Can indoor positioning system work outdoor? – Yes, of course!
There is no fundamental difference between indoor local positioning systems and outdoor local positioning systems. They use the same architectures and even very similar beacons. But there are important details that have to be taking into account.
The biggest difference between indoor and outdoor is in the environmental conditions. Indoor is typically office and warehouse/factory like environment – reasonable 10…40C, no rain, no moisture, no dust, i.e. no special protection is required for the equipment.
Outdoor can be significantly harsher:
– -40C… +50C or even wider
– Salt water
– Chemical exposure
– Explosives in the air
So, depending on the conditions, different protection methods shall be applied. They may affect performance (weight, size, power consumption, etc.) and almost always affect the costs.
Be very vigilant about IPxx marks, for example, IP67. It is a great mark, where:
– No ingress of dust; complete protection against contact (dust-tight). A vacuum must be applied. Test duration of up to 8 hours based on airflow
– Immersion, up to 1 meter (3 ft 3 in) depth
– Ingress of water in harmful quantity shall not be possible when the enclosure is immersed in water under defined conditions of pressure and time (up to 1 meter (3 ft 3 in) of submersion)
– Test duration: 30 minutes. Tested with the lowest point of the enclosure 1,000 mm (39 in) below the surface of the water, or the highest point 150 mm (5.9 in) below the surface, whichever is deeper
Impressive, right? Well, there is a fine print. What the test actually says is that when such conditions are applied, the device must withstand them and not be killed. The IP67 guarantee that the system will work in those conditions.
There is even more fine print:
– Immersion to what? – distilled water. Salt water or other liquids – not applicable
– Dust – what is dust? – it is a fine powder talc. If you have some other types of dust or even metal dust – not really applicable
Thus, when IPxx mark is seen, it shall be interpreted and used with all underlying assumptions and often those assumptions are simply not applicable in real life.
Therefore, environmental conditions is the most challenging part for the outdoor local positioning system.
It is relatively easy to get power supply for stationary beacons indoor: regular power grid + converters, unprotected batteries, PoE – many options.
It is more difficult outdoor:
– Sources maybe simply not available
– Bringing power can be expensive than beacons themselves particularly for explosive environment (oil/gas refineries)
Wind can affect ultrasound based systems in two ways, at least:
– Wind produces noise that can reduce signal to noise ratio and limit the distance or decrease accuracy
– Since wind is a moving air, the speed of air is not accounted in the system and the measured location can be distorted
Luckily, these factors are not severe unless the wind is strong.
Drops of rain produce noise similar to ultrasound pulses. Thus, it is important to protect ultrasound receiving beacons from the noise produced by the drops. All kind of special shields and umbrellas usually do the job well.
Typically, outdoor is an open space. It is possible to cover the same territory with a fraction of stationary beacons required indoor.
It is possible to do sensor fusion with GNSS systems, for example, GPS or RTK GPS for the best performance.