Some Early Observation Techniques (Classical Methods)

The satellite tracking methods of the first years after 1957 originated from before the launch of the first artificial satellites, or were based on existing techniques. This is true for the methods of stellar triangulation, which follow from astronomy with the Moon as a target, and for the visual, photographic, and electronic tracking of rockets. Only the satellite laser ranging technique can be regarded as an original development of the early satellite era (Henriksen, 1977).

After the launch of SPUTNIK-1 on October 4, 1957, the satellite signals, which were continuously transmitted on frequencies of 20 MHz and 40 MHz, could be re-ceived all over the world with existing antennas. The Doppler shift [6.1] of these signals was measured (mostly by observing the behavior of Lissajou figures with os-cilloscopes), and could be used for tracking purposes. More precise radio-tracking systems (e.g. Minitrack [4.4.2]) were already under development for the anticipated national American space program, and were used after 1958 for the observation of a large number of satellites. For monitoring and tracking of passive, non-transmitting, satellites, powerful cameras were used, the so-called tracking cameras. The Smithso-nian Astrophysical Observatory (SAO) initiated, within the International Geophysical Year (1957–1958), the development of the Baker–Nunn Camera, which could be used for the photography of small, sun-illuminated satellites (Pearlman, 1983).

The primary motivation during the first years of satellite observation was focussed on the development of improved models for the orbital motion of near Earth satellites, and to the determination of substantial geometrical and physical Earth models. Many observations of a large number of satellites were included in the determination of the early Earth models [12.2]. These data from the “classical period” of satellite geodesy still contribute to current Earth models. In this respect, the classical observation methods retain their importance, and are briefly discussed in this book. The present practical importance of the classical observation techniques in their original form, however, is very small.

The photographic determination of directions led to a remarkable early result in satellite geodesy, namely the establishment of the first worldwide geometric network [5.1.5]. Directional methods are still of high significance in satellite geodesy and found a remarkable new perspective with CCD technology. This is why the method is described in a particular chapter [5].

The TRANSIT technology also belongs to the classical observation techniques.

The underlying Doppler method, however, is still an important observation tool in satellite geodesy, and has a modern realization in the DORIS concept. Furthermore, the methodology developed along with TRANSIT has considerably influenced the geodetic use of GPS. This is why the Doppler method is treated in a particular chap-ter [6].

4.4.1 Electronic Ranging SECOR

The development of electronic ranging techniques began rather early. To implement two-way ranging capability, the satellite had to be equipped with dedicated receivers and transmitters, so-called transponders. The SECOR technique was developed par-ticularly for geodetic application. SECOR means SEquential COllation of Ranges.

One of the first SECOR transponders was flown on ANNA-1B (1962). A total of sixteen satellites with SECOR equipment were launched into near polar orbits of 1000 to 4000 km altitude between 1964 and 1970 (NGSP, 1977, Vol.1, 221), among them GEOS-1 and GEOS-2.

The basic idea of SECOR is that four ground stations and one satellite form a group, the so-called Quad. Three of the four ground stations are considered to be at known positions, the fourth station is the new pointN, to be located. This is the trilateration principle, a purely geometric method of coordinate determination which is illustrated in Fig. 1.2, p. 3. At least three well-selected satellite positions are determined through simultaneous ranging from the three “known” ground stations. Based on the three determined satellite positions the coordinates of the unknown stationN are derived by spatial resection. Further “quads” of groundstations can be added to form larger networks, up to a worldwide girdle of stations. In addition to the purely geometric simultaneous method, the orbital method of SECOR was used (see Fig. 1.3). A short portion of the orbit (short arc) was determined from at least three known ground stations, and was then extrapolated for the determination of unknown ground stations.

In practice, combined evaluations have also been used.

Note that the basic principles which have been developed for the technique of point positioning with SECOR are also applicable to modern ranging methods in satellite geodesy.

SECOR used a phase comparison technique for the determination of ranges. Mod-ulated signals on a carrier frequency of 420.9 MHz were transmitted from the ground station to the satellite. They were transmitted back to the ground stations via satellite–

borne transponders on two different frequencies (449 MHz and 224.5 MHz, for esti-mation of an ionospheric correction). The ranging signal had a frequency of 585 MHz, corresponding to a resolution of 25 cm. Three additional modulation frequencies were used for solving the ambiguities. One of the four stations was designated the mas-ter station, and synchronized all measurements. The inmas-terrogation period for all four participating stations was 50 ms.

The main purpose of the SECOR system was the geodetic connection of isolated local reference frames (datum connection), in particular between North America, Aus-tralia, Japan, and several islands in the Pacific (Rutscheid, 1972). To achieve this objective an equatorial network, consisting of 37 stations, was observed between 1964 and 1966. The pure SECOR solution showed a rather weak geometry and was affected by large systematic errors. The standard deviation of a single range measurement (in-ternal accuracy) was about± 3 m; however, the systematic differences, when compared with other solutions (BC4, Doppler), were up to 50 m (external accuracy). A com-bined final adjustment, including short arc techniques and orientation control from

BC4 azimuths, resulted in a position accuracy of± 10 to 15 m (NGSP, 1977, Vol.1, 203).

4.4.2 Other Early Observation Techniques

During the first years of the satellite era some other electronic observation techniques were used for orbit control and orbit determination. Some of the results were used for geodetic purposes. The systems and techniques in question were:

GRARR (Goddard Range and Range Rate) for the determination of ranges,

MINITRACK for the interferometric determination of directions, and C-BAND RADAR for the simultaneous determination of directions and

ranges.

Many of the satellites, that were launched between 1960 and 1970 carry the appropriate equipment for these techniques.

GRARR is a two-way ranging technique. A phase-modulated carrier frequency (2.27 GHz) is transmitted from the ground to the satellite, where it is shifted in fre-quency by a transponder, and sent back to the ground station on 1.70 GHz. The distances are determined from phase measurements with up to 8 modulation frequen-cies (λ ≈ 0.6 to 37 500 km); the range rate is derived from the Doppler shift of the carrier. The precision of the ranging signals is about±10 m, and that of the range rate signals about± 3 cm/s. The system was operated by the Goddard Space Flight Center (NASA) and was successfully used on GEOS-2 (NGSP, 1977, Vol.1, 433). The results have been included in the Goddard Earth Models (GEM) [11.2]. Orbital arcs up to 7 days duration were observed .

PRIME MINITRACK is an interferometric one-way technique; it was used by NASA for the orbit determination of many satellites. A beacon on the satellite transmits a continuous carrier signal at 136 MHz which is received at a pair of crosswise arranged antennas. Interferometric phase differences are measured [4.2.6] and transformed into direction information. Because of the rather long wavelength (λ ≈ 2.2 m), compared with the extension of the interferometer (≈125 m), the angular resolution is only about

±20^. Minitrack observations of single orbital arcs have contributed to the GEM computations [12.2].

C-BAND RADAR uses the 5 to 6 GHz domain ($= 5 cm). It is a ground-based two-way technique. The radar signals are reflected from the satellite surfaces without using transponders. This is why the system is particularly suitable for the orbital control of satellites, rockets, and parts thereof. The ground station is rather large and uses an 8.8 m parabolic dish. The range is derived from pulse travel times. The orientation can be read at the two-axis mounting. The ranging accuracy is about ± 2 to 5 m and the angular accuracy about±20^. With modified equipment coherent phases and phase changes can also be observed. When a transponder is used (as is the case on GEOS-3) the range and accuracy can be increased considerably. The C-band radar was intensively used for the determination of GEOS-3 orbits.

of Directions

The determination of directions from the ground to satellites based on optical obser-vations, is one of the early methods of satellite geodesy that led to remarkable results.

In addition, optical tracking of satellites is of fundamental importance because it is the only technique in satellite geodesy which directly establishes access to the inertial reference frame (cf. [2.1.2.1]). All other methods (like GPS [7] or SLR [8]) only indirectly provide a link to the frame through the equation of motion.

Unfortunately the optical era in satellite geodesy came to a sudden end with the development of satellite laser ranging (SLR) and the use of the Doppler technique for positioning soon after about 1975. The reason is well understood. A directional accuracy of±0.^1 corresponds to 3 m for a satellite at 6000 km (e.g. LAGEOS). The optical method was not competitive compared with the cm accuracy available with laser ranging

Recent progress made in the development of Charge Coupled Device (CCD) tech-nology has led to a revival of optical satellite observations. This development is promising and interesting because directional observations, besides the direct link to the inertial frame, still provide important contributions to satellite geodesy and satellite tracking, e.g. (Hugentobler, 1998):

− optical observations are the most reliable and accurate source of information for small, passive, and remote objects, like inactive satellites or space debris, in particular in the geostationary belt,

− geostationary or GPS satellites show characteristic resonances with Earth’s ro-tation which can be accurately determined with optical observations,

− other than VLBI or SLR, only optical observations from single stations can provide important information, and

− optical observations are an independent tool to control and calibrate other ob-servation techniques.

The classical photographic determination of directions contributed significantly to the early development of satellite geodesy. The basic methodological foundations of this method, in particular the technique of plate reduction, are still of value and can also be applied to the analysis of CCD images. This is why a review of the photographic method is given first.