GPP can not be measured directly due to complexities involved in the representative estimates of autotrophic and heterotrophic respiration terms. It can only be derived from some indirect methods of measuring NEE (Net Ecosystem Exchange) and Heterotrophic (soil) respiration. NEE can be estimated by eddy flux towers that measure carbon flux which can be used to estimate net amount of Carbon fixed over a given time period by a target ecosystem. Soil respiration is estimated using night time measurements of NEE. Similarly, automated gas exchange chamber instruments can be utilised to get estimates of autotrophic and soil respiration terms.
However, these fluxes including photosynthesis process are complex and very sensitive to micro-meteorological conditions. Hence these indirect measurements are prone to large errors depending on measurement protocols and heterogeneity of the sampling area.
Flux towers measure fluxes of CO2, water vapour with the help of InfraRed Gas Analyzer (IRGA) system. They are also equipped with sonic anemometers to measure temperature fluctuations. These data are later used in the calculations of NEE. Post-processing of Eddy flux data is also complicated and have to be done carefully under expert guidance. Some examples of flux tower systems: LI-7200RS Enclosed CO2/H2O Analyzer, IRGASON by Campbell scientific.
Some efforts have been done to establish a network of flux towers globally. It is called FLUXNET. More than 500 towers have been deployed globally and dataset can be accessed here.