Technologies based on assessing position relative to a number of "beacons" formed by routers and hubs using standard WiFi technologies. A particularly lightweight and innovative approach from the SCYP project makes use of purely signal strength information from unauthenticated beacons. Commercially, Nokia has been working on this technology, demo video here, useful for illustrating the concept.
Pluses of this scheme:
Dedicated Bluetooth "beacons" may be attached to key points within the museum space, and recognised by special hardware in the phone.
Advantages and disadvantages largely as for Bluetooth - however, an interesting "inverse" system run by the Exploratorium instead tagged visitors rather than exhibits. In this way, the cheap "passive" receivers could be given out, and the more expensive active sensors retained as part of the museum infrastructure.
An interesting extension of this model would be to associate the identity of the tag with a cookie, etc. for the duration of a visit, providing a "round-the-back" connection enabling a traditional mobile device owner to gain access to location-awareness via detection of their passive tag.
It is unclear the number of RFID masts would be required to give a decent granularity of location coverage across an institution, or the cost of such a solution. The "Mojix eLocation Solution" is a recent commercial system which insists that it is the "market's first UHF passive RFID Real Time Location System" - which suggests that affordances for any form of fine-grained location over RFID (above simple "swipes" at specially positioned kiosks) may be crude and/or expensive. However, another promising report of passive RFID in a museum context (Skyetek sensors - 1m range) is by Baldwin & Kuriakose at Melbourne.
This is a commercial system for location of objects, which seems to be used most in hospitals at the present time. Here's a video demonstration of the tech.
A recent publication on Local positioning systems (Google books link - Kolodziej & Hjelm) surveys the overall accuracies possible with the radio technologies. The following verdict: (p.121) "Most systems using radio technologies such as Bluetooth or Wi-Fi can only pinpoint an item within about 10 to 16 ft (3 ft with site surveying) in an indoor environment. With UWB, Ubisense is able to track to an accuracy of 6in."
UWB, "ultra-wide band" is another radio modality which achieves higher accuracy due to its wide bandwidth, solving the problem of multipath in an indoor environment. UWB works in the unlicenced frequency of the RF spectrum commonly used for garage door openers, portable telephones and baby monitors. Since UWB is built around sending short (~ 1ns) discrete pulses, positioning can be more accurate than other technologies. The Ubisense system referred to in the text is available as a Research Package - overall cost is unknown, but one estimate puts tag costs at between $50 and $100.
This system could be useful for making an "initial survey" of a museum space using one or more cameras taking a large number of images with a UWB sensor attached – for later use via an image recognition based visitor system (see below).
A variety of 2-dimensional barcode systems are prevalent - some front-runners are
A useful software platform reading a variety of codes is 2dsense, however this is delivered for the iPhone environment only.
These attempt direct recognition of either objects present in the scene or else entire scenes.
A front-running research group in this area is the Media Faculty at the Bauhaus-University, Weimar. In particular, a live system, PhoneGuide has been demonstrated running live in two museums (Senckenburg Museum of Natural History, Frankfurt and Weimar Museum of Ancient and Early History) during working hours, delivering fairly fast recognition performance on the Nokia J2ME platform. In particular, the PhoneGuide publication reports an interesting range of user testing as well as some more details of the platform. In terms of recognition/location times, "11 percent of all subjects would prefer a recognition time of less than 1 second,
50 percent of 1 to 2 seconds, 33 percent of 2 to 4 seconds, and 1 subject would accept 4 to 6 seconds"
A particularly valuable affordance of recognition systems are for vision-impaired users. This may allow them to "see"/register exhibits that otherwise might be completely imperceptible.
Ultrasound positioning systems work in a similar way to radio-based positioning, except that the communication channel is one-way (as opposed to two-way on Bluetooth or WiFi). A device on the individual (or object whose position is being tracked) emits a unique high frequency ultrasound which is picked up by a network of strategically placed microphones.
Because the device itself has no knowledge of its position (only the system behind the microphones does), one would need a seperate companion device (e.g. PDA, iPhone, etc.) connected to the backend system and query it for the ultrasound device's position.
SciAm has an article (A Positioning System That Goes Where GPS Can't) on the topic, and Sonitor Technologies (based out of Norway) appears to be a forerunner in the field.
Notes on ultrasound, as well as a survey of all other positioning technologies, is in Global Positioning: Technologies and Performance (covers indoor, despite the title).