to a PC and up/download maps and routes. Full  mapping is available allowing your position to be displayed live on a map on your phone. These apps recognise coordinates.

This category can actually be divided into two sub-categories, being:

Offline Maps

These apps that have 100% off-line mapping, i.e. maps are stored in the phone. You will know whether your app has off-line maps as you will have bought and copied or downloaded the maps to your phone.

Viewranger is a good app for Android, iOS and Nokia platforms. There are others.

Viewranger, for example, allows you to track your route as you go and store that track on your phone. At the end of the route various stats are available, such as distance, heigh gained, speed etc, which can be good information to use for future route planning.

Having your location displayed real time is great, Use such apps to check your position, and maybe record your track too. They are valuable addition for navigation and safety.

Online Enhanced Maps


These apps have maps served directly to the phone by a mobile data connection, meaning that there are no map files stored on the phone,thus if no mobile signal, there are no maps.

This sub-category is the dangerous one from a mountain navigation perspective, as the user can be lulled into a sense of having rich OS maps on their phone, only to find that they have nothing when their mobile phone signal is lost. Mountain Rescue teams have given warnings about using such apps. See news below.

If you are unsure whether your app has online or offline maps, you probably have online maps that will not work when out of a mobile phone signal.

The screen shot opposite if from an app called OS Atlas Lite, which is great for areas of the country with a good mobile data signal, especially where you don’t have the OS map for that area. However, for your own safety you should always assume that mobile data coverage will be non-existent in the hills & mountains.

Do not rely on apps that require a mobile signal to function. You should ALWAYS take a map onto the hill. ALWAYS.


Position Only Apps


Limited in functionality, these apps do one thing, which is to display your position as calculated from GPS satellites, as a BNG grid reference. If you have a smartphone, you should have one of these apps installed as a backup to your GPS unit. If you don’t have a dedicated GPS receiver, this type of app is better than nothing.

There are a number of such apps available, with the one opposite & detailed below being one that I’ve used. Basic, but useful, and takes hardly any phone memory (this good for battery life) and it’s accurate, in my experience.

Grid Reference App – click here to view on Google Play

A word of warning: most of the apps are free downloads, so you would be well advised to test out the accuracy of them whilst at known locations, before needing your position in a situation when you are mis-located. Licence conditions for the software will effectively say that the software has no warranty to actually work.

Remember also that your smartphone will be of limited use in the wet, or if dropped. A dedicated GPS unit is more robust in that respect, but again, a GPS app is better than nothing.

GPS Fix Problems

If your smartphone is either not acquiring a GPS fix or taking a long time to do so, check that you do have the GPS facility switched on. iPhones refer to this as ‘location services’.

It has been known also for phones to become unable to get a GPS fix or take a long time to do so, and in that case it could be that the almanac and/or ephemeris data is corrupted.

For Android phones, you could try using GPS Doctor (link opens Google Play), which clears all GPS data (including almanac and ephemeris data) from your phone. There may be iPhone & other smartphone equivalents, but you’ll have to find them yourself!


Reasons to have a Stand Alone GPS Receiver

A smartphone GPS app is great, but never a replacement for taking a map and compass into the hills. If you are intent on using GPS location capability, a stand alone GPS receiver has some clear benefits over using a smart phone app.

  • Designed for use in the outdoors, are rugged and able to withstand the odd drop without breaking


  • Waterproof. Smartphones are not.


  • Big buttons that can be used with gloved hands. Smartphone apps tend to be touch screen. Navigational difficulties most often occur when the weather is poor, meaning that your smartphone will be susceptible to water ingress and your fingers may not work well due to cold.


  • The sole purpose of the devices are to provide accurate location data. Anyone can write a smartphone app and add licence conditions to the extent that it may not actually work


  • You can carry spare batteries for your GPS receiver, which are normally the AA type. Some smartphones don’t allow a battery swap and many people don’t have a spare battery anyway