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6 APRS IN ACTION
TM-D710A/E CONTENTS 23
6.1.5.2 Corner Pegging (Transmission after Heading Change)
When the mobile station’s speed is above LOW SPEED, Corner Pegging begins operating. A
Corner Pegging position beacon will be transmitted when the heading difference exceeds the
TURN THRESHOLD value. The heading difference is the compass heading of the present
direction of travel (in degrees) subtracted from the compass heading transmitted in the previous
position beacon. (Technical note: only the absolute value of the heading difference, normalized to
the first and second quadrants, is used.) The threshold value called TURN THRESHOLD
combines a fixed angle threshold, called TURN ANGLE, with a constant called TURN SLOPE,
times ten, divided by the present speed. You can use the formula below to calculate TURN
THRESHOLD.
By changing the value of TURN SLOPE, you can adjust how Corner Pegging responds as the
mobile station executes curves. When the value of TURN SLOPE is increased, a large heading
angle change is necessary to cause a position beacon transmission, especially at low speeds.
When the value of TURN SLOPE is reduced, the effect of speed on Corner Pegging is reduced, as
the constant TURN ANGLE becomes the controlling term in the equation. A value for TURN
SLOPE of 110 (in units of 10 degrees per mph) has been experimentally determined to be a good
value for vehicles on typical roads.
Corner Pegging Formula:
TURN THRESHOLD = TURN ANGLE + 10 x TURN SLOPE / SPEED
The above formula is based on the reality of how a mobile station executes turns: when traveling
at slow speed, it can turn acute angles, but when traveling at high speeds, it turns only shallow
angles. In other words, TURN THRESHOLD varies inversely with speed: at a low speed, the
threshold becomes large, while at a high speed the threshold becomes small.
To prevent the Corner Pegging beacon from transmitting too often and causing QRM, there is a
timer value called TURN TIME. The Corner Pegging position beacon is not transmitted until TURN
TIME seconds have elapsed since the previous beacon transmission, regardless of which method
(Corner Pegging or Variable Rate Beaconing) caused the previous beacon.
Table 6-4 Conditions for Corner Pegging Operation
Table 6-5 Configuration Example
Speed Corner Pegging
Over HIGH SPEED Yes
Under HIGH SPEED
Over LOW SPEED
(Only when HIGH SPEED ≥ LOW
SPEED)
Yes
Under LOW SPEED No
Menu Item Configured Value
LOW SPEED 5
TURN ANGLE 28 degrees
TURN SLOPE 26 (x10 deg/speed)
TURN TIME 30 seconds