How do GPS systems keep tracking without a signal?
14 Oct 2010 Jason 2
To know its position, a GPS device relies on being able to receive signals from at least three satellites. Without going into too much detail, a GPS unit uses the three satellites to triangulate its location* on earth (learn more about how GPS systems work).*Referring to a 2-dimensional position (latitude and longitude). To determine a user’s 3-dimensional location (latitude, longitude and altitude) a unit requires contact with four or more satellites.
GPS satellites transmit a low-powered radio signal that travels by line-of-sight (LOS). This means it will pass through clouds, glass or plastic but will not go through most solid objects such as buildings or dense foliage. Electric interference can also affect the quality of the GPS signal, thus increasing the chance of the GPS unit not being able to receive the satellite’s signal correctly.
A GPS device’s ability to pick up a clear and accurate signal from a satellite can also be negatively affected by something known as satellite geometry or shading. This is when a number of satellites are ‘bunched’ together in relation to the device, such as all lined up in a row. GPS satellites work best when positioned at wide angles from each other, from the receiver’s point of view.
GPS won’t work GPS will work GPS will work best
- Inside a building
- Covered car park buildings
- Beneath dense foliage such as forests
- Outside (even on a cloudy day)
- Behind glass (e.g. by a window or on a vehicle dashboard)
- Behind plastic
- When at least four satellites are in view
- When the satellites are at the widest angle from the receiving unit
- When a GPS device uses a supplementary location system such as WAAS or DGPS
It’s probably fair to say that most users of GPS devices have at one time or another lost sufficient signal strength so that they can no longer determine their current location with any certainty. Or they have powered up a GPS device (normally inside a building) and wondered why the unit is unable to get a lock on its current location.
Of course sometimes a GPS unit may not be receiving a satellite signal because it’s faulty, in which case it needs to be returned for servicing.
No GPS signal, now what?
So what if a GPS unit doesn’t have a clear LOS to at least three satellites? Does it just stop working?
Basically, yes. It will no longer be able to provide the user with a current location, until it can resume a link to at least three satellites and then update its position. So what does this mean for navigation systems that are guiding someone to a destination? For example, a driver may be driving through a tunnel and no longer have LOS to any GPS satellites. How does the driver continue to receive directions?
Some GPS systems such as Garmin® use a process known as dead reckoning.
Dead reckoning has a long history, particularly with seafarers who would calculate their estimated position by using their current speed and direction. It is thought the word comes from ‘deduced reckoning’. It works in a similar way to how some utility companies will estimate a consumer’s power usage based on historical usage, when an accurate reading is not available.
Using a combination of the driver’s destination, the vehicle’s last known position as well as its current speed and direction and maps loaded into the GPS unit, relatively accurate routing can be maintained even where there is GPS satellite signal.