# ODOP
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> Source: https://en.wikipedia.org/wiki/ODOP
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{{Short description|Radar tracking system}}
[[File:Odop.png|thumb|ODOP [Doppler radar](/source/Doppler_radar) tracking system functional block diagram.]]
The '''ODOP''' (Offset DOPpler) radar tracking system is essentially the same as the [UDOP](/source/UDOP) system used for many years at the [Atlantic Missile Range](/source/Atlantic_Missile_Range), but ODOP operates at different frequencies. It is a phase-coherent, multistation [Doppler](/source/Doppler_radar) tracking system which measures the position of a vehicle equipped with the ODOP transponder. ODOP stations are located at and around [Cape Kennedy](/source/Cape_Canaveral). The ODOP transponder is carried in the first stage ([S-IB](/source/S-IB) or [S-IC](/source/S-IC)) of the [Saturn vehicles](/source/Saturn_(rocket_family)) and, therefore, ODOP tracking data is limited to the flight of the first stage only. The ODOP tracking system provides data immediately following lift-off while other tracking systems cannot "see" the vehicle or their accuracy is reduced by [multipath propagation](/source/multipath_propagation) during the early phase of the flight.
The ODOP system is a radar [interferometer](/source/Interferometry) tracking system which determines the position of a vehicle-borne transponder. The ground transmitter radiates a [CW signal](/source/Continuous_wave) of 890 MHz to the transponder in the vehicle. The transponder shifts the received signal in frequency by 70 MHz and retransmits it to the receiving stations (R1, R2, R3). The signal from the transponder received at the ground stations contains a 2-way [Doppler shift](/source/Doppler_effect) fD which is extracted by mixing the received signal (fi = 960 MHz + fD) with the reference frequency (fR = 960 MHz) derived from the transmitter frequency. Actually, a reference frequency of 53.33 MHz is transmitted over a [VHF](/source/Very_high_frequency) link to each transmitter station and then multiplied by a factor of 18, yielding 959.94 MHz. When this frequency is combined with the signal received from the transponder, the Doppler shift is obtained with a 60 kHz bias frequency (60 kHz + fD). The UDOP system used a transmitter frequency of 450 MHz which was doubled in the transponder (900 MHz). The higher frequency in the ODOP system (890 MHz versus 450 MHz) is less affected by the [ionosphere](/source/ionosphere) and the result is increased tracking accuracy.
The Doppler frequencies, fD, (including the bias frequency) from all receiving stations are transmitted to the central station and recorded on [magnetic tape](/source/magnetic_tape). Integration of the Doppler frequency received at a particular station provides the range sum, i.e., the distance transmitter-transponder receiver. At least three range sums (for three different stations) are necessary to compute the position of the vehicle (transponder). The ODOP system uses 20 receiver stations around Cape Kennedy for redundancy and optimum tracking geometry. ODOP tracking data is not available in real time but is obtained from post-flight evaluation.
==ODOP transponder==
thumb|ODOP transponder functional diagram.
The ODOP transponder is a modified version of the transponder used by the [Jet Propulsion Laboratory](/source/Jet_Propulsion_Laboratory) in the [Ranger vehicles](/source/Ranger_program). Separate antennas are used for the receiver and the transmitter. The transponder consists of a double [superheterodyne receiver](/source/superheterodyne_receiver) (890 MHz) and a transmitter (960 MHz). The signal transmitted from the transponder is phase-coherent with the signal received by the transponder. Phase coherence is accomplished by an automatic phase tracking loop. The transponder is completely transistorized.
==ODOP system characteristics==
{|class=wikitable
|+ODOP system characteristics
!colspan=2|Ground Transmitter
|-
| Frequency || 890 MHz
|-
!colspan=2|Transponder
|-
| Receiver frequency || 890 MHz
|-
| Receiver [noise figure](/source/noise_figure) || 14 [dB](/source/Decibel)
|-
| Receiver threshold sensitivity || −132 [dBm](/source/dBm)
|-
| Predetection bandwidth || 100 kHz
|-
| Threshold noise bandwidth || 600 Hz
|-
| Strong signal noise bandwidth || 1400 Hz
|-
| Transmitted frequency || 960 MHz
|-
| Transmitter power || 1 watt
|-
| Required 28 VDC power || 36 watts
|-
| Weight || 10.25 kg (23.0 lb){{Inconsistent|date=October 2024|reason=Verbatim from source, but {{cvt|10.25|kg|lb|disp=x|{{=}}}}; {{cvt|23.0|lb|kg|disp=x|{{=}}}}.}}
|-
| Size || 7370 cm3 (448 in3){{Inconsistent|date=October 2024|reason=Verbatim from source, but 7370 cm³ = 450 in³; 448 in³ = 7341 cm³.}}
|-
!colspan=2|Ground Receiving station
|-
| Frequency || 960 MHz
|}
==References==
*[https://ibiblio.org/apollo/Documents/MSFC-IV-4-401-1-AstrionicsSystemHandbookSaturnLaunchVehicles.pdf#page=217 Astrionics System Handbook], revised ed., NASA MSFC No. IV-4-401-1, NTIS Doc. N70-70002, 1 November 1968, 418 pp. (International Business Machines Corporation working under NASA Contract NAS8-14000).
Category:Radar
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Adapted from the Wikipedia article [ODOP](https://en.wikipedia.org/wiki/ODOP) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/ODOP?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.