Mobile Geospatial Applications: A New GIS Paradigm

Wireless communications
The ability to connect to a corporate Intranet or the World Wide Web from anywhere without the requirement of physical wires is what makes wireless communications so valuable and mobile GIS applications possible. Essentially wireless communications involve the usage of radio waves as the transport layer. Many possible choices exist ranging from dedicated wireless LANs to two-way wide area packet data networks. Table 1 summarizes wireless communications options.

Type	Description	Network/Technology
Dedicated wireless LANs	Allows roaming in limited areas typically a building or campus while maintaining a wireless connection to the regular network	Wired network is usually an Ethernet backbone.
Wireless circuit switched	Data transmitted over existing cellular network, (session based)
Two-way wide area packet data networks	Available in all major metropolitan areas in US, Europe, and Australia	ARDISS (DataTac Motorola) RAM (Mobitex)
CDPD – Cellular Digital Packet Data	Overlays the regional cellular network, shares the network with voice.	All the major cellular providers offer CDPD
GSM – Global System for Mobile Communication	European digital cellular system primary purpose was to allow international roaming	Requires a SIM (Subscriber Identity Module)
Paging (one-way /two-way)	Available everywhere. Most common form of wireless.	EMBARC
SMR – Specialized mobile radio	Uses high powered mobile transmitter and one base transmitter for an entire metropolitan area. Vertical markets specialized application areas.	Nextel (ESMR Motorola)
Satellite based digital networks	Ideal for work in remote areas (oil and mineral exploration, research stations). High cost.	IRIDIUM
HDR –High Data Rate	To be offered soon. Data rate is comparable to cable modem 2megabytes/sec. Drawback is must have dedicated bandwidth.	HDR (Qualcomm)

GPS/DGPS
The advent of GPS/DGPS has done more for Navigation and GIS than any other technology. GPS (Global Positioning System) was developed by the Department of Defense as a tool for accurate navigation and positioning. It utilizes 24 satellites whose orbits are accurately known and can act as reference points. GPS utilizes 3 dimensional triangulation by measuring the travel time between the satellites and the ground receivers (the GPS receivers) whose position on the surface of the earth is not known. Essentially if the receiver locks on to one satellite one can describe a sphere whose radius is the distance based on the travel time of the signal emitted by the satellite. The position of the receiver is somewhere on the surface of that sphere. If the receiver locks on to two satellites then an elliptical circle where the spheres intersect describes the potential solution. To narrow the solution to a point, 4 satellites are required, although in practice, 3 satellites could suffice since a two point solution would most likely result in one of the points lying beyond or below the surface of the earth. The solution is given in terms of latitude, longitude and altitude.

DGPS: Differential GPS is a technique for increasing the accuracy of GPS. Without DGPS the best measurements are at best
+/- 150 feet; with DGPS accuracy increases to within a couple of feet. DGPS works by subtracting out the effects of atmospheric and other transient effects by comparing the computed position of a well know reference point and comparing it to the actual position of the reference point. From this discrepancy a correction to the travel time can be computed which is then applied to our GPS measurements. The differential correction could then be applied in near real time if a communication link is present between the reference station and the GPS receiver, or as part of a post processing operation, if
The measurements are to be stored and used later.