The ECVs addressed in the GAIA-CLIM project (temperature, water vapour, aerosols and atmospheric composition) are measurable by a diverse range of instruments. For some non-satellite instruments, there are geophysical limitations as to when measurements can be undertaken, e.g., FTIR requires direct line of sight to the sun under clear-sky conditions. However, other instruments (e.g., GNSS-PW and microwave radiometers) can, in principle, operate on a continuous basis.

The primary benefits of sustained and continuous operations are two-fold: Firstly, the opportunities to achieve spatiotemporal match-ups with satellite measurements - if this is the primary approach to validation - are maximised; and secondly the validation of higher level data products (spanning the full range from retrieved products, through gridded products, to global reanalysis-based products) is enhanced through the use of continuous, or almost continuous, datasets.

The measurement techniques potentially available to serve as reference measurements for the relevant ECVs include: ground-based microwave radiometry and infrared spectrometry; differential optical absorption spectroscopy (DOAS and Pandora), lidar (including Rayleigh, Raman, rotational Raman and differential absorption lidar), Brewer/Dobson spectrometers, and sunphotometers. There are a number of reasons why, in practice, many measurements are not made on a continuous basis:

• Technical - instruments may require frequent maintenance, adjustment, calibration, or retuning requiring manual intervention, which may not be available on a continuous basis; data acquisition and analysis may still require too many manual interventions;
• Scientific – particular site-specific conditions may prevent measurements being made. For example, cloud conditions may preclude certain measurements (e.g., FTIR – for composition measurements, or for passive measurements of temperature and humidity, also rotational Raman lidar for temperature).
• Operations / logistics – the site may not be manned continuously and instruments cannot, as yet, operate in an automated way; also, the data analysis may not be sufficiently automated.
• Financial - funding authorities often neglect the importance of the non-satellite observing system, whereas it is indispensable for ca/val of the space segment of the observing system and as a transfer standard between successive satellites. 

Funding, clearly, plays a key role in determining the capacity for a given instrument to make (continuous) measurements and to rapidly deliver the data. Targeted funding support to meet multiple stakeholder needs including, but not limited to satellite cal/val, could ensure that a station/instrument is capable of more continuous operations and more rapid delivery of the data through higher levels of manning. Funding could also support technical development work to improve the degree of automation of the instrumentation across entire national or international networks and of subsequent data analysis, thereby lowering the cost for continued operations and rapid data delivery.

The purpose of this gap is to recognise this general deficiency in many observing networks, and to encourage support to rectify these deficiencies. A funding mechanism (or mechanisms) needs to be instigated that recognises the costs to be covered by those communities which shall benefit from such sustained operational capabilities (including but not only satellite applications). Such targeted support would ensure sustainability, recognising the substantial diversity of competing demands on resources of in-situ measurement assets.