  We present results from a large global VLBI(Very Long Baseline
Interferometry) survey of compact radio sources at 86 GHz which started in
October 2001. The main goal of the survey is to increase the total number of
objects accessible for future 3mm-VLBI imaging by factors of 3-5. The survey
data reach the baseline sensitivity of 0.1 Jy, and image sensitivity of better
than 10 mJy/beam. To date, a total of 127 compact radio sources have been
observed. The observations have yielded images for 109 sources, and only 6
sources have not been detected. Flux densities and sizes of core and jet
components of all detected sources have been measured using Gaussian model
fitting. From these measurements, brightness temperatures have been estimated,
taking into account resolution limits of the data. Here, we compare the
brightness temperatures of the cores and secondary jet components with similar
estimates obtained from surveys at longer wavelengths (e.g. 15 GHz). This
approach can be used to study questions related to mechanisms of initial jet
acceleration (accelerating or decelerating sub-pc jets?) and jet composition
(electron-positron or electron-proton plasma?).
