EXPLORER 36 (GEOS 2)

About This Satellite

GEOS 2, also called Explorer 36 or GEOS-B, launched on January 11, 1968. NASA lists three linked objectives: compare tracking-system accuracy, investigate Earth's gravity field, and improve geodetic reference data. The mission used carefully measured satellite motion to study Earth, rather than producing the kind of surface photographs associated with an imaging mission.

Six experiments provided different views of the same motion

NASA's inventory lists an optical beacon, radio Doppler, range and range-rate transponders, a C-band transponder, and a laser corner reflector. The spacecraft had an extendable nine-metre boom. The historical summary records data returned by all experiments; that is a result from the mission, not a claim that its transmitters still work today.

Range and Doppler are not the same measurement

Range concerns separation from a tracking station; Doppler concerns a radio-frequency shift associated with relative motion along the line of sight. Combining observations constrains an orbit in ways that a single type cannot. A latitude and longitude marker is an output of an orbit model, not a substitute for these original tracking observations.

Mission reference: NASA Historical Data Book: Explorer 36 (GEOS-B), Table 3-101. Editorial review: 2026-09-14.

Why investigators brought tracking stations together

A NASA intercomparison report describes placing laser, SECOR, and TRANET equipment near C-band radars at Wallops Island. Simultaneous GEOS-2 observations in April through June 1968 were used to compare tracking data while reducing uncertainties associated with survey, gravity modelling, and timing. The experiment design helped separate differences between instruments from differences in where and when they observed.

Reference: NASA: GEOS observation-systems intercomparison at Wallops Island.

A reflection point is not necessarily the centre of mass

A NASA-supported retroreflector study included the GEOS 2 array when calculating effective reflecting areas and corrections from laser ranges to satellite centre-of-mass ranges. Those corrections depend on array geometry and illumination direction. This is why centimetre-level ranging discussions need a spacecraft model, rather than assuming that every detected photon came from the point used in an orbital calculation.

Reference: NASA ILRS archive: retroreflector transfer functions and centre-of-mass corrections.

Reading this orbital snapshot

Based on TLE epoch: 2026-09-05. Estimated altitude ranges from 1,081 km at perigee to 1,573 km at apogee, a difference of 492 km. The orbital period is about 112.2 minutes. Eccentricity 0.03194 shows why a single altitude cannot describe the whole path. In a two-body approximation, perigee speed is 1.07 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 105.78 degrees implies approximate geocentric ground-track limits of 74.2 degrees north and south. This is a retrograde orbit, moving against the direction of 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 3093 · GCAT: S03093 (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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