# Why was the 100m Green Bank dish needed together with DSN's 70m Goldstone dish to detect Chandrayaan-1 in lunar orbit?

The Phys.org article New NASA radar technique finds lost lunar spacecraft describes the use of radar to relocate two spacecraft that were in orbit around the moon but who's orbit had not been actively tracked for a while. (See also the JPL version.)

"We have been able to detect NASA's Lunar Reconnaissance Orbiter [LRO] and the Indian Space Research Organization's Chandrayaan-1 spacecraft in lunar orbit with ground-based radar," said Marina Brozovic, a radar scientist at JPL and principal investigator for the test project. "Finding LRO was relatively easy, as we were working with the mission's navigators and had precise orbit data where it was located. Finding India's Chandrayaan-1 required a bit more detective work because the last contact with the spacecraft was in August of 2009." (emphasis added)

The article goes on to mention the use of powerful radar signals broadcast by the Deep Space Network's 70m Goldstone dish and received by the Green Bank 100m dish.

Question: Since the deep space network can perform ranging on spacecraft much farther away (tens of thousands of times farther than the moon) by itself, why was it necessary to use a non-colocated, non-DSN dish to receive signals in this case?

Later in the article:

Radar echoes from the spacecraft were obtained seven more times over three months and are in perfect agreement with the new orbital predictions. Some of the follow-up observations were done with the Arecibo Observatory in Puerto Rico, which has the most powerful astronomical radar system on Earth. Arecibo is operated by the National Science Foundation with funding from NASA's Planetary Defense Coordination Office for the radar capability.

...which suggests to me at least that the Arecibo dish could perform the measurement alone, without the need of a second dish.

Edit: In both cases a pseduo-random coded radio signals are broadcast at the satellite. For spacecraft in deep space it is received, amplified, and simultaneously and coherently rebroadcast back, while for radar ranging the return signal is passively reflected back. Here coherently means that the carrier signal for the transmission is carefully phase-locked with the incoming signal's carrier so that even though it is at a different frequency, the doppler shift can be recovered and analyzed much in the same way as in radar.

Due to the $1/r^4$ loss of signal intensity, radar detection of spacecraft can not be used much past a few lunar distances, so for much longer distances the amplification and coherent rebroadcast is required. From a signal processing point of view, delay and doppler information are recovered by correlating the received signal with the transmitted code. However from an operational point of view there may be substantial differences.

above: "Radar imagery acquired of the Chandrayaan-1 spacecraft as it flew over the moon's south pole on July 3, 2016. The imagery was acquired using NASA's 70-meter (230-foot) antenna at the Goldstone Deep Space Communications Complex in California. This is one of four detections of Chandrayaan-1 from that day." Credit: NASA/JPL-Caltech. From here

above: "This computer-generated image depicts the Chandrayaan-1's location at time it was detected by the Goldstone Solar System radar on July 2, 2016. The 120-mile (200-kilometer) wide purple circle represents the width of the Goldstone radar beam at lunar distance. The white box in the upper-right corner of the animation depicts the strength of echo. Inside the radar beam (purple circle), the echo from the spacecraft alternated between being very strong and very weak, as the radar beam scattered from the flat metal surfaces." Credit: NASA/JPL-Caltech. From here

above: Cropped section of the previous figure, with an arrow added to draw attention to "The 120-mile (200-kilometer) wide purple circle represents the width of the Goldstone radar beam at lunar distance." Credit: NASA/JPL-Caltech. From here

above: Cropped section of the previous figure to draw attention to "The white box in the upper-right corner of the animation depicts the strength of echo." Credit: NASA/JPL-Caltech. From here

• original article on the JPL site has a bit more information, but no explanation: jpl.nasa.gov/news/news.php?feature=6769 – Hobbes Mar 10 '17 at 10:11
• @Steve I think you could be on to something there. While a given DSN site has many dishes, there is only one of the large, 70m variety per site. – uhoh Mar 10 '17 at 14:18
• @Steve this is a pretty friendly site - why not post this as a tentative answer? Since it is passive reflection, there is no band shift like there is for normal radio doppler ranging, so the receiver would have to be at the same frequency as the transmission, whereas ranging spacecraft allows for different bands for transmit and receive (at least some of the DSN dishes have band-splitting elements in the beam paths inside dishes 'radio shack'.) So it's beginning to sound quite difficult to do with one DSN dish. – uhoh Mar 10 '17 at 14:28
• So they used Goldstone to bounce microwaves off the satellite and Green Bank was used to detect the signal. My guess is that they used Green Bank simply because it's a larger telescope. This would give them more sensitivity and higher resolution. After all, if they're unsure of whether or not they'd get a detection, they'd want to use the largest telescope possible. And that would be GB. – Phiteros Mar 10 '17 at 22:36
• @Phiteros That makes some sense. Looking at the delay-doppler histogram for Chandrayaan-1, they had quite a nice signal/noise, and in hind sight could have pulled an identification out of a much weaker signal with more analysis (see inset: 140$\sigma$), but as you point out the large diameter provides better resolution which in this case may be necessary to avoid picking up much stronger reflections of any spill-over of the broadcast beam that hit the moon. I think the beauty of this result lies to large extent in the tightly focused beam past the edge of the moon (the purple circle). – uhoh Mar 11 '17 at 2:21

The explanation has to do with the operation of the radar transmitters and the round trip light travel time.

It takes about 3 seconds for a radar pulse to travel from the Earth to the Moon and back. The planetary radar transmitters are high power; the Goldstone transmitter (at full strength) is 500 kW, the Arecibo transmitter is nearly 1000 kW. By contrast, the radar return is quite weak. It is difficult (essentially impossible) to design a system that could simultaneously transmit this much power and receive a weak echo. Only a small amount of the transmitted power would have to leak into the receiver for it to swamp the received echo.

Accordingly, if the configuration is monostatic (i.e., the same antenna transmits and receives), the transmitter has to be switched on and off. Switching the transmitter this rapidly can be damaging, either to the transmitter or to associated components.

By contrast, with a bistatic configuration (i.e., one transmitting antenna and one receiving antenna), the transmitter can be left on and long tracks can be obtained, which also can be valuable to build up signal-to-noise ratio.

• Sounds good, and welcome to Stack Exchange! You've answered "why were separate transmitting and receiving dishes necessary?" or simply "why was bistatic radar used?" but I'm really asking "Why was the 100m Green Bank dish needed together with DSN's 70m Goldstone dish..." so you might add something. DSN has three 70 meter dishes, and also each is surrounded with many smaller dishes that can be combined, whereas Green Bank is an entirely separate organization. See Why does DSN sometimes uses two dishes at the same time to receive Voyager-1?. – uhoh Aug 25 '18 at 7:19
• So I think a complete answer will have to investigate issues of received signal strength as well as diffraction limit since the target is so close to a much much larger object (the Moon!) You might also be interested in Has DSS-43 ever been used in high power mode (>>20 kW) for an emergency situation? – uhoh Aug 25 '18 at 7:21
• Steve's answer and my comment there address the use of a monostatic radar. – Hobbes Aug 25 '18 at 16:00

All the radar systems I've worked with used a single antenna to both transmit and receive. The power of the transmitter is very large compared to the return echo that needs to be received, and the very sensitive receiver needs to be disconnected from the antenna when the radar pulse is generated. The receiver is then connected to the antenna until the next pulse. This is performed by a duplexer, which in many radars consists of a T/R tube.

Switching from transmit to receive operation takes time. This effects the Minimum range of the radar. Perhaps this is why a different antenna/receiver was necessary.

Also, as Phiteros points out in a comment, it could also be the size of the receive antenna was necessary to get the sensitivity needed.

• It's hard to imagine a radar with minimum range extending all the way to the Moon. Radio waves take more than 2 seconds to get there and back, orders of magnitude than any reasonable technology needs. – Lesser Hedgehog Mar 11 '17 at 3:02
• Naval radar systems can use transmitters in the MW range, and still manage to have a minimum range of a few km. So I'd be surprised if the duplexer was the limiting factor. – Hobbes Mar 11 '17 at 8:45
• You, you're right, the time it takes to get an echo is an eternity in this case. – Steve Mar 12 '17 at 22:41

Since the deep space network can perform ranging on spacecraft much farther away (tens of thousands of times farther than the moon) by itself, why was it necessary to use a non-colocated, non-DSN dish to receive signals in this case?

The ranging you're referring to is cooperative radio ranging: The DSN sends a signal to the spacecraft, the spacecraft receives it, and sends it back at maximum gain after a predetermined time. I think signal strength reduces with 2r2 in this case.

Radar ranging, in contrast relies on an echo of the transmitted signal, which is much weaker. Signal strength reduces with r4.