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The VDML series of Tracking Units The VDML SERIES of mobile unit are GPS-based tracking devices for use in radio-based communication networks. The unit is configurable for connection to any radio used in simplex, repeater, or trunking systems. The units operate from either +12 or +24 Volts DC and at +12V the current draw is 130 mA. This document is designed to familiarize the user with the many available features of the VDML Series unit. The details for setting the various operational parameters of the unit are provided as part of a separate programmer's manual. The main features of the VDML Series unit are as follows:
Periodic Position Reporting Periodic position reporting (also known as 'broadcast' position reporting) is a mode where the VDML is programmed to automatically send its position over the air at pre-defined intervals. The unit will calculate time and automatically key the radio to send the position report in its programmed interval. Periodic reporting is best for small fleets where each unit is assigned a scheduled time to report and the reporting period is short. Each unit in a fleet is assigned its own time slot so that no two occupy the same time slot. Each time slot must be large enough to account for timing jitter and communication interface delay. The time slot in which the unit transmits is a function of its assigned ID and the programmed size of the fleet. The minimum period at which the unit can transmit its location depends on the number of units in the fleet and the width of the time slots assigned to the fleet. For example, if there are 30 units in a fleet and the time slots for the fleet are one second wide, then the period is 30 seconds. Increasing the programmed fleet size for a fixed actual fleet size results in longer periods. The programmed fleet size must always be equal to or larger than the actual fleet size or some units may transmit in the same time slot. Periodic reporting mode is preferred for small fleets where the programmed fleet size results in an acceptable transmission period. For larger fleets the 'Polled' mode is recommended. For applications that provide only one-way communication to the base station so that polling can not be used and have a large number of units and want to provide faster updates for some of them, an 'Overlapped' periodic reporting mode may be used. Overlapped periodic reporting involves breaking a fleet into two or more sub-fleets, each with a different update period. One of the sub-fleets provides a short update period and therefore a higher update rate. Units are then assigned to one of the sub-fleets and programmed accordingly. For example, a shuttle-van company may use this mode and always assign rookie drivers to the higher update rate fleet. This mode requires that the timing parameters for each sub-fleet be computed to overlap exactly. It is recommended that the user contact CTI if this mode is desired. Corp Ten will work with each customer to assure that the most appropriate method of position reporting is chosen and implemented. Polled Position Reporting Polled mode is useful for larger fleets when only a fraction of the total fleet will be active at any given time. In this mode, the unit remains silent until a poll is received via the radio network. If the unit finds its assigned ID in the poll, it responds in a time slot that is dependent on its position in the list of polled units. The exact order in which the units in the fleet are polled is controlled entirely from a central location. Custom polling algorithm design services are available from CTI. Report Scrambling To prevent eavesdropping on position reports sent from the unit, reports are scrambled when a scramble-key is programmed into the unit. The receiving location must then unscramble the reports. Unscrambling of the position reports is done in the AutoFind™ AVL System tracking program. The user is responsible for managing and using the proper scrambling keys. A key can be from 0 to 32 printable ASCII characters in length. A null key (zero-length) disables scrambling. The AutoFind™ AVL System program is capable of handling several keys at once to help operation during times of key changing or rollover. Panic Reports In response to a user-activated panic button switch, the unit enters a 'panic' mode. In this mode, the unit transmits a position report every 7 seconds ± 3 seconds for a period of 45 seconds. The state of the panic signal is contained in the position report for use at the base station. When used in conjunction with the AutoFind™ Tracker AVL System, the AVL system centers the map on the panicking unit and alerts the user to the unit in question. Fence Alarms If the unit moves outside of a configurable geographic region, the unit enters a 'fence-alarm' mode. In this mode, the unit transmits a number of position reports just like in 'panic' mode but with a status bit set that indicates that the fence was broken through. Entering 4 geographic points into the unit configures the Fence Alarm. The 4 points define the "boundary" outside of which the unit will send position reports with an alarm notification. Optional Location History With this option, the unit periodically stores its current location in non-volatile memory to maintain a history of up to several thousand positions. The period between stored reports is adjustable. When the history memory becomes full, it rolls over and new locations overwrite the oldest locations in the history. To extend the length of time covered by the history memory, a threshold may be set that indicates a speed below which only a single report will be stored. For example, with the threshold set at 1 MPH, only the first report will be stored after the unit stops. Periodic report storage resumes after the vehicle again exceeds the 1-MPH threshold. A threshold of 0 MPH tells the unit to periodically store locations regardless of the speed of the vehicle. The location history is read from the unit using the VDML Interface program and must be read via direct RS-232 connection to the unit. When used in conjunction with the AutoFind™ Tracker AVL System, downloaded history files may be plotted on a map and the history data viewed in a table. The data available for each position report are the date and time, speed and heading, and latitude and longitude. Unit Programming The VDML Interface program is a graphical user interface program that is used to configure the unit. It runs on Windows 9x or NT. Setup of the unit must be accomplished via direct RS-232 connection. 
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