GPS Technology

For those unfamiliar with Global Positioning System (GPS) technology, here is a quick resume.

GPS was developed by the Americans to provide all-weather round-the-clock navigation capabilities for the military. It consists of two components. The primary one is a constellation of 24 satellites orbiting the earth at a height of over 10,100km. The secondary component is the users product which comes in many forms - GPS receiver, in-car satellite navigation, remote satellite tracking etc.

The spacing of satellites in orbit is so arranged that a minimum of five satellites will be in view to at any time. The Russians developed their own system - the Global Navigation System (GLONASS). But this is outside the scope of this article.

A recent development has been the introduction of the Galileo GPS project, which is a European version of GPS. The first of 30 satellites was launched early in 2006, but delays to the project mean that operation is now unlikely before 2013. It is hoped that there will be international co-operation to allow harmonisation and compatibility of the different systems.

The second part of the GPS system is the mobile receiver; this uses the signals from the satellites to continuously calculate position. Accurate time from the satellites atomic clock is used as a basis for calculation of position.

In the early days, RPS members started using handheld GPS receivers such as the Garmin GPS 12 to verify a trains speed. Today, there are a number of different devices that allow detailed recording of acceleration, braking and can compile an automated log of the journey. Other developments have included Bluetooth GPS receivers which can wirelessly connect to a handheld or laptop computer and provide position and speed data to mapping software.

GPS receivers need to have good reception from a minimum of four satellites in order to calculate position. The more satellites in view, the strength of each signal and the spread of satellites around the sky all affect the accuracy of the calculated position. Trees, tall buildings, tunnels, railway cuttings, overhead catenary and the coach body of the train interfere with the signal and hence the accuracy.

Based on simple positional information, and the ability to store information from previous calculations the GPS receiver can work out your current, average and maximum speed. Also, since it is receiving time information, it will display the current time and some models also have a stopwatch style timer.

We consider thefeatures of GPS Receivers and explain Waypoints, Routes and Tracks and show how tracks can be downloaded to PCs, and their format. We explain that maps can be downloaded to GPS receivers. We explain the uses and benefits of using GPS receivers, but also point out the problems with using these machines in a railway environment and list the RPS recommendations. We give tips and explain the methodology of using the technology.

Obviously, GPS technology is continually improving, and developments are being frequently introduced. We will try to ensure that readers are kept up to date by updating the website, and if you require further information, please contact appropriate members of the committee.

Features

Most receivers provide information on pages; you choose what page to display and what appears on the page. This is where the description gets a bit more difficult, as new receivers are regularly released with new facilities. So please take the following section as what a receiver may contain.

Waypoints, Routes and Tracks

Most have the ability to enter information about locations – known as Waypoints and Routes, which are a collection of Waypoints. If you want to set up a new route you can either enter the Waypoints manually, on the PC, or actually be at the location and get the device to store its current position. You can then get a more accurate estimate of its position by averaging a number of readings over a period. However standing on an overbridge with no footpath and traffic rushing past is perhaps not the most fun thing to do on a Sunday afternoon.

The GPS will give the position in many different position formats; perhaps the National Grid is most useful, which it will give a reference to the nearest metre (e.g. Swindon Station – SU 14982 85215). If it is intended to produce logs automatically, based on the proximity of the waypoint to a downloaded track, it may be better to use latitude and longitude in the format dd.ppppp (where d=degrees and p are decimal of a degree (East of Greenwich readings would be negative).

The receiver can also store information every few seconds (or specified distance) about the track you have taken – a Track log. With the right cable and software, Waypoints, Routes and Track logs can be moved to and from a PC. This will enhance the ability to store information only on the GPS receiver, which typically allows 1000 Waypoints, 50 Routes made up of 30 Waypoints and a Track log of 10,000 readings. Creating and editing routes and waypoints is easier on a PC than the GPS Receiver due to its lack of conventional keyboard.

Downloading

Waypoints, routes and tracks can be moved between the GPS Receiver and PCs.

An example of a downloaded route is shown:

Transferring files from GPS to PC requires the “Open from” command from the GPS software. The “Export” command is used to transfer files from the GPS software, and is saved as a “text file” To convert to Excel, open the text file and import using “delimited” option.

Sample waypoints, routes and software to create RPS-type logs from downloaded tracks are available from the Editor.

Maps

Some receivers come with inbuilt Maps, or the ability to load map information from a CD and software running on a PC. Because of the limited storage on the GPS receiver on the one hand, and the distance travelled by the train timer on the other, map data is of limited value, and is more relevant to walkers or cyclists who cover shorter distances, for whom a small extract of the map can be very useful.

Uses and Benefits

The GPS technology is being used (or could be used) in a variety of ways. Listed below are some uses and benefits:

• Speed. From its instantaneous readout brief minima and maxima can be obtained

• Waypoints and Routes set up before a timing run will give a countdown (in miles at first, and then feet) to each Waypoint as you progress along the route.

• Using the tracking facility can be the basis for a detailed log.

• Average position can be used to confirm information for new mileage charts

• Night timing far more accurate than alternatives

• Accurate log can be compiled in poor visibility conditions (dirty windows, low sun, heavy rain/deep shadows)

• Accurate log can be compiled where mileposts are difficult/poorly maintained and where milepost side seats are unavailable

• You don’t have to keep pressing the stopwatch button, so recording a log is not so onerous

Problems

The problems with GPS chiefly relate to the consequences of poor signal strength due to the fact that a railway carriage is not an ideal environment to receive GPS signals. They manifest themselves in the following ways:.

• The signal is actually lost, and may have been inaccurate for the previous 30 seconds

• The displayed speed is inaccurate but the recorder is unaware of it. This is due to the operation of a filter in the software which suppresses what it considers to be significantly inaccurate speeds. Unfortunately, this may also suppress acceleration/deceleration. The data on the screen will “freeze” until realistic speeds are re-established. Consequently GPS speeds should only be quoted to the nearest 1mph and ignored when braking severely. As with other timing methods, if you suspect the accuracy of the data, exclude it.

• There are also other occasional time lags or errors in the display of correct speeds

• Readers should be aware that conventional GPS receivers do not work in some of the recently-introduced rolling stock, mainly Virgin stock and Desiros. It does appear that the new Sirfstar equipment will work in Desiros, Virgin Pendolinos, but only in the vestibule areas of Virgin Voyagers.

RPS Recommendations:

Corroboration of GPS outputs is important. Members are strongly recommended, where possible, to continue recording stopwatch milepost readings, noting the GPS speed at each post. Comparing average speeds between timing points with GPS speeds is just as relevant for a GPS log. GPS units are a useful tool and they fulfil a role supporting conventional train timing equipment. A perfect data source does not exist, so train timers should ensure that the technology available is used to maximise the accuracy of the data.

Whilst lost signals cannot be eliminated they can be minimised by adopting simple actions:

• A machine with an external antennae facility is believed to provide improved signal acquisition and retention. Sirfstar-fitted equipment seems to acquire and hold signals better than conventional receivers

• Turn the machine on and obtain a GPS signal before boarding the train

• If it is not possible to view mileposts, sit on the side of the train with the highest density of satellites. However, the combination of time elapsed and distance travelled may necessitate re-positioning if the signal subsequently deteriorates.

Methodology

For best results, there are semi-professional style GPS datalogging units available from Racelogic and Race Technology. These are recommended for any RPS member wishing to use GPS to compile a journey log or measure the performance of a train. These units cost from around £350+VAT, which is only slightly dearer than a top of the range Garmin device.

GPS tips and tricks

Turning the GPS receiver on will start the satellite acquisition process; this takes anything from 30 seconds to 5 minutes. The receiver stores information about satellite orbits, so it will try and acquire the satellites it thinks should be in the sky above it, i.e. it remembers where it was when it was turned off. If it has been some time since you turned it off, or you have move location it will start its general search of looking for any satellite it can find. It will take longer for the device to initialise if it is moving or keeps losing the satellite signal.

• Switch your GPS device on at least 10 minutes before joining train.

• Manipulate the receiver to pick up as many satellites as possible. At some stations better readings might be obtained outside the station e.g. on Euston Road at some London termini.

• Join the train as late as possible. Ensure the antenna has an unobstructed view of the window. Use an extrenal antenna bluetacked to the window if possible. Keep the receiver as still as possible.

• If there is no speed reading after movement consider changing sides even if only for long enough to get an initial reading.

• When reading speeds watch the figures and allow for the fact that they can cycle around the true readings especially when the signal is poor in deep cuttings and urban jungles. Watch for phantom maxima.

• If the machine says it requires 2D altitude input an altitude to help it. As accurate a one as possible helps.

• If it says to power down and reinitialise, do so. If it says it needs you to select initialisation method, input country or auto-locate.

• Resist the temptation to turn it on and off every time you change trains just to save batteries.

Power

This last point brings us on to power for your receiver.

Because of the different way NiCad and NiMH rechargeable batteries lose power you may not get a warning that the power is likely to go. Alkaline rechargeables seem to have overcome these problems and give about 10-12 hours of use, but after several recharges this drops to near 6 hours. NiMH rechargeables are also better giving about 40% more that NiCad batteries.

Hint - keep a spare set ready to be used. Invest in a multimeter to check the condition of batteries. A good NiCAD or NiMH should be around 1.3v per cell when fully charged. For long battery life, ensure batteries do not drop much below 1.0v per battery. If you are using rechargeble batteries, ensure they are recharged as soon as possible. Use a multimeter to check that batteries do not fall below the crucial 1.0 volt.

Also remember that all rechargebale batteries suffer an amount of self-discharge , even when sitting out of use on a shelf or in a drawer. This can be as much as 25% dicharge in the first day. So the best time to recharge these for a full day's use is the night before a days use.

Copyright Railway Performance Society Ltd ©2008 Web Design Scotland