Sierra Wireless MP750 modem GPRS con GPS: FAQ

FAQ

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Hardware | Software | Communications | Integration | Maps

Hardware:

What is the difference between the various AVL devices out there?

The differences in AVL hardware usually relate to the cost, intelligence, versatility, and supported communication protocols offered by the device.

What are the costs?

AVL hardware typically varies between $500 and $2000 per vehicle. As you would expect, the higher end devices offer more functionality, ruggedness, and versatility.

What is meant by device intelligence?

Intelligence refers to the sophistication level of the software and hardware on board the device itself. Some devices may only output information when requested by a host application such as an attached laptop or a command from the remote dispatch center. An intelligent device can be programmed to output information based events such as speed thresholds, entering or exiting regions, time elapsed, distance traveled, status changes, and a whole host of other events. In addition, an intelligent device will store information when communications are lost.

What is meant by device versatility?

Versatility refers to the number of different applications a particular device can be used in. For example, a truly versatile AVL box will be small in size, rugged, expandable intelligent, communications independent, able to support (but not require) mobile data computer interfaces, and equipped with multiple input and output options.

How is GPS used in Automatic Vehicle Location?

GPS receivers located in the vehicle continuously monitor the location, speed, and heading of the vehicle and transmit the data to a remote computer using wireless technology. GPS is a satellite based global positioning service offered by the U.S. Department of Defense ideally suited for AVL and Location Based technologies.

What is Differential GPS?

Differential GPS is technique used to enhance the accuracy of the standard GPS signal to better than 5 meters in most cases and better than 1 meter for higher end receivers. The technique makes use of a stationary GPS receiver called a Base Station that logs the errors of the GPS signal. Roving receivers can use these error corrections to significantly enhance their accuracy. DGPS is often unnecessary for AVL applications because autonomous accuracy is usually better than 10 meters without the aid of differential GPS.

What type of hardware is right for my application?

The answer depends on several factors such as cost, wireless communications, size, preference, and ruggedness. The driving factor is often the choice of wireless technology. For example, if you plan on using CDPD as your communications options you may want to consider an integrated CDPD/GPS combination device. If you have access to a radio network, as a city or government organization would own, there are specific devices suited to the task. The bottom line is that no one device can be all things to everyone. It is important to focus on your own specific application and think about what you truly want to accomplish with AVL technology.

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Software:

What does the software do?

In short, everything. The software represents the core of AVL technology - it is what gives an AVL system most of its functionality. At a minimum, AVL software will display your vehicle locations on a digital map. In advanced applications the AVL software integrates with other systems such as dispatching aids or fleet analysis tools.

What is client / server architecture?

Most AVL software comes in two parts - a server and a client. The server is the main repository of AVL data from the fleet. The client (or clients) connect to the server to get a realtime or post-processed view of the fleet.

What is meant by scalability?

Some software solutions do not work well when the vehicle count gets large. A scalable AVL server application can be implemented to handle the volume of 10's, 100's, and even 1000's of vehicles. It is important to use AVL software that you can grow into.

What is the AVL database?

In most applications, each AVL record is stored in a database of some type. Common DB applications in use are MS Access, MS SQL Server, and Oracle. The database is used primarily to generate fleet analysis reports and to allow AVL replay.

What is thick and thin client architecture?

Thick AVL clients are applications that you install onto your local hard drive. Thin clients are typically Web-based, so that all you need is your browser. The advantage of a thick client is speed of operation. A computer can execute applications fastest when stored locally. Thin clients have the advantage of not requiring separate software licenses for each instance of use, but may not be "snappy" enough for some users in terms of panning, zooming, and controlling map layers.

What about fleet analysis reports?

AVL software should come with an option for running queries against the database for historical fleet analysis. Such fleet reports may include mileage, speed, number and length of stops, time spent under special conditions (sirens on/off, lights on/off), and violations of geographic boundaries. It is important to remember that an AVL application can ONLY report and summarize data that exists in it's database. Information that is not collected can not be queried.

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Communications:

What are some communications options for AVL?

Several common choices include CDPD, GPRS, Private Mobile Radio, Digital Radio, Cellular, and Satellite.

What's the deal with CDPD and GPRS?

The main advantage of using a public data network such as CDPD or GPRS is that you do not have the up front costs of building and maintaining your own wireless network. There are monthly reoccurring costs that average $30 - $50 per month per vehicle, though. CDPD and GPRS networks are owned an d operated by the cellular phone companies. As such, the coverage is typically limited to major urban areas. Another significant advantage of public packet data networks is the tremendous bandwidth they can support. The real time data volume of a 1000 vehicle fleet can easily be handled by CDPD technology.

What's the deal with Private Mobile Radio (PMR)?

Typically, only cities and other government agencies own and operate their own private radio networks. Depending on the technology, AVL may or may not be supported. For instance, a city with a 10 channel 800Mhz radio system already loaded to capacity with the existing voice traffic will find it very difficult to squeeze additional bandwidth for AVL. However, an agency with frequencies that can be dedicated to AVL may be in good shape. The main advantage of using Private Mobile Radio is that customers can leverage the money already spend on their radio network. The disadvantages usually relate to AVL performance. PMR based AVL systems are very difficult to scale up to hundreds of vehicles unless the customer can live with slow update rates.

What's the deal the Digital Radio?

Digital radio networks are built to handle data - not voice traffic. They are optimized to allow much greater over the air bandwidth than a PMR type system. Examples of digital radio networks include, Motorola Data Tac, RF Spread Spectrum, and Data Radio. The up front costs associated with building a digital radio network may be significant depending on how "big" the system is.

What's the deal with Cellular?

Standard cellular based AVL system s have the advantage of being available in most locations. The primary disadvantage is the low performance and sometimes high cost of using the cellular airtime. With cellular based AVL systems, the customer pays by the minute. The more vehicles report, the more minutes are used. In addition, the performance of such system is usually fairly low. Fundamentally the problems of performance are because each cellular AVL device must physically make a phone call to the server to pass it's data. Unless you have many phone lines attached to your server, only one device can communicate at a time.

What's the deal with Satellite?

Satellite based communication networks offer continental (or even global) coverage. Some systems, such as SatCom, allow full blown TCP/IP connectivity to a remote device. The trade off is usually price - satellite communications costs more per minute (or per byte) than terrestrial based networks. In addition, they have limited bandwidth. However, satellite technology is getting better and cheaper each year. Some say it is only a matter of time before satellite data networks compare in price and performance to ground based systems.

Which communications method should I use for AVL?

If you don't have access to a digital radio infrastructure you may want to seriously consider CDPD or GPRS. If you can't justify the re-occurring costs of public digital networks, a conventional radio system may suffice (as long as your vehicle count is below 100). Customers with existing radio networks need to find out what their current loading situation is - if the system is already running at capacity, adding AVL is usually not a good idea. One thing to remember - not all AVL devices support all communication options. The choice of your wireless network often dictates the type of AVL device you must use.

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Integration:

What is really involved with AVL integration?

In short, system design, configuration, installation, testing, acceptance, and support. These tasks require skilled professionals and as such incur costs. As much as everybody would like a "shrink wrapped" system, it rarely happens. The problem lies with the fact that everybody has their own definition of what AVL is supposed to do for them and how it will work. The best way to keep integration costs to a minimum is to have a very focused project scope - don't implement functionality that you don't need. It's always a good idea to have people on your staff that understand the technology.

How long does an AVL system take to setup?

Depends on the scale of the project. In some cases, customized software solutions must be written before integration can start. If your project calls for off the shelf software and hardware, integration should be relatively quick. Once again, the key to a quick and painless integration is to have a focused scope.

What about vehicle equipment installation?

The physical installation of AVL equipment is typically not very difficult - anybody that can install a car stereo can probably install AVL devices. Some applications require the AVL to connect to various devices in the vehicle (status heads, laptops, vehicle data, etc...). Installation is a more involved process in these situations. Expect, on average, about 2-4 hours of labor for each vehicle.

What are some common system failures?

The three most common system failures are: bad in-vehicle equipment, network issues, and improper administration of the system. Vehicle components can fail because of loose connections, loss of antennas, blown fuses, and normal wear and tear. It is a good idea to hold onto a spare system or two in case you need to rapidly replace parts. Network failures involve the communications system and the internal LAN/WAN of your agency. Wireless problems occur when tower sites or the links to them fail for whatever reason (lightning strikes, construction, flood, etc...). Internal network issues can come up any time. The client/server aspect of most AVL software requires network connectivity between the mobiles, the clients, and the server. Routers, firewalls, and network switches all have to be on the same page to ensure data packets are routed properly. Improper administration of the server, clients, or mobile equipment can also cause trouble (user error).

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Maps:

What's so important about the maps?

Without good maps, the AVL displays inaccurately. Maps can be described by two characteristics - accuracy and completeness. Accuracy involves the spatial "reality" of the map data. If you maps are spatially out of whack it will be apparent when vehicles show up driving in rivers or through shopping malls. Incomplete maps are missing features and data. A classic example of an incomplete map would be a road centerline file without street names in the database.

What are the minimum specifications for map data in AVL?

At minimum, your map data should be no more than a couple of years old (1 year if you're in a high growth area). The spatial accuracy should be better than 50 ft. In addition, the map data should contain street names and address ranges for ALL segments.

What is Geocoding?

Geocoding is the process of determining a latitude and longitude from an address. Reverse geocoding is the process of determining an address from a latitude and longitude. Unless the street map layer contains an address database, geocoding is impossible. An example of geocoding in public safety would be the automatic plotting of an E911 call in a dispatcher's AVL screen. Geocoding for commercial customers involves plotting the location of a service site and dispatching the closest technician.

What's the deal with coordinate systems and map projections?

It turns out that there hundreds of coordinate systems in use today. In the general sense that's ok, but if you try to overlay a streets layer in one coordinate system with a rivers layer in another coordinate system, they will not line up. Each layer may be just fine independently, but you can not compare apples to oranges. GPS natively returns a position as a Latitude and Longitude referencing the WGS84 ellipsoid (for you geodesy minded folks, WGS84 and NAD83 are functionally interchangeable down to better than a meter in most cases). If the maps you wish to use for AVL reference a coordinate system that is not in a latitude and longitude format, something has to be transformed - either the GPS coordinates have to be changed or the maps have to be changed to make both sets of coordinates line up.

Where do the maps come from?

There are three sources of map data for AVL - Commercial, GIS, and TIGER. Commercial map data can be purchased from companies such as GDT, Navigation Technology, and Tele Atlas. This data is fairly high quality but usually lags new road construction by a year or two. GIS data comes from an agency's GIS department. The quality of GIS data ranges from extremely good to very bad. Different GIS departments have different standards for accuracy and completeness. TIGER data is free from the US Census Bureau. It is adequate for non-critical AVL applications. As a first option, you would want to consider your own GIS data. Time and money have been spent making these maps so you should try and use them. If the GIS is not adequate or doesn't exist, consider the commercial sources of data. As a last resort use TIGER files. Expect to pay several thousand dollars for high quality, state-wide digital map layers.

What about aerial or satellite imagery?

As long as the coordinate system and projection of your aerial or satellite images matches the digital map layers there is no reason you shouldn't be able to include them. It is important to note that while not all AVL display software supports aerial image display ( AVL clients, Compass Trac® and Compass Trac Mobile®, do indeed support aerial image display).