The Science of Tracking - Revision history

Paul: /* Angle: ''Relative–phase measurement'' */

Paul — Wed, 07 Feb 2007 02:20:32 GMT

Angle: ''Relative–phase measurement''

←Older revision		Revision as of 02:20, 7 February 2007
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	Tracking antennas have two drive systems which act perpendicular to each other so that an antenna can be pointed to any point in the sky using the right combination of instructions to its two drive motors. The Acquisition Aid (Acq Aid) was a typical example. Its array of eighteen small turnstile antennas was evenly spread over a wire-screen ground plane. The signal outputs from each quadrant of the screen were summed so that they could be compared in specific ways to create the two sets of error signals. Each quadrant sum was split in two and each split was summed again in four pairs to form top-half, bottom-half, left-half, and right-half outputs. The phase difference between the ‘top’ and the ’bottom’ outputs, generates an ‘up-down’ error signal and that between the ‘left’ and ‘right’ outputs generates a ‘sideways’ error signal. The angle measurement technique on the GRARR VHF was essentially the same.		Tracking antennas have two drive systems which act perpendicular to each other so that an antenna can be pointed to any point in the sky using the right combination of instructions to its two drive motors. The Acquisition Aid (Acq Aid) was a typical example. Its array of eighteen small turnstile antennas was evenly spread over a wire-screen ground plane. The signal outputs from each quadrant of the screen were summed so that they could be compared in specific ways to create the two sets of error signals. Each quadrant sum was split in two and each split was summed again in four pairs to form top-half, bottom-half, left-half, and right-half outputs. The phase difference between the ‘top’ and the ’bottom’ outputs, generates an ‘up-down’ error signal and that between the ‘left’ and ‘right’ outputs generates a ‘sideways’ error signal. The angle measurement technique on the GRARR VHF was essentially the same.

-	The Q6, VERLORT, USB and GRARR S-band antennas were all parabolic dish reflectors focussing the received signal via a sub-reflector onto a four-aperture horn in the centre of the dish [14]. Signals from the four apertures generated error signals in the same way, as did the four quadrants of the Acq Aid and the GRARR VHF antennas.	+	The Q6, VERLORT, USB and GRARR S-band antennas were all parabolic dish reflectors focussing the received signal via a sub-reflector onto a four-aperture horn in the centre of the dish [14]. Signals from the four apertures generated error signals in a similar manner to the four quadrants of the Acq Aid and the GRARR VHF antennas.
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Paul: /* Range Rate: ''Phase-rate measurement'' */

Paul — Wed, 03 Jan 2007 07:54:02 GMT

Range Rate: ''Phase-rate measurement''

←Older revision		Revision as of 07:54, 3 January 2007
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	Any movement of the spacecraft towards or away from the ground station will cause the returned carrier signal to show an apparent frequency shift, known as Doppler. The Doppler shift is directly proportional to the rate of change of path length from the transmitter to the spacecraft, i.e. range rate – the direct measurement of the first derivative of range. When the ground transmitter and receiver are at the same location the extracted frequency shift is ± 2D<sub>α</sub> where D<sub>α</sub> is the Doppler shift on the uplink signal.		Any movement of the spacecraft towards or away from the ground station will cause the returned carrier signal to show an apparent frequency shift, known as Doppler. The Doppler shift is directly proportional to the rate of change of path length from the transmitter to the spacecraft, i.e. range rate – the direct measurement of the first derivative of range. When the ground transmitter and receiver are at the same location the extracted frequency shift is ± 2D<sub>α</sub> where D<sub>α</sub> is the Doppler shift on the uplink signal.

-	On both the USB and GRARR systems, digital and tone codes were removed once ranging had been achieved. The range of the spacecraft could then be maintained by integrating the range rate (Doppler) measurement and summing it with the already-measured range value.	+	On both the USB and GRARR systems, digital and tone codes were removed once ranging had been achieved. The range of the spacecraft could then be maintained by integrating the range rate (Doppler) measurement with the already-measured range value.

	[[Image:Triangulation.jpg\|left\|thumbnail\|150px\|Triangulation with three USB stations]]		[[Image:Triangulation.jpg\|left\|thumbnail\|150px\|Triangulation with three USB stations]]

Paul: /* Range Rate: ''Phase-rate measurement'' */

Paul — Tue, 02 Jan 2007 00:21:53 GMT

Range Rate: ''Phase-rate measurement''

←Older revision		Revision as of 00:21, 2 January 2007
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	==Range Rate: ''Phase-rate measurement''==		==Range Rate: ''Phase-rate measurement''==

-	Any movement of the spacecraft towards or away from the ground station will cause the returned carrier signal to show an apparent frequency shift, known as Doppler. The Doppler shift is directly proportional to the rate of change of path length from the transmitter to the spacecraft, i.e. range rate – the direct measurement of the first derivative of range. When the ground transmitter and receiver are at the same location the extracted frequency shift is ± 2D<sup>α</sup> where D is the Doppler shift on the uplink signal.	+	Any movement of the spacecraft towards or away from the ground station will cause the returned carrier signal to show an apparent frequency shift, known as Doppler. The Doppler shift is directly proportional to the rate of change of path length from the transmitter to the spacecraft, i.e. range rate – the direct measurement of the first derivative of range. When the ground transmitter and receiver are at the same location the extracted frequency shift is ± 2D<sub>α</sub> where D<sub>α</sub> is the Doppler shift on the uplink signal.

	On both the USB and GRARR systems, digital and tone codes were removed once ranging had been achieved. The range of the spacecraft could then be maintained by integrating the range rate (Doppler) measurement and summing it with the already-measured range value.		On both the USB and GRARR systems, digital and tone codes were removed once ranging had been achieved. The range of the spacecraft could then be maintained by integrating the range rate (Doppler) measurement and summing it with the already-measured range value.

Paul: /* Range Rate: ''Phase-rate measurement'' */

Paul — Tue, 02 Jan 2007 00:20:21 GMT

Range Rate: ''Phase-rate measurement''

←Older revision		Revision as of 00:20, 2 January 2007
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	==Range Rate: ''Phase-rate measurement''==		==Range Rate: ''Phase-rate measurement''==

-	Any movement of the spacecraft towards or away from the ground station will cause the returned carrier signal to show an apparent frequency shift, known as Doppler. The Doppler shift is directly proportional to the rate of change of path length from the transmitter to the spacecraft, i.e. range rate – the direct measurement of the first derivative of range. When the ground transmitter and receiver are at the same location the extracted frequency shift is ± 2D&supα where D is the Doppler shift on the uplink signal.	+	Any movement of the spacecraft towards or away from the ground station will cause the returned carrier signal to show an apparent frequency shift, known as Doppler. The Doppler shift is directly proportional to the rate of change of path length from the transmitter to the spacecraft, i.e. range rate – the direct measurement of the first derivative of range. When the ground transmitter and receiver are at the same location the extracted frequency shift is ± 2D<sup>α</sup> where D is the Doppler shift on the uplink signal.

	On both the USB and GRARR systems, digital and tone codes were removed once ranging had been achieved. The range of the spacecraft could then be maintained by integrating the range rate (Doppler) measurement and summing it with the already-measured range value.		On both the USB and GRARR systems, digital and tone codes were removed once ranging had been achieved. The range of the spacecraft could then be maintained by integrating the range rate (Doppler) measurement and summing it with the already-measured range value.

Paul: /* Range Rate: ''Phase-rate measurement'' */

Paul — Tue, 02 Jan 2007 00:18:56 GMT

Range Rate: ''Phase-rate measurement''

←Older revision		Revision as of 00:18, 2 January 2007
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	==Range Rate: ''Phase-rate measurement''==		==Range Rate: ''Phase-rate measurement''==

-	Any movement of the spacecraft towards or away from the ground station will cause the returned carrier signal to show an apparent frequency shift, known as Doppler. The Doppler shift is directly proportional to the rate of change of path length from the transmitter to the spacecraft, i.e. range rate – the direct measurement of the first derivative of range. When the ground transmitter and receiver are at the same location the extracted frequency shift is ± 2D where D is the Doppler shift on the uplink signal.	+	Any movement of the spacecraft towards or away from the ground station will cause the returned carrier signal to show an apparent frequency shift, known as Doppler. The Doppler shift is directly proportional to the rate of change of path length from the transmitter to the spacecraft, i.e. range rate – the direct measurement of the first derivative of range. When the ground transmitter and receiver are at the same location the extracted frequency shift is ± 2D&supα where D is the Doppler shift on the uplink signal.

	On both the USB and GRARR systems, digital and tone codes were removed once ranging had been achieved. The range of the spacecraft could then be maintained by integrating the range rate (Doppler) measurement and summing it with the already-measured range value.		On both the USB and GRARR systems, digital and tone codes were removed once ranging had been achieved. The range of the spacecraft could then be maintained by integrating the range rate (Doppler) measurement and summing it with the already-measured range value.

Paul: /* Angle: ''Relative–phase measurement'' */

Paul — Mon, 01 Jan 2007 23:55:29 GMT

Angle: ''Relative–phase measurement''

←Older revision		Revision as of 23:55, 1 January 2007
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	==Angle: ''Relative–phase measurement''==		==Angle: ''Relative–phase measurement''==

-	The third technique relies on relative or interferometer-type, rather than absolute, phase measurement. This technique can be the basis of tracking from a number of long base-line stations but at CRO it was used solely as an angle measurement technique.	+	The third technique relies on relative or interferometer-type, rather than absolute, phase measurement. This technique can be the basis of tracking from a number of long base-line stations. But at CRO it was used as a monopulse technique where every tracking antenna independantly compared four different off-axis beams simultaneously.

	Consider a tracking antenna that is pointing directly at a spacecraft. Each part of the Antenna’s surface will be equally distant from the spacecraft so there will be no apparent phase difference. If the antenna is pointing slightly away from the spacecraft, one edge of the antenna will now be closer and the opposite edge will be further away – a difference in signal path length. This will produce a ‘relative’ phase difference between the signals received by the two sides of the antenna: this difference can be used to generate an error signal that will drive the antenna to restore the phase balance.		Consider a tracking antenna that is pointing directly at a spacecraft. Each part of the Antenna’s surface will be equally distant from the spacecraft so there will be no apparent phase difference. If the antenna is pointing slightly away from the spacecraft, one edge of the antenna will now be closer and the opposite edge will be further away – a difference in signal path length. This will produce a ‘relative’ phase difference between the signals received by the two sides of the antenna: this difference can be used to generate an error signal that will drive the antenna to restore the phase balance.

Paul: /* The Nature of Tracking */

Paul — Mon, 01 Jan 2007 23:42:58 GMT

The Nature of Tracking

←Older revision		Revision as of 23:42, 1 January 2007
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	== The Nature of Tracking ==		== The Nature of Tracking ==

-	A tracking signal [4] is a propagated electromagnetic (radio) wave that travels at the speed of light; nearly 300,000 Km/sec. Several parameters - frequency, wavelength, velocity, amplitude, and directivity - can be determined for such a wave. [5] Measurement of these reveal several ‘particulars’ that are modified as a consequence of the path travelled by the wave: propagation time, phase delay, frequency shift and amplitude.	+	A tracking signal [4] is a propagated electromagnetic (radio) wave that travels at the speed of light; 299,792,458 m/sec. Several parameters - frequency, wavelength, velocity, amplitude, and directivity - can be determined for such a wave. [5] Measurement of these reveal several ‘particulars’ that are modified as a consequence of the path travelled by the wave: propagation time, phase delay, frequency shift and amplitude.

	The amplitude of a signal, apart from telling us whether an antenna is pointed directly at the source of the signal, has no practical use in measuring range. The other three particulars are closely related, e.g. “a measurement of propagation time is actually a direct time measurement of the phase delay of the signal.” [6]		The amplitude of a signal, apart from telling us whether an antenna is pointed directly at the source of the signal, has no practical use in measuring range. The other three particulars are closely related, e.g. “a measurement of propagation time is actually a direct time measurement of the phase delay of the signal.” [6]

Paul: /* ''Continuous-wave USB'' */

Paul — Mon, 01 Jan 2007 23:33:26 GMT

''Continuous-wave USB''

←Older revision		Revision as of 23:33, 1 January 2007
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	===''Continuous-wave USB''===		===''Continuous-wave USB''===

-	The MSFN USB PRN [12] ranging system only ever used digital codes. These consisted of five pseudo-random codes, mostly of prime number length, combined to generate a 5,456,682-bit pseudo-random code transmitted at 100kbs and giving a maximum unambiguous range of 804,650km, nearly three times the distance to the Moon. The USB range resolution was ±1m, however system jitter and ground instabilities downgraded this to about ±15m. The code-stepping correlation technique for the USB ranging codes was essentially the same as for GRARR.	+	The MSFN USB PRN [12] ranging system only ever used digital codes. These consisted of five pseudo-random codes, mostly of prime number length, combined to generate a 5,456,682-bit pseudo-random code transmitted at 100kbs and giving a maximum unambiguous range of 804,650km, nearly three times the distance to the Moon. The USB range resolution was ±1m, however system jitter and ground instabilities downgraded this to about ±15m. The code-stepping correlation technique for the USB ranging codes was essentially the same as for GRARR ARC sub-system.
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Paul: /* ''. . . Continuous-wave USB'' */

Paul — Mon, 01 Jan 2007 23:30:20 GMT

''. . . Continuous-wave USB''

←Older revision		Revision as of 23:30, 1 January 2007
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-	===''. . . Continuous-wave USB''===	+	===''Continuous-wave USB''===

	The MSFN USB PRN [12] ranging system only ever used digital codes. These consisted of five pseudo-random codes, mostly of prime number length, combined to generate a 5,456,682-bit pseudo-random code transmitted at 100kbs and giving a maximum unambiguous range of 804,650km, nearly three times the distance to the Moon. The USB range resolution was ±1m, however system jitter and ground instabilities downgraded this to about ±15m. The code-stepping correlation technique for the USB ranging codes was essentially the same as for GRARR.		The MSFN USB PRN [12] ranging system only ever used digital codes. These consisted of five pseudo-random codes, mostly of prime number length, combined to generate a 5,456,682-bit pseudo-random code transmitted at 100kbs and giving a maximum unambiguous range of 804,650km, nearly three times the distance to the Moon. The USB range resolution was ±1m, however system jitter and ground instabilities downgraded this to about ±15m. The code-stepping correlation technique for the USB ranging codes was essentially the same as for GRARR.

Paul: /* ''. . . Continuous-wave RARR '' */

Paul — Mon, 01 Jan 2007 23:28:16 GMT

''. . . Continuous-wave RARR ''

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-	=== ''. . . Continuous-wave RARR ''===	+	=== ''Continuous-wave RARR ''===

	A GRARR station consisted of a VHF system for near-Earth scientific missions and an S-band system for cis-lunar [10] space.		A GRARR station consisted of a VHF system for near-Earth scientific missions and an S-band system for cis-lunar [10] space.