EXPLORER 27

About This Satellite

Explorer 27, also called Beacon Explorer C, launched on April 29, 1965 to investigate ionospheric electron content and support geodesy. Its equipment combined radio beacons, a Doppler system, an electrostatic probe, and passive laser reflectors. These techniques addressed the atmosphere and the measurement of Earth, rather than providing weather photographs.

No recorder meant measurements depended on ground-station visibility

The satellite had no tape recorder, so telemetry could be received only while a ground station was in range. A magnet and damping rods aligned the spacecraft with the local magnetic field after despinning. This was magnetic alignment, not a camera-pointing system maintaining a fixed view of one place on Earth.

Turning off the radios did not remove the laser target

ILRS records that the 162 and 324 MHz transmitters were switched off on July 20, 1973 because of interference with other spacecraft. The 160 corner-cube reflectors were passive. Subsequent laser observations must therefore be distinguished from a restarted radio mission or new ionospheric telemetry.

Mission reference: NASA-hosted ILRS: Beacon-C objectives, reflectors and transmitter shutdown. Editorial review: 2026-09-14.

A later tracking campaign studied changes in gravity

The ILRS 2000 annual report describes renewed Beacon-C tracking after a long gap. A six-month campaign began in July 1999 and was extended through December 2000 following successful results. It supplemented other satellites used to study secular and long-period tidal variations in Earth's gravity field. Reactivating ground-based tracking was not the same as reactivating the spacecraft's transmitters.

Reference: ILRS 2000 Annual Report: renewed Beacon Explorer-C tracking campaign.

Tracking precision belongs to a particular analysis

A 2004 ILRS workshop presentation reports an average of seventeen edited passes per day from forty-two stations and a weighted fit residual of about 9.5 cm. Beacon-C observations were combined with those of LAGEOS, Starlette, Ajisai, and Stella for gravity-variability studies. The reported residual describes that tracking analysis; it is not an accuracy guarantee for this website's TLE-based position or for every later laser measurement.

Reference: 14th International Workshop on Laser Ranging: Beacon-C tracking statistics and gravity studies.

Reading this orbital snapshot

Based on TLE epoch: 2026-09-05. Estimated altitude ranges from 933 km at perigee to 1,294 km at apogee, a difference of 361 km. The orbital period is about 107.5 minutes. Eccentricity 0.02406 shows why a single altitude cannot describe the whole path. In a two-body approximation, perigee speed is 1.05 times apogee speed. These are mean-element estimates, not instantaneous measurements. Distance per orbit uses an ellipse approximation.

Saved build snapshot. If a valid TLE is retrieved, this explanation and the orbit panel update together. Otherwise this dated snapshot is retained.

Ground track is not sensor coverage

An inclination of 41.18 degrees implies approximate geocentric ground-track limits of 41.2 degrees north and south. This is a prograde orbit, moving in the same general direction as Earth rotation. The plotted line marks the point beneath the object. It does not represent camera coverage, radio reception, or a guaranteed visible pass.

Accuracy and missing information

The introductory record describes the build snapshot. The orbital snapshot explanation and orbit panel share any successfully retrieved TLE; check the displayed epoch. Reentered objects retain historical data only. Mission history is not rewritten by TLE updates. TLE/SGP4 positions are predictions and can change after maneuvers or updated observations. The 14-day stale-data cutoff is a display safeguard, not an accuracy guarantee. Local passes are geometric predictions, not a visibility forecast. No verified operator or mission status means unknown, not active. Do not use this catalog for collision avoidance or operational flight decisions.

Catalog sources: CelesTrak SATCAT: NORAD 1328 · GCAT: S01328 (source update 2026 Sep 4 2345:57). Supplementary metadata: SatNOGS DB, retrieved 2026-09-05T12:57:25.375Z. Page assembled 2026-09-15. GCAT phase codes describe trajectory phases, not mission health.

Learn more: TLE data · Perigee and apogee · Ground tracks · JOT (Jewawud Live Orbital Map)

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