On June 29, Sentinel-1A, the first satellite of Europe’s Copernicus programme, officially ended its operational mission. On July 2, China’s Haiyang-2E (HY-2E) satellite was launched to take over from HY-2B, which entered orbit in 2018. One in the West, one in the East; one a radar, the other a microwave radiometer suite. Both satellites worked for more than a decade and have now passed the baton to a new generation. Whether by coincidence or not, today I’d like to introduce these two veteran remote sensing satellites that have brought us tremendous benefits.
Sentinel-1A
Sentinel-1A was launched in April 2014 as the first satellite of the Copernicus programme. Its core instrument is a C-band synthetic aperture radar (SAR), operating in a sun-synchronous orbit at approximately 693 km, with a single-satellite revisit cycle of 12 days. For GIS professionals, the most commonly used mode is the Interferometric Wide Swath (IW) mode, which provides a 250 km swath width and 5 m × 20 m resolution. It has been widely applied in flood mapping, land subsidence monitoring and glacier observation. The data are distributed free of charge, and many remote sensing graduation theses in China have relied on this IW dataset.
Sentinel-1A was designed for a 7-year lifespan but actually operated for 12 years – a true workhorse. After Sentinel-1B entered orbit in 2016, the two satellites formed a constellation, reducing the revisit time to 6 days. However, in 2021, Sentinel-1B was lost due to a power system failure, and 1A continued to shoulder the mission alone for nearly three more years. On June 29 this year, ESA officially announced the end of its operational mission.
Will there be a data gap? There is no need to worry. Sentinel-1C and Sentinel-1D are already in orbit, phased 180° apart. Their final orbital configuration was completed on June 24, restoring the 6-day interferometric revisit cycle. Their imaging modes are compatible with 1A, and they also carry an additional AIS payload for ship identification. Looking further ahead, the Sentinel-1 NG contract was signed at the Berlin Air Show, with the potential to improve resolution to 5 m × 5 m. However, the first satellite is expected no earlier than 2034 – for more details, see my previous article: Sentinel-1 Next Generation: €1.1 Billion Contract Signed for New SAR Satellite.
HY-2B
Haiyang-2B (HY-2B) is China’s second marine dynamic environment satellite. It was launched on 25 October 2018 from the Taiyuan Satellite Launch Center aboard a Long March 4C rocket. Designed for a 5-year lifespan, it actually served in orbit for nearly 8 years. It carried a suite of four microwave remote sensing instruments – a radar altimeter, a scatterometer, a radiometer and a calibration radiometer – dedicated to measuring sea surface wind fields, wave height, sea surface height and sea surface temperature.
Compared with the experimental HY-2A launched in 2011, the biggest change on HY-2B was the addition of an AIS ship identification payload and a data collection subsystem. After HY-2C was launched in 2020 and HY-2D in 2021, the B, C and D satellites formed China’s first marine dynamic environment monitoring network, reducing the global coverage revisit time from 24 hours with a single satellite to about 6 hours. Data from this constellation play a role in storm surge warnings, El Niño monitoring and fishery forecasting.
The HY-2E satellite, launched on July 2, takes over from HY-2B. It retains the same four main payloads, but its capability for observing nearshore waters has been upgraded.
Summary
Two satellites, two different technical paths, and the same outcome – the veterans bow out, but the constellation service continues without interruption. For GISers and remote sensing professionals, I believe two points are worth noting. First is data continuity. The historical archive of Sentinel-1A remains usable, but new operational planning should gradually migrate to Sentinel-1C/1D. Likewise, with the launch of HY-2E, attention should be paid to product transitions and algorithm updates. Second is the diversification of data sources. International open data and domestic operational satellites are developing in parallel. When choosing thesis topics or designing operational systems, there is no need to rely solely on a single constellation – multi-source data fusion has already become the norm.
Finally, let us salute the veteran remote sensing satellites!