Mobile trackers. Thanks to the clever designs of Larry Bush W5NCD, HOTARC has managed to acquire, modify, and configure a collection of inexpensive 2-meter handheld radios to function as APRS trackers. HOTARC members were able to acquire these radios at roughly half price: the Club and the members shared the cost, with the understanding that they would be made available for use in Club special events. These low-cost transmitters are also low-wattage, which presents some challenges for use as trackers, as discussed below.
Tracking mobile stations. We install APRS trackers into as many support (SAG) vehicles during special events as we can, and this greatly aids our coordination efforts. The special event Net Control is thereby able to track in real time the locations for many of the assets as they move around (or not). The movements might be over a few city blocks (e.g., on foot), or over a county (e.g., in vehicles). Typically, we set up a tall antenna for Net Control to receive the many tracker signals. Digipeaters and i-gates strategically located can help low-wattage trackers be reliably heard, too. (See below.)
Avoiding interference. APRS trackers are normally configured to beacon position and speed data dependent on the tracker's motions—such as when turning a corner or traveling a specified distance. However, several trackers using that strategy will surprisingly often need to transmit at nearly the same instant. Although trackers are generally able to wait for "quiet" before transmitting, it is easy to understand how several distant low-wattage trackers will not be heard, and "step on" each other. To avoid such inadvertent radio-collisions by a set of managed transmitters we use time slotting.
Time slotting. The TinyTrak used in most of our trackers supports two configuration choices, so we have HOTARC trackers set to employ time slotting when their secondary configuration is selected. With time slotting, the tracker sends out a position report 1) every X minutes, 2) at a specific "slot" of time (seconds) 3) during each transmit cycle (an agreed-upon number of minutes). This "time slotting" totally eliminates the radio collisions from coordinated multiple trackers used in a special event, and still allows for "uncoordinated" trackers (which typically wait for a quiet moment to transmit) to slip in. The table below details an example plan of "time slotting" for an event, using a 3-minute cycle time and 8-second intervals. Three-minute updates are quite adequate for this event. Each packet burst is less than a second, so that leaves 7 seconds of "quiet" during every 8-second interval, during which uncoordinated trackers (e.g., mobile rigs with built-in APRS capability) can squeeze in their reports.
Frequency choice. For such events we operate APRS on the much quieter 147.500 MHz rather than the usual APRS 144.39 MHz. If necessary, we set up several digpeaters to hear remote low-wattage trackers—especially those that are far away from Net Control or blocked by dense buidlings. We also have an i-gate listening on the frequency to upload our packets to the APRS database. This last step allows anyone with Internet access to follow our Trackers—even Waco Bicycle Club members using their smartphones! (Click map image below for an example of how thoroughly our trackers monitored movements of the various event vehicles across the county over the course of several hours of a special event.)
|APRS Trackers using 3-minute
with time slotting (supported by TinyTrak)
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