The AIRS and AMSU-A instruments mostly operate autonomously, collecting and sending science and engineering data to the Aqua instrument without intervention from the ground. All channels are used at all times, and the instruments remain on at all times.

AIRS operations group

The AIRS instrument suite operations group resides at JPL and is responsible for commanding the instruments, monitoring instrument health, and responding to instrument anomalies. Because of the autonomous operation of the instruments, the operations group normally has little need to actively intervene in instrument operation. Occasionally, the operations group does interrupt the usual operating mode of these instruments. For instance, the operations group intervenes when an anomaly – an unexpected interruption in normal operation, often attributed to a radiation hit that affects instrument electronics – occurs. AIRS has two other categories of operations, both of which are considered routine and not anomalous: calibration and spacecraft maneuvers. (There are no routine AMSU-A operations; that instrument is only commanded in cases of anomaly.)

Calibration tests

Two AIRS calibration tests are run monthly: the “guard test” and a photometric calibration test. The purpose of the guard test is to evaluate the response of the IR detectors under controlled conditions: first, only the “A-side” detectors for each dual-redundant channel are turned on, then only the B-side detectors. This gives the calibration team the information needed to give optimal weights to the A- and B-side for each channel.

The photometric calibration test turns on each of the three calibration lamps in succession to measure the response of the Visual/Near Infrared (VIS/NIR) detectors.

Other calibration tests expose the IR detectors to space or to the onboard blackbody, either by stopping the scan mirror at the desired target or by rotating the scan mirror profile so that the mirror’s motion is slowest over the target rather than when pointed at the Earth.

Anomaly table

A table describing AIRS/AMSU-A anomalies is here.

End of Aqua Prime Mission – Free Drift Constellation Exit and Perigee Lowering Maneuvers

The Aqua spacecraft uses propellant to maintain orbital inclination within the “A-Train” constellation of satellites. The orbit is a sun-synchronous, nearly polar orbit with a mean local time (MLT) at each ascending node (northward equator crossing) of 1:35 p.m. The last set of Inclination Adjust Maneuvers (IAMs) took place in the Spring of 2021. Since then, Aqua has not had enough fuel to maintain its previous inclination and its orbit has been drifting away from the A-Train constellation. Without further IAMs, the MLT drifts in such a way that the ascending node occurs later and later in the afternoon. As shown in the plot below, by January 2025 the MLT will be about 2:30 p.m.

Chart titled "Aqua: Mean local time at ascending node" with mean local time on the y axis and Date on the x axis
Inclination Adjust Maneuvers (IAMs) make it possible for Aqua to maintain a sun-synchronous orbit with a tightly controlled Mean Local Time (MLT) at its ascending node of about 1:35:45 p.m. After late 2021, fuel depletion on Aqua made it impossible to perform IAMs and the MLT has been drifting as shown in this plot. This drift will also result in Aqua’s solar panels generating less power due to increasingly unfavorable angles of solar incidence. The CEMs referred to in the explanation are Constellation Exit Maneuvers, which were not performed in favor of a “free drift” constellation exit. Source: Aqua Flight Dynamics Team.

Aqua plans to execute a series of Perigee Lowering Maneuvers (PLMs) in 2024 to ensure that Aqua meets the requirement to fall out of orbit within 25 years of the end of the science mission. Aqua’s orbital drift will cause decreased solar panel power generation until by mid-2026 there will not be enough power for the spacecraft and its instruments.