User's Manual

P330 Data Sheet / User Guide 39
DRAFT
Two-Way Time-of-Flight
Max Range:
30-400 meters (See Section 6.2)
Precision (3 Standard Deviations)
10.0 cm (See Section 6.4)
Unit to Unit bias error (uncalibrated):
~30 cm (See Section 6.4)
Bias error (-40 to 85) (uncalibrated):
~20 cm (See Section 6.4)
Bias error (if fully calibrated):
+/-2cm (See Section 6.4)
Range Measurement Rate
20-182 Hz (See Section 6.3)
Table 6-1: P330 performance characteristics
From time to time changes will be made to the P330 design. Section 6.8 describes how the version
number of any given board can be determined and describes the version to version differences.
6.2 Maximum Operating Range of a P330 Radio
Operating range in any given application will be a function of parameters the user can control and
ones that are beyond control. Controllable parameters include transmit power (normally set to the
maximum value that the regulations allow), the antenna height, and the RF channel/message
configuration parameters (such as PRF, data rate and preamble length). The channel and parameters
are usually set either for maximum ranging rate (which provides a shorter operating range) or the
maximum operating range (which results in a slower ranging rate).
Parameters which affect range but which are not controllable include blockages from RF reflective or
impenetrable structures and attenuation due to propagation through foliage and walls. Interference
from other RF systems operating in the area is also a possibility but in practice it rarely happens.
While all of these factors are well understood (if not obvious) the most important parameter to
consider (and the one most frequently overlooked) is constructive and destructive Fresnel cancellation
and enhancement.
Section 6.2.1 will describe why Fresnel is such an important factor and Section 6.2.2 will offer
empirical data to illustrate operational range performance and Fresnel effects under a wide variety of
conditions.
6.2.1 Fresnel – Key Limit to Operational Range
Basically, when operating over open ground, transmissions will be received from the direct path as
well as from the forward scattered reflections from the ground. Since the ground reflection will
arrive at the receiver a bit later than the signal following the direct path, the summation of the two
signals can be either destructive or constructive. Destructive cancellation can be so severe that at
some separation distances the signal will not be received. At some distances constructive interference
will significantly increase the received signal, in which case the operating range will be increased. At
other distances the delay time between the two signals could be so large that neither has a meaningful
effect on the other.
While the Fresnel effect is frequently overlooked, in most outdoor applications it will be the primary
factor limiting the operating range. To illustrate this phenomenon consider Figure 6-1. In this
example, two radios were placed approximately 1.7 meters above the ground and data was taken
while the distance between the units was increased. The first unit was mounted on a tripod and the
second mounting above the roof of a small car. The test started with a separation distance of 8 meters
and the car slowly drove off until the signal disappeared. Figure 6-1 shows the results as a plot of
First Path Power (a measure of the received signal strength) as a function of the distance.