Receiving LRO and LCROSS

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This page documents my attempts to receive the S-band downlink of LRO and LCROSS using mostly what I already have in stock. This includes using the USRP and GNU Radio as software receiver. It is important to note that we are only talking about detecting the presence of the signal from the spacecraft and not decoding the signal. Decoding would require a much higher G/T than what can be achieved with small antennas.


Contents

System Overview

Functional overview of the receiver system

Budget / BOM
60cm Dish + patch feed 165 €
Photo rack
Various mounting hardware
HEMT LNA 250 €
Bias-T 47 €
Coax cables & conncetors
USRP v1 500 €
DBSRX 110 €
Computer
Software GPL
Total

Link Budget

Complete link budget is available here: File:AMSAT-IARU Link Model Rev2.5.2-LRO-1.ods.

A summary and some notes can be found below.

Parameter LRO LCROSS
Frequency 2271.2 MHz 2248.5 MHz
TX power 37 dBm (5W) 38 dBm (7W)
Antenna gain 0 dBi 22 dBi 0 dBi 10 dBi
EIRP 37 dBm 59 dBm 38 dBm 48 dBm
Distance 380.000 km
Free space loss -211 dB
Atmospheric, pointing and polarisation losses -5 ... -1 dB
Signal at receiver antenna -177 dBm -155 dBm -176 dBm -166 dBm
Receiver G/T 2.6 dB/K
S/N0 22 dBHz 44 dBHz 23 dBHz 33 dBHz
SNR @ 1 MHz BW -37 dB -15 dB -35 dB -15 dB
SNR @ 500 Hz BW -4 dB 18 dB -2 dB 17 dB

Notes:

  • For the S-band, LRO has omni (assumed 0 dBi) and a HGA[1]. The HGA is shared with the Ka-band transmitter and is a 75 cm dish[2] → 22 dBi gain assumed.
  • LRO transmission modes for S-band downlink: 3M40G2D, 4M57G2D, 5M00G1D[3].
  • LRO transmission modes for Ka-band downlink on 25.650 GHz: 57M25G1D, 114M50G1D, 229M00G1D[3].
  • LRO downlink data volume is 461 Gb per day[4] @ 100 Mbps
  • LCROSS transmission modes for S-band downlink: 3M41G2D, 5M00G1D
  • LCROSS Telemetry: Spacecraft communications are provided through two medium gain antennas operating at 1.5 Mbps (nominal), two omnidirectional antennas operating at 40 Kbps (nominal), and a 7-watt S-band radio frequency transponder. [4]
  • SNR @ 1 MHz is for USRP+DBSRX, which has a programmable channel filter that can be as narrow as 1 MHz.
  • SNR @ 100 Hz would be for a standard ham radio receiver in narrow CW mode.


System Noise Temperature

System noise temperature calculation. See complete link budget

I do not have any inline devices between the antenna feed and the LNA. In fact, I will mount the LNA directly on the antenna feed via an adaptor that has a loss of ~0.1 dB.

The LNA is a HEMT-based amplifier with gain 30 dB and typical noise figure 0.5 dB corresponding to an LNA temperature of 35.4 K.

From the LNA I will have a few meters of low loss coax running to the receiver. Between the receiver and the cable coming from the LNA there is a bias-T I will use for injecting DC voltage into the coax to feed the LNA. This has an insertion loss of 1 dB.

The tuner is a wideband receiver covering 800 MHz to 2.4 GHz and has a typical noise figure between 3-5 dB. I used 4 dB which is roughly 400 K.

The total loss between LNA and receiver, including all connectors and bias-T is estimated to 3.2 dB.

The sky temperature is the noise coming from the sky. Empty regions of the sky have a temperature of 2.73 K (the cosmic background radiation), but since I will be pointing my antenna at the moon I will have to include that too.

I will use a 60 cm dish, which has a field of view (FOV) of 15.4°. The size of the Moon is ~0.52° meaning that it covers 3.4% of the antenna FOV. I will assume 250 K for the part of the sky covered by the Moon and 2.73 K for the rest:

T(sky) = 0.034 * 250 + .966 * 2.73 = 11.14K

Of course, this is just an approximation and proper calculation would use integral calculus and take the antenna pattern and temperature variations over the sky into account. For this experiment, the above approximation is good enough. Also note that I assumed that the Sun is not in the field of view of the antenna. If it was, the performance would be degraded by ~2.5 dB (measured value).

The result above is not complete - we should add about 12 K for noise coming from the atmosphere [5]:

T(sky) = 23 K

An additional contribution to the sky temperature that we have to take into account is the terrestrial noise coming into the system, for example via the side lobes of the antenna. I decided to leave that contribution at 0 K for now because I do not expect any interference at the receiving site.

A system noise temperature of 65 K is excellent for amateur equipment. Interestingly, the assembly instructions of the antenna mention that when using a good LNA one can expect a system noise temperature around 90 K that includes about 25 dB "spillover" that I didn't take into account. The 25 K difference is not much considering that the system noise temperature can quickly increase by several hundred degrees if using a lossy coax cable or bad connectors.

Detailed Component Descriptions

To be written.


Antenna

The antenna consists of a 60 cm dish equipped with a patch feed. It is the S-band antenna system by James Miller. The dish and patch have been specially manufactured for satellite operations and has better performance and e.g. standard satellite TV dishes in term of side-lobes. The gain of the antenna is 21 dBic. Comparing with the formula for a parabolic dish, we can see that the aperture efficiency is around 60%, which is very good.

60cmDish.jpg
Specifications
Frequency 2250-2450 MHz
Gain 21 dBic
-3dB beamwidth 16°
-10 dB beamwidth 28°
SWR < 1.2:1
Axial ratio 1.05:1
Polarisation RHCP
Connector N-male [Option: N-female]
Impedance 50 ohm
Overall diameter 590 mm (23")
Weight 1.4 kg (3 lb)

Dish

The dish has 590 mm diameter, 119 mm deep, 1.2 mm thick (18swg). This gives an f/d ratio of 0.31, and is virtually identical to the dish described in Oscar News issue No. 100, Amsat Journal, Amsat-DL Journal, and many others as A 60 cm S Band Dish Antenna. The dish can be ordered from G3RUH. Package deals include the RHCP/LHCP patch feed and mounting kit.

The maximum usable frequency of the dish is TBD.

Feed

The feed can be used as an antenna on its own having a gain of 8.5 dBic. It is suitable for illuminating a dish with f/d ration between 0.3 and 0.5, therefore it works very well as a feed for the 60 cm dish. The feed polarization is LHCP (RHCP as option), so that the antenna becomes RHCP when the patch is mounted as a feed for a dish.

SbandPatchFeed.jpg
PatchPolar.gif
Specifications
Frequency 2250-2450 MHz [Option 2150-2350 MHz]
Gain 8.5 dBic
-3db beamwidth 85°
-10 dB beamwidth 125°
SWR < 1.2:1
Axial ratio 1.05:1
Feed polarisation LHCP [Option: RHCP]
Suitable dish f/d 0.3 to 0.5
Connector N-male [Option: N-female]
Impedance 50 ohm
Overall diameter 120 mm
Depth 17 mm excl. connector
Weight 130 grams

Note that the gain of the patch feed can not be added directly to the gain of the dish. The gain of the dish is determined by its size. The efficiency of the feed – in particular how it illuminates the dish – has influence on the antenna efficiency, which of course has influence on the effective gain of the antenna.

Mast and mount kit

The antenna is not heavy so it can be mounted on a photo rack.

To be written.

Improvised antenna mount kit from the local hardware store.

Low Noise Amplifier

Kuhne KU LNA 222 AH HEMT super low noise amplifier is mounted close to the antenna feed and is used to improve the receiver performance by increasing the figure of merit (G/T).

Expected performance gain TBD.

Kulna222ahhemt.jpg
Specifications
Type KU LNA 222 AH-HEMT
Frequency range 2200 ... 2400 MHz
Noise figure @ 18 °C typ. 0.5 dB @ 2250 MHz
Gain typ. 30 dB
Output IP3 typ. 27 dBm
Supply voltage +9 ... 15 V DC
Current consumption typ. 80 mA
Input connector SMA-female, 50 ohms
Output connector SMA-female, 50 ohms
Case milled aluminium
Dimensions (mm) 73 x 30 x 20
Weight 90 g

The LNA will be mounted directly onto the dish feed.

ExpectedLnaMount.jpg

Bias-T

Kuhne KU BT 271 N 10–3000 MHz bias-T is used to inject DC supply voltage needed by the LNA into the coax cable (thereby save a DC cable from shack to antenna).

Kubt271n.jpg
Specifications
Type KU BT 271 N
Frequency range 10 ... 3000 MHz
Insertion loss

typ. 0.1 dB @ 150 MHz
typ. 0.5 dB @ 1300 MHz
typ. 1.0 dB @ 3000 MHz

Voltage range 0 ... +15 V DC
Current max. 1 A
DC connector DC socket 2.1 mm
Input connector (DC output) N-female, 50 ohms
Output connector N-female, 50 ohms
Case German Silver
Dimensions (mm) 37 x 37 x 30
Weight 90 g


RF Front-end (tuner)

This component is the RF daughter-board that plugs onto the USRP. It converts the high frequency RF signal to I/Q baseband that is is passed to the USRP ADCs. The options for this include the RFX2400 and DBSRX.

RFX2400

Initially, this option was considered; however, since the RFX only covers 2.3 to 2.9 GHz it is not suitable for this experiment. Even if it was possible to go down to 2.25 GHz, the noise figure of this receiver is worse than the DBSRX.

DBSRX

The DBSRX is a 800 MHz to 2.4 GHz receiver with a 3-5 dB noise figure and a software controllable channel filter that can be programmed between 1 MHz and 60 MHz[6].

It contains an MGA82563 wide band LNA followed by a MAX2118 DBS direct conversion tuner chip, followed by an AD818x (TBC) VGA. Note that according to the MAX2118 data sheet, the tuner is specified to work in the 850-2175 MHz range.

  • DBSRX block diagram (incl gains, AGC, dyn range)
  • DBSRX schematics
  • DBSRX PCB
  • MGA82563 data sheet
  • MAX2118 data sheet
  • AD818x data sheet

USRP

Software Receiver

The software receiver is implemented using GNU Radio. Since we only want to detect the signal (but not decode), the signal processing can be very sinmple and consist of some basic filtering, down-sampling and display of the baseband data coming from the USRP.

Wiring

To be written.


Adaptors


Detecting the Signal

Without any detector, we can only observe the spectrum of the passband. How do we know that we are actually receiving the spacecraft and not just some background noise?

There are two possibilities:

  1. Indirect method — Point the antenna towards the moon; we should see increased noise level in the passband. to check whether it is satellite signal or moon-noise, tune to a frequency where the satellite is not transmitting.
  2. Direct method — We can try to demodulate (maybe even decode) one of the sub-carriers.



System Tests

To be written.


OTA Results

To be written.


References

  1. NASA Solicitation: Lunar Reconnaissance Orbiter Mission Communication System, see Talk Page
  2. K-Band TWTA for the NASA Lunar Reconnaissance Orbiter, http://gltrs.grc.nasa.gov/Citations.aspx?id=3385
  3. 3.0 3.1 NASA Long Range EM Spectrum Forecast, October 2007
  4. 4.0 4.1 http://www.scribd.com/doc/16551697/Main-LRO-LCROSS-Presskit2
  5. Ippolito, Radiowave Propagation in Satellite Communications, Figure 7-7
  6. Datasheet for the BasicRX, BasicTX, LFRX, LFTX, TVRX, and DBSRX daughterboards http://www.ettus.com/download