As discussed above, the major hadronic leading-order contribution to 
$a_{\mu}^{\rm had}$ comes from the energy range below 1 GeV, where in turn
the $\pi^+\pi^-$ channel gives the dominant contribution. Direct scan at VEPP-2000 will
deliver huge statistics at the experiments CMD-3 and SND, but the accuracy of the cross
sections will be determined by systematic errors. Therefore, any other
possibility to measure the pion form factor, for example with ISR,
will be a valuable tool to provide a cross check for better understanding
the scale of systematic effects.

The design luminosity of $\sim 10^{32}$cm$^{-2}$c$^{-1}$ is expected at 
$\sqrt{s}$ = 2 GeV. 
The luminosity recalculated to the $\rho$-peak will be close to the one obtained 
with CMD-2.
% so a  statistics similar to CMD-2 will be collected.
Let us recollect that the ISR method provides a continuous ``low energy scan'',  while
taking data at fixed high energy. The threshold region, $2m_{\pi}$ -- 0.5 GeV,
gives about 13\% of the total contribution to the muon anomaly. As a rule, 
the collider luminosity dramatically decreases at low energies. To overcome the lack of
data in the threshold region, the ISR method can serve as a very efficient
and unique way to measure the pion form factor inside this energy region.

Today, the theoretical precision for the cross section of the process $e^+e^-
\to \pi^+\pi^-\gamma$ is dominated by the uncertainty of the radiator function
(0.5\%), and there is hope to reduce it to a few per mill in the future.
 In the case of the pion form factor extraction
from the $\pi^+\pi^-\gamma / \mu^+\mu^-\gamma$
ratio, the dependence on theory will be significantly reduced, since the
main uncertainty of the radiator function and vacuum polarisation effects
cancel out in the ratio. With the integrated luminosity of several inverse
femtobarn at 2 GeV, one can reach a fractional accuracy on the total error
smaller than 0.5\%.

In direct scan experiments the data are collected at fixed
energy points. Thus, some ``empty'' gaps without data naturally arise.
The experiments with ISR will cover the whole energy scale, filling any existing gaps.
Trigger and reconstruction efficiencies, detector imperfections and many other factors
will be identical for all data in the whole energy range. Therefore, some systematic 
errors will be cancelled out in part. Comparison of cross sections for the process 
$e^+e^- \to \mu^+\mu^-$, measured both with ISR and direct scan, can serve as a 
benchmark 
to study and control systematic effects. It should confirm the validity of this 
method and 
 help to determine the energy scale. A fit of the $\omega$ and $\phi$ resonances
will also provide a calibration of the energy scale -- an important feature to achieve a
systematic accuracy of a few per mill for the pion form factor.

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