The Environmental Research Satellite (ERS, alternatively Earth Resources Satellite) program was a series of small satellites initially operated by the United States Air Force Office of Aerospace Research. Designed to be launched "piggyback" to other satellites during launch, detaching once in orbit, they were the smallest satellites launched to date—what would today be classified as microsatellites. 33 ERS satellites in six different series were launched between 1962 and 1971, conducting scientific research and serving as test beds to investigate the reliability of new spacecraft components.

The TRS (Tetrahedral Research Satellite) Mk. 1 was developed by Space Technology Laboratories, a subdivision of TRW Inc., as an inexpensive, miniaturized "off-the-shelf" satellite that customers could use to perform simple experiments in orbit. The Mk. 1 was a regular tetrahedron measuring 16 cm (6.3 in) on a side, each face mounted with sufficient solar cells to operate the experiments and the telemetry system when the satellite was in the sun. Transmission of data was constant at that time as the spacecraft included neither internal battery nor command system. A transistor-based system provided eight channels of data: five for experiments, two for telemetry calibration, and one for the spacecraft temperature. A 40 in (1.0 m) antenna transmitted data. on 136.771 Mhz.

Each TRS satellite was estimated to cost only $25,000 (equivalent to $213,888.89 in 2020) to build, excluding development, launch and mission operations costs.

The Air Force Space Systems Division (AFSSD), then headed by Col. T. O. Wear, was the first and only customer for STL's TRS Mk. 1 satellites, initially purchasing six for its Environmental Research Satellite (ERS) program. Ten TRS Mk. 1 satellites were ultimately produced, designed to research radiation and micrometeoroid flux in Earth orbit. All were launched from Vandenberg Air Force Base attached to primary payloads.

At the time it was launched, TRS 1 was the smallest satellite ever placed into orbit. The aluminum spacecraft carried 140 solar cells, producing 600 milliwatts of power, and five radiation-detecting cells. Though the satellite did not separate from its primary satellite, it returned eight minutes of data per orbit to tracking stations below. It was designed to turn off after 90 days of operation. TRS 1 circled the Earth in a Low Earth orbit.

Data returned from these three fully successful TRS satellites, circling the Earth in Medium Earth orbits (within the Van Allen Belts), returned valuable data on the effects of orbital radiation on solar cells. Of significance, it was determined that p-on-n silicon cells deteriorated five times more quickly than n-on-p cells. While protective covers did not affect n-on-p degradation, they were shown to be helpful for the more sensitive p-on-n cells. An unexpected result of the solar cell experiment was that, while it had been observed in ground tests that exposure to radiation of the cells' quartz covers and the epoxy adhesive that held them to the cells reduced the light they transmitted (and thus the power generated) to the cells by 15%, such was not observed in orbit.

TRS 2 and 3 marked the first time two satellites were deployed into orbit simultaneously. Data was obtained by NASA's Minitrack communications network in cooperation with the USAF.

The TRS Mk. 2 design was a tetrahedron measuring 21 cm on a side. Four were produced: ERS-11 through ERS-14, though only two were launched, both pick-a-back with primary USAF payloads. They were designed to operate for one year, at which point, an onboard timer would shut the satellites off.