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The PRARE system development was initiated in 1982 as a response to ESA/ESTEC's "announcement of opportunity" to participate in the evaluation of Europe's first remote sensing and Earth watch satellite ERS-1. Initially, the goal of this completely new development was to provide a system capable of measuring highly accurate ranges (= radial distances) and range-rates (= relative velocities) between space and ground at the same time and fully synchronously.The system should work continuously and autonomously, and it should avoid the disadvantages due to other microwave satellite tracking systems available at that time period.

The measurement principle of the system is based on a fully coherent two-way signal flow (space -> ground -> space) using low power signals in the Gigahertz-band, which is mostly unaffected environmentally and capable to operate independent from seasonal and local lighting conditions. The signals are structured as a combination of high frequency carriers (X- and S-band), truncated pn-codes (10 and 1 Mcps), and spread spectrum binary data, modulated in two channels (16 bps = time code signal, and 2/4/10 kbps = measurement and housekeeping data).


X-band downlink

carrier = 8489 MHz,
pn-code = 10 Mbps BPSK (bandwidth = 10 MHz),
LowRate data = 16 bps code-biphase mod., 
HighRate data = 2/4/10 kbps mod(2)-add.,
crossed dipole antenna, 1 W transmission power


S-band downlink

carrier = 2248 MHz,
pn-code = 1 Mbps BPSK (bandwidth = 1 MHz),
LowRate data 16 bps code-biphase mod., 
HighRate data 2/4/10 kbps mod(2)-add.,
crossed dipole antenna, 1 W transmission power


X-band uplink

carrier = 7225.296 MHz (= 749/880 X-down), 
pn-code = 10 Mbps BPSK (bandwidth = 10 MHz), 
LowRate data = 16 bpscode-biphase mod.,
[HighRate data = 1 kbps mod(2)-add.],
60 cm asymetric dish antenna, 5 W transmission power

PRARE measurement signal characteristics

The bandwidth-spread PRARE signals serve simultaneously for the primary ranging purposes and for transmission of all necessary data among the system components. Both the user ground station network and the control segment can decode these superimposed data and react according to the information contained. Additionally, a 1-pps (one pulse-per-second) syncronization signal assures system coherency and data time tagging with sufficient precision. The PRARE system is, therefore, fully independent from the host satellite (except power) and easily adaptable to different missions and purposes. The combined PRARE communication/ranging signal is summarized in the following picture.

image

PRARE measurement signal structure

Based on this appropriately designed signal, the precise range and range-rate measurements are generated as follows: derived from the system's very heart, an ultra-stable BVA-quartz oscillator (VCXO, voltage-controlled crystal oscillator), the space segment generates permanently the X- and S-band signals in coherent mode and disseminates them quasi-simultaneously via the dipole antennas down to ground. Any PRARE ground station which is actually in view of the satellite receives the signals, demodulates the pn-codes and downconvertes the carrier with a fixed phase relation (749/880). Up to four preselected ground stations remodulate their uplink signal with one out of four orthogonal pn-codes, and retransmit the X-band signal back to the space segment.

The four independent space segment receiver channels track the selected stations automatically as long as visibility is maintained and carry out the range and range-rate measurements. Ranging data is acquired within a Delay Locked-Loop (DLL) determining the delay between the outgoing and the incoming signal (pn-code correlation method, 91 averaged measurements per second, effective resolution = 1/1000 of code length = 3 cm), range-rate data within a Phase Lock-Loop (PLL) counting the cycles of the difference frequency of outgoing and incoming carriers (1 measurement per second, effective resolution = 1/10 of carrier wavelength = 0.4 mm/s). The high measurement precision is mainly driven by the fully coherent two-way principle of the system, involving only one control oscillator, the high code and carrier frequencies, and the resolution of the receiver counters.

image

PRARE measurement techniques (one receiver channel out of four)

Additional data, so-called correction or secondary data, is gained by:

  • individual ground station measurements of the relative ionospheric delay of the quasi-simultaneously transmitted X- and S-band signals, which is a measure for the Total Electron Content (TEC) of the slant signal path,
  • internal test transponder loops inside the space segment and inside each ground station for periodic determination of the hardware signal delay, and
  • meteorological measurements for computation of the tropospheric refraction index. Those meteo data are acquired at most of the ground station sites by compact external sensors, available readily adapted to the PRARE ground station electronics from the private company TimeTech GmbH, Stuttgart.

All of these data are binary coded following a dedicated multiplexing schedule controlled by the data transfer channels of PRARE. They are automatically collected and sorted by the space segment operational software and stored highly compressed in the space segment's data memory, together with further correction data as operational temperatures and voltages, signal transmission power etc. During visibility of the satellite in Germany, these data are read out of the memory and dumped down the PRARE Control Segment superimposed on the routine tracking signal. This process is initiated and controlled upon telecommand from the Monitoring and System Command Station (MSC) Stuttgart. On ground, these data are forwarded automatically to the PRARE Master Station (MS) Oberpfaffenhofen, where the data preprocessing is located.

Data preprocessing is activated daily, so the collected data from one day are readily available in the morning of the day after. This way, the time delay between actual measurement at the worldwide ground station sites and data availability is very short. Moreover, complete information about the ground stations' performance and local status is available, independent from the station's location and infrastructural accessibility.

As not only the primary measurements' precision is very high, but all the necessary secondary informations are available with a very good precision as well, the PRARE measurement products show a very high overall accuracy. It is equally important that all measurements are collected system-internally reducing any systematic errors probably induced by data conversion or different reference systems. In the following table, the precision of the primary measurements and of the most important corrections is given, followed by the operationally gained system accuracy.


Noise Values

1.5 ... 3.2 cm (X-band ranging, 1 measurement/s, 
corrections due to measurement principle included)

0.05 ... 0.1 mm/s (X-band doppler, 30 s integration time, 
corrections due to measurement principle included)


Bias Values

< 1 cm (X-band ranging, calibration data applied)

< 0.01 mm/s (X-band doppler, calibration data applied)


Correction Values

residual ionospheric error: 1 cm
residual calibration error: 1.5 ... 2 cm
residual tropospheric error: 2 ... 5 cm
residual phase and mass centre error: 1 cm

< 0.01 mm/s for X-band doppler error


Ranging Accuracy (root squared sum): 2.5 ... 6.5 cm


Range-Rate Accuracy (root squared sum): 0.1 mm/s

  PRARE measurement accuracy (two-way data)

From the data in the table, which are - to say it again - not theoretical values, but values gained from daily operational preprocessing including all types of station locations, signal disturbances, or unexpected events, it can be recognized that the system's measurement capabilities are well tuned w.r.t. the typical noise influences and residual biases. Especially the primary range and range-rate measurements show noise values which are of the same order as all other influences, guaranteeing a suitable system performance under all circumstances.

The main reason for the good range-rate measurement precision of around 0.1 mm/s is the high carrier frequency of around 8 GHz. For the range measurements, the precision of around 4.5 cm originates basically from the well-suited measurement principle, the so-called pn-code correlation. The principle of this technique is shown below: the reference pn-code sequence which is generated inside the receiver DLL (bottom) is tuned in time and frequency in such a way that this pn-code comes into perfect correlation with the received one (top). When full correlation is achieved, the green correlation indicator (far right) shows its maximum amplitude.

image

The principle of pn-code correlation
(by courtesy of P.H.Dana, University of Texas, Austin)


Frequency tuning within the DLL is necessary because of the (varying) Doppler-shift of the received signal during correlation, the time tagging tuning is a direct measure for the delay of the received code. Because the received code is the same one which has initially been transmitted down to ground and transponded by the ground station, this measured time tag shift is exactly the value that is to be gained: the two-way signal travel time or two-way range between transmitter (PRARE space segment) and ground station.

Doppler tracking is done simultaneously within the adjacent PLL by bringing the reference carrier (i.e the transmitted signal) and the two-fold doppler-shifted reception carrier (i.e. the signal transponded by the ground station) into correlation. By simply counting the cycles plus fraction of cycles of the resulting difference frequency, the desired range-rate is gained: the variation of the range, which is expressed by these difference cycles, is a measure for the relative velocity of the satellite (SV) w.r.t. the tracked ground station. The following picture produced at the University of Texas is showing this technique nicely.

image

The principle of carrier phase tracking (by courtesy of P.H. Dana, University of Texas, Austin)
Note: for PRARE, the remote receiver is identical to the reference receiver, providing two-way carrier cycles

As PRARE is providing both two-way range and two-way doppler - at the same time, fully synchronously, and with a very high accuracy -, the system is able to fulfill all tasks which are important for precise satellite control (i.e. precise orbit determination), independent from the type and characteristics of the mission which actually is supported. This feature should be considered when designing any future satellite mission which is relying on very precise orbit monitoring.

 

Courtesy of GeoForschungsZentrum Potsdam (GFZ),