IRA General Information

IRA is a general-purpose receiver platform for conducting radio astronomy observations of several types:

System Requirements

IRA requires a modern Linux platform to run on, Science Radio Laboratories (http://www.science-radio-labs.com) currently offers software installation “kits” for:

The available compute and memory resources will determine the maximum available observing bandwidth for use with IRA.

A recent install of Gnu Radio is also required—see http://www.gnuradio.org for build and installation instructions. IRA requires access to a source tree, since it makes some minor customizations to Gnu Radio to facilitate IRA performance.

Other Hardware Requirements

IRA requires a USRP1 (http://www.ettus.com) for its radio interface, along with the daughter-card(s) that are appropriate for the operating frequencies. The DBS_RX, for example, allows observing over the 950MHz to 2300Mhz frequency range, which happens to include both the Hydrogen line, and several of the Hydroxyl lines. The TV_RX card allows observing in the 50MHz to 800Mhz region. A high-quality USB cable rated for USB 2.0 use is required between the USRP1 and the computer platform.

In some circumstances an external down-converter can be used, with the IF output from the down-converter feeding a BASIC_RX or LF_RX card. Several modern all-band receivers have wide IF outputs that allow them to be used to down-convert UHF and microwave frequencies to 10.7MHz or some other intermediate frequency that is in range for the BASIC_RX and LF_RX cards (approximately 100Hz to 30Mhz). It is vitally important that the down-converter has a feature that disables Automatic Gain Control (AGC). Without the ability to disable AGC, radio astronomy continuum observations cannot be usefully conducted, since the down-converter/receiver will tend to compress-out any slight variations in received signal power (which is precisely what a continuum observation is all about).

Antenna Requirements

The type and size of antenna required for successful observations is determined largely by the types of observations that are to be performed. Solar observations at UHF frequencies and higher, for example, can usually be conducted using smaller parabolic-dish antennas. For example, at 12GHz, using a standard TVRO Low Noise Block Down-converter Feed-horn (LNBF), successful solar observations can be conducted using a dish of approximately 60cm diameter, at lower frequencies a larger dish is required, since the thermal component of the solar radio noise increases with increasing frequency.

For observing weaker sources, including pulsars, spectral lines, etc, larger antennas are required. For example at 21cm, the Hydrogen Line can be observed using a dish of as little as 1.3M diameter, while sources such as Cygnus A, Casseopia A, Sagittarius A require a dish of at least 2 to 2.5M diameter.

For pulsars at low UHF frequencies (300MHz to 500Mhz), a dish of at least 3.8M diameter is required to successfully observe a handful of the most powerful pulsars.

Observing Site Requirements

Just as optical astronomical observations require clear access to the sky, so do radio observations. Trees, hills, buildings, and other obstructions will completely block most of the radio waves that are arriving from the cosmos. It is important, therefore to choose an observing site that is clear of obstructions, to within at least 20 degrees of the horizon, but less horizon blockage is always better!

Most observations cannot be conducted within large built-up areas such as large cities, due to the overwhelming amount of local-produced Radio Frequency Interference (RFI). It is sometimes possible to conduct observations at the higher microwave frequencies (4GHz and higher) from within a built-up area, but generally over a smaller section of sky, that avoids local terrestrial interference sources.

Your observing site should have stable power, both for the computer, and the USRP. For some types of instruments, such as a Meridian Transit radio telescope, observations may be conducted over very long time periods--having a power failure interrupt such an observing “run” can be very frustrating.

Other things to consider

Front-end Gain and noise figure

It is typically the case that the receiving apparatus (USRP and computer) are located at some distance from the dish itself (indeed, proper RFI hygiene virtually commands that this be so). For this reason, the gain of the low-noise amplifier (LNA) chain out at the antenna needs to be able to adequately compensate for RF cable loss between the antenna and other receiving apparatus. Typically, amplifiers with gain totalling 25dB or more are used in situations with long cable runs.

Since the signals arriving from the cosmos are extremely weak, it is important that the first gain stage in the receiving system (whether an LNA or an LNBF) have a very low noise figure. Noise figures below roughly 0.8dB provide acceptable performance. Sometimes the performance of a low-noise device is quoted in terms of noise temperature. In this case, temperatures below roughly 60K are considered adequate for radio astronomy observations for small observatories. Very often manufacturers will quote a noise temperature that seems impossibly low—they'll quote the lowest temperature they were able to achieve in the lab, rather than the typical or average temperature. If a device manufacturer claims a noise temperature below 25K, it is very likely that “typical” devices will have a noise temperature 8-15K higher.

Gain stability

Apart from the noise issues, and the overall gain issues there is another extremely important issue when considering front-end hardware for a small radio observatory. The overall system stability, in terms of gain or detected signal strength determines the quality and reliability of observations made of the weaker sources. Indeed, the variation in gain of a system can often exceed the detector variation caused by a source! The two factors that dominate gain stability in low-noise amplifiers are power-supply stability, and temperature stability.

Ensure that whatever power-supply is being used for the front-end components (LNAs and line amplifiers) is very, very stable. Use a supply that is larger than is required for the anticipated load, by a factor of three or four. Generally, linear-regulated power supplies, with large filter capacitors are preferred over switching power supplies, which tend to produce unwanted RF noise.

If possible, the front-end electronics should be mounted to a heavy aluminum plate, with the entire assembly, including the feed-horn(s) enclosed in an insulated enclosure. White polystyrene foam of 22mm or greater thickness is an excellent material for this purpose. Polystyrene foam is also quite transparent to most RF, so it can be safely used in front of the feed-horn. Packing the spaces inside the enclosure with loose polystyrene packing material can also help with thermal stability of the front end.

In some circumstances, active thermal control of the entire front-end can be used to achieve excellent system gain stability, at a cost of adding complexity to an already complex system. Both heating and cooling systems can be considered. Heaters, for example, are easier to use, and if actively controlled with a temperature set-point a few degrees above the seasonally-adjusted ambient temperature, they can be very useful. Heating does add a small penalty in noise figure, but if you start out with an already-excellent noise temperature, it will only be degraded very slightly by using a heater to stabilize the LNA temperature above ambient by a few degrees.

Feed-line loss

Feed-lines are lossy, more so at higher frequencies than lower frequencies. Keep the feed-lines as short as is practical, and consider burying them, to improve their temperature stability. It is the case that feed-line loss factors change with temperature, so stabilizing the temperature of your feed-line, by burying it, can be a very wise investment.

With adequate consideration given to weather-proofing, the USRP system can be moved out to the antenna itself, drastically reducing the amount of feed-line (and therefore, loss) that must be used. Systems using this technique must use a USB-2.0 extender. Contact sales@science-radio-labs.com for more information about our recommended USB-2.0 extender, and its limitations.

Lightning Protection

In many parts of the world, lightning is hazard that must be considered when designing an observing system. If RG-6 feed-lines are used, high-quality in-line lightning arrestors are available from nearly-any do-it-yourself TVRO satellite store, and there are a number of such stores that have on-line ordering available. These units are rugged, and can asborb a large number of induced-current “hits” without needing to be replaced. Furthermore, they're usually available for under USD$5.00 each.