Making Waves
Steve White G3ZVW takes a look at radio systems that require extremely accurate timing.
Steve White G3ZVW takes a look at radio systems that require extremely accurate timing.
Collectively, such radio systems are known as Time of Arrival (ToA). They rely on really accurate timing, to perform a function that would not otherwise be possible. I am going to detail two such systems; one which a lot of people know about and one which not a lot of people know about.
First though, a few nuggets of information about the Global Positioning System (GPS). This American system works by transmitting extremely accurate time signals from a constellation of satellites orbiting Earth at a height of 20,200km. This height is known as Medium Earth Orbit (MEO), as opposed to Low Earth Orbit (LEO) or Geostationary (Geo). The accuracy of the time signals is crucial to the accuracy of the system as a whole – and in the case of GPS is to within 10 nanoseconds. Currently there are 24 GPS satellites in use, plus spares, all of them in orbit. There are other satellite navigational systems that work in a very similar way, e.g. Glonass (Russian) and Galileo (European).
Blitzortung
There are various live lightning sites on the internet, perhaps the best known of them being Blitzortung (German for Lightning Location). A not-for-profit ‘community’, it works by people buying its kits, assembling them and connecting them to the internet. A kit will contain a very low frequency (VLF) radio receiver, a GPS receiver and a good bit of logic. Purchasers are expected to provide their own antenna, a magnetic loop being popular.
A lightning strike is an electrical discharge and when one occurs it results in a wideband pulse of radio frequency energy. On a radio the pulse will sound like a brief crunching sound. They are often known as static crashes. Normally the crashes would be heard over the station you might be listening to, but if you tuned a receiver to a frequency on which there was no activity you would just hear the crashes. A Blitzortung receiver detects the exact time it hears every static crash, then sends it and the receiver’s exact coordinates (derived from the GPS receiver) to a central server. The central server uses all the incoming reports to build up a picture of lightning activity, which it superimposes on maps and then makes available on the internet. It requires a minimum of four receivers for the server to determine the location of a lightning strike, but more reports from more receivers makes for a more accurate fix. Blitzortung’s maps contain two hours of historical data too, so you can easily see in which direction lightning activity is moving. The whole collection and distribution process takes only a few seconds. Blitzortung works worldwide, but there is no meaningful coverage in some parts of the world because very few Blitzortung receivers are there. Europe, the Far East and North
America are all well covered. Arctic regions, Africa and South America aren’t.
So how accurate is Blitzortung? In theory it should be able to pin down a lightning strike to three metres, but in reality the accuracy is less because some receivers
will hear a static crash via ground wave propagation and some will hear it via sky wave. Sky wave signals – signals that have gone up to the ionosphere and been refracted back to Earth – take a longer path than ground wave signals, so are received slightly later than they would otherwise be. Blitzortung tries to overcome this by using a VLF receiver, because signals at VLF travel very long distances by ground wave. The worst case accuracy might be a couple of hundred metres. Blitzortung certainly puts you in the picture if a storm is coming your way, Fig. 1.
Security
The 2012 Olympics Games took place in London. Suppose someone with a VHF transceiver decided to deliberately interfere with one of the frequencies used by security guards at the main site of the Games in Stratford, East London. Let’s compare traditional and ToA methods of locating the illicit user.
Traditional
You could certainly use traditional direction-finding techniques to locate the illicit transmitter. In theory it could be done by one person, but to save a lot of time you would use a team of people experienced in direction finding, each of them in a vehicle. Say you had five of them, located at the yellow stars in Fig. 2. Each member of the team would need to use a receiver and a directional antenna. They would need to wait for a transmission to take place and be certain that it was not legitimate, then plot bearings on maps and liaise with one another to come to a consensus on where the transmitter was. Taking bearings is never precise, so there is never going to be complete agreement on where the illicit transmitter is.
To minimise false bearings, they are best taken from locations that are clear of clutter (buildings, trees, etc). In London the top of a block of flats should be good, but even if a list of buildings was available they would take time to access. Say the direction finders decide that the transmission is coming from where the red star is. They would then need to converge on that location, listen again and take further bearings. They would have to do this several times before they finally homed in on the target. Radio reflections from buildings and structures make for a difficult task, because sometimes the strongest signal might not be taking a direct path.
The average speed of road traffic in
London is 12mph, so I show the straight line distances and driving times in Fig. 2. The real-world distances would, of course, be more. Eventually it would be possible to find the source of the transmission, but it would be likely to take hours. The illicit user would probably have stopped and gone away long before the exact location was known, let alone before something was done to put a stop to it.
Now, if you think things are difficult enough so far, consider how difficult the task becomes if the illicit user is mobile and/or making occasional brief transmissions. And what about if there is more than one illicit user? Essentially it becomes practically impossible.
Where this traditional method of direction finding scores is when a transmission is permanent, not intermittent or occasional, but finding the transmitter will still take time.
Time of Arrival
Although little was spoken or written about it, a commercial Time of Arrival radio monitoring system was deployed for the Olympics Games in London. It was put in place to quickly identify the location of any transmitters causing interference to radio systems being used at the Games. Such a system works in a similar way to the live lightning locating system, but instead of listening for static crashes it can be set to monitor specific frequencies.
How such a system works is that a frequency is identified and sent via a secure radio link or the internet to a network of ToA monitors, each of which are located in an advantageous location. For convenience, in Fig. 3 I show them at the same locations that were used in Fig. 2. Rather like Blitzortung, multiple monitors are needed to triangulate a transmitter. Three is the minimum requirement, but more work better.
The monitors don’t use directional antennas, because they don’t know in which direction a transmitter is. When instructed, they all switch to the frequency in question. The instant a transmission starts on that frequency they each report their location (from GPS) and the precise time that the transmission started. Just like the lightning detectors they use GPS clocking, so they know the time that a transmission starts to within ten nanoseconds. That’s one hundredth of one millionth of a second. I also show in Fig. 3 how many microseconds (wS) the signal takes to get to each monitor. Collectively, the ToA system can pinpoint every transmission made on the frequency that is being monitored, so someone in a control room can see where everyone using the frequency concerned is, be the transmissions legitimate or illegitimate. In a street you could walk straight up to the building. In a line of cars you could walk straight up to the vehicle. You might want to do a local search or listen when you get close, but it should soon be a case of ‘job done’. In traffic, a ToA system enables a vehicle’s progress to be plotted.
Because the radio systems at the Olympic Games were largely VHF and UHF, transmissions were pretty-much line-of-sight. This means no ionospheric reflections, which in turn means the accuracy of the system was better than the lightning monitors. That said there will be local reflections, which will diminish the accuracy.
You might imagine that ToA systems don’t work well when a transmitter is on all the time, because there is no definable time that a transmission starts. That’s not so, because as long as there is modulation present they work by performing timed analysis on it. That said, the accuracy isn’t as good.
Where ToA systems don’t work is when a transmission is permanent, but with no modulation.