FASTRAK MANUAL
Rev. G
E-4
JUNE 2012
may be interpreted as the error in degrees per “v” degrees/second in
orientation input, or the error in inches per “v” inches/second of translation. Note that in
either case the units are in seconds.
As an example, suppose that the update rate of the tracker is 120 Hz, thus
∆
t = 1/120
second. Suppose also that the receiver is slewing in azimuth at 90 degrees per second and that
α
is a constant 0.95, a value that can be attained by either fixing both the upper and lower limits of
α
to 0.95, or by setting just the upper limit to 0.95 and letting the adaptive filter push
α
to this
maximum limit which is what would happen for slew rates of this magnitude. The filter lag for
this example is calculated to be 0.44 ms. The correct interpretation of this figure is that the
receiver coordinates output at t=5.5 ms correspond to where the receiver was at t =
τ
/2 - 0.44 ms
= 2.2 ms; this would increase apparent latency to T -
τ
/2 + 0.43 ms = 4.2 ms.
The next example demonstrates what can happen when the filter constant is set too low
producing extremely heavy filtering. Suppose
α
is set to 0.05 and all other conditions are the
same as in the above example. In this case the filter lag calculates to 158 ms, and the
interpretation is that the coordinates output at t=5.5 ms corresponds to where the receiver was at t
=
τ
/2 - 158 ms; this yields an apparent latent period of T-
τ
/2+158 = 162 ms. Obviously, low filter
settings must be avoided if any reasonable dynamic response is desired.
RECAPPING
This technical note has discussed the latency in the application of a Polhemus 3SPACE™
FASTRAK product. As pointed out, sync-to-output and effective latencies are measures of
tracker throughput and cannot be changed, while apparent latency and filter response are
controlled to a degree by the interface and application environment. To derive best performance
the FASTRAK product should be synchronized and data records should be reduced to the
minimum required. Also, use the fastest baud rates available, consider the use of binary formats,
and use the continuous print mode.