  A radio transmitter which is accelerating with a non-zero radial component
with respect to a receiver will produce a signal that appears to change its
frequency over time. This effect, commonly produced in astrophysical situations
where orbital and rotational motions are ubiquitous, is called a drift rate. In
radio SETI (Search for Extraterrestrial Intelligence) research, it is unknown a
priori which frequency a signal is being sent at, or even if there will be any
drift rate at all besides motions within the solar system. Therefore a range of
potential drift rates need to be individually searched, and a maximum drift
rate needs to be chosen. The middle of this range is zero, indicating no
acceleration, but the absolute value for the limits remains unconstrained. A
balance must be struck between computational time and the possibility of
excluding a signal from an ETI. In this work, we examine physical
considerations that constrain a maximum drift rate and highlight the importance
of this problem in any narrowband SETI search. We determine that a normalized
drift rate of 200 nHz (eg. 200 Hz/s at 1 GHz) is a generous, physically
motivated guideline for the maximum drift rate that should be applied to future
narrowband SETI projects if computational capabilities permit.
