Measuring the gain of a dish

Here I want to show a technique for measuring the gain of a dish that I first learned from an article by Christian Monstein about the Moon’s temperature at a wavelength of 2.77cm. The technique only uses power measurements from an observation of a radio source, at different angles from the boresight. Ideally, the radio source should be strong and point-like. It is also important that the angles at which the power measurements are made are known with good accuracy. This can be achieved either with a good rotator or by letting an astronomical object drift by on a dish that is left stationary.

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Trying to find the DSLWP-B crash site

As you may well know, DSLWP-B, the Chinese lunar orbiting Amateur satellite crashed with the Moon on July 31 as a way to end its mission without leaving debris in orbit. I made a post with my prediction, which showed the impact point southeast of Mare Moscoviense, in the far side of the Moon. Phil Stooke was more precise and located the impact point near the Van Gent crater.

Our plan is to get in contact with the LRO team and try to find the crash site in future LRO images. We are confident that this can be done, since they were able to locate the Beresheet impact site a few months ago. However, to help in the search we need to compute the location of the impact point as accurately as possible, and also come up with some estimate of the error to define a search area where we are likely to find the crash. This post is a detailed account of my calculations.

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Transmitting through QO-100 with the LimeNET Micro and LimeRFE

A couple weeks ago, I did a demo where I showed the LimeRFE radio frequency frontend being used as an HF power amplifier to transmit WSPR in the 10m band. Another demo I wanted to do was to show the LimeNET Micro and LimeRFE as a standalone 2.4GHz transmitter for the QO-100 Amateur radio geostationary satellite.

The LimeNET Micro can be best described as a LimeSDR plus Raspberry Pi, so it can be used as an autonomous transceiver or remotely through an Ethernet network. The LimeRFE has a power amplifier for 2.4GHz. According to the specs, it gives a power of 31dBm, or a bit over 1W. This should be enough to work QO-100 with a typical antenna.

You may have seen the field report article about the QO-100 groundstation I have in my garden. It is based around a LimeSDR Mini and BeagleBone Black single board ARM computer. The groundstation includes a driver amplifier that boosts the LimeSDR to 100mW, and a large power amplifier that gives up to 100W. The LimeSDR Mini and BeagleBone Black give a very similar functionality to the LimeNET Micro, but the LimeNET Micro CPU is more powerful.

The idea for this demo is to replace my QO-100 groundstation by the LimeNET Micro and LimeRFE, maintaining only the antenna, which is a 24dBi WiFi grid parabola, and show how this hardware can be used as a QO-100 groundstation.

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More DSLWP-B lunar surface images identified

In my last posts about DSLWP-B, I have been showing all the images of the lunar surface that were taken by the satellite during the last weeks of the mission, and tried to identify to which area of the Moon each image corresponded. For several of them, I was able to give a good identification using Google Moon, but for many of the latest images I was unable to find an identification, since they show few or none characteristic craters.

Thus, for these images I only gave a rough prediction of which area of the Moon was imaged by using GMAT and the published ephemeris from dslwp_dev. This doesn’t take into account camera pointing, orbit or shutter time errors.

Phil Stooke has become interested in this and he has managed to identify many of the images, even some containing very little detail, which I find impressive. No wonder, Phil is the author of several atlases of space exploration of the Moon and Mars, so he knows a lot of lunar geography.

Phil tells me that he has used Quickmap, which is a very nice tool that I didn’t know of. It is much more powerful than Google Moon. He recommends to switch to an equidistant cylindrical projection and set as a basemap layer the “WAC mosaic (no shadows) map”, which contains images with the sun directly overhead. This resembles the images taken by DSLWP-B better, since these are always taken with the sun at a high elevation, because the camera always points away from the sun. It is interesting to see how the appearance of the surface changes between the “no shadows” and “big shadows” maps.

In this post I show the locations of the images identified by Phil.

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