NICER / ISS Science Nugget
for July 1, 2021

Successful Verification Test of OHMAN Software

During the week of June 22, 2021, members of the NICER team were at the Sonny Carter Training Facility performing ISS requirement verification tests for the OHMAN target-of-opportunity (TOO) enhancements for NICER and MAXI. OHMAN (Orbiting High-energy Monitor Alert Network) is a software payload in development that will connect JAXA's MAXI payload on the JEM-EF to the NICER payload at ELC2 (see figure) using an Express Laptop Computer for Express Rack 5.

The OHMAN flight software will scan live data from MAXI before it is fed into the telemetry multiplexer (LEHX) inside the JEM. The OHMAN software finds X-ray enhancements observed by MAXI, taking advantage of the huge (~900 square degree) instantaneous MAXI field of view (FOV) to monitor many well known as well as unknown celestial sources. If an enhancement is seen, the OHMAN software checks whether NICER has an unobstructed view of its location on the sky. This visibility calculation takes into account the ISS orbit ephemeris, attitude, and the joint angles of the ISS solar and radiator panels, as well as NICER's Earth, Sun, and Moon avoidance constraints. If the visibility is good, the OHMAN software generates an alert that is sent to NICER for prompt follow up. The alert is encoded in a Health and Status data packet sent through the Express Rack Computer to the Payload MDM on ISS. The Payload MDM then passes this along to NICER via the Unique Ancillary Data (UAD) channel on the ISS 1553 communication network.

The recent verification tests confirmed that the OHMAN software meets many of the ISS requirements for payload software running on an Express Laptop Computer. These tests also confirmed that the required UAD data were produced as needed by NICER's flight software. On June 30, 2021, the first delivery of software for integrated verification testing was completed.

OHMAN will demonstrate, for the first time, the integration of two external payloads built by different nations using ISS infrastructure. It will also provide a unique and powerful capability for NICER to respond to MAXI-generated TOOs in less than 5 minutes. This rapid followup capability will enable NICER and MAXI to reap a scientific windfall in a variety of astrophysics areas where timely response is key to understanding the physics that drives highly dynamical celestial events.

OHMAN is expected to come on-line in March 2022 as ISS prepares for its next increment change.

NICER 5ight curve showing the brilliant rise of 4U 1543-47's outburst.

Figure: OHMAN will connect MAXI on the JEM EF to NICER on ELC2 using an Express Laptop Computer inside the JEM and the existing ISS communications infrastructure.

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