HAM-TRAILS

Health-&-Welfare messages for thru-hikers

Bob Bruninga, WB4APR@amsat.org, Annapolis, MD

THIS PAGE IS THE OLD IDEA ABOUT USING SPECIAL "CALL BOXES" ALONG THE TRAIL. See the new idea

OLDER HARDWARE IDEA: Placing PC's along the trail was the easy quick-start way to bring APRS tracking to the trails. But the long term objective could be to place small low power solar powered devices in each of the isolated shelters along the trail where the hikers rest each evening. See the complete APRSdos TEXT file.

The rest of this page addresse this original long range plan... HOWEVER, we douibt it will ever fly, so it remains here for historical purposes but is OBSOLETE. Only the idea of PC's at protected locations (all of the above) seems viable at this point.

HIKER DATA: The APRS-TRAILS device reports the following items for each hiker in a single 5 byte ABCDE field. Up to 16 fields can be transmitted in a single report. On receipt the hikers ICON and position are plotted on APRS maps.

POSITION: Since there can be no local clock at these sites, the time elapsed since entry is combined with the speed to predict a distance traveled in the direction indicated. The transmission is canceled when the hours-to-go has expired. Thus these two entries can be used to give almost 5 bits of distance in 1-mile increments (32 miles maximum) Since each device knows the exact trail directions, it translates the hiker's rough cardinal direction into a more precise vector so that the future predicted direction can be fairly accurate (within 5.1 degrees).

DATA ENTRY: The data entry device must be very robust and simple. For this reason a single 16 character by two line PROMPT window is used along with UP/DOWN and BACK/NEXT(OK) push buttons. The hiker is prompted through his data entry and he can also query the system about others along the trail.

PLACARD: The placard associated with each HAM-TRAIL device is the HAM RADIO payback benefit. It gives local clubs the opportunity to proselitize about HAM radio. The sample text above might work for your club. In general, the entire device is probably no bigger than a 6" utility box and should be secured from physical damage and weather as much as possible.

LOCATIONS: For the physical sercurity and best utility of these devices, the following guidelines should be considered:

Locate them in the clear to minimize vandalism

Mount them to maximize southern sun exposure on solar panel

Northern facing LCD panel might be easier to read

Locate them on hill tops and crests to maximize comms

Locate them far from roads and camps and public access. They are for through-hikers and not day hikers and kids

Camoflage or hide the conduit and solar panel so that the device blends with nature except for the placard and display unit.

POWER BUDGET: This system must be very low power to operate forever on the average sun from a 1/4 watt (3" square solar panel). This yields an average 24/7 available power of about 40 milliwatts. If the processor and display run on 40 mA at 10 volts but sleep most of the time when not in use, this gives the following power budget.

CUSTOM DATA: The following site-specific data will need to be programmed into each device:

APRS Position packet

Distance to next device in each direction (if known)

Transmit rate (may need to be adjusted to match solar power)

Device NAME and CALLSIGN

Exact trail vectors in the two (or more) directions so that dead-reckoning of positions works out well

Notice that the trail direction is actually resolved to 5 bits of precision, 3 bits from the hiker entry and two bits computed from this table to give 11.25 degree resolution. These offsets can be pre-computed and stored in this table, not the full angle to save math in the PIC. To give distance, the device PIC processor only has to multiply the elapsed time by speed to get the distance.

APRS PACKET FORMAT: The format for these packets will come from the experimental APRS data format "{HT" and uses the PRINTABLE subset of the ASCII code. That is, 8 bit values are permitted, but no bytes will be control codes. The overall packet format is as follows:

Notice that the TOCALL may be just about anything to match the application and that this does not correspond with the standard APRS-TOCALL filters. This was done intentioanlly since this format is new and is not backward compatible with any existing code, and cannot be displayed meaningfully without NEW code. THus, new code can ignore the TOCALL filter for this new data type.

WEATHER DATA: If additional instruments are added, then the once every 12 minute report can also report local weather data. The battery Voltage Temperature and Solar flux will be in every report (since they are only a thermister and a zenith pointing solar cell. These bytes use a (tbd) set of base 91 characters to give roughly 1% precision in the data.

HIKER DATA: Hiker data consists of a 3 byte ID field and then two other bytes as follows:

This is a representative APRS map of on-air APRS activity in the MD, NJ, PA area. Such maps can easily represent the locations of Hikers along the trail... ZOOM in or select a detail camp map if you see activity at or near camp.

**javAPRS Commands (Case Insensitive)**
U or D	zooms up/down (you may also use PGup/dn)
L, S, or M	List stations, Show Status or Messages to Java console
CTRL or ALT-click	Centers or Zooms map on clicked location
Arrow keys	scrolls map

You are visitor number: Since 22 April 2002. .

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The Naval Academy is a registered user of APRS and WinAPRS. The purpose of this web page is to show several applications currently in use at this site and should not be considered as an advertisement or an endorsement of any commercial product.