EO·Atlas
Differential absorption lidar (DIAL / IPDA) for trace gas
Differential absorption lidar transmits two closely-spaced wavelengths (online, on a gas absorption line; offline, off it) and retrieves trace-gas column or profile from the differential return attenuation. Acquisition: dual-wavelength on/off-line pulses (integrated-path IPDA against the surface return, or range-resolved DIAL). Retrieval: column-integrated dry-air mixing ratio of the target gas. Example instrument: MERLIN (methane, IPDA, ~1645 nm, CNES/DLR, phase D, launch ~2027).
Differential absorption lidar (DIAL) and its integrated-path variant (IPDA) retrieve atmospheric trace-gas concentrations by transmitting two closely spaced laser wavelengths: an online wavelength tuned to a strong absorption feature of the target gas and an offline reference wavelength positioned just outside that feature. The ratio of the backscattered signals at the two wavelengths carries the round-trip absorption, from which column-integrated or range-resolved dry-air mixing ratios are derived.
IPDA targets the surface return or a hard reflector to accumulate the full column signal, making it particularly suited to global greenhouse gas monitoring from orbit. MERLIN, a joint CNES-DLR mission, uses an IPDA lidar at approximately 1645.552 nm, placed on a methane R6 absorption line, with a closely offset offline pulse. Its target retrieval precision for column-average methane (XCH4) is 1% (approximately 18 ppb), sufficient to detect regional flux signals against the global background.[1][2][3] As of available mission documentation, MERLIN is in phase D, with a planned launch on a MYRIADE Evolutions platform; schedule details may have evolved after the research date.[2]
China's ACDL instrument, operating at 1572 nm via an IPDA-DIAL channel for column CO2 (XCO2), flew on DQ-1 (Daqi-1) and demonstrated global XCO2 column retrieval from spaceborne IPDA lidar, confirming the technique's maturity for carbon cycle monitoring.[4][5] The same ACDL instrument is also carried on DQ-2 (Daqi-2), which launched in April 2026 and continues CO2 column measurements via IPDA.[6]
Key data products are column-average dry-air mixing ratios (XCH4, XCO2), supporting greenhouse gas flux estimation and carbon cycle monitoring from orbit.
- [1]MERLIN: A French-German Space Lidar Mission Dedicated to Atmospheric Methane - Remote Sensing 2017peer reviewed2026-06-05Peer-reviewed mission overview; IPDA DIAL principle, online wavelength ~1645.552 nm on CH4 R6 absorption line, offline reference, XCH4 column retrieval, 1% (~18 ppb) precision target; CNES platform + DLR payload
- [2]MERLIN - eoPortalcommunity2026-06-05Mission profile: phase D, launch ~2027, sun-synchronous orbit, nadir-pointing IPDA lidar; MYRIADE Evolutions platform
- [3]First global XCO2 observations from spaceborne lidar: methodology and initial result - ScienceDirect 2025peer reviewed2026-06-05Peer-reviewed; confirms ACDL 1572 nm IPDA-DIAL technique demonstrated operationally for CO2 column retrieval on DQ-1
- [4]ACDL/DQ-1 calibration algorithms Part 1 - AMT 2025peer reviewed2026-06-05Confirms ACDL 1572 nm IPDA channel for XCO2 and 532 nm HSRL for aerosol on DQ-1 launched April 2022
- [5]MERLIN (Methane Remote Sensing Lidar Mission): an overview - ResearchGatethird party2026-06-05Conference paper confirming DIAL/IPDA dual-wavelength measurement principle and methane science case
- [6]WMO OSCAR - ACDL instrument record (DQ-1 and DQ-2)agency doc2026-06-06Agency-doc: ACDL listed on both DQ-1 (Jul 2022) and DQ-2 (Apr 2026); CO2 mole fraction listed as primary measurement variable for both missions; 1572 nm dual-wavelength IPDA channel described. Confirms ACDL on Daqi-2 measures CO2 via IPDA, backing the daqi-2 flies_on relation.