RADAR

RADAR#

Planetary radar observations are supported by the JPL radar backend.

Introduction#

The GBT participates in radar observations of near-Earth asteroids and comets, as well as Lunar and planetary mapping and rotation studies. These are done in collaboration with JPL/Goldstone at X-band or C-band}, and formerly with the Arecibo Telescope, which could transmit at 2380 MHz (S-band) or 430 MHz (P-band) before its unfortunate collapse in 2020.

If you wish to do radar studies you should collaborate with scientists at NASA/JPL to plan the experiment. Observing time with a transmitting antenna should be secured independently from a proposal to receive with the GBT. Opportunities for radar observations can arise on short notice, in which case you can make use of DDT proposals if the normal proposal process is not timely enough. Use the NRAO proposal submission tool to submit all proposals, and indicate the proposal is for DDT; these proposals will be reviewed and responded to within a few working days.

Data Acquisition Backends#

Table 24 Radar data acquisition backends#
Backend	Sample rates	Bandwidths
JPL	6.25-400 MHz	0.31-73 MHz
VEGAS baseband modes	100-800 MHz	100-800 MHz

At present, the best choice is the JPL system which can be configured flexibly under computer control for a wide choice of bandwidths and sampling rates. Sample rates and bandwidths are listed in Table 24. For the rest of this chapter, we will explain the usage of the JPL Radar backend. The VEGAS baseband modes will function similarly to the incoherent pulsar modes described in VPM, but consult with your project friend to ensure correct and efficient usage.

GBT Scheduling Blocks#

The following configuration works for the JPL backend. It should be noted that the data recording is not controlled through the GBT user interface AstrID. The SB tracks the object, but you have to run the data acquisition process independently. Consult with your project friend for specific instructions about using the JPL backend data acquisition process.

Here is an example script for 8560 MHz observations.

# Astrid setup script for X-band planetary radar
ResetConfig()
Catalog('/home/astro-util/GBTog/cats/asteroidephemexample.astrid')
obj = '1999JV6'

Xsetup = '''
receiver  = 'Rcvr8_10' # select receiver
obstype   = 'Radar'    # select observing type
backend   = 'Radar'    # select type of backend
nwin      = 1
restfreq  = 8560       # observing frequency
bandwidth = 80         # see note below
swmode    = 'tp_nocal'
swper     = 0.2
tint      = 0.2        # see note below
vframe    = 'topo'     # see note below
vdef      = 'Radio'
noisecal  = 'off'
pol       = 'Circular'
'''

Configure(Xsetup)
Slew(obj)
AutoPeakFocus()
Break('Check peak')

Configure(Xsetup)    # need to configure after the AutoPeakFocus
Slew(obj)
Balance() # adjust power levels
Break('Set Radar Levels')

# when tracking the object, adjust power levels in the back end.
Track(obj, None, 3600) # track object for one hour
Track(obj, None, stopTime='2016-01-09 07:00:00') # Track until UT stop time

The ephemeris file referred to in the Catalog() command, above, gives the coordinates for the object, as described in the next section. The object name, in this case 1999JV6 is defined in the file referred to in the Catalog() command.

The bandwidth is applied before the optical driver step in the signal path, and can take on the values listed in Table 9.2, with the caveat that the final filter going into the JPL backend is 500 MHz wide. The JPL backend itself has an output filter that can be configured to be between 0.31 and 73 MHz wide. The integration time does not have any affect on data acquisition, and can be kept at 0.2. The velocity frame should be kept as topocentric, as doppler shifting is typically done by the transmitting telescope.

Todo

Reference table 9.2 in the observer guide.

Please refer to Configure the GBT system for more information on GBT configurations and SBs.

Tracking moving objects#

Here is an example of an ephemeris file for an asteroid. Please refer Ephemeris Catalogs for a detailed description of the Ephemeris format.

# ephemeris format example for Astrid
FORMAT = EPHEMERIS
VELDEF = VRAD-TOP
COORDMODE = J2000
HEAD = date utc ra dec dra ddec
# 1: soln ref.= JPL#178 
NAME = 1999JV6
2016-Jan-09 04:00 07:15:34.38 -23:41:33.7 -317.1984 1123.6330 
2016-Jan-09 04:01 07:15:34.02 -23:41:15.0 -317.2251 1123.6110 
2016-Jan-09 04:02 07:15:33.67 -23:40:56.3 -317.2518 1123.5900 
2016-Jan-09 04:03 07:15:33.31 -23:40:37.6 -317.2763 1123.5680 
2016-Jan-09 04:04 07:15:32.96 -23:40:18.8 -317.3008 1123.5460 
2016-Jan-09 04:05 07:15:32.60 -23:40:00.1 -317.3231 1123.5250 
2016-Jan-09 04:06 07:15:32.25 -23:39:41.4 -317.3454 1123.5030 
2016-Jan-09 04:07 07:15:31.90 -23:39:22.7 -317.3667 1123.4820 
2016-Jan-09 04:08 07:15:31.54 -23:39:04.0 -317.3868 1123.4610 
# etcetera ... 

Consult section 5.3.5.2 for a description of obtaining ephemeris data from the NASA/JPL Horizons website and converting it for use with AstrID. Here is a brief description of the process:

Access the JPL Horizons web interface: url{http://ssd.jpl.nasa.gov/horizons.cgi}
Set up Horizons web-interface as follows:
- ephemeris type: Observer Table
- target body: [select the object]
- Observer Location: Green Bank (GBT) – click Edit, then type -9 in the search bar and press Enter.
- Time Specifications: [put in desired values]
- Table Settings: QUANTITIES=1,3,20newline (1) Astrometric RA&Dec, (3)rates in RA&Dec, and (20) Range and range rate
Click Generate Ephemeris
Use the web browser file menu to save the output file as (for example) cometfilename.txt
Run the program jpl2astrid cometfilename.txt. A new file with an .astrid extension will be created. An example of such a file is shown in the script above.

The resulting .astrid file is used as an argument to the AstrID Catalog() command.

If you wish to track the velocity, use:

jpl2astrid cometfilename.txt vel

This will put the velocity in the .astrid file. This option is usually not necessary because the relative velocity of the object is compensated by the transmitter, i.e., the transmitted frequency at Arecibo or Goldstone is programmed to result in a constant frequency received at Green Bank.

Note

the coordinate rates, columns 5 and 6 in the above example, as given by the Horizons listing, are:

\(dRA*\cos{D}\)
\(d(DEC)/dt\)

In converting to the .astrid result, jpl2astrid divides the RA rate by cosine(Declination) so that it is the rate in the RA, rather than in \(RA*\cos(Dec)\). The units in both coordinates are arcseconds per hour.

The jpl2astrid program often does not fill in the object’s name correctly. One should edit the NAME in the .astrid file to be something meaningful, and one should make sure the object name in the SB matches that in the ephemeris table.