Laser Ranging From The Chihuahuan Desert

Laser ranging played a core role in the birth of modern space geodesy, and during the seventies the McDonald Observatory in West Texas established a virtual hegemony over these types of observations, beginning with the transformation of the moon into a high-precision geodetic reference target. In space geodesy, the Earth is understood as a highly dynamic system rather than a rigid sphere. Calculating the highly precise round-trip travel time of a laser pulse to the moon or an artificial satellite requires an extraordinarily accurate mathematical model. To isolate the distance measurement, researchers must account for a vast array of Earth's rotational dynamics, including its overall rotation rate, precession, nutation, and polar motion. With the placement of the first laser retro-reflector arrays on the lunar surface, particularly during the Apollo program, researchers were able to revolutionize lunar distance measurement. The Apollo era retro-reflectors enabled centimeter-level measurements from McDonald, replacing radar uncertainties of 0.7 miles during the sixties, and early laser experiments from 1965 that achieved only 180 meter accuracy.

The moon was a difficult target for laser ranging because of both its distance and the relative speeds involved. The phenomenon of velocity aberration dictated that smaller retro-reflector cubes would provide higher intensity for a single ground-based receiver, so that arrays of smaller cubes were preferable. These physical constraints necessitated a permanent tracking station capable of capturing the few photons available after the enormous losses compared to retro-reflectors on near-Earth satellites. The small signal called for a large telescope, and this was one of the key motivations for NASA to fund construction of the new 107 inch telescope at McDonald in support of the Apollo program. The 107 inch telescope served as the world’s only routine lunar ranging facility for over fifteen years, achieving its first successful returns on August 19, 1969. 

During the seventies, satellite laser ranging at the McDonald Observatory developed in parallel with the lunar laser ranging program. While the 107 inch telescope is most famous for its post-Apollo lunar tracking, its observing time was also shared with artificial-satellite ranging. UT Austin's Center for Space Research utilized the SLR data collected to make pioneering advances in precision orbit determination and geophysical measurement. Key satellite missions and SLR milestones included the GEOS-3 Geodynamics Experimental Ocean Satellite which was used to study precision orbit determination and to develop regional gravity models. With LAGEOS in 1976, a high-density passive reflector in medium Earth orbit, CSR researchers were among the first to demonstrate that SLR could be used to measure tectonic plate motion. SLR tracking of LAGEOS also allowed CSR to provide definitive results for Earth's polar motion, calculate the value of Earth's mass, and make the first measurements of climate-induced time-variable gravity. And with the early Earth-observing satellite SeaSat in 1978, CSR led the precision orbit determination team, using tracking data to support radar altimetry processing for accurate ocean surface topography and gravity measurements. 

As the demand for artificial satellite tracking grew, the engineering and operational footprint at McDonald Observatory expanded. In the late seventies, UT Austin developed and tested the first Transportable Laser Ranging System at the McDonald site, validating coordinate determination algorithms and demonstrating the viability of mobile laser tracking. This effort evolved into the dedicated McDonald Laser Ranging Station, sited near the 107 inch. Furthermore, starting in 1979, various NASA-supplied Earth satellite ranging stations began operating continuously at the observatory.

The connection between the Transit satellite system and McDonald Observatory as a geodetic station was made in the computational modeling at UT’s CSR and ARL. These models combined Transit's radio-frequency Doppler tracking with the optical SLR networks anchored at McDonald Observatory, creating an early end-to-end tie between the celestial frame, terrestrial frame, and the Earth’s position and rotation. In some sense CSR was tying together McDonald and McMurdo via UT's TRANET Station 019. The Transit satellites continuously broadcasted two coherent carrier frequencies at 150 MHz and 400 MHz to allow ground receivers to eliminate first-order ionospheric refraction. The global tracking network supporting these geodetic investigations, TRANET, consisted of stations equipped with ultrastable rubidium or cesium frequency standards. UT Austin served as a vital institutional node linking this Doppler-based satellite network with high-precision laser ranging. ARL developed specialized mathematical packages to compute ionospheric and tropospheric refraction corrections for the TRANET network. To standardize geodetic data archiving and coordinate exchange, ARL developed the FICA Floating Integer Character ASCII format, which served as the standard for geodetic GPS and Transit data before the universal adoption of RINEX.

The direct operational connection between the Transit system and McDonald Observatory was forged through modeling and orbital synthesis at CSR. Precision orbit determination for major oceanographic altimetry missions required integrating data from the 48 station TRANET network with optical tracking data. CSR researchers merged the TRANET Doppler measurements with SLR data collected by McDonald and similar observatories. To resolve scale discrepancies, CSR executed precise coordinate ties between the TRANET receivers and the SLR network. This synthesis successfully tied Doppler-determined geocentric coordinates to the highly accurate SLR reference system, defined by targets such as LAGEOS, reducing radial orbit errors to approximately 20 cm. ARL and CSR also evaluated prototype GPS geodetic receivers by comparing GPS pseudorange and phase observations against legacy Transit integrated Doppler phase accumulations. These field tests demonstrated sub-meter baseline measurements, validating the transition from Transit-based datums to the modern WGS 84 World Geodetic System and the ITRF International Terrestrial Reference Frame, a framework to which McDonald Observatory continuously contributed.

As McDonald developed into a key geodetic reference site across the seventies, the nature of its West Texas geology was crucial. For one thing, it is onboard one of the major cratons that have played major roles across geologic deep time, though near a margin or boundary zone, rather than in an ultra-stable core region. In other words, it is in a fairly stable and ancient continental plate region. Likewise, it's sitting on deeply stable rock, at the opposite end of the spectrum from a site on top of subsurface reservoirs which can expand and contract. The Davis Mountains represent the largest contiguous volcanic remnant of the mid-Tertiary Trans-Pecos Magmatic Province, a massive alkalic volcanic field formed over forty million years of tectonic reorganization. Between 39 and 35 million years ago, a violent series of caldera collapses and explosive eruptions buried the region in vast sheets of ash-flow tuffs and flood rhyolites. The resulting stratigraphic architecture produced a landscape characterized by low-viscosity, high-temperature silicic magmas that cooled into exceptionally dense, structurally rigid igneous rocks. These precise geomechanical properties served as the foundational bedrock for the world-class optical and geodetic instrumentation at the McDonald Observatory.

The volcanic rocks supporting Mount Locke and Mount Fowlkes are highly fractured. Rainwater rapidly seeps deep into these fractures, accumulating as "rock moisture" within the unsaturated zone. During heavy monsoon seasons, the increased water mass within the fractured trachyte and tuff increases the local gravitational pull. This localized subsurface water storage mimics the gravimetric signature of actual tectonic crustal subsidence. The deep infiltration of water into the observatory's bedrock plays a critical role in the broader regional hydrogeology of Far West Texas. The precipitation that falls on the Davis Mountains percolates underground, migrating through highly porous volcanic pathways and underlying Cretaceous limestone layers to recharge artesian desert spring systems, most notably the San Solomon Springs in Balmorhea. While the continuous baseflow of these springs originates distally from the Salt Basin Bolsons to the west, the system is augmented multifold by local stormflow recharge moving through the subsurface from the Davis Mountains. The fractured volcanic layers, particularly the basal conglomerate of the Huelster Formation and associated vesicular lavas, act as rapid transport conduits, though some volcanic layers may also retain water and release it slowly over months, sustaining the springs long after precipitation events.

While the interior of the Davis Mountains is relatively stable, the region is bounded by major active fault zones associated with the Rio Grande Rift and the Texas Lineament, a major crustal boundary separating the stable North American craton from the active Chihuahuan borderlands. The persistent seismic risk of the subsurface was demonstrated by the 1931 Valentine Earthquake, the largest in Texas history, centered just 25 miles west-southwest of the observatory. Although the dense, consolidated volcanic sequences of Mount Locke and Mount Fowlkes act as a rigid, dampening block that mitigates catastrophic ground failure, the continuous, low-frequency seismic stress of the basin-and-range faults must be continuously accounted for in the observatory's geodetic and astronomical measurements. McDonald Observatory’s enduring scientific legacy is inextricably linked to its geology and subsurface environment. In recognition of this intrinsic connection, its mission has evolved to include geological, hydrological, and ecological research across its 650 acres of Chihuahuan Desert terrain. [1]

The telescope site’s geomechanical stability was paramount. High-precision astronomical instrumentation requires structural foundations entirely decoupled from wind shear, thermal expansion, and human-induced vibrations. The Mount Locke Formation, restricted to a 93 meter thick flow of coarse, highly porphyritic metaluminous trachyte, provided an exceptionally dense and unyielding bedrock. By anchoring the 107 inch telescope’s massive 160 ton structure directly into this rigid volcanic substrate, engineers achieved the absolute dimensional stability necessary to isolate the optical path from environmental noise. The rigid bedrock of the Mount Locke Formation provided the exceptional load-bearing strength necessary for the telescope's foundations and allowed for the site’s calibration to the International Terrestrial Reference Frame and the International Celestial Reference Frame with zero margin for error. The seventies data boom allowed for the first empirical measurements of Earth Orientation Parameters and the determination that the North American Plate drifts southwestward across the mantle.

The data boom resulting from the network of lunar reflectors and the unyielding trachyte foundation of Mount Locke allowed McDonald’s LLR measurements to serve as a highly stable anchor for the ITRF, ICRF, and IERS. Through continuous tracking, researchers successfully utilized the LLR data to empirically measure Earth Orientation Parameters, including polar motion, nutation, and variations in the length of day. By definitively linking terrestrial coordinates to the Earth's center of mass, the observatory tracked the continuous drift of the North American Plate across the mantle, validating the mechanics of plate tectonics and cementing the site's legacy in the geosciences. McDonald Observatory and the Center for Space Research were the first laser ranging group to provide operational observations of these Earth Orientation Parameters, notably polar motion and length of day. Through the accumulation of SLR and LLR data, researchers have been able to isolate variations in the Earth's principal figure axis and key geopotential coefficients across timescales ranging from sub-daily to decadal.

Along with SLR, researchers tracked the precise distances between different global ground stations. Over time, these inter-station baseline rates reveal the slow, continuous drift of the Earth's tectonic plates with a precision of better than one centimeter. McDonald Observatory played a critical role in this endeavor and accumulated a continuous legacy of providing reference positions that accurately tracked the motion of the North American Plate from the seventies onwards. These measurements are a cornerstone of the ITRF for tracking the dynamic movement of the Earth's crust.

Understanding continental motion is an absolute prerequisite for accurately measuring global sea levels. CSR led the Precision Orbit Determination efforts for satellites monitoring variations in sea surface height using radar altimeters such as the TOPEX/Poseidon and Jason series missions. These altimeters measured the distance between the satellite and the ocean surface. To determine the actual height of the sea, CSR tracked the satellite's exact altitude and position in space relative to the center of the Earth. This required tracking the satellite from ground stations. Because these ground stations are constantly moving due to tectonic drift, their exact coordinates must be continuously updated using the ITRF. If continental motion were not accounted for, the changing position of the ground stations would introduce significant errors into the satellite's orbit calculations, which would in turn corrupt the sea level measurements. While altimeter missions track the total height and volume of the continental and ocean surfaces, CSR has also led efforts to directly track subsurface and oceanic mass variations.

Observations from McDonald Observatory and the global geodetic network reveal that Earth's rotation is altered by three primary mechanisms: external gravitational torques, internal mass redistribution, and the transfer of angular momentum between the solid Earth and its fluid layers. The gravitational attraction of the Sun and Moon generate oceanic and solid-body tides, and the friction from these tides causes the Earth's rotation rate to slowly decrease. To conserve the total angular momentum of the Earth-Moon system, this deceleration forces the Moon to spiral away from the Earth at a rate of approximately 3.8 centimeters per year.

As the Earth displaces its mass, its moment of inertia changes, subsequently altering its spin rate and the position of its poles. Variations in the Earth's length of day on decadal scales are heavily influenced by the internal structure of the planet. Gravitational and hydrodynamic coupling between the fluid outer core, the solid inner core, and the mantle exerts torques at the boundaries of these layers, leading to persistent oscillations in Earth's rotation rate. The precision of LLR is so extraordinary that even biospheric cycles are detectable. As tree sap rises or falls seasonally in the heavily forested Northern Hemisphere, the corresponding change in Earth's moment of inertia minutely alters the planet's rotation rate, a signature captured in the ranging data. A certain young astronomy undergrad was in the audience when Pete Shelus described this research in 1987 during one of Harlan Smith's weekly astronomy seminars. Tying tree sap to continental plates, subsurface reservoirs, satellites, the moon, and the stars was an extremely effective lecture, needless to say.

[1] Chihuahuan Desert Nature Center in Fort Davis.

McMurdo Sound Antarctica And Austin

UT Austin operated the Transit satellite system TRANET Station 019 at McMurdo Sound Antarctica for much of the seventies. This was the critical polar node for the Transit system, the predecessor to modern GPS. Because Transit satellites were in polar orbits, high-latitude tracking data from McMurdo was essential for accurately determining the satellites' inclination, nodal progression, and overall global orbital models. A hallmark of the McMurdo station was that it was staffed and operated by two-person teams consisting of a grad student and an undergrad, usually from electrical engineering. These students lived in extreme cold, maintained complex Doppler receivers, managed HF radio communication links, and ensured continuous data collection. [1] 

Gear inside TRANET Station 019 operated by UT Austin at McMurdo for much of the 1970s.

The Transit system was driven by the US Navy’s need for high-precision navigational updates for its submarine fleet. To ensure the accuracy of inertial guidance systems onboard submarines, the Transit satellite system was deployed, utilizing Doppler frequency measurements to provide all-weather positioning. The first Transit satellite was launched into orbit on 13 April 1960. Among other information, it provided confirmation of the Earth's asymmetrical shape and highlighted the inadequacies of contemporary knowledge of the Earth's gravitational field for the prediction of satellite orbits and other important near-Earth ballistic trajectories. Such prediction was essential for Transit's navigational role, in which receivers would determine their own position by monitoring the Doppler shift from a satellite of known orbit.

Already in May 1961 it was noted that “Meeting the ultimate program goals for Transit thus requires considerable improvement in the present knowledge of these factors (roughly the shape and mass distribution of the earth). This is the primary remaining development challenge of the Transit program.” Navigators have always relied on the stars for the purposes of terrestrial positioning. Artificial satellites revolutionized this paradigm by providing radio signals, available day and night and regardless of weather conditions. However, this shift introduced a significant challenge. Unlike the natural stars, these artificial stars are moving rapidly and are subject to complex orbital perturbations. 

A satellite is essentially a body in free fall influenced by forces such as gravity anomalies, atmospheric drag, and solar radiation pressure. Variations in the Earth’s gravitational field mean that the satellite’s trajectory is a direct reflection of the planet's irregular mass distribution. The concept of using satellites as test particles for probing the Earth’s gravitational field quickly developed and the Center for Space Research at UT Austin was on the frontier of this research. The convergence of astrodynamics and geophysics established a computational grand challenge, where the requirements of gravity modeling and subsurface modeling drove a new scale of computing problems. Through the lens of computational geodesy, the satellite serves as a remote sensor for the Earth's interior, linking orbital mechanics to subsurface exploration and the management of global resources.

Transit system accuracy. UT Austin played important roles in these results. Note that the time span fits exactly with our 1958 to 1982 focus.

[1] Tracking operations at McMurdo (initially designated as Station 019) began on February 5, 1965, within a National Science Foundation building, and were originally managed by New Mexico State University's Physical Science Laboratory (PSL). After PSL withdrew in December 1966 due to administrative issues, the University of Texas at Austin's Applied Research Laboratories (ARL:UT) officially reopened the station on October 10, 1968. ARL:UT maintained continuous management of the site for a quarter-century.

Casscam Image Dissector Star Tracker

This is an experimental post. It's the first to expand the scope to cover the history of computing at UT's McDonald Observatory and Center for Space Research, as discussed in the About. And it's related to an excellent new post on Ken Sherriff's Blog. Ken's blog is a direct inspiration, much like the TCHC Blog. Ken's post explores a historical star tracker. Star trackers are a fascinating topic and played a role in the history of computing at UT. This post will focus just on the image dissector star tracker used in the McDonald Observatory 82-inch telescope Cassegrain Camera. Later posts will discuss the CCD star trackers used by the Center for Space Research for the NASA ICESat and ICESat-2 missions, and the associated computational modeling and data analysis. It's a long and complex story, covering different eras of computing at UT, and best explored gradually over time. There are also three good books for background reading on these topics [1], [2], and [3]. 

Around 1990, the 82-inch telescope at McDonald Observatory was used to make glass plate photographs of asteroids for the Texas Minor Planet Project (TMPP) and the Hubble Space Telescope Astrometry Team. It was the Indian Summer of traditional analog glass plate imaging, digital CCD imagers had not yet taken over this niche. The heart of the TMPP system was the suitcase-sized Casscam and its integrated star tracker. The Casscam was among the last and most advanced plate cameras. It's possible that the star tracking control loop was entirely analog electronics. Based on operational experience and the environment at McDonald, it's also possible that at least the encoders and logic were digital. It was directly descended from the first instruments used on the 82-inch. Below is an old photo of a direct ancestor, probably from the thirties. [4] The knife-edge focus frame and glass lens were still used in 1990, as discussed below.

An 82-inch telescope instrument and direct ancestor of the Casscam. The knife-edge focus frame lies to the left and has the basic outer dimensions of a glass photographic plate. The cone mounted on top is a heavy glass lens. Peering through the lens, celestial objects appeared much as in a long-exposure full-color astrophotograph.

Even the largest asteroids were small and faint, requiring a relatively long exposure to build up an adequate spot in the photographic emulsion on the glass plate. While building up an asteroid image spot, the Casscam had to track the asteroid’s apparent motion to hold the spot still on the glass plate. This apparent motion was principally from the Earth’s own motion and parallax effects. Against the background of effectively fixed stars, an asteroid moved appreciably when viewed from the Earth, especially when imaged with the magnification of the 82-inch telescope. The Casscam had to nullify this apparent motion during asteroid tracking. The photographic plate holder was rotated to align the asteroid's motion along the Casscam's primary axis. Then, during asteroid tracking, the Casscam moved the plate at the same speed as the asteroid's apparent motion, nullifying it. This was open-loop tracking, without feedback or active error correction.

Star tracking was also needed, separately from asteroid tracking. The stars in the image near the asteroid were also faint, and it was essential to build up adequate spots in the photographic emulsion for them as well, as they were the means to computationally tie the asteroid to the celestial reference frame. The computational modeling and data analysis aspects of this are subjects for later, dedicated posts. In this post, the focus is purely on the Casscam’s capability to track the stars and hold their image spots still on the glass plate. It did this using a combined image intensifier and image dissector tube star tracker locked onto a guide star. The image intensifier was a close relative of a photomultiplier tube. Incoming photons initiated a cascade of electrons down a cylindrical tube, roughly twelve inches long and a few inches in diameter. The circular end of the tube was a glass phosphorescent screen with a cross-hair etched on it. In normal operation the green fuzzy ball of a star image was kept centered in the cross-hair. Below is an example with a much lower magnification and wider field of view. Imagine this zoomed in on the central bright star, with a cross-hair on it.

Stars in an image intensifier. This one has a much lower magnification and wider field of view than the one on the Casscam.

Image dissector tubes seem to have been named to suggest dissecting or taking apart an image, in other words sampling an image. Image sampler may be a more suggestive name to modern eyes. An image is formed using electrons, and that image is then sampled, all within the tube. In the picture below, the lens on the right forms an electron image on the photo-electric plate while the aperture and valve samples the electron image.

An image dissector tube in its early role as a television camera. [9]

The Casscam's combined image intensifier and dissector was sampling the electron image just at the center of the field of view, around the cross-hair. Once a star was placed in the cross-hair, the sampling would output an error signal whenever the star began to drift away. The control loop would then move the plate holder to zero the error signal and correct the drift. The control loop was running at about 1 Hz, and produced a loud clicking noise every second. This soon became a familar sound in the darkness of the 82-inch dome, a steady click click click while the star tracker control loop was active.

In note [5] below there's mention of a 64x64 image dissector at McDonald in the seventies, so clearly something like sampling of a pixel grid was possible and a viable technology until solid-state CCD imagers became available in the eighties and nineties. The transition to CCD star trackers will be explored in a future post about the Center for Space Research and the NASA ICESat star trackers.

Since this post includes a photo showing the knife-edge focus frame, its use can also be described. At the beginning of an observing run, early steps included preparing the Casscam and focusing the telescope. The Casscam was a heavy instrument, about the size of a suitcase. The McDonald operations staff would mount it onto the back of the 82-inch using a lift and heavy bolts. The combined system then needed to be focused by moving the secondary mirror, which was roughly fifty feet overhead. The secondary mirror was moved by an electric motor controlled from a control paddle on the observing platform. Focus was achieved using a knife-edge technique within the focal plane. By placing a metal frame with a straight knife-edge into the Casscam’s plate holder, the observer could adjust the secondary mirror until the light from a star was cut off instantaneously rather than gradually. On occasions when time permitted, the knife-edge focus frame could be replaced by the massive glass eyepiece also shown in the photo. Needless to say, star-gazing through the 82-inch telescope was something very special. 

Notes and photos

[1] MacKenzie, Donald. Inventing Accuracy: A Historical Sociology of Nuclear Missile Guidance. Cambridge, MA: MIT Press, 1990.

[2] Spinardi, Graham. From Polaris to Trident: The Development of US Fleet Ballistic Missile Technology. Cambridge: Cambridge University Press, 1994.

[3] Grewal, Mohinder S., Angus P. Andrews, and Chris G. Bartone. Global Navigation Satellite Systems, Inertial Navigation, and Integration. 4th ed. Hoboken, NJ: John Wiley & Sons, 2020.

[4] Evans, David S., and J. Derral Mulholland. Big and Bright: A History of the McDonald Observatory. Austin: University of Texas Press, 1986.

Direct ancestor of the star tracker discussed in Ken's post. [1] 

Another direct ancestor.

[5] Though the image dissector is capable of scanning a two-dimensional image, it is difficult to do so before the phosphor of the last intensifier has decayed substantially. McDonald observatory did indeed build an area photometer which scanned a 64x64 two-dimensional array (P. M. Rybski, G. W. Van Citters & G. F. Benedict, IAU Coll. 40 Astronomical Applications of Image Detectors with Linear Response, 1976). Bull Astr Soc India, 406-423 December, 1985. This could very well have been related to the Casscam star tracker. Fritz Benedict was a member of the Hubble Space Telescope Astrometry Team into the nineties.

[6] Image dissector tubes have found widespread use in astronomy, beginning with the pioneering work of L. Robinson & J. Wampler in the early seventies. Though occasionally used as imaging devices for either recording extended fields or guiding in automatic/remote-manual mode, the more popular usage has been in intensified scanning spectrometers. Such a system was first developed at Lick Observatory (Robinson & Wampler, Publ. Astr. Soc. Pacific 84, 16 1972), who subsequently duplicated it at the Anglo-Australian Observatory. Kitt Peak National Observatory, European Southern Observatory and Ohio State University have subsequently built similar instruments, some of which are still maximally used. The introduction of more sensitive detectors like the image photon counting systems and charge-coupled devices, and resultant shift in the emphasis of observing programs to fainter limits, have rendered the image dissectors less popular in recent years. However, the image dissector remains the most useful detector at intermediate light levels where avenues remain open for astronomical research. Ibid.

[7] The Intensified Image Dissector Scanner has been in routine use at Kitt Peak National Observatory for two years ... In this instrument, the output phosphor of a three-stage image intensifier is used as a temporary storage medium for incoming photon events. An image dissector tube is used to rapidly scan this output phosphor. Instrumentation in astronomy III, Proceedings of the Society of Photo-optical Instrumentation Engineers, v172, p86, 1979.

[8] An image dissector, also called a dissector tube, is a video camera tube in which photocathode emissions create an electron image which is then swept up, down and across an anode to produce an electrical signal representing the visual image. It employs magnetic fields to keep the electron image in focus, and later models used an electron multiplier to pick up the electrons ... they continued to be used for imaging in early weather satellites and the Lunar lander, and for star tracking in the Space Shuttle and the International Space Station. Wikipedia

[9] https://www.earlytelevision.org/baird_and_farnsworth.html