VSOP Data Reduction | HALCA calibration | GRT calibration
HALCA Calibration
Ground Telescope calibration is described on a separate page.
Last modified: 23 July 1999
System Temperature Measurements
HALCA system temperatures are measured using noise diode on/off power ratios and a pre-launch model for the noise power transmission and response through the observing system. As the noise diode roughly doubles the system temperature, system temperature measurements have been conducted in the orbits immediately before or after the actual observation (i.e., not during the VLBI recording). Experience has shown that the variation from orbit to orbit is small and so using this system temperature data for the observation is a good assumption. There can be some variation within each orbit, of the order of 5%, due mostly to changing thermal conditions near eclipses and a larger earth (nearer the main beam) near perigee. HALCA calibration files are produced by the VSOG for each observation and are available for nearly all General Observing Time experiments observed to the end of 1998. Calibration files for 1999 experiments and earlier Survey Program observations will be available in the near future: the use of the nominal values given below is however a good approximation. If you have any difficulties with the system temperature data, use the nominal HALCA system temperature values: 75K at 1.6 GHz and 95K at 5 GHz.There is a slight difference between channels: at 1.6GHz ch.1 is typically 2K lower than ch.2, and at 5.0 GHz ch.1 is typically 4K lower than ch.2. At 5GHz there is also a change across the frequency band: nominal ch.1 system temperature is 88K near 4820 MHz, 90K near 4870 MHz, 92K near 4950 MHz and 95K near 4990 MHz, with, as mentioned above, ch.2 system temperatures 4 degrees higher. Putting all that in tabular form....
| 1.6 GHz | 75 | 77 |
| ~4820 MHz | 88 | 92 |
| ~4870 MHz | 90 | 94 |
| ~4950 MHz | 92 | 96 |
| ~4990 MHz | 95 | 99 |
HALCA Gain
Unlike ground radio telescopes, which can have significant gain variation with elevation, HALCA has no elevation effects. The analogous effect for HALCA is a gain variation with Sun angle (the angle between the Sun and the radio source being observed as viewed from the satellite). The effects of differential heating on the antenna were expected to be strongest at 22 GHz, however at 1.6 and 5 GHz the effects appear to be negligible. The nominal HALCA gain values are 0.0043 K/Jy at 1.6 GHz and 0.0062 K/Jy at 5 GHz.
Phase Calibration Tones
By default, phase calibration tones every MHz throughout the observing band are continuously switched on for continuum observations and switched off for spectral line observations. The phase calibration signals are, like the noise diode signals used for measuring system temperature, injected through 25 dB couplers into the Low Noise Amplifiers (LNAs). The phase calibration tones are continuously injected into the LNAs at 1 MHz intervals (the sum of the power in these tones is only 0.4% of the system temperature power). The relative phases of these tones are extracted at the tracking stations and/or correlators and are currently being tested as a means of measuring the offsets between the two channels. The phase cal tones may also be useful for bandpass calibration within each channel. Currently, the phase calibration data for HALCA is not yet available for application within AIPS. Users are encouraged to attempt a manual phase calibration derived from a strong time range (as described in the AIPS Cookbook) if the full band width synthesis is required to detect in time ranges with weaker fringes.