Gav's Blog

I wrote this article for the 6 December 2007 issue of The Box, it also appeared in Stuff. I was also interviewed for a geocaching article that ran next to this GPS article, and managed a mention under Adventure Caches!

When I purchased my first Global Positioning System (GPS) receiver in 2001, it was rare to see someone else with them. These days, you can find them just about everywhere – particularly suckered to car windscreens. But how does the GPS work?

It’s quite appropriate that in-car GPS receivers are usually located close to the radio, as the GPS is similar in concept to a radio. Instead of ground-based aerials to transmit radio stations with voice and music, the GPS uses around 30 GPS satellites orbiting 20,200km above the Earth’s surface to transmit signals that GPS receivers listen to, and use these signals to calculate their position.

Each satellite accurately knows the time and its own position in space based on its orbit and onboard atomic clocks. This information is then broadcast from space for any receiver to hear. The GPS receiver uses the precise time and position information broadcast from each satellite to trilaterate and determine its location based on how far it is from known reference points – the satellites. Whereas triangulation uses angles to calculate location, trilateration uses distance.  Listening to one satellite, the receiver could be located anywhere in a circle around that satellite. If the receiver knows how far it is from a second satellite, then the number of possible locations of the receiver is reduced to two. The known location of a third satellite would identify a unique location, and reduce the calculation error to an acceptable level to generate an accurate location.

A GPS receiver is capable of listening to 12-20 satellites at the same time. Generally, the more satellites the handheld can listen to at the same time, the more accurate it is at calculating your latitude, longitude and to a lesser extent altitude. The receiver needs to be listening to at least 3 satellites to calculate a location. In good conditions the location is accurate to 3-5 metres.

The default model of the Earth used for calculation is the World Geodetic System 1984 (WGS-84). This is used on all new GPS receivers. Most receivers allow alternative display systems to be selected – in New Zealand the combination of Geodetic Datum ’49 and New Zealand Map Grid should be selected when using a receiver with the current 260 series topographic maps.

GPS receivers aren’t always able to report accurate positions – this is usually due to the receiver not being able to ‘hear’ the satellite signals correctly. When the receiver has a good clear view of the sky – for example standing on top of the Port Hills – there is nothing stopping the signals reaching the receiver. Problems arise when you place the receiver under thick bush canopy or in buildings where the signals have trouble penetrating. Similar problems can be found in gullies and valleys, or urban canyons in high-rise cities. In these situations, the receiver can only produce approximate locations and often will fail to even produce a location.

Over the past 2-3 years, the processing technology within receivers has reached the point where they can produce accurate locations even in many demanding environments. With a mixture of low and high sensitivity receivers on the market, this can be an important purchasing decision if you are going to be using the receiver in challenging locations.

Change is also afoot in space, with Japan, China, India, Russia and Europe announcing new or updated navigation satellite systems. The EU’s Galileo system recently announced an agreement that will make their system compatible with the US GPS. More satellites and updated receivers will make it easier to get a signal and more accurate location!

I just heard today that the Department of Internal Affairs is consulting on an opt-in single sign-on identity verification service (IVS) that may be used by government agencies to identify us online when interacting with said agencies.

I have included my submission below for reference.

We would like to know whether you are likely to use the Internet to verify your identity with a government agency.

Yes – but it must work on any operating system and web browser. I use a variety of operating systems and web browsers including:

• Operating Systems – Apple OS X, Fedora Core Linux, and Microsoft Windows
• Browsers – Firefox and Safari

I will not be able to use the service if it is tied to Microsoft Internet Explorer/Windows platform. I expect that all the good work that the State Services Commission has been doing on standards and interoperability will be applied to IVS as well.

We would like to hear from you regarding the type of services you might want to access that require you to verify your identity.

• Inland Revenue for management of personal/business taxes, KiwiSaver?
• Government Electronic Tender Service (GETS)
• NZ Qualifications Authority for NZQA Learner’s Record
• Local Government

We would like to know what you think of being able to verify your identity with businesses and other organisations.

I would support the service being made available to local government.

I am initially dubious about IVS being made available to businesses until such time as more details are made available. I trust the Government to run their IT systems to a higher level of security than most businesses. I am also concerned that if the IVS was made available to non-governmental users, that uptake may well make the IVS to be more than an opt-in service – businesses may use incentives that Government cannot to strongly promote registration and use of the service.

I would however support a limited number of business sectors to utilise the IVS – in particular those that provide online financial services such as banks, fund managers and sharebrokers. It is preferable to have them using a national framework rather than having a token for each organisation AND government on my keyring. Note that this would present some risks – in particular the risk of a distributed-denial-of-service (DDOS) attack against the IVS infrastructure. If the IVS does grow to become widely used, and includes the financial sector, then a DDOS against poorly planned IVS infrastructure may have significant negative consequences – even if just in perception of the service. Naturally, as IVS grows in usage, it would have the potential to become national critical infrastructure and would need to be managed as such.
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