No one who knows the answer would risk the jail time to divulge it. Fortunately, for me, I don't know the answer, but I know enough about other naval weapons to spitball here. This is all open source, available on Wikipedia and other platforms.
Missiles have few possible guidance systems, falling into four broad categories:
Active
Semi-active
Passive
Location
Active seeking is an onboard RADAR transceiver.
Passive seeking relies on the target's own electromagnetic radiation. This category contains heat (infrared) seeking missiles as well as seekers that home on RADAR and radio emissions. It also contains video guided missiles, although those are not historically very common.
Semi-active missiles contain a RADAR receiver, but no transmitter of their own. The transmitter is based on a friendly platform (usually the ship it was launched from), and is typically called a director or illuminator. The missile will either fly along the transmitted beam or the director will illuminate the target and the missile will seek it. The US's Standard Missile series is an example of this type.
Location is either GPS (or equivalent) or Inertial Navigation (gyros and accelerometers). These aren't useful for attacking ships, as they're not stationary, but it can get a long range missile within range to turn on it's seeker.
Capable anti ship missiles use a combination of the above.
An aircraft carrier is always in a strike group and surrounded by other ships.
Viewed from above, a carrier has a large, flat surface that can't be coated with radar absorbant material, due to the extreme wear and tear flight operations put on the flight deck. That makes it an enormous RADAR reflector, especially compared to the surrounding ships, many of which are designed to have reduced radar cross sesctions. While the ocean is also a large reflector, the signal wouldn't be nearly as strong due to the irregularity of the sea surface, especially in rough places like the South China Sea.
American super carriers are nuclear powered, which means they don't have big exhaust stacks, so Infrared seekers are out.
Each type of ship has different radio emissions, based on the types of radars and communications that are onboard. This can be used to discriminate between classes of ships, and can even be used to identify specific ships (ships operate RADARS on slightly different frequencies to prevent interference. Also, each RADAR and radio transmitter has unique irregularities in it's signal which can be analyzed and used to determine its source). With how the physics of electromagnetic radiation in the radio spefctrum works in the atmosphere, a passive receiver can detect and identify a transmitter at twice the RADAR's effective range, so passive detection is an extemely effective way to locate a target. The downside is that it only gives you direction, not range.
A semi-active seeker is extremely unlikely.
GPS satellites would be very juicy targets in a war between the US and China. Both countries have demonstrated the capability to destroy satellites. I am not aware of a China based GPS-like system, and the US controls the GPS system, to the point where, in wartime, the system would shift to encrypted only, making it useless to all non-NATO receivers. But, an Inertial Navigation System (INS) is pretty simple and effective with modern components.
So, to conclude, the likely DF-21D guidance is:
Inertial navigation to get in the area
Passive EM seeking to get near the target and discriminate between targets
Active RADAR seeker in the terminal phase
Sorry if you already know this stuff. To get into any further depth, I'll have to find what else is freely available.
No one who knows the answer would risk the jail time to divulge it. Fortunately, for me, I don't know the answer, but I know enough about other naval weapons to spitball here. This is all open source, available on Wikipedia and other platforms.
Missiles have few possible guidance systems, falling into four broad categories:
Active seeking is an onboard RADAR transceiver.
Passive seeking relies on the target's own electromagnetic radiation. This category contains heat (infrared) seeking missiles as well as seekers that home on RADAR and radio emissions. It also contains video guided missiles, although those are not historically very common.
Semi-active missiles contain a RADAR receiver, but no transmitter of their own. The transmitter is based on a friendly platform (usually the ship it was launched from), and is typically called a director or illuminator. The missile will either fly along the transmitted beam or the director will illuminate the target and the missile will seek it. The US's Standard Missile series is an example of this type.
Location is either GPS (or equivalent) or Inertial Navigation (gyros and accelerometers). These aren't useful for attacking ships, as they're not stationary, but it can get a long range missile within range to turn on it's seeker.
Capable anti ship missiles use a combination of the above.
An aircraft carrier is always in a strike group and surrounded by other ships.
Viewed from above, a carrier has a large, flat surface that can't be coated with radar absorbant material, due to the extreme wear and tear flight operations put on the flight deck. That makes it an enormous RADAR reflector, especially compared to the surrounding ships, many of which are designed to have reduced radar cross sesctions. While the ocean is also a large reflector, the signal wouldn't be nearly as strong due to the irregularity of the sea surface, especially in rough places like the South China Sea.
American super carriers are nuclear powered, which means they don't have big exhaust stacks, so Infrared seekers are out.
Each type of ship has different radio emissions, based on the types of radars and communications that are onboard. This can be used to discriminate between classes of ships, and can even be used to identify specific ships (ships operate RADARS on slightly different frequencies to prevent interference. Also, each RADAR and radio transmitter has unique irregularities in it's signal which can be analyzed and used to determine its source). With how the physics of electromagnetic radiation in the radio spefctrum works in the atmosphere, a passive receiver can detect and identify a transmitter at twice the RADAR's effective range, so passive detection is an extemely effective way to locate a target. The downside is that it only gives you direction, not range.
A semi-active seeker is extremely unlikely.
GPS satellites would be very juicy targets in a war between the US and China. Both countries have demonstrated the capability to destroy satellites. I am not aware of a China based GPS-like system, and the US controls the GPS system, to the point where, in wartime, the system would shift to encrypted only, making it useless to all non-NATO receivers. But, an Inertial Navigation System (INS) is pretty simple and effective with modern components.
So, to conclude, the likely DF-21D guidance is:
Sorry if you already know this stuff. To get into any further depth, I'll have to find what else is freely available.
China has the BeiDou Navigation Satellite System as an alternative to GPS.