Windmill Software Ltd
Windows Engineering Software

December 2005

The Newsletter for PC-Based Data Acquisition and Control
Issue 89       December 2005
--------------------ISSN 1472-0221----------------------

A very happy new year to you.  This year's final issue 
contains an example of how Windmill is being used in a 
project to locate moving vehicles, and an article on 
the differential global positioning system.  The Excel 
Corner, however, is having a Christmas holiday.  

We hope you find Monitor useful, but should you wish to 
cancel your subscription you can do so at

* Windmill News: Locating Moving Vehicles
* What is a Differential Global Positioning System?
* What next for Satellite Navigation?

Windmill News: Locating Moving Vehicles

David Wilkinson Associates Ltd, experts on the safe and 
efficient movement of trains, have recently been 
working on a DGPS project to locate moving vehicles. 
(DGPS stands for Differential Global Positioning System.) 
Whilst developing this system they used the free 
Windmill software to extract data from a Garmin GPS18 
receiver to a laptop computer.
They found it easy to set up Windmill to extract the 
data they needed, using the LabIML Help facility.  Once 
the required settings were entered, the data appeared in 
the Windmill DDE programme.  They then exported that data 
to Excel using DDE (dynamic data exchange).
David Wilkinson is happy with the Windmill software, but 
he  has experienced some difficulties with Windows and 
the GPS drivers.
   "The Windmill software works without any problem.  The 
    main problem I have experienced so far is due to the 
    fact that when you plug the GPS18 receiver into a 
    laptop, Windows sometimes detects the RS232 data as 
    indicating that a serial mouse has been plugged in. 
    This causes the screen to behave erratically and you 
    have to wait till the Windows system has settled down 
    before trying again."
The settings the company used with Windmill were:
* Reading Protocol: Continuous Flow
* Instrument Idle or Wait Time: 0
* Returned Message Length: 60
* Instrument Initialisation String: none
* Baud rate: 4800
* Flow control: None

David Wilkinson Associates Ltd was formed in 1997 as an 
independent railway engineering and safety consultancy. 
They can be reached at

For a later version of the software see

Further Reading
Interfacing a Garmin GPS

What is a Differential Global Positioning System?

A differential global positioning system, or DGPS for 
short, provides very accurate location information.  It 
does this by correcting errors in the GPS errors.

The original GPS was developed by the US Department of 
Defense.  Briefly, GPS satellites broadcast radio 
signals containing their position and time.  A 
GPS receiver picks up signals from at least four 
satellites.  The receiver knows exactly where in the 
sky the satellites are, it just doesn't know exactly 
where on earth it is.  Until, that is, it determines 
its distance from the satellites.  It does this by 
calculating the time it took for the signals to reach 
it.  However, signals may be delayed as they bounce 
off obstructions like mountains, buildings and 
particles in the atmosphere.  Differential GPS 
compensates for these timing errors.

With DGPS a reference is fixed in a known position.  
Instead of using timing to calculate position, it 
uses its position to calculate timing.  It works out 
the difference between the actual time a GPS signal 
takes on its journey and the calculated time it should 
have taken.  It then broadcasts this error correction 
information to mobile GPS receivers in the area.

This error correction technique results, in moving 
situations, in a 5 m accuracy 95% of the time (source: 
Northern Lighthouse Board).  Much better than the 20 m 
accuracy of GPS.  It also means that a DGPS can check 
the integrity of each satellite's signal and report 
whether it is good or bad.

Further Reading

The GPS: History, Function and PC Interfacing 
Monitor 30, January 2001

Connecting a GPS Receiver to a PC

What Next for Satellite Navigation?

The first test satellite of Europe's Galileo navigation 
system has just been launched.  Galileo is a civilian 
project and promises the permanent provision of a 
navigation system.  The rival US (GPS) and Russia  
GLONASS) systems are both run by the military and 
could in theory be turned off at any time. 

You will be able to take a position with the same GPS 
receiver from any of the satellites in any combination. 
The system is designed to deliver real-time positioning 
accuracy down to one meter.  Better than the existing 
GPS and DGPS systems. 

The Galileo system will guarantee service under all 
but the most extreme circumstances and will inform 
users within seconds of a failure of any satellite.  

By placing satellites in orbits at a greater 
inclination to the equatorial plane than GPS, 
Galileo aims to achieve better coverage at high 
latitudes. This will make it particularly suitable 
for operation over northern Europe, an area not 
well covered by GPS. 

Galileo is scheduled to be fully operable in 2008 - 
but you will need a new GPS receiver to make use of it.

Further Reading
European Space Agency

European Commission

* Copyright Windmill Software Ltd
* Reprinting permitted with this notice included
* For more articles see

We are happy for you to copy and distribute this 
newsletter, and use extracts from it on your own web site 
or publication, providing the above notice is 
included and a link back to our website is in place.

An archive of previous issues is at
and an index of articles at

Windmill Software Ltd, PO Box 58, North District Office,
Manchester, M8 8QR, UK
Telephone: +44 (0)161 833 2782
Facsimile: +44 (0)161 833 2190
E-mail: [email protected]


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