Navigation Camera (Navcam)
There are 2 Navigation Cameras (Navcams) on each rover. The Navcams are located on the Pancam Mast Assembly (PMA). The Navcams have the exact same optical design as the EDLcam and the Hazcams. The Navcams are used to acquire images of the terrain and landscape on Mars, which can be used to help in the navigation of the rover.
The Navcams are made of a detector head and an electronics box. The detector head houses a lens assembly and a Charge Coupled Device detector (CCD). The electronics box contains the CCD driver electronics, the 12-bit Analog to Digital Converter (ADC), camera/rover interface electronics, and a heater resistor that will keep the box above the minimum operating temperature of 218 K.
Information in this instrument description is taken from the Mars Exploration Rover Engineering Cameras paper [MAKIETAL2003]. See this paper for more details.
The chief scientific objectives of the Navcams are:
- to support the operation of the rover on the surface of Mars by acquiring images of the terrain and landscape,
- to acquire images for support of auto-navigation, IDD operations, and pointing of the Pancam and Mini-TES,
- to investigate the landing sites at cm/ pixel resolution,
- to acquire images of the terrain that will help identify evidence of water based on morphology of rocks and soil,
- to measure the distribution of rocks and soil around the rover as it moves from site to site,
- to calibrate and validate orbital remote sensing data, and
- to place rock and soil type encountered by the rover in a geological context.
The Navcam has been calibrated over the flight range of temperatures. They were all calibrated with respect to: geometric flat field response, detector dark current, camera absolute responsivity, detector noise performance, and detector gain.
The Navcams use 1024 x 2048-pixel CCD with 12 micron-square pixels and a 100% optical fill factor. The CCDs operate in frame-transfer mode, dividing the detector into two regions. One of the regions is a 1024 x 1024 pixel photosensitive imaging region where the image is recorded. The other region is a 1204 x 1204 shielded storage region where the recorded image is shifted and stored during detector readout. It takes 5.1 msec to transfer data from the imaging region to the storage region, and 5.4 seconds for readout of data from the storage region. Each CCD includes 32 non-imaging pixels in the serial readout registers, which allow the monitoring of the CCD electronics offset and detector noise performance. Both the Navcam and Hazcam CCD pixels have full well capacities of approximately 160,000 electrons and are digitized at 12 bits/pixel. The RMS read noise at cold temperatures (218 K) is approximately 20 electrons, and the detector systems have gain values of approximately 50 e-/DN, which results in a system with approximately .5 DN of RMS read noise.
The absolute CCD quantum efficiency (QE) of the CCDs has been measured between 400 and 1000 nm at four operating temperatures ranging from 218 K and 278 K. The QE of the MER CCDs is typical of a silicon CCD detector, with sensitivity peaking at 700 nm with a QE value of approximately 43%. The SNR of the detector system is essentially Poisson-limited because of its low readout noise and small dark current rates in the Martian operating environments.
The Navcam CCDs use a 'clocked antiblooming' readout technique, instead of having anti-blooming circuitry. The proper choice of autoexposure parameters prevent the blooming effect.
The Navcams also have the capability for onboard image processing. The Navcams can do pixel summation in which they sum the rows and columns of an image, returning the results in a one-dimensional array of 32-bit integers whose length is the image height. These are very useful when the spatial content of an image has relatively low scene entropy and the emphasis is on the radiometry. The flight software (FSW) can also calculate and return the histogram of an image, which can be useful for atmospheric observations. MER images can be spatially downsampled to a user-specified image size using one of a few techniques; nearest neighbor computation of the mean, computation of the mean with outlier rejection, and median averaging. The FSW also allows the option to downlink a subframed region of an image so full resolution can be downlinked at a lower data volume. The FSW is also capable of lossy and lossless compression of MER images. The rovers will use a ICER wavelet image compressor for the lossy and lossless compression.
The Navcams use a combination of three filters (Schott OG590, KG5, and ND1.3) to create a red bandpass filter centered at 650 nm and a FWHM of approximately 140 nm.
The Navcams have f/12 cameras with a 14.67 mm focal length. They have a 45 x 45 degree field of view, with a 67 degree diagonal. They have an angular resolution at the center of the field of view of .82 mrad/pixel. The field depth of the Navcams ranges from .5 meters to infinity. The nominal exposure time for a noontime image on Mars is approximately .25 seconds. This time is 50 times the frame transfer time of 5.1 ms. This ensures that the image signal is significantly than the image smear acquired during the frame transfer.
The Navcams are attached to a titanium bracket which is mounted on the Pancam Mast Elevation Assembly.