DSN Ground Station Configuration and Calibration for Bistatic Radar 70-m NASA Deep Space Network (DSN) stations are preferred as the ground element in bistatic radar (BSR) experiments because they can receive right- and left-circular polarization (RCP and LCP, respectively) at both S- and X- Band simultaneously. 34-m antennas can receive two or more bands (though the choices are not arbitrary) but only in a single polarization. The four Radio Science Receivers (RSRs) at each DSN complex (six at Goldstone, of which any four are selected) capture and record the BSR signals for later processing. The configuration and calibration description which follows ensures the proper end-to-end signal path for each channel (X-RCP, X-LCP, S-RCP, and S- LCP) and provides calibration information in the experiment data stream so that absolute amplitudes of the signals can be determined during the data processing. This document should be sufficient for a scientist to understand the steps and to complete the calibration during the post-experiment data processing; it is NOT expected to be sufficient to carry out the configuration and calibration. Instead, the Network Operations Project Engineer (NOPE) compiles a "briefing message" prior to each experiment; station personnel, working with the NOPE and staff of the Radio Science Systems Group (RSSG), execute the steps in the briefing message. Microwave Configuration: The microwave configuration is specified by a table identified by the NOPE in the briefing message. The X-RCP and X-LCP signal paths are essentially identical; there is no known reason for the downstream scientist to be concerned about the X-Band microwave configuration. For S-Band there are several choices. First, the dichroic reflector is extended so that S-Band signals are reflected to the S-Band feed. Second, S- LCP signals are routed through the "low noise" path to Low Noise Amplifier 2 (LNA-2), and S-RCP signals are routed through the diplexer to LNA-1. For normal S-Band tracking and telemetry support, S-RCP would be routed through the lower noise path; but the choice for BSR brings the system temperatures for S-LCP and S-RCP to more nearly equal levels. In experiments where S-RCP is expected to be unusually weak, the "telemetry" configuration might be preferred; but this would require that the NOPE issue a different briefing message and (possibly) that a new microwave configuration table be created. The microwave configuration is set by the time Step-01 of the pre-calibration (pre-cal) has been completed. Pre-Calibration: Initial End-to-End Configuration Test The initial configuration test allows RSSG personnel to confirm that known antenna feeds are connected to their designated RSRs. The station is configured initially with all channels looking at cold sky and the antenna pointing to zenith. The X-Band ambient load is then switched in and the corresponding RSRs should see approximately 10 dB increases in noise power. It is not possible to distinguish X-RCP from X-LCP at this point, but X-Band is easily distinguished from S-Band. Then the S-LCP ambient load is attached, and finally the S-RCP ambient load is connected. The most common configuration error is that S-LCP and S-RCP are swapped. This part of the configuration test is not recorded. Pre-Calibration: Noise Diode Calibration: With all RSRs looking at ambient loads (AMBs), the RSR attenuators are set using an automated procedure which calculates the signal power and adjusts the attenuators so that the analog-to-digital converter (ADC) amplitudes are approximately -10 dB. The attenuators are not changed during the remainder of the observation. The RSR recordings are started after the attenuators have been reset. Through a series of 14 steps, the output from each channel is recorded (nominally for 5 minutes at each step) with the ambient load only, with the ambient load plus a noise diode (ND) (12.5K nominal value), with the receiver looking at cold sky (SKY) (still pointing to zenith), and with the receiver looking at cold sky plus the noise diode. The steps in the briefing message and the receiver states are shown in the table below; see also Figure 1. Step-01 through Step-04 are part of the initial configuration test above. Step S-LCP X-LCP S-RCP X-RCP ---- ----- ----- ----- ----- 05 AMB AMB AMB AMB 06 AMB AMB AMB AMB+ND 07 AMB SKY AMB SKY+ND 08 AMB SKY AMB SKY 09 AMB SKY+ND AMB SKY 10 AMB AMB+ND AMB AMB 11 AMB AMB AMB AMB 12 AMB SKY AMB SKY 13 AMB SKY AMB+ND SKY 14 AMB SKY SKY+ND SKY 15 AMB SKY SKY SKY 16 AMB+ND SKY SKY SKY 17 SKY+ND SKY SKY SKY 18 SKY SKY SKY SKY Comparing the output power when each receiver is connected to its ambient load with the power when it is looking at cold sky yields the receiver system temperature. Comparing the cold sky plus noise diode power with cold sky alone gives the effective temperature of the noise diode. Comparing the ambient load with the ambient load plus noise diode is an alternate way of finding the effective temperature of the noise diode; but this measurement is better used to estimate the linearity of the receiving system over the dynamic range of interest. The 14 step procedure provides several redundant measurements, but the sequence is optimized in the sense that only the minimum number of states change at each step and there is never a case when the input signal to an RSR is undefined (certain combinations of S-Band switches leave RSRs looking at open circuits). Mini-Calibrations: Once the antenna begins tracking, the noise diodes can be used to provide additional calibrations without requiring use of the ambient loads (and loss of all spacecraft signals and surface echoes). Mini-calibrations (mini-cals) are scheduled before the surface observations begin and after they have been completed. For especially long bistatic radar tracks it may be desirable to include additional mini-calibrations. Beginning with the experiment on 2008/060, ground antenna pointing was changed from "spacecraft" to "planetary" before the first mini-cal and then was switched back to "spacecraft" after completion of the last mini-cal. This means the planet's center is on the 70-m boresight for the mini-cals and the surface observations. For spacecraft in highly elliptical orbits observed at close Earth-planet ranges, a 70-m antenna following the spacecraft can lose several dB of surface echo (e.g., Venus Express at inferior conjunction in August 2007). For Mars Express and Venus Express, the spacecraft slew to/from the BSR attitude requires as much as 36 minutes. The mini-cals were scheduled to fit within the slew intervals, biased toward times when the signal propagating directly from the spacecraft to Earth was weakest. Steps in the mini-cal take 3 minutes each and are shown in the table below. Mini-Cal Step S-LCP X-LCP S-RCP X-RCP ------------- ----- ----- ----- ----- 01 SKY SKY SKY SKY+ND 02 SKY SKY SKY+ND SKY 03 SKY SKY+ND SKY SKY 04 SKY+ND SKY SKY SKY 05 SKY SKY SKY SKY Post-Calibration: The post-calibration is an abbreviated version of the pre-calibration; steps are combined and the nominal time spent at each step is 3, rather than 5, minutes. It is assumed that the post-cal will simply confirm the results of the pre-cal and that experience gained during the pre-cal and mini-cals will permit its more rapid execution. These assumptions are not always valid; crews at the stations rotate, and weather changes can increase or decrease system temperatures by factors of 3 or more. The post-cal contains all of the states visited in the pre-cal, so its results can be substituted if the pre-cal was defective; but the uncertainties in the post-cal values will be slightly larger. The steps in the post-cal are shown in the table below. Step S-LCP X-LCP S-RCP X-RCP ---- ----- ----- ----- ----- 01 AMB AMB AMB AMB 02 AMB AMB AMB AMB+ND 03 AMB AMB AMB+ND AMB 04 AMB AMB+ND AMB AMB 05 AMB+ND AMB AMB AMB 06 SKY+ND SKY SKY SKY 07 SKY SKY+ND SKY SKY 08 SKY SKY SKY+ND SKY 09 SKY SKY SKY SKY+ND 10 SKY SKY SKY SKY Physical Conditions: Before, during, and after the pre-cal station personnel report the ambient load physical temperatures. S1, S2, and X1 correspond to the S-RCP, S-LCP, and combined X-Band ambient loads); the temperatures are given in degrees Celsius. The ambient load temperatures are also reported at the beginning and end of the post-cal. The local temperature, humidity, sky condition, and (sometimes) barometric pressure are reported by the station at the beginning of the pre-calibration and at the end of the post-calibration. Temperature may be given in Celsius or Fahrenheit, relative humidity in percent, sky conditions in a convenient narrative form, and pressure in millibars. Other Calibrations: While the ground antenna is tracking the spacecraft before and after the bistatic radar experiment, it may be possible to measure the carrier level, its frequency, and the stability of both. To interpret these values, it is necessary to know the modulation state and the frequency reference for the spacecraft transmitter; these can sometimes be found in DSN Keyword Files (DKFs), Sequence of Events (SOE) files, or equivalent files. The spacecraft signal needs to be unmodulated and referenced to an on-board oscillator for bistatic radar. The modulation can only be controlled by the command sequence which has been uploaded. The frequency reference should default to the on-board oscillator if there is no uplink; if an uplink exists, it may still switch to the on-board oscillator once the spacecraft antenna slews to the BSR attitude. | | | ----- ANTENNA \ / \ / | | -------------- ----O ----------- | INTERMEDIATE | ---------- \ | MICROWAVE | | FREQUENCY | | RADIO | O---------------->| FRONT |--->| AMPLIFIERS |--->| SCIENCE | ----- | END | | AND | | RECEIVER | ----O | | ----------- | SWITCHES | ---------- / / -------------- \ AMBIENT \ / LOAD / NOISE \ \ DIODE / / | | ----- ----- / / / / / / Figure 1: Simplified schematic of end-to-end receiving system for 70-m DSN stations. The Microwave Front End (LNA) is shown connected to the antenna.